fast starting hollow cathode test pallet · flow to the tether, through the power supply, and back...

FAST Starting Hollow Cathode Test Pallet 2006-2007

Meghan Capra – Structures

[email protected]

Matthew Larson – Real-Time Systems

[email protected]

Daniel Strawn – Manufacturing

[email protected]

Steen Vecchi – Thermo/Fluids

[email protected]

Jeffery Wyant – Dynamics

[email protected]

Advised by:

Dr. Binyamin Rubin

Dr. John Williams

4 May 2007

Colorado State University

Department of Mechanical Engineering

Senior Practicum Projects Program

Table of Contents

Summary......................................................................................................................................... 8

Introduction..................................................................................................................................... 9

Hollow cathode basics .............................................................................................................. 12

Problem Statement ........................................................................................................................ 14

Problem Description ..................................................................................................................... 15

Goals ......................................................................................................................................... 15

Objectives ................................................................................................................................. 15

Constraints and Criteria ................................................................................................................ 17

Constraints ................................................................................................................................ 17

Objective-quantitative........................................................................................................... 17

Subjective-qualitative ........................................................................................................... 17

Criteria ...................................................................................................................................... 19

Objective-quantitative........................................................................................................... 19

Subjective-qualitative ........................................................................................................... 20

Preliminary Design ....................................................................................................................... 23

Design Criteria .......................................................................................................................... 23

Design Selection ....................................................................................................................... 25

Mock-Up ................................................................................................................................... 27

Prototype Design....................................................................................................................... 28

Design Flaws and Redesigns of Prototype System................................................................... 31

Design Details ............................................................................................................................... 33

Manifold/Bracket Design and Manufacture.............................................................................. 33

Base Plate.............................................................................................................................. 33

Top Plate ............................................................................................................................... 34

Shell ...................................................................................................................................... 35

Fill and Drain Valve Manifold.............................................................................................. 36

Tank Bracket......................................................................................................................... 37

Tank Connecting Bracket ..................................................................................................... 39

T-Swagelok Bracket.............................................................................................................. 41

Pressure Transducer Manifold .............................................................................................. 41

Solenoid Valve Manifold...................................................................................................... 43

Hollow Cathode Mounting Bracket ...................................................................................... 44

Electronics System.................................................................................................................... 46

Gas Flow Control .................................................................................................................. 46

Hollow Cathode Activation .................................................................................................. 50

Hollow Cathode ........................................................................................................................ 52

Ground Support Equipment (GSE)........................................................................................... 62

Hollow Cathode Test Facility ............................................................................................... 62

Thermal Vacuum/Cycle Facility........................................................................................... 63

Fill/Drain System.................................................................................................................. 64

Thermal Testing ........................................................................................................................ 66

Cathode testing...................................................................................................................... 66

Budget ........................................................................................................................................... 69

Conclusions and Recommendations ............................................................................................. 71

3

Technical References .................................................................................................................... 72

Engineering Drawing Package...................................................................................................... 73

Appendix A................................................................................................................................. 100

Appendix B ................................................................................................................................. 101

Appendix C ................................................................................................................................. 102

Table of Figures Figure 1 - An electrodynamic tether shown de-orbiting a LEO satellite........................................ 9

Figure 2 - Schematic of bare electromagnetic tether experiment ................................................. 10

Figure 3 - Hollow cathode ............................................................................................................ 10

Figure 4 - Simplified hollow cathode schematic .......................................................................... 13

Figure 5 - Flight altitude profile.................................................................................................... 23

Figure 6 - Pressure profile............................................................................................................. 24

Figure 7 - Random vibration profile ............................................................................................. 24

Figure 8 - Circular casing design .................................................................................................. 25

Figure 9 - Rectangular casing design............................................................................................ 26

Figure 10 - Hexagonal casing design............................................................................................ 26

Figure 11 - Initial component layout design ................................................................................. 27

Figure 12 - Initial pallet design..................................................................................................... 27

Figure 13 - Mock-Up of pallet ...................................................................................................... 28

Figure 14 - Prototype design......................................................................................................... 29

Figure 15 - DC-DC Converter ...................................................................................................... 29

Figure 16 - Pressure Transducer ................................................................................................... 30

Figure 17 - Solenoid Valve ........................................................................................................... 30

Figure 18 - Bottom of Base Plate.................................................................................................. 33

Figure 19 - Base Plate ................................................................................................................... 34

Figure 20 - Top Plate .................................................................................................................... 35

Figure 21 - Shell............................................................................................................................ 36

Figure 22 - FEA stress on fill and drain manifold ........................................................................ 37

Figure 23 - Fill and drain bracket ................................................................................................. 37

Figure 24 - Tank bracket............................................................................................................... 38

Figure 25 - Tank Bracket Test Setup ............................................................................................ 39

Figure 26 - Tank connecting bracket ............................................................................................ 40

Figure 27 - Tank and tank connecting bracket.............................................................................. 40

Figure 28 - T-Swagelok Bracket................................................................................................... 41

Figure 29 - Pressure Transducer bracket ...................................................................................... 42

Figure 30 - Pressure transducer with manifold ............................................................................. 43

Figure 31 - Solenoid Valve manifold............................................................................................ 43

Figure 32 - Cathode mounting bracket ......................................................................................... 44

Figure 33 - Cathode mounting bracket clamps ............................................................................. 45

Figure 34 - Cathode assembly static analysis ............................................................................... 46

Figure 35 - Cathode assembly modal analysis.............................................................................. 46

Figure 36 - Microcontroller digital blocks.................................................................................... 47

Figure 37 - Microcontroller analog blocks ................................................................................... 47

Figure 38 - Solenoid valve control schematic............................................................................... 49

Figure 39 - Op amp schematic ...................................................................................................... 50

Figure 40 - Keeper power schematic ............................................................................................ 51

Figure 41 - Keeper power analysis ............................................................................................... 52

Figure 42 - Hollow cathode at operating temperature .................................................................. 52

Figure 43 - LabView program ...................................................................................................... 53

Figure 44 - First cathode test ........................................................................................................ 54

Figure 45 - Cathode with radiation shielding ............................................................................... 54

5

Figure 46 - Pro-E model of cathode and heater coil ..................................................................... 56

Figure 47 - Close up of heater coil................................................................................................ 57

Figure 48 - Modified Pro-E model................................................................................................ 58

Figure 49 - Close up of modified heater coil ................................................................................ 58

Figure 50 - Pro-E displacement results ......................................................................................... 59

Figure 51 - Pro-E stress results ..................................................................................................... 60

Figure 52 - Close up of max stress................................................................................................ 60

Figure 53 - First Pro-E modal result ............................................................................................. 61

Figure 54 - Second Pro-E modal result......................................................................................... 62

Figure 55 - Cathode Test Facility ................................................................................................. 63

Figure 56 - Thermal Vacuum/Cycle Facility ................................................................................ 64

Figure 57 - Fill and drain schematic ............................................................................................. 64

Figure 58 - Fill/Drain System ....................................................................................................... 65

Figure 59 - Thermal model of hollow cathode (scale in degrees F) ............................................. 66

Figure 60 - C-type Thermocouples on orifice plate...................................................................... 67

Figure 61 - LabView program ...................................................................................................... 68

Figure 62 - Pallet Assembly.......................................................................................................... 73

Figure 63 - Fill and Drain bracket................................................................................................. 74

Figure 64 - Pressure Transducer bracket ...................................................................................... 75

Figure 65 - Solenoid Valve manifold............................................................................................ 76

Figure 66 - Swagelok T housing bracket ...................................................................................... 77

Figure 67 – Tank connecting bracket............................................................................................ 78

Figure 68 - Alumina Plate............................................................................................................. 79

Figure 69 - Cathode Mounting Bracket ........................................................................................ 80

Figure 70 - Keeper Back Plate ...................................................................................................... 81

Figure 71 - Keeper Cap................................................................................................................. 82

Figure 72 - Keeper Face Plate....................................................................................................... 83

Figure 73 - Keeper Tube ............................................................................................................... 84

Figure 74 - Shell 1......................................................................................................................... 85

Figure 75 - Shell 2......................................................................................................................... 86

Figure 76 - Top Plate .................................................................................................................... 87

Figure 77 - Base Plate 1 ................................................................................................................ 88







Figure 84 - Base Plate 10 .............................................................................................................. 97



Figure 87 - Raytheon S550 Laser Welder/Driller....................................................................... 103

Figure 88 - Butt Weld ................................................................................................................. 103

Figure 89 - Corner Weld ............................................................................................................. 103

Figure 90 - Fillet Weld................................................................................................................ 104

Figure 91 - Lap Joint Weld ......................................................................................................... 104

6

Figure 92 - Edge Weld................................................................................................................ 104

Figure 93 - Fillet Weld Strength Potential .................................................................................. 104

Figure 94 - Nd: YAG Laser Internal Components...................................................................... 105

Figure 95 - Partially Reflective Lens and Shutter Control ......................................................... 106

Figure 96 - Rear Focus Lens....................................................................................................... 106

Figure 97 - Water to Water Heat Exchanger .............................................................................. 107

Figure 98 - Weld Geometry ........................................................................................................ 108

Figure 99: Bramson Equation ..................................................................................................... 108

Figure 100 - 16 Gage Stainless Steel Sheet Metal Weld ............................................................ 109

Figure 101 - Weld Properties...................................................................................................... 110

Figure 102 - 0.020" Cylinder Laser Welded Butt Joint .............................................................. 111

Figure 103 - Industrial CNC Laser ............................................................................................. 112

7

List of Tables Table 1 - Maximum stress and displacement................................................................................ 60

Table 2 - Maximum Pulse Rate .................................................................................................. 109

Table 3 - Rated Maximum Pulse Energy.................................................................................... 110

8

Summary

The Japanese Aerospace Exploration Agency (JAXA) is conducting a sounding rocket mission to study the collection of ionospheric electrons on a bare tether. The experiment needs a reference to ground the power supply used to bias the tether. This ground allows one to ‘complete’ the circuit and allow current to flow to the tether, through the power supply, and back into the Earth’s ionosphere. The ground connection can be made using a hollow cathode-based plasma bridge formed between the sounding rocket and the space plasma. It is the objective of the Fast Starting Hollow Cathode Test (FAST) Pallet team to build the sub-system that will act as the electrical ground for the sounding rocket. This sub-system will control the gas flow and power to the hollow cathode assembly. The hollow cathode must start within the first two minutes of the mission to allow enough time to conduct the sounding rocket mission, which is estimated to last about 6 minutes.

The design process for the F.A.S.T. pallet began by understanding the key components of a hollow cathode assembly and the gas flow and electronics processes. The hollow cathode needs a heater, keeper, and a porous tungsten insert. The gas feed system needs a fill/drain valve, gas tank, pressure regulator, pressure transducer and two solenoid valves. The activation and control of both systems must be automated and needs a microprocessor. These components were all ordered with a vacuum rating and (for the gas control system) the ability to withstand an internal pressure of 500 psi or higher. Once all the components were received, a circular volume with a diameter of 10 inches was chosen as the external casing dimensions. This diameter is sufficiently small to allow the pallet to fit inside the diameter of an S-310 sounding rocket. A mock-up of all the components was completed and allowed for an understanding of the volumetric constraints the system was to be held to. After a general layout was planned, all of the brackets and manifolds for the different components were designed. A prototype was completed which accurately represented the final deliverable. At which point the ground support equipment design and fabrication effort was implemented (GSE).

Three ground support systems were designed and built to enable testing of the prototype. They included (1) a Fill/Drain system, (2) a Thermal/Cycle Facility, and and (3) a Cathode Test Facility. The Fill/Drain System was used to leak test the pallet, set the pressure regulator, and fill/drain the pallet. The Thermal/Cycle Facility simulated the temperature cycle that the support structure of the sounding rocket payload will induce on the FAST pallet. The Cathode Test Facility included all of the support equipment required to test individual components, sub-systems, and the entire system for thermal properties and overall functionality in a vacuum environment.

The prototype was constantly under test, throughout the build process on individual components, on sub-systems, and with the overall system. These tests included static load tests, thermal testing, startup, and long-term functionality. Dynamic testing may be completed by Marshall Space Flight Center, including but not limited to sine sweeps and random vibrations. The final deliverable has few modifications from the prototype. These include mass reduction on both the top and bottom plate, a laser welded shell and a modified circuit board. At present the hollow cathode can be activated at one minute and forty five seconds, however final testing on the hollow cathode will continue through the next twelve months at which point the final deliverable will be checked again for functionality and delivered to the Japanese prior to the summer of 2009 launch.

9

Introduction A tether experiment is planned by the Japanese Aerospace Exploration Agency (JAXA) on a sounding rocket. The goal is to test the theory of orbit-limited-motion collection of electrons on a positively-biased, bare, conducting tether in the Earth’s ionosphere. Applications that will benefit from the data collected incudle electrodynamic tether systems planned for use in low Earth orbit can be for removal of orbital debris, satellite re-positioning, power generation, science investigations, and related tasks.

Figure 1 - An electrodynamic tether shown de-orbiting a LEO satellite

An electromagnetic tether consists of a long conducting wire deployed from a satellite in a gravity-gradient orientation with the tether either above or below the satellite. When a satellite with a metallic tether orbits around the Earth or another planet with a significant magnetosphere (more specifically, the ionosphere around the Earth), the tether crosses magnetic field lines and a voltage is generated between the ends of the tether as shown in Fig. 1 and Fig. 2. If the electrical circuit is closed, an electrical current will be induced in the tether.

The circuit can be closed through contact with the natural space plasma environment by collecting free electrons from the plasma along the length of a bare tether and expelling them back into plasma at the other end of the tether.

While the positively charged end of a bare tether will readily collect elections from the space plasma, the electron emission at the negatively charged end of the tether is more difficult to achieve in a passive manner. An active electron emitter, such as a hollow cathode, however, can be used for this purpose. The current in the tether will, in turn, interact with the magnetic field and produce a force that will either slow down the orbital motion of the tether and the satellite attached to it or speed up the orbital motion depending upon (1) the relative velocity of the magnetic field relative to the tether/satellite system and (2)

10

the direction of current flow in the tether. Consequently both generator and motor modes of operation are possible.

Figure 2 - Schematic of bare electromagnetic tether experiment

In the generator mode of operation, the kinetic energy of orbital motion is converted into electrical energy that can be used to operate electrical loads on the spacecraft. In the motor mode of operation, a power supply is used to overcome the motion-induced emf and drive a current in a direction opposite to the direction of normally induced current flow. The resulting force will boost the orbit of the spacecraft in a low Earth orbital environment. The unique advantage of this technique compared to other space propulsion systems is that it doesn't require any propellant other than the small amount of gas used by the hollow cathode plasma emitter. A hollow cathode is shown in Figure 3 where it is being used to emit ~40 A of electrons into a low pressure vacuum environment.

Figure 3 - Hollow cathode

As a consequence of passive electron collection and efficient electron emission from a hollow cathode-based plasma contactor, electrodynamic tether systems can provide nearly propellant-less propulsion, dramatically reducing costs of high delta-V space missions, such as formation flying, low-altitude station

11

keeping, orbit raising, and end-of-mission de-orbit operations. Recent electrodynamic tether application studies include periodic re-boosting of the International Space Station, removal of orbital debris from low Earth orbit, and scientific missions to Jupiter to name a few.

Selection of electrodynamic tether systems for difficult missions and the details of the tether design are reliant on the validation of Orbital Motion Limited (OML) theory and how Low Earth Orbit (LEO) plasma conditions and orbital parameters affect the predictions of this theory.

The sounding rocket experiment planned by JAXA requires a plasma device (a plasma contactor) to produce a low impedance connection between the space plasma and the mother payload end of the tether. The electrical connection to the space plasma will allow electrical biases to be applied to the tether enabling the investigation of electron collection on the tether versus bias.

Due to the short time duration of the sounding rocket mission, the plasma contactor will need to become operational within 120 seconds after launch using a minimal amount of electrical power. It is noted that conventional power efficient plasma devices can sometimes require tens of minutes or more to become operational—times that are longer than typical sounding rocket flights of six to ten minutes.

The goal of the plasma contactor development effort described herein is to design, analyze, and fabricate all of the sub-systems and ground-support equipment that are required to quickly start and operate a plasma contactor. In addition, this task includes integration of the sub-systems into a system that is self-contained, autonomous, and straightforward to mate to the sounding rocket payload. Critical supporting sub-systems include gas storage and delivery, power conditioning and delivery, transducers and sensors, and a micro-controller.

Orificed hollow cathodes have been developed for a wide variety of applications, including plasma electron supply for ion and Hall thrusters, plasma contactors for electromagnetic tethers, and for spacecraft charging control. Hollow cathode devices have been used in both orbital and sub-orbital (or sounding rocket) missions. The major difficulty in adapting the hollow cathodes for sounding rocket experiments is the problem of quickly starting the hollow cathode discharge. In contrast to that, orbital experiments provide more time for cathode conditioning, and afford many chances to start and re-start the cathode. Some of the experiments where hollow cathodes were used are described below.

In the Spacecraft Charging Sounding Rocket (SCSR) experiment in January, 1978, a plasma source with a hollow cathode was used on a payload that was flown to an apogee of 257 km [1]. The primary purpose of this experiment was to create charging on the payload by emission of both positive ions and electrons and to determine the relationship between environmental parameters and changes in the vehicle potential during periods of emission. The fast starting problem was solved by placing the plasma/ion source and hollow cathode within a small vacuum chamber that was pumped to low pressures prior to launch. The hollow cathode device was prepared for operation within the vacuum chamber and was pre-heated for a period of ~0.5 hour before the launch. The hollow cathode and ion beam system was turned on 103 s after the launch and was operated over a period of 340s, from 139 km on ascent to 111 km on descent.

In the Artificial Radiation and Aurora at Kerguelen and Soviet Union (ARAKS) experiment (1975) the interactions between charged particles and the Earth’s magnetosphere were studied [2]. Specifically, high-energy electrons (in tightly focused beams) were injected into the space plasma at different pitch angles relative to the local magnetic field direction. Cesium hollow cathodes capable of 10A flux were used for beam neutralization.

In the framework of Space Power Experiment Aboard Rocket (SPEAR) program several sounding rocket experiments were carried out. The purpose of the experiments was to determine the feasibility of exposing high voltage systems to ionospheric plasma. A hollow cathode plasma contactor was used in the experiments to control the rocket potential. In the SPEAR-1 mission (conducted in 1987), the sealed cover of the contactor did not come off at launch and the cathode did not operate [3]. The SPEAR-II

12

mission was terminated because of an attitude control failure, but the SPEAR-III mission (1993) was successful in operating the hollow cathode-based plasma contactor [4].

A hollow cathode plasma contactor was used in Space Experiments with Particle Accelerators (SEPAC) onboard the Shuttle Orbiter Atlantis during the ATLAS-1 mission in 1992 [5]. In the Plasma Motor Generator (PMG) mission, launched to Low-Earth orbit (193 km x 869 km) in 1993, a 500-m electromagnetic tether was deployed and hollow cathodes were used to provide a low impedance bipolar electrical current between a spacecraft and the ionosphere [6]. The Thermal Ion Dynamics Experiment/ Plasma Source Instrument (TIDE/PSI) launched onboard POLAR satellite in 1996 included a hollow cathode plasma source [7]. Other notable hollow cathode plasma contactors include those used for charge control on the International Space Station [8] and on-board the DSCS-III B-7 satellite used by the Air Force to study the efficacy of plasma contactor operation for use in the elimination of differential and net charging of spacecraft in geosynchronous orbit [9].

The Propulsive Small Expendable Deployer System (ProSEDS) experimental mission also included Hollow Cathode Plasma Contactor (HCPC), but was canceled [10]. The HCPC utilized a hollow cathode component that required on-orbit conditioning in order to prepare it for operation (similar to many of the studies mentioned earlier). The total conditioning time was on the order of ten minutes, and the subsequent starting time was on the order of 150 s. Additionally, approximately 70 W of heater power was required between periods when the hollow cathode discharge was shut off. This relatively large energy requirement placed stringent demands on the ProSEDS battery system.

Hollow cathode basics A hollow cathode assembly consists of a cathode tube, a low work function insert, a heater, and a keeper (see Figure 4). An internal (to the hollow cathode) plasma is generated by flowing propellant through the cathode and heating the low work function insert to temperatures where electrons can be thermionically emitted from the cathode insert and nearby surfaces. Specifically, when gas flow is supplied to the cathode and a positive bias is applied to the keeper, electrons can be accelerated from the insert surface. When the electrons collide with neutral atoms, it is possible that an ion will be created. As this process cascades and many electrons are emitted and ions are created, a dense plasma (10

13 cm

-3) is formed

within the hollow cathode. Electrons that are extracted from the interior plasma through the cathode orifice are accelerated again through a sheath located between the interior plasma and the region just downstream of the orifice. A portion of these electrons will ionize neutrals outside the orifice region resulting in the formation of a second plasma region. A simplified schematic of an orificed hollow cathode operation is presented in Figure 4 where the processes described above are shown. The plasma flow field created by the hollow cathode is used to electrically connect the cathode to the ambient space plasma in such a way that electrons can readily flow from the cathode to the space plasma under biases on the order of tens of Volts.

13

Figure 4 - Simplified hollow cathode schematic

Before a hollow cathode discharge can be started, it is necessary to prepare it for operation. This process is called conditioning and can take up to several hours. The purpose of conditioning is the removal of contaminants like water, oxygen, carbon dioxide etc from the insert. This goal is achieved by slowly ramping the cathode temperature, which gradually drives off the contaminants. The slow ramping is necessary because, if cathode is heated too fast, the low work function insert will be poisoned by the outgassing contaminants. It is also important to note that the cathode should not be heated in the presence of these same contaminants since the same undesirable (and irreversible) chemical reactions will occur. As a consequence, conditioning should not be started when the cathode is exposed to air.

After the conditioning is performed, the hollow cathode can be activated. The minimum requirements to activate the cathode are the following: (1) Temperature of approximately 1100 degrees Celsius, (2) Gas flow in the range of 2-10 sccm, and (3) Keeper potential biases in the range of 100 to 200 V.

As mentioned above, the main challenge of using a hollow cathode in a sounding rocket experiment is the problem of quickly starting the hollow cathode discharge. In the following section the approach to the development of a fast-starting hollow cathode test pallet is presented. The status of the system design, fabrication, and testing is also described.

14

Problem Statement The F.A.S.T. Pallet is part of a large experiment headed by the Japanese Space Agency (JAXA) that is being launched on a sounding rocket in order to demonstrate the viability of an electro-dynamic tether system to be used in lieu of a standard propulsion system. This entails deploying a tether based system to create a positive and negative bias between the mother and daughter portions of the rocket within the magnetosphere. The sole responsibility of the F.A.S.T. Pallet is to create a viable plasma field to connect one portion of the rocket to the natural space plasma in 120 seconds after launch in order allow for directional current flow.

15

Problem Description

Goals JAXA has employed CSU engineering students to develop a test pallet that can successfully operate a hollow cathode during a sounding rocket tether experiment. The primary goal of this project is to better understand the characteristics of a hollow cathode and the sub-systems required for activation.

The team goal is to provide JAXA with a test pallet that will successfully activate a hollow cathode that meets their requirements while forming a relationship with an international company. This will also enhance the reputation of CSU in the professional engineering community and provide a hands-on learning experience for engineering students.

Objectives • Consult with JAXA regarding requirements for the test pallet

Rocket selection

Geometry required for the test pallet

Input power provided by rocket for the F.A.S.T. pallet

• Select space flight approved components necessary for the test pallet

• Design layout for all of the components on the test pallet

• Research flight environment and create models to simulate space flight

Consult with JAXA to determine flight details and mission profile

Research and model launch dynamic loadings and shock during rocket separation

Create thermal profile for entire mission

Create power profile for entire mission

• Design and manufacture casing, manifold, and bracketry for components

Design in Pro Engineer and analyze with FEA

• Design electronics to control system

Program a microcontroller to control components of the system

Design keeper power schematic

• Hollow cathode testing and design

Create LabView program to record temperature and current data

Analyze heater power of cathode assembly

16

Analyze and design geometry of cathode

Implementation of radiation shielding

Design and manufacture keeper of cathode assembly

• Design, manufacture, and assemble Ground Support Equipment

Fill and Drain system

Hollow cathode test facility

Thermal vacuum/cycle facility

• Test components and bracketry for flight environment

Create LabView program to record temperature data of components

• Assemble all components and test for functionality and flight environment

Run full-systems test

Static testing

Dynamic testing

• Perform data processing and analysis in order to present results to JAXA and the Mechanical Engineering Management Team

• Design and manufacture shipping container for test pallet shipment

• Deliver final pallet to Japan

17

Constraints and Criteria

Constraints

Objective-quantitative

• Must activate hollow cathode operation within 120 seconds of launch

• Entire mass of unit must be 5 kg or less

• The F.A.S.T. Pallet will be launched on a Sounding Rocket S-310 or S-520

The diameter of the pallet will be limited to the payload capacity of the rocket 26 cm.

• Pallet must be 139.7 mm (5.5 in.) in height or less

• Power consumption from the spacecraft buss must be 100 W and 24V or less at startup of hollow cathode tube and gas purge process

• Pressure transducer requires 10-18 VDC with maximum current of 200 mA

• Input buss requirement for the pallet is 100 W at 24 V

• After 120 sec, the buss power draw will decrease to 60 W

• Withstand 60 sec of 10g’s RMS loading

• Operate within a frequency range of 20-2000 Hz

• Withstand an impact load of 35g’s for 15 milliseconds

Subjective-qualitative • Tank must be installed properly to ensure safety of unit to prevent explosion

• All materials selected for the F.A.S.T. Pallet must outgas to a minimum Colorado State University Vacuum Chamber clean environment

Electrical components

Adhesives must be less than 2% TMC

• Xenon gas leaks must be less then 1*10-4

SCCM

Component design must be designed sufficient enough to not leak under a compressed Helium scenario

Test must be performed to ensure that no gas leaks exist using a helium “sniffing” device

• Must ensure a constant flow rate between 3 and 10 SCCM through the plasma source

• Flow variations must be less than +/-1 SCCM

18

• Total amount of heat dissipated needs to be calculated and tested to provide data to JAXA team to account for F.A.S.T. Pallet teams thermal contribution to the sounding rocket

• Need to determine which component(s) of the sub-assemblies are International Trade Acts Regulation (ITAR) controlled before shipment

19

Criteria

Objective-quantitative

• Develop/build a light weight and rigid case to incorporate components

Case cannot exceed 65% of the total mass

Case must be built such that resonance cannot occur during flight and destroy hardware

Deflections of the case structure must be within 0.5% or less of original position under the worse case simulated dynamic loading of 30G’s

Prototype F.A.S.T. Pallet may be heavier (+5kg) than final design in order to conserve funds (Prototype must be functional as describe in the ‘statement of work’)

• Pallet must survive environment of space flight up to a maximum of 600 km from the surface of the Earth

Vacuum tests need to be conducted to simulate pressure profile that will occur in flight

All sub-systems need to be tested and pass in a vacuum test

All systems will need to withstand a vibration frequency of up to 1 kHz

All systems will need to have a resonance frequency that are not seen for extended periods of time during flight vibrations, if unavoidable, the system will need to be strong enough to withstand resonance vibrations

• Power Consumption may be allowed to drop to ~60 W after initial startup approximately 120 seconds after initial start

• Ensure that the cathode does not initiate the startup process until it is in a vacuum pressure of 0.063 Pa

Critical to the functionality of the Hollow Cathode Tube

Design Real Time (RT) system to activate system at desired pressure

The pressure transducer will be monitored by a microcontroller device which will begin operation of the F.A.S.T. Pallet

Develop a backup sub-system that will act as a fail safe if pressure transducer fails

• The output of the pressure transducer is in voltage of 0-5 VDC

Use of a pull up resistor circuit will be used to prevent a low fault trigger

• Pressure transducer must have less than 5% Full Scale accuracy

• Pressure regulator must have a Full Scale accuracy of 1% or less

• The solenoid valve requires 12-24V and a current range of 42-170mA

20

• All construction materials used must be nonmagnetic

• Structure needs to sustain spin rates of up to 6-10 cycles per second.

• Draw, interpret, and inspect all drawings and parts to ASME Y14.5M standard

• Parts must be free of burs (edge break to a minimum of 0.001”) and clean procured/manufactured in house per following:

Clean with mild detergent (Dawn Dish Soap with cool water and dried using ambient air or equivalent from compressed air source)

Clean Part using Simple Green once cleaned with mild detergent

Finish cleaning process using Acetone and allow to air dry

All processes need to be done wearing latex gloves or equivalent to limit oil transfer to part

• All material used needs to comply to ISO standards for purity and material property

Exact thicknesses, densities, etc will be determined pending material selections

• Material selection for external and internal parts need to allow for a non-operating temperature range of -20

oC to 70

oC

Test will be conducted under a slow bake process and tested throughout process for functionality

• Fatigue test for up to 1000 hours of operating time.

• Impact testing for shipping must sustain a drop of a minimum of 3m and the F.A.S.T. Pallet must remain fully operational

• F.A.S.T. Pallet can be run continuously if securely attached to a 20oC mounting plate

Subjective-qualitative

• Design of the F.A.S.T. Pallet needs to be built and designed to interface with control document (not yet received from JAXA) units to ensure proper mating with JAXA rocket components

Internal components that do not directly connect to the JAXA rocket do not need to be constrained to being built in SI units

• Design sub-systems to be modular to allow for ease of installation and testing

• A fill and drain valve needs to be connected to a manifold attached to the Xenon gas tank to allow for ground support filling and draining of the tank

• Xenon gas flow will be controlled by a latch valve, a pressure regulator, a flow restrictor and a solenoid valve

• A solenoid valve will be placed downstream of the pressure regulator followed by a flow restrictor

• A microcontroller device will be used to control all component operations

21

• Approximately 7+ DC-DC converters will be used to provide power to all components including the hollow cathode device and it’s elements

• The computer program Lab VIEW will be used to measure, test and analyze components and their outputs

• Use of redundant pressure transducers will be used to ensure reliability

• The solenoid valve will be controlled by the microcontroller device inputting either a high or low voltage

• All fasteners need to be secured with either Locktite or pre-loaded washers to limit possible failure caused by vibrations

• Possible implementation of a independent battery source or integration to JAXA sounding rocket provided power buss

• Must construct ground support systems to demonstrate, validate and prepare F.A.S.T. Pallet

Test facilities and Analysis Facilities

Vibration Analysis

Mass/Static Analyses

Thermal Analysis

Pressure Analysis

Electrical Analysis

Fill and Leak tests

Shipping Container

• Pallet will include a ram head engraving located on the top or bottom of the case

• All ordered components must include manufacturer specification sheets

• F.A.S.T. Pallet components cannot adversely affect one another due to heat generation

• The brackets created for the Hollow Cathode will need to dissipate and spread the heat to the base plate in order to keep the average temperature in the optimal range (actual temperature to be determined)

• The interior of the F.A.S.T. Pallet needs to represent as close as possible the emissivity of a black box body (ε = 1); or after experimental data is obtained to determine that radiative heat generated is inconsequential

• Design component structure to allow for a majority of heat transfer to occur through conduction

• Pressure regulation internal to the pipe structure of the pallet will be modeled and analyzed for functionality and safety

22

• Simulate in flight dynamics to ensure proper design

Sine Sweeps to determine Resonance Frequency

Quasi-Static Tests

Random Vibe Test

Acoustic Vibe Test

• Mass/Static Property testing

Create CAD models of Center of Gravity (CG) of overall test pallet in a Wet and Dry Scenario

CG should be within +/-20 mm of the intersection centerlines of the pallet

• Adjust CG using ballast and test configuration of a Dual Axes Test

• Determine required material for ballast(s) based on error present

• CG of the Final Pallet needs to correspond to an accuracy of +/- 5% of predicted CG of computer models

• Safety Testing

Xenon gas pressure must remain below 500psi

Battery components under range of temperatures and pressure changes

• Material Handling in conjunction to EPA and OSHA standards

23

Preliminary Design

Design Criteria To first understand the criteria that must be followed in the design selection, profiles characterizing the sounding rocket mission were developed. The sounding rocket used for this experiment will separate and reach a maximum height of 300 km (as shown in Figure 5). This maximum altitude will be reached approximately 250 seconds after launch.

Figure 5 - Flight altitude profile

A pressure profile of the mission was generated to gain some understanding of the flight environment the pallet will undertake. In order for the hollow cathode to activate safely in vacuum, the ambient pressure must be less than 0.06 Pa, which from Figure 6 occurs at an elevation of approximately 100 km. Using this pressure profile, the mission can be simulated using a vacuum chamber and tests run accordingly.

24

Figure 6 - Pressure profile

Dynamically, there will be accelerations as well as vibrations throughout the rocket due to firing of the boosters. The acceleration due to shock will be 35 G’s for 15 hundredths of a second and spinning of the rocket will produce a 4 G maximum acceleration. The vibrations during launch will last for approximately one minute and are defined by a random vibration profile as shown in Figure 7.

Figure 7 - Random vibration profile

25

The profile starts at a frequency of 20 Hz and a power spectral density of 0.01 g2/Hz and increases to

1000 Hz and 0.1 G2/Hz at a rate of 1.8 dB/Octave. It will then stay at a power spectral density 0.1 g

2/Hz

and increase to 2000 Hz.

The GRMS for this test can be calculated using the area under the profile line.

23

8.1

1 4.62)20)1000

20(1000(

8.13

1.03GA =×−×

+

×=

2

2 1001.)10002000( GA =×−=

Where A1 is the area under the sloped part of the profile and A2 is the area under the flat part of the profile.

2

21 4.162 GAAAREA =+=

GAREAGRMS 7.124.162 ===

Design Selection From initial design concepts, three casing designs were selected as a starting point for concept selection. The first considered design was creating a circular pallet similar to the one shown in Figure 8. This was chosen because the sounding rocket is circular in shape and thus provides the most efficient spatial design. One concern with this is design is the ease of manufacturing. Creating a circular base plate, side wall, and top of the pallet adds more complexity to the manufacturing process. This design also adds more mass to the pallet which is one of the main concerns regarding this project.

Figure 8 - Circular casing design

To help solve the concerns with this design, a rectangular casing design was developed as shown in Figure 9. Although this design does create a much more easily manufactured product, some major concerns were discovered. Placing a rectangular pallet in a circular sounding rocket requires a significant

26

decrease in space available within the pallet. With the number components needed within the pallet, this design was abandoned.

Figure 9 - Rectangular casing design

Another concept was a hexagonal pallet as shown in Figure 10. This design allowed for more space within the pallet than the rectangular design and would possibly weigh less that the circular design. A major concern was that the hollow cathode needs to create a strong contact with the natural space plasma outside and with this design that would be difficult with the cathode having to be placed more towards the center of pallet. This idea also applies to the rectangular design. Since this concept is the most important to the success of the sounding rocket mission, the circular casing design was selected for further investigation.

Figure 10 - Hexagonal casing design

Once the design was selected, it was necessary to determine the components required for the pallet and possible placements. Figure 11 shows the components required in order to successfully operate the hollow cathode. The biggest component shown is the tank, which will take up the most space on the pallet and will hold the Xenon gas. The fill and drain valve is needed to be able to fill and drain the tank with Xenon gas. The manifold houses the solenoid valve which controls the flow of the gas to the rest of the system. The pressure transducer is also in the manifold which reads the pressure of the pallet.

27

Finally, the regulator regulates the rate of flow of gas going to the hollow cathode assembly. Some space is left for the electronics to control the different components and also the control box and bus connectors to obtain power from the sounding rocket. Figure 12 shows the components within the circular pallet with the top plate and outside shell included.

Figure 11 - Initial component layout design

Figure 12 - Initial pallet design

Mock-Up Once the components were selected and obtained, a mock-up was built to help determine the component layout and spatial constraints. A ten inch plastic base was used to simulate the base of the pallet. As shown in Figure 13, the tank takes up a large percentage of space in the pallet. This will prove to be a

28

difficult aspect of the project. DC-DC converters are required to power different components of the project and are considerably large in size. The regulator was also bigger than initially planned.

Figure 13 - Mock-Up of pallet

From this mock-up, it was determined that manifolds and brackets for the different components must be very carefully designed in order to minimize the amount of space they take up. Tubing for the gas flow and the wiring to the components also needs to be considered in the layout of the parts.

Prototype Design Upon completion of the mockup, a prototype design was constructed using Pro/Engineer (seen inError! Reference source not found.). Each of the individual components, brackets, and manifolds were all constructed in Pro/Engineer prior to the creation of the prototype design. This allowed for multiple component layout iterations to be performed, and only required minimal changes to the parts designed exclusively by the F.A.S.T. Pallet team. The Computer Aided Model (CAM) prototype design contains all of the necessary components with the addition of the tubing used for the gas delivery system.

29

Figure 14 - Prototype design

While working in parallel on the electronic control portion of the project, it was determined that five DC-DC converters were required to provide power to the components, including the hollow cathode. The necessity of the additional DC-DC converters posed a unique problem to the team for the reason that these converters are rather large and could no longer be spread out around the base of the pallet. For spatial purposes these converters are stacked on top of each other. This was done using standoffs place between the DC-DC converters to provide ample room for the electronic connections to be made and not interfere with the converter directly above. Included are two pressure transducers, one to monitor the pressure of the ambient air and one to obtain the pressure of the gas within the tank.

Figure 15 - DC-DC Converter

30

Figure 16 - Pressure Transducer

The overall gas flow system of the prototype design is rather unique, in terms of a typical gas flow system for more common hollow cathode assemblies. In order to meet the objective of the overall experiment, gas is used as a multi-purpose tool. First, it provides the necessary component to produce plasma. Additionally, and unique to the experiment, it uses the gas to purge the system. This in turn allows for the hollow cathode to be heated faster, and is also used to flush out contaminants from within the cathode. As discussed previously it is imperative to the health of the hollow cathode that the heater does not causes oxidation to occur on the interior components of the hollow cathode. Oxidation will occur if the cathode’s temperature reaches temperatures over 200 C while at atmospheric pressure. In order to develop a system that would provide to different levels of gas flow to go through the hollow cathode, two solenoid valves were placed on the pallet, one to purge the hollow cathode before activation and the other to control the main flow to the hollow cathode. In order to route the gas from the gas tank it was determined that four Swagelok T’s were required for the main flow and purging processes.

Figure 17 - Solenoid Valve

31

The structural integrity of the components is crucial to the success of this experiment. In order to ensure that each component is secure, brackets and manifolds were created. Each manifold and/or bracket for the components was individually designed, analyzed and tested using Finite Element Analysis in Pro/Engineer before under going real world test. The real world tests included the individual testing of the component for functionality and then integrated in to the system as a whole where it was tested to operate as designed in conjunction to the other parts. In order to maintain the weight constraint and ease of manufacture 6061-T6 aluminum was used throughout the pallet on parts that were produced uniquely to the F.A.S.T. Pallet project.

A preliminary tubing design was developed on Pro/Engineer Piping. The tubing used is 1/16” diameter stainless steel. In order to ensure that the tubing bends could be manufactured a 0.25” radius of curvature was used for all tubing design. With this preliminary knowledge in hand the tubing design was laid out using the CAM model. This design provided the basis for manufacture and allowed for all pipes to be double checked on their clearance relative to their surroundings.

To encapsulate the entire pallet, two base plates were designed out of 6061-T6 aluminum. These plates provided a structural base to secure parts, and were designed with a 0.030” groove in order to provide some structural support to a thin surrounding shell that is used to protect the components internal to the pallet.

Design Flaws and Redesigns of Prototype System

The prototype contains numerous flaws; most of the drawbacks within the prototype were caused by the necessity to produce a product rapidly. For instance, many of our components are not designed specifically for the pallet, but for a much more broad purpose. The regulator for example is extremely massive and much more capable than the pallet actually needs. The functionality of the prototype caused the team to proceed using components that were proven to be effective for the final design. With additional financial resources and more time to re-design for a final product individual components could have been re-selected and implemented. This follows the design paradox that as time progresses the knowledge of the project increases, but the design freedom decreases.

The final deliverable contains changes that were able to be implemented without affecting the functionality of the pallet’s primary objective of activating the hollow cathode. The base plate was engineered to be of a lowered mass while maintaining the structural integrity to survive the flight dynamics. This redesign was implemented around the existing features of the prototype base; this allowed the same machine programs for the top portion of the final base plate to be reused. The base plate is a more efficient use of mass because of this design change.

The shell of the pallet for the final design contains a stronger, cleaner weld. The prototype weld was produced by hand, and in doing so produced a structurally detrimental heat affected zone (HAZ). The final design of the shell incorporates a laser weld. In doing this the material properties of the shell remain almost constant throughout the entire shell, and accounts for much greater accuracy in comparing the computer analysis to the actual structural tests of the shell. The overall design of the shell remained unchanged, only the manufacturing process was changed.

The circuit board design for the final deliverable contains a variety of improvements. The board itself is of a higher space grade quality. This raises the resonance frequency of the bolts that hold the circuit board in place above the tank within the pallet. It is a small difference, but the change provides an additional safety factor to the stability of the circuit board during flight. For the actual circuitry, the most significant change is in the location of the voltage regulators on the circuit board. The original orientation of the regulators relies of thermal radiation to cool the regulators; the new design flips the regulators

32

downward and places them on to a conducting copper plate to maximize the conduction of heat out of these components. All final deliverable circuit board components are held to a higher space flight quality.

After analyzing the resonance frequencies within the pallet, there was a potential failure of the top plate and base plate separating from the shell. For the prototype this did not cause great concern; the overall goal of the prototype was for the hollow cathode functionally, not as much for the flight dynamics. The final design will be potted (space grade epoxy) around both edges of the shell. This will provide adequate strength to the rim of the pallet to prevent separation of the exterior components.

33

Design Details

Manifold/Bracket Design and Manufacture

Base Plate The base plate was the starting point for the entire F.A.S.T Pallet. Everything is mated in some way or fashion to the base plate. The plate was selected to be produced out of 6061-T6 Aluminum, and was chosen to be 3/8” thick. The components, brackets, and manifolds discussed throughout this paper all require screws. This resulted in certain screws to be countersunk into the base plate; due to the size of such countersinks the plate was required to be a minimum of 3/8” thick in order to prevent the parts from being ripped out during flight. With the overall external dimensions and thickness pre-determined, the features unique to the base plate were continuously designed as all the parts were designed.

The base plate started out as a circular disc, and as new brackets, manifolds, or even components were added a resulting feature was designed in to the base plate. All of these features helped define the principle characteristics of the plate. The leftover, unused areas of the base plate were then just additional mass to the part. The base plate being the most massive contributor to the mass of the pallet needed to be reduced in weight. With additional superficial pockets placed in the base plate, the mass was reduced significantly. The base plate can be shown below in Figure 19, but it is important to note that all the other components were designed first before the shape of the base plate took place.

Figure 18 - Bottom of Base Plate

34

Figure 19 - Base Plate

One of the unique features to the base plate was the necessity for an extremely thin groove (0.030”) to be placed near the out edge of the plate. This would provide the support and guiding surfaces for the shell that is to encompass the entire pallet. With verification using Finite Element Analysis on Pro/Engineer Mechanica and all of the above information the base plate was finalized in design and manufactured using Computer Numerical Controlled (CNC) machine.

Top Plate The top plate of the pallet is very similar to the base plate, but does not need to be as thick as no screws are directly bolted to the top plate. The top plate requires a 0.030” groove just as the base plate required to hold the shell of the pallet. The top plate was proofed using similar FEA analysis and produced using a CNC machine, as shown in the Figure 20.

35

Figure 20 - Top Plate

Shell The shell’s primary goal is to provide protection to the internal components of the pallet. It is comprised of stainless steel that is approximately 0.020” thick. The shell provides minimal structural support to the entire pallet; the openings for the hollow cathode and for the fill and drain manifold diminish the potential strength of the shell as a structural member. The most unique feature of the shell is the rather obscure technique required to fasten the shell in to a uniform piece. The shell, being so thin of material, is fastened using a laser welder that is explained in further detail in Appendix B. Upon the final design of the shell, FEA analysis was performed to determine to structural support potential to the compress forces that will be exerted on the pallet during flight. This analysis determined that additional internal supports are necessary to help support the structure of the pallet. In order to minimize the required size of these supports they were incorporated in to the structure for the electronic components. The final shell design can be seen in Figure 21

36

.

Figure 21 - Shell

Fill and Drain Valve Manifold As stated earlier, the fill and drain is the first component in the gas flow process. The valve is composed of a Schrader valve that allows for filling and draining of the Xenon tank. The Schrader valve is not the typical valve found commonly on most tires, but is rather more complex in order to guarantee to no leaks. The valve consists of the typical Schrader portion with a spring load pin valve, but also includes a threaded conical valve portion that makes the more secure seal. This works by force a conical portion of the valve in to a mating port that is clamped together using a threaded locking system. Upon the decision to use this valve as the entry point of the gas in to the entire pallet a manifold needed to be created in order to mate the valve to a 1/16” NPT Swagelok fitting.

The manifold had to allow for a proper seal with the face surface of the Schrader valve assembly, and needed to accommodate the Swagelok fitting. With this basic set of criteria the manifold was designed. The structure of the manifold was minimized to the smallest size possible in order to maintain a low mass, yet contain all the components. The manifold is secured to the pallet base using for 6-32 screws in order to break breakout of the manifold during flight. The external dimensions were chosen to accommodate these screws, but the rounded top portion of the manifold was designed as such to minimize its mass.

In conjunction with the 3D solid model analysis was performed to ensure that the manifold would survive the in-flight conditions. The tank needs to be filled to a pressure of 500psi which in turn means that the manifold must be capable of withstanding these forces on its internal surfaces. From here the manifold was loaded additionally under a 35g load. This Finite Element Analyses (FEA) was all performed using Pro/Engineer Mechanica. The results of the FEA can viewed in Figure 22.

37

Figure 22 - FEA stress on fill and drain manifold

When the part survived the virtual simulations it was manufactured as shown in Figure 23.

Figure 23 - Fill and drain bracket

Tank Bracket The tank bracket is the largest bracket on the entire pallet. This in turn is due to the tank being the most massive and bulky component to the entire system and therefore is one of the largest concerns. Upon completion of the calculations of the expected force the tank will exert under flight conditions the bracket was designed to hold the tank from moving and colliding with other components. The brackets primary objective is to hold the tank down and prevent it from reaching a detrimental resonance frequency. The secondary objective is to prevent the tank from moving along its center axis. As with all of the components designed, maintaining a minimal mass is crucial as well.

38

Using the above criteria the bracket was designed as shown in Figure 24. The approach discussed in designing the fill and drain manifold was used in the design of the tank bracket. The part was first modeled and analyzed in Pro/Engineer, and upon successful simulation the part was procured.

The features of the tank bracket are rather unique. The clamping force is provided by compressing a C-shape clamp at the top of the bracket. This design causes a lot of localized stress to occur at or near this point of loading. The stresses were minimized using large radii of curvature to re-direct the stress to the bulk of the bracket. This in turn causes a low level hoop stress that directs the stress to the large base portion of the bracket. The stress concentrations are then minimized in the base portion of the bracket. Again, any point of possible localized stress was minimized using large radii of curvature.

Figure 24 - Tank bracket

After designing the bracket to withstand the clamping stress imposed upon it the design process then required to ensure that the bracket would survive flight environments, as well as not to rip-out of the base plate. The part was subjected to 35g loads in a multitude of directions to ensure that the bracket would survive. Actual rip-out tests were performed to become confident that the ¼”-20 screws were strong enough to maintain a rigid connection to the base plate. As well as equivalent static load scenarios. These tests all used a mock-up of the actual tank in order to obtain a more rigid structure, as well as not damaging an actual tank as shown in Figure 25 (this was necessary to not ruin a real tank due to budget constraints).

39

Figure 25 - Tank Bracket Test Setup

Tank Connecting Bracket The tank connecting bracket was crucial in maintaining a consistent gas flow throughout the system. This bracket connected the nozzle of the tank to the 1/8”-27 NPT for the tubing of the pallet. Any leak between these two components would end in failure of the pallet. It was crucial that the bracket be secured tightly to the base plate to avoid any vibrations that could end up in bending of the pipe. An O-Ring was also placed on the bracket to ensure a tight seal and prevent leaking. The O-Ring relief was designed to provide an optimal thirty percent compression of the O-Ring to the tank. This was done through reverse engineering of a product produced by the same manufacturers of the actual tank.

Using the general goals for all of the parts designed the bracket was created and analyzed under the same conditions as all of the parts previously discussed. Once it was determined the design was ready for manufacturing, the bracket was machined and can be seen in Figure 26Error! Reference source not found..

40

Figure 26 - Tank connecting bracket

The bracket had to be externally threaded in order to screw the tank into the bracket. This portion of the part and the part as a whole is a very complex part to manufacture. The entire part is one uniform piece to provided the largest possible strength (any additional joints would weaken the part). In order to cut the external threads necessary to mate the bracket to the tank, a fairly complicated function was performed using a three axis CNC machine. This is known as thread milling, and requires the three axes (X, Y, and Z) to operate in conjunction together. The table moves in a circular path as the Z axis lowers down at the same pitch of the threads in order to create the part. This unique machining capability allowed the F.A.S.T Pallet team to produce this part with much greater ease, and additionally made it possible to produce spare brackets to perform the physical test on.

The bracket contains two ¼”-20 UNC threaded holes on the bottom to connect to the base plate. After completion, the seal between the bracket and the tank was tested to ensure the connection was air-tight. A static load test was also performed with the tank connecting bracket and the main tank bracket connected to the tank to demonstrate the three components together could withstand a 35G static load.

Figure 27 - Tank and tank connecting bracket

41

T-Swagelok Bracket The brackets for the T-Swageloks are a small part of the test pallet brackets that are required. Upon the initial calculations to determine the resonance frequencies within the pallet, it was determined that the T-Swageloks are massive enough to produce detrimental forces upon resonating that a bracket would be required to contain these components.

With this in a mind a bracket was designed at a minimal weight to contain the movement of the T-Swageloks. The brackets were designed in order not to overcome large stresses, but rather to keep the vibrations that are experienced during flight to a minimum such that they will not affect the tubing. If adverse conditions result in the tubing failure a catastrophic failure to the mission could occur. The brackets themselves are extremely light weight, but provide well over a safety facture of three for the stress they will be subjected to. The actual manufactured brackets can be seen in Figure 28.

Figure 28 - T-Swagelok Bracket

Pressure Transducer Manifold The pressure transducer manifold was designed very uniquely with the dynamic scenarios the pallet will undergo. The pressure transducers are rather large components of the pallet, but more concerning is the height to mass ratio of the actual part. This posed a unique problem to this part. The part had to be secured in such fashion to avoid detrimental resonance frequencies occurring during the flight, but as is the case with all manifolds needed to allow for the fitting of a 1/8”-27 NPT Swagelok fitting to connect to the piping system. It is important to note that only one pressure transducer is connected to the pipe system, the other is used to monitor the ambient pressure of the surrounding environment.

With the overall goals of the manifold set, the design was created to give the team confidence of its success during flight. The final design shown below in Figure 29, is rather bulky, and after completion of the FEA analysis maintains a safety factor well beyond what the manifold will experience during flight.

42

Figure 29 - Pressure Transducer bracket

The decision behind this design came from a variety of factors. The most important was the ease at which this part could be manufactured. This part became crucial to critical tests being performed early on and was needed rapidly. In conjunction the ease of manufacturing, calculations were performed on the manifold to determine how much mass was gained on the part by producing it in this fashion. With a more complicated design the mass savings could not be justified versus the ease of manufacturing. The net savings was approximately 18g per part; the size constraints of the two intersecting holes did not allow for sufficient savings to justify the additional complexity of the design. The part is to be secured using four 6-32 UNC screws to the base plate. The part was produced and passed all tests to date.

43

Figure 30 - Pressure transducer with manifold

Solenoid Valve Manifold

The solenoid manifold is one of the most crucial of all the manifolds. This small manifold is required to control the flow throughout the entire system. The actual size of the manifold is extremely small, and requires extreme precision to mate to a manifold for the flow system. The manifold requires two sets of intersecting holes placed at a precise distance apart in order to control the gas. The solenoid valve requires approximately 1W of power to operate, and according to the manufacture only guaranteed to operate below 40°C. This thermal contribution added to the complexity of the solenoid manifold.

Upon the gathering of constraints for this manifold a variety of decisions were created. The shapes ranged from squares, rectangle, circles, and finally the hexagonal shape shown. This shape was chosen above of all the others based purely on two factors: ease of installation and mass. The hexagonal shape is much lighter than even a circular shape because it utilizes on the necessary area and the rest of the material is not present. The hexagonal shape also allows for the part to be secured to the base plate with much more ease. The 7/8” hexagonal stock is a common size and can easily be grasped using the proper wrench.

The hole locations and surface finish of the part is of utmost importance on the design of the solenoid manifold. The valve required that a 0.030” hold be placed precisely from the larger 6-32 UNC hole. The close proximity of the two holes posed a unique problem in how to create intersecting holes to attach the necessary 18”-27 NPT Swageloks to the manifold. Add to the design that a thirty micron finish was required to ensure that the O-Ring on the solenoid valve would seal and the part was near design completion. The part underwent numerous FEA analyses to proof the design. Upon the successful completion the part was produced.

Figure 31 - Solenoid Valve manifold

As stated above, the solenoid requires 1W of power to operate. This was not recognized as a

problem until the solenoid had underwent thermal testing. The solenoid was not dissipating heat

to the manifold as quickly as would be required to qualify the parts for flight use. It was

determined that an additional feature would be necessary for the solenoid manifold. As seen in

Figure 31, two additional holes were added to the side of the manifold. These act as a contact

point for two screws to bolt a copper sheathing to the manifold. The sheathing is wrapped

44

around the solenoid to conduct heat out and re-direct the energy to the manifold. This helps

guarantee that the solenoid will be within the acceptable operating temperature ranges set forth

by the manufacturer.

Hollow Cathode Mounting Bracket The hollow cathode mounting bracket is responsible for holding the hollow cathode as well as the keeper for the hollow cathode. It connects the ¼” hollow cathode to the 1/16” piping via a Swagelok. The bracket is to be made out 1/8” thick stainless steel since the Swagelok, which is also made out of stainless steel; both assemblies are then welded together. Stainless steel was used over aluminum for its higher yield strength.

Figure 32 - Cathode mounting bracket

The bracket was first designed and drawn in Pro-Engineer. It originally was designed so the cathode was only a few centimeters off the pallet but when developing the piping layout it was discovered that piping would need to be run underneath the cathode. Since the cathode is the largest source of heat in the pallet it was decided that the cathode needed to be raised in order to minimize the heat transfer to the piping. It was crucial to reduce the heat transfer to the piping because the pipe underneath the cathode was connected directly to the solenoid valve which cannot have gas above 40 degrees Celsius flowing through it via manufacturer specifications.

The cathode mounting bracket also has two clamps that will hold the heater coil as well as act as the electrical connection for the heater coil. The lower clamp will hold the smaller inner wire of the heater coil and will be biased positive while the upper clamp will hold the outer part of the heater coil and electrically ground the outside of the coil.

45

Figure 33 - Cathode mounting bracket clamps

Raising the mounting bracket caused structural concern because this caused larger stresses in the bracket as well as causes vibrational concerns since it lowers the resonance frequency of the cathode assembly. Gussets where added to the bracket in order to strengthen the bracket and raise the resonance frequencies of the bracket. After the analyses were run the maximum Von Mises stress calculated was a 3.1 ksi, which is well below the yield stress for stainless steel.

46

Figure 34 - Cathode assembly static analysis

The modal analysis found four modes in the frequency range of 5 to 2000 Hz. This range was used because it is the suggested sine sweep range from the European Space Agency. The modes occurred at 190 Hz, 500 HZ, 1170 Hz and 1650 Hz. The lower frequencies cause some concern but the bracket is strong to withstand resonance.

Figure 35 - Cathode assembly modal analysis

Electronics System

Gas Flow Control

Microcontroller Program

The gas flow system is controlled primarily by a microcontroller. The specific microcontroller chosen was a CY8C29466-24PXI by Cypress. The device contains 16 digital blocks and 12 analog blocks. The following modules were used to control the system.

47

Figure 36 - Microcontroller digital blocks

Figure 37 - Microcontroller analog blocks

A-D Converters (ADC) and Programmable Gain Amplifiers (PGA)

Two analog-to-digital converters were needed in order to correctly read the analog output of the pressure transducers and store that reading as a digital signal in the microcontroller. For testing purposes, the digital signal converted by the microcontroller was then converted to decimal format and displayed on an LCD. Using this reading, the height of the rocket can be found knowing the pressure of the ambient air. The components can be controlled while ensuring the safety of the hollow cathode by not letting the cathode get too hot at too high of pressure.

The third A-D converter was placed in order to read a pulse that will signify when the sounding rocket has been launched. Knowing this allows time-based control of components by using the altitude vs. time graph as discussed in the design criteria.

48

The Programmable Gain Amplifiers are a requirement for the operation of the A-D converters. These act as a bridge between the module in the analog block and the converted module in the digital block.

Pulse-Width Modulators (PWM)

Pulse-Width Modulators (PWM’s) enable a pulse to be sent to an output pin at any specified period and duty cycle. One PWM used was for the heating process of the hollow cathode. This is a 16-bit converter which allows a period up to approximately 180 seconds with the 366 Hz clock input. The output pin of the microcontroller designated to the PWM output was connected to the Gate In pin of the DC-DC converter that powers the heater. The Gate In pin serves as the enable pin. Setting the duty cycle on the PWM is the duty cycle that the heater will be run at by the converter.

Another 16-bit PWM was used for the solenoid valve that is in place for the purging process. The solenoid valve will be pulsed at a very low duty cycle in order to allow a controlled amount of flow through the valve to purge the hollow cathode prior to launch.

The 8-bit PWM was placed to control the activation of the hollow cathode. Since this is a one-time occurrence and a high period is not needed, the 8-bit PWM was chosen. Once the hollow cathode is ready for activation, this PWM will activate the main solenoid to allow full gas flow to the hollow cathode. This will also cut power to the purge solenoid and heater.

16-bit Counters

In order to implement time-based control to the system, two 16-bit counters were integrated into the microcontroller program. A 16-bit counter is capable of counting up to a period of just less than 180 seconds using the 366 Hz clock input to the counter. One of the counters was used to control the heating process of the hollow cathode assembly. The heater must be carefully heated in order not to oxidize the cathode. The counter can be set to count down to zero in a certain amount of time and when it reaches a specified value, it will perform the function needed. When the counter reaches zero, the duty cycle of the Pulse-Width Modulator is increased. As the sounding rocket increases in altitude, the duty cycle would be increased to efficiently increase the hollow cathode temperature without damaging the cathode itself.

Solenoid Valve Control

The solenoid valves obtained required an input voltage of 24 V, which was the same voltage as the input buss coming from the sounding rocket. This saves the need for a converter but a different way to control the valve was needed. The transistor was the ideal component to act as a switch in order to turn the solenoid valves on and off when desired. The final schematic of this can be seen in Figure 38.

49

Figure 38 - Solenoid valve control schematic

Using the output of a PWM from the microcontroller, the output pin could be connected to the base of the transistor. A resistor was also placed in series with the solenoid valve to limit the amount of power consumed by the solenoid. Also included was a capacitor in parallel with the solenoid and resistor.

Pressure Transducer Reading

The pressure transducers had an output of 0-100 mV and the microcontroller reads an analog signal from 0-5 V, so an operational amplifier had to be implemented in order to get more accurate readings from the pressure transducer. The operational amplifier chosen was a LM324N, which is a quad operational amplifier (op amp). This allows for both transducer outputs to be connected to one integrated circuit. The op amp will be powered by a 12V source powered by a voltage regulator, so the op amp output can be 5V. Appropriate resistors were set up to create a non-inverting op amp circuit to increase the analog signal to the microcontroller to 0-5V. The final schematic of this can be seen in Figure 39.

50

Figure 39 - Op amp schematic

Hollow Cathode Activation

Heater Control

Before the hollow cathode can be successfully activated, it must be heated to temperatures of approximately 1000

OC. A heater is wrapped around the hollow cathode in order to reach necessary

temperatures. Due to certain elements of the cathode, the assembly can not exceed 200OC while on the

Earth’s surface or the cathode will be severely damaged. The hollow cathode will be slowly heated prior to launch by using a Pulse-Width Modulator which will pulse the DC-DC converter powering the heater. It was determined that a 12% duty cycle will cause the hollow cathode to reach a steady-state temperature of 145

OC. This will act as part of the conditioning process of the cathode. Once the rocket has been

launched, the power applied to the heater will be slowly ramped up by increasing the duty cycle to the converter. Once the hollow cathode has been activated, the heater will be turned off to prevent any excess current to flow through the cathode.

Hollow Cathode Keeper Schematic

The hollow cathode requires a 100-200 V across the keeper in order for the hollow cathode to activate. This needed to be accomplished without consuming very much power. Multiple DC-DC converters were used with a resistor and diodes to get this high initial voltage biased. The finalized schematic is shown in Figure 40.

51

Figure 40 - Keeper power schematic

The schematic contains two 95 V output converters in series with a diode and a 20 kΩ resistor. In parallel is a 36 V output in series with a resistor. The two 95 V converters provide the initial high voltage needed for activation. The resistor is in place to limit the amount of power consumed by the keeper. Once the converters are turned on, the keeper resistance is initially infinite and drops over time. As the resistance decreases, the voltage applied decreases as current increases as seen in Figure 41. Very little current is needed to activate the cathode so this keeps the power consumption low.

52

Figure 41 - Keeper power analysis

Hollow Cathode

Figure 42 - Hollow cathode at operating temperature

The hollow cathode is the most crucial aspect of the pallet. The first step was to gain a basic understanding of what the cathode does and what is required to start the cathode (all of which is discussed in the Hollow cathode basics section in the introduction of the report).

The second step was to get one of the cathodes in the Ion Propulsion lab up and running so preliminary tests could be run to could get a preliminary start-up time of the hollow cathode. The best way to know if the cathode will start is by measuring the health of the cathode. This is done by measuring the emission

53

current from the keeper to the cathode. When this current reaches ten micro-amps, then the cathode is healthy and will start easily.

Before testing the cathode a LabView program was developed to measure the time and emission current then saved to an excel file. Figure 43 shows the lab view program developed for testing the emission current.

Figure 43 - LabView program

Once the LabView program was developed testing of the cathode could begin. Figure 44 shows the results of the first test of the hollow cathode.

54

First Cathode Test: Time Vs Emission Current

-10

0

10

20

30

40

50

60

0 50 100 150 200 250 300 350

Time (seconds)

Em

issio

n C

urr

ent

(mic

ro A

mps)

Figure 44 - First cathode test

The cathode reached the required emission current in about 190 seconds, 70 seconds longer than needed. This result was not surprising since no modification had been do to the cathode in an attempt to start it faster, this would be the next step.

In order to get the cathode to start faster, the amount of heat radiation from the heater coils needed to be reduced. This could be achieved by wrapping the heater coils with tantalum radiation shielding. The shielding was cross-hatched with an exact-o knife. This would result in raised lines in the shielding thus separating the different layers of radiation shielding to allow less heat transfer through the radiation shielding.

Figure 45 - Cathode with radiation shielding

55

The hollow cathode and heater coil also needed to be dynamically tested. This was done analytically as well as with Pro-Engineer Mechanica software program.

The following calculations were performed to estimate the natural frequency for the hollow cathode, without the heater coils. For these calculations, the hollow cathode was treated as a simple cantilever beam and fundamental natural frequency equations could be applied. Using the results from these equations it gave a good idea of what the natural frequencies should be. The fundamental natural frequency equation is as follows:

st

n

gf

δπ×=

2

1

Where:

EI

Lwo

st8

4

=δ (static deflection)

Since:

gAdensitywo ××= (distributed load)

Adensity

g

wm o

o ×== (distributed mass)

This can be simplified to:

4

8

2

1

Lm

EIf

o

n ×=π

Where:

4

)(44

io rrI

−=

π

(moment of inertia) W = weight

L = length

E = modulus of elasticity

m = mass

ro and ri = cross-sectional outer and inner radii

wo = weight per unity length

mo = mass per unit length

A = cross-sectional area

56

Using these equations on the hollow cathode:

4104344

1047.71079.14

)21.255(.minI

−− ×=×=−

=π

E = 186 GPa for tantalum

L = 4.13 cm

m = 7.51 g

mo = .177 kg/m

Hz

cmm

kg

mGPaf n 7.1502

)13.4(177.

1047.71868

2

1

4

410

=

×

××××=

−

π

This result shows that a natural frequency around 1500Hz should be expected.

Pro-Engineer static and modal analyses were also run on the hollow cathode and heater coil. A model of the hollow cathode and heater where developed in Pro-Engineer as shown in Figure 46.

Figure 46 - Pro-E model of cathode and heater coil

A close-up view of the heater coil and its components can be seen in Figure 47.

57

Figure 47 - Close up of heater coil

The heater coil is composed of a very thin Tantalum wire that conducts the heat to the hollow cathode. This is insulated by a magnesium oxide insert. Wrapped around that is a tantalum housing

In order for the program to be able to mesh this part the part had to be modified with some less complex features. This includes making the 10 heater coil wraps around the hollow cathode into one long cylinder and changing the outer cross section of the heater coil from a circle into a square as shown below. This allowed for an easier and less time consuming analysis process.

58

Figure 48 - Modified Pro-E model

Figure 49 - Close up of modified heater coil

The first analysis performed was a static analysis to determine how the hollow cathode would react under accelerations due to the spinning and propulsion of the rocket. According to the European space agency,

59

the maximum acceleration of a rocket similar to ours was ten g’s, or 3862.2 in/sec2 (note: S-310

information could not be found so European Space Agency information from a similar sounding rocket is being used for now). For the spin acceleration, a spin rate of 2.8 rev/sec was used to calculate a tangential acceleration of 1547.6 in/sec

2. The Pro-Engineer displacement analysis can be seen in Figure

50. The stress results are also shown in Figure 51.

Figure 50 - Pro-E displacement results

60

Figure 51 - Pro-E stress results

Figure 52 - Close up of max stress

Table 1 - Maximum stress and displacement

Max Von Mises Stress 881.2 psi

Max Displacement .000126 inches

Figure 52 shows a close up of where the maximum stress occurs on the cathode. The stress occurs at the very base of the cathode where it is constrained. A good secure connection was made to ensure that this maximum stress does not cause any failure.

61

The results shown in Table 1 are within an acceptable level such that structural failure was not expected.

In the modal analysis, a frequency range of 5Hz to 2000Hz was used since this is the sine sweep range suggested by the European Space Agency. The analysis resulted in two modes in this frequency range, 1061.3Hz and 1176.54Hz. The two mode results are shown in Figure 53 and Figure 54.

Figure 53 - First Pro-E modal result

62

Figure 54 - Second Pro-E modal result

When compared to the computational analysis done previously, the results are fairly similar. This leads to confidence that the resonance frequency of the hollow cathode will be in the 1 to 1.5kHz range.

Ground Support Equipment (GSE) Three ground support equipment systems were designed and built to enable the team to prepare and test the pallet prior to launch. A Cathode Test Facility, Thermal Vacuum/Cycle Facility, and Fill/Drain System are the three support systems.

Hollow Cathode Test Facility The Cathode Test Facility is primarily a vacuum chamber that is located in the Ion Propulsion Lab. The main purpose of this facility was to run hollow cathode tests and also thermal tests on individual components of the pallet. The chamber had to be cleaned out and prepared for operation of a hollow cathode. A hollow cathode stand was developed and a cathode was secured to the stand. Electrical connections were made in order to power the heater and the keeper of the cathode assembly. Wires were run from the assembly to connectors that ran outside of the chamber. These were connected to a power supply to prepare for testing. Additional connectors had to made and run to the outside of the chamber in order to prepare for thermal tests. Piping also had to be run from a gas tank under the chamber into the hollow cathode for the gas flow. The test facility in its final stage can be seen in Figure 55.

63

Figure 55 - Cathode Test Facility

All of the tests with the hollow cathode were performed in this facility. Connections were made a computer nearby where the LabView programs were generated and testing was recorded.

Thermal Vacuum/Cycle Facility

The thermal vacuum/cycle facility was created in order to cycle the pallet through different temperature ranges that it will encounter throughout flight. The pallet was attached to the target shown in Figure 56. The target then had a glycol water mixture pumped through. This allowed the pallet to see temperature ranges from -30°C to 130°C. Through this testing a more accurate thermal profile of the pallet was generated. From the models the thermal requirements needed for the pallet can be better addressed by JAXA.

64

Figure 56 - Thermal Vacuum/Cycle Facility

Fill/Drain System The fill and drain system was primarily created for filling and draining the Xenon tank on the pallet. The system also serves as a way to leak test the pallet gas flow system. While hooked up to the pallet, the system can also check the mass flow through the gas system and check the pressure.

The system consists of a roughing pump, a flow meter, a Baratron and a gas tank. The schematic of the entire system is shown in Figure 57. The complete system developed and tested for functionality can be seen in Figure 58. The entire system can be attached to a vacuum chamber to further testing.

Figure 57 - Fill and drain schematic

65

Figure 58 - Fill/Drain System

The system runs through a series of pipes all of which are connected by five valves. At any point in time all or none of the components can be interconnected. Initially the system was used to leak check the completed prototype. This is done by connecting the fill/drain valve on the pallet through a pipe to the fill/drain system. The first step is to turn on the roughing pump to alleviate the pallet of air at standard atmosphere, and then the vacuum chamber valve is opened. There is a pressure gauge on the chamber that shows pressure down to 10micro Torr, this gauge will rise with a leak in the system. Major leaks can be found by listening for a gushing noise coming from the pallet. Once the pallets leaks are reduced to the level that is capable to pump the pallet (internally) down to 10

-5 Torr, the next step is to test with

Helium. A residual gas analyzer (RGA) can be used to do this. The RGA is hooked into the vacuum chamber and on a computer screen; the readout will show different gas partial pressures by atomic number. On an additional screen a read out shows a flat time dependent readout of the overall gas pressure.

In order to find leaks, Helium was added to specific locations on the pallet while watching a readout that monitors the presence of helium. If the readout jumps up once the helium is sprayed on a certain location, the location of the leak can be detected. Helium testing gets into the 10 micro torr range, the final small leaks can be found by dropping acetone onto the connections of the pallet. If acetone enters a leak the pressure gauge will spike momentarily. Once the leak testing is finalized the system is prepared for gas to begin a total system check in vacuum. In order to do this, the vacuum chamber is closed off from the pallet, along with the roughing pump. An Argon filled gas tank is connected to the regulator and will flow in to the pallets gas tank until full. By closing off the Fill/Drain tank, the pallet is almost ready. The next step is to set the pressure regulator inside the pallet; this will be done by opening the valve for

66

the flow meter and watching the read out which reads the rate at which the gas in the pallet is flowing. By turning the pressure regulator, the desired value of 500psi can be set. Once this is complete the pallet can be disconnected from the Fill/Drain system and ready for testing.

Thermal Testing

Cathode testing In order to better understand the thermal properties of the hollow cathode a transient thermal model was created in Pro/Engineering and ePhysics (donated by Ansoft Corporation). With the model predictions it was possible understand the hollow cathode temperature varies with the heater power. It was determined that at the tip, the cathode would see temperatures over 300 at a power level of 8 W shown in Figure 59. For the nominal power level of 65 W temperature of the cathode tip will reach approximately 1100 degrees C (Figure 58 B).

Figure 59 - Thermal model of hollow cathode (scale in degrees F)

67

Figure 58 b Thermal model of Hollow cathode (scale in degrees C)

The next step was to conduct tests using thermocouples to measure the temperature at various locations on the Cathode. Two C-type thermocouples were spot welded on the orifice of the cathode shown in Figure 60 (The C-type thermocouple was used due to its high temperature tolerances).

Figure 60 - C-type Thermocouples on orifice plate

68

The wire used was .003 inches. A small diameter wire was used to reduce the tare errors that would be found with such a measurement. A K-type thermocouple was also placed at the base of the cathode to define how much temporal radiation will be given off within the pallet, since the base is located inside the pallet.

The next step was to create a LabView program that would record the temperatures during a set time range. The program that was used is shown in Figure 61.

Figure 61 - LabView program

The reason such extensive thermal testing was done on the cathode is due to the fact that the cathode should reach certain temperature to be started. In addition the cathode is the largest heat source on the pallet and much of the thermal energy will need to be dissipated.

69

Budget

$50k Marshall Space

Flight Center Grant Received Donated

$10k Colorado Space Grant Ordered Unknown

$5k department alotment

Test Pallet

Component Part Number Quantity Cost Per Unit Total Cost Shipping Company

Pressure Transducer 422-H3-08-P7 2 $235.00 $470.00 $8.92 Fiero Fluid Power

Pressure Transducer 422-H3-01-A-P7 2 $203.50 $407.00 $8.92 Fiero Fluid Power

Flow Regulators BB-13AH2XV99-005 4 $245.00 $980.00 $9.29 Fiero Fluid Power

Xenon Tank Luxfer P07A 2 $167.00 $334.00 $12.33 Nitro Duck

PSoC Kit CY3214 2 $116.46 $232.92 $6.44 Digi-Key

Wire 22 19/34 Type E white 1000 $0.16 $164.00 Allcable

Cable Tie 4in Tefzel 400 $0.12 $48.00 Allcable

1/16" Stainless Steel Tubing ASTM A-269/A-213 20 $6.24 $124.80 $4.87 PAC S.S. LTD

Stainless Steel 1/16''

Swage Lock w/ 1/8'' NPT SS-100-1-2 10 $9.60 $96.00$4.00

Swagelok

S.S. 1/16" Swage Lock T's SS-100-3 8 $33.30 $266.40 $4.00 Swagelok

Unsheathed, Tungsten-Rhenium Wire WW26-003 10 $22.00 $220.00 Omega

Sub Mini Ferrite Connector HFMPW-C-M 4 $3.45 $13.80 Omega

Mini Hitemp Connector HMPW-C-F 4 $2.45 $9.80 Omega

Solenoid Valve Anger 407 am 6 $83.00 $498.00 $8.63 Flow Solutions

Flow Restrictors 5190-1/8-1-16-ss-20 sccm 2 $80.00 $160.00 Mott Corporation

Filters 5190-1/8-1/16-ss-10 Micron 8 $80.00 $640.00 Mott Corporation

Fill and Drain Valves H-4361 3000psi 3 $48.84 $146.52 $0.00 McCoy

DC-DC Converters VI-JWM-CX 2 $153.00 $306.00 Vicor

DC-DC Converters VI-JW1-EX 1 $112.00 $112.00 Vicor

DC-DC Converters VI-JWB-CX 4 $153.00 $612.00 Vicor

DC-DC Converters VI-JWJ-CX 2 $153.00 $306.00 Vicor

Thermocouples SA1-K-SRTC 1 $75.00 $75.00 Omega

Tank Manifold 616889 1 $88.40 $88.40 $11.65 Nitro Duck

SS 1/16" swagelok to npt elbow SS-100-2-2 2 $18.50 $37.00 $4.00 Swagelok

Flat head socket screw 1/4"-20-5/8" 1-560-08 26-29 (10pk) 2 $7.02 $14.04

Flat head socket screw 1/4"-20-1/4" 1-560-05 21-40 (10pk) 2 $6.90 $13.80

Flat head socket screw 6-32-3/8" 1-560-04 06-51 (10pk) 2 $4.69 $9.38

Xenon 99.999% 1 $1,287.50 $1,287.50 $80.00 Spectra Gases

Reducing union ss-400-6-1ZV 2 $17.30 $34.60 $4.00 Swagelok

Reducing union SS-200-6-1 2 $12.80 $25.60 Swagelok

Ferrule Set SS-100-set 10 $3.10 $31.00 Swagelok

prototype circuit board components 1 $19.56 $19.56 $0.00 radio shack

prototype circuit board components 1 $4.99 $4.99 $0.00

fill valve 1 $74.12 $74.12 $0.00 McCoy

connectors Ms27484T12F35s 1 $278.15 $278.15 $5.38 spacecraft comp.

converter bracket spacers 48 $0.37 $17.76 $0.00 McMaster-Carr

all threads 6-32,6" length 2 $9.66 $19.32 McMaster-Carr

screw nuts ss 6-32 1 $4.32 $4.32 McMaster-Carr

Hollow Cathode 1 Donated $0.00 Plasma Lab

Total $8,181.78

$7.72

$4.68

$5.00

Proposed Budget

Accumulated budget

MSFC grant will be used for administrative costs i.e Dr. Rubin (Co-Advisor)

Test Pallet Budget

$5.39

$4.75

McMaster-Carr

$4.00

$4.75

70

Keeper Assembly

Alumina Plate P99003-12//N 5 $17.52 $87.60 Sup. Tech. Ceramics

Cathode mounting bracket 1 $80.00 $80.00 Alpha engineering

Keeper Face Plate 1 $25.50 $25.50 Alpha engineering

keeper back plate 1 $25.50 $25.50 Alpha engineering

keeper cap 1 2 $10.00 $20.00 Alpha engineering



Alumina plate 1 $60.00 $60.00 Alpha engineering

Mica spacer 10 $2.00 $20.00 Alpha engineering

Total $358.60

Case and Assembly Materials

Casing Al6061-T6 12" x 24" x 3/8" 1 $32.00 $32.00 $0.00

Casing Al6061-T7 12" x 12" x 3/8" 1 $48.00 $48.00 $0.00

1" Al hex stock 1ft 89845K141 1 $11.70 $11.70 $4.47 McMaster-Carr

SS sheet metal 301 ss shim roll 2 $25.70 $51.40 $6.00 McMaster-Carr

Brackets and Case Shell $1,000.00

Tape HSS Taper Pipe 1/8"-27 2553A12 1 $17.20 $17.20 McMaster-Carr

Black-Oxide Jobber twist 1/32" , 1-3/8" 12 $0.94 $11.28 McMaster-Carr

Two-Flute Carbide End Mill 21/64" 8876 A39 1 $21.00 $21.00 $4.50 McMaster-Carr

Steel Two-Flute End Mill 3/8" 2983A27 1 $12.95 $12.95 McMaster-Carr

Steel Spiral Point Tap 1/2"-20 2523A435 1 $11.27 $11.27 McMaster-Carr

Tapered pipe thread guge 2365A111 1 $71.07 $71.07 $4.50 McMaster-Carr

shell cutting 2 $15.00 $30.00

Plastic casing 12*12*7 1 $38.10 $38.10 Ft Collins Plastics

Total $1,355.97

Ground Support Equipment

Fill/Drain System

Tank Mounts Cylinder Bracket SG6203 2 $46.40 $92.80 General Air

1/8" valves SS-2H 4 $139.10 $556.40 Swagelok

VCR to 1/8" pipe fitting SS-4-VCR-6-200 2 $18.10 $36.20 Swagelok

1/8" S.S. Cross SS-200-4 2 $37.00 $74.00 Swagelok

1/4" to 1/8" reducer SS-400-6-2 4 $9.80 $39.20 Swagelok

Ferrule set SS-200-SET 20 $1.94 $38.80 Swagelok

Gasket SS-4-VCR-2 20 $1.20 $24.00 $4.00 Swagelok

Cable CB270-1-10 2 $78.32 $156.64 $6.13 MKS

1/8" pipe Donated $0.00 Plasma Lab

Test Facilities (VAC Chamber) 1 Donated $0.00 Plasma Lab

Total $1,018.04

Packaging

Wood 1 $50.00 $50.00

Springs/Shock Absorbers 4 $375.00 $1,500.00

Packaging Supplies 1 $50.00 $50.00

Total $1,600.00

Shipping and Handling Costs Total $255.32

Total Budget Costs

Total Spent to date $12,514.39

TOTAL $12,769.71

$4.00

$4.00

CO Iron & Metal

$4.50

$4.50

71

Conclusions and Recommendations In conclusion, the F.A.S.T. Pallet team successfully developed and built a fully operational system which activates and sustains a hollow cathode. All specifications described in the constraints and criteria were met. The prototype and final deliverable were designed and manufactured before graduation. The pallet will remain at CSU for further testing and improvements before launch out of Japan in the summer of 2009. Along with building the pallet a ground support system was designed and built to fill, drain and test operational functionality of the entire system. By analyzing the prototype the F.A.S.T. Pallet team made improvements to the final deliverable by improving the case structure, new circuit board design, hollow cathode modifications and mass reduction.

The pallet is largely autonomous and designed to survive launch in an S-520 sounding rocket. The pallet is also capable of receiving signals from the rocket to indicate when to initiate pre-heating of the hollow cathode and when the launch sequences has started. Tests have shown the pallet is capable of starting the hollow cathode in less than two minutes and can sustain the plasma plume created by the hollow cathode through out the entire mission.

For future work, leak testing should be done in a systematic manner starting with the fill and drain valve and working through the system testing each component individually. In addition add more redundancy to electronic system to improve reliability. Continued testing of the hollow cathode could be performed to reduce the amount of time required to start. Considerations in to more advanced composite tanks should be considered to further reduce mass of the pallet. New pressure transducers with a lower profile, and with a larger voltage output to reduce complications that arouse in construction. The design of the pressure transducer manifold could be modified to reduce mass and still maintain a high safety factor. The pressure regulator is also extremely oversized for the application it is being used for; a smaller and better sized regulator should be considered.

Group member contributions:

• Meghan Capra developed and manufactured the fill and drain system for the pallet as well as did structural testing. She acted as team leader for the group throughout the year.

• Matt Larson developed circuitry and wrote the programs required to operate entire pallet. He also optimized the electronics system to reduce the activation time of the hollow cathode.

• Dan Strawn manufactured the parts for the pallet as well as designed and built parts in Pro-Engineer. He also performed FEA and physical structural testing.

• Steen Vecchi performed thermal testing of the pallet and sub-components as well as testing of the hollow cathode. He maintained the vacuum chamber as well as set-up tests for inside the vacuum chamber.

• Jeff Wyant assisted with CAD design to ensure launch survival as well as designed and built hollow cathode support assembly. He performed the analysis for the flight dynamics to be used in analysis and test.

72

Technical References

1. Cohen, H. A., Mullen, E. G., Huber, W. B., Masek, T., “The Sounding Rocket Flight of a

Satellite Positive Ion Beam System”, AIAA-1979-2068 , 14th International Electric

Propulsion Conference, Princeton, N.J., Oct 30-Nov 1, 1979.

2. G.A.Gusev, Yu.V.Kushnerevsky, S.A.Pulinets, V.V.Selegey, “Wave Phenomena Under

Injection of Energetic Electrons into the Ionosphere in ARAKS Experiment”, Physics of the

Ionosphere and Magnetosphere, Nauka Publ., Moscow, 1978, p.134-143.

3. G. A. Jongeward, “Spacecraft Charging in the Spear I”, International Conference in

Spacecraft Charging, Livorno, Italy, Sept. 1991.

4. Rustan, P., Garrett, H., and Schor, M. J., “High Voltages in Space Innovation in Space

Insulation”, IEEE Transactions on Electrical Insulation Vol. 28 No. 5, October 1993, p. 855-

865.

5. Taylor, W. W. L., Moses, S. L., Neubert, T., and Raganatan, S., “Beam Plasma Interactions

Stimulated by SEPAC on - ATLAS 1: Wave Observations”, XXIVth General Assembly of the

International Union of Radio Science, Kyoto, Japan, August 25-September 2, 1993.

6. McCoy, et al., "Plasma Motor-Generator (PMG) Flight Results," Proceedings of the Fourth

International Conference On Tethers In Space, Science and Technology Corp., Hampton,

VA, Apr. 1995, p. 57-82.

7. Moore, T.E., Chappell, C. R. Chandler, M. O. et al., “The Thermal Ion Dynamics Experiment

and Plasma Source Instrument”, Space Science Reviews, v. 71, 1995, p. 409-458.

8. Carpenter, C.B., “On the Operational Status of the ISS Plasma Contactor Hollow Cathodes”,

40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, 11 - 14 July 2004,

Fort Lauderdale, Florida, AIAA 2004-3425.

9. Krause, L.H. Cooke, D.L. Enloe, C.L., “Survey of DSCS-III B-7 Differential Surface

Charging”, IEEE Transactions on Nuclear Science, Dec. 2004, Vol. 51, Issue 6, Part 2, p.

3399- 3407.

10. Vaughn J. A., Curtis, L., Gilchrist, B. E., et al, “Review of the ProSEDS Electrodynamic

Tether Mission Development”, 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference

and Exhibit, 11 - 14 July 2004, Fort Lauderdale, Florida, AIAA-2004-3501.

73

Engineering Drawing Package

Figure 62 - Pallet Assembly

74

Figure 63 - Fill and Drain bracket

75

Figure 64 - Pressure Transducer bracket

76

Figure 65 - Solenoid Valve manifold

77

Figure 66 - Swagelok T housing bracket

78

Figure 67 – Tank connecting bracket

79

Figure 68 - Alumina Plate

80

Figure 69 - Cathode Mounting Bracket

81

Figure 70 - Keeper Back Plate

82

Figure 71 - Keeper Cap

83

Figure 72 - Keeper Face Plate

84

Figure 73 - Keeper Tube

85

Figure 74 - Shell 1

86

Figure 75 - Shell 2

87

Figure 76 - Top Plate

88

Figure 77 - Base Plate 1

89


90


92


93


94


95

Figure 80 – Base Plate 8

96


97


98


99


100

Appendix A

Maximum Force Expected on Tank Brackets

m1830.83

1000kg:= mass of the tank

m2158

1000kg:= large tank bracket

m379

1000kg:= tank manifold

-Assuming the connecting to the base plate is perfectly rigid: -Maximum force during mission is a 35g shock

g 35 9.81⋅m

s2

:= g 343.35m

s2

=

-Largest force will occur on the unconnected end of the tank -Use only half the tanks mass to calculate the largest mass being accelerated

Fmax

m1

2g⋅:=

Fmax 142.633N= Fmax 32.065lbf=

-There is a potential moment occurring around the large tank bracket

d1 3.85in:= d1 0.098m=

Tmax Fmax d1⋅:=

Tmax 13.948N m⋅= Tmax 123.451in lbf⋅=

101

Appendix B

Loads on load bearing surfaces:

mtotal 5.11575kg:= mtotal 11.278lb=

Ftotal 35 g⋅ mtotal⋅:=

Ftotal 1.756 103

× N= Ftotal 394.74lbf=

abolt 8 .0683⋅ in2

:= abolt 0.546in2

=

atb 0.25in2

:=

ashell 0.625in2

:=

Atot abolt atb+ ashell+:= Atot 1.421in

2=

Ptot

Ftotal

Atot

:= Ptot 277.712psi=

Pbolt

abolt

8Atot

Ptot⋅:= Pbolt 13.344psi=

Pbolttotal

abolt

Atot

Ptot⋅:= Pbolttotal 106.755psi=

Ptb

atb

Atot

Ptot⋅:= Ptb 48.845psi=

Ptbhalf

Ptb

2:=

Ptbhalf 24.422psi=

Pshell

ashell

Atot

Ptot⋅:= Pshell 122.112psi=

Ptotal Pbolttotal Ptb+ Pshell+:= Ptotal psi=

Fshell Pshell ashell⋅:= Fshell lbf=

102

Appendix C Manufacturing

Industrial Use of Laser Welding Dan Strawn

F.A.S.T. Test Pallet

Copyright © 2006 Department of Mechanical Engineering, Colorado State University

ABSTRACT

The use of lasers in the field of welding is not a new up and coming technology, but it is finally becoming a main stream welding technique. Laser welding has grown exponentially over the past few years, and is widely used in the production of electronic circuit boards, heavy machinery and even the jewelry business. The use of lasers in manufacturing is still young in terms of its potential capability. Though, through continued development, lasers will be able to re-define how products are made. The principles behind laser welding and related innovations with laser manufacturing are described herein, along with the devices used to produce these laser beams.

Of the devices discussed, there will be an emphasis on the machines that are capable of producing extremely small and intricate welds on thin materials.

INTRODUCTION

A laser, Light Amplification by Stimulated Emission of Radiation, is most commonly thought of as a science fiction weapon, but the application of real world lasers is as real as the sun or sky. Lasers are capable of providing an immense amount of heat that is higher than the temperature of the sun itself. With such a large potential of power, the use of lasers in industry is rather limitless.

In recent years, the use of lasers to perform complex or difficult welds has grown. Lasers, utilizing the large heat potential, make fusion welds of many different materials in a variety of sizes possible. A fusion weld requires no additional filler material to be added to the weld, which is contrary to most common forms of welding. In laser fusion welding there is a large reduction in the heat affected zone (HAZ) around the weld, thus maintaining the bulk material properties. In addition, the fusion weld allows for unlike metals to be joined with much more success than conventional methods.

Lasers are classified in two general categories: solid state lasers and gaseous state lasers. The gaseous state laser is created by passing a current through a gas to generate light, which in turn is then focused in to a high intensity beam. The actual entire process of the laser is much more involved, but the focus of this paper will be on solid state lasers. The solid state laser of discussion is a Neodymium Yttrium Aluminum Garnet (Nd: YAG) laser, specifically the Raytheon S550 Series Precision Laser Welder/Driller.

103

Figure 87 - Raytheon S550 Laser Welder/Driller

As in any manufacturing process, there is great consideration in the selecting a particular method to create a weld. Thickness of material, material properties of the weld and application of the welded part are three key criteria to consider when choosing the proper technique to create a weld. The diversity of a laser welder allows for all three characteristics to be optimized for the given situation.

PRINCIPLES OF WELDING

In order to gain insight to the extra dimension that laser welding offers to the field of conventional welding, it is important to address the basics of traditional welding. The technique of welding involves bonding two materials together through the use of intense heat. The metals are joined together either through the use of a filler rod or through the fusion of the metals.

There are many types of welds, but the butt, corner, fillet, lap, and edge weld will be the focus. These are the most common forms. Although there are many variations of weld types, it is easiest to relate to these four welds.

Figure 88 - Butt Weld

Figure 89 - Corner Weld

104

Figure 90 - Fillet Weld

Figure 91 - Lap Joint Weld

Figure 92 - Edge Weld

When a butt weld is formed properly, the strength of the weld is comparable to that of the plates being joined. This relates to a weld efficiency of 100% under static loading for a butt weld. In comparison, a fillet weld under parallel static loading, as well as a fillet weld under transverse static loading, is not as strong as the metals themselves that are joined together.

Figure 93 - Fillet Weld Strength Potential

105

The above equations illustrate that a fillet weld is much weaker than the plates it joins. This is apparent in the fact that the areas of the plates are (typically) much larger than the weld. Also in addition to the area, the plates do not have a correction factor of 0.58 imposed upon it.

In relation, a corner weld and a lap joint weld are very similar to a fillet weld, both in appearance and in their physical properties. Whereas an edge weld is similar to a butt joint weld, both in appearance and physical properties. All four forms of these welds can be performed in a variety of positions, meaning that the weld can be performed vertically, horizontally, flat, and even inverted. Granted that the typical laser welder is confined to the apparatus in which it is mounted, the most common position for traditional laser welding would be the flat position. In addition, it is important to note that the corner, fillet, and lap joint welds require the laser beam to be focused at the corner where the two pieces meet. This allows for both materials to be fused simultaneously.

When utilizing a laser welder, the same equations hold true, in most cases. A laser uses a fusion weld and does not impose a filler material into the metal being welded. A laser weld maintains the material properties of the plates, which is one of the main benefits of laser welding.

Nd: YAG LASERS

The use of solid state lasers, particularly the Nd: YAG laser, has been around since the early 1960s. Even though the technology is rather dated, it is important to understand the principles that drive the operation of the laser in order to recognize the potential of these welding systems.

An Nd: YAG laser operates by emitting a highly concentrated beam of light from a crystal. The crystal is made up of Neodymium Yttrium Aluminum Garnet, and is the key to generating the laser beam. The crystal lies within a highly reflective (typically gold plated) cavity and is surrounded by krypton cathode lamps. The shape of these cavities is critical to the power of the laser. The krypton lamps emit an incoherent light, and the cavity shape is designed to focus this light on to the crystal.

Figure 94 - Nd: YAG Laser Internal Components

The krypton lamps, being the source of energy to the crystal, are the limiting factor of a solid state laser. The lamps require large amounts of power to be transmitted through them in order to release a high strength incoherent light. The light, after being focused by the reflective cavity, strikes the Nd: YAG crystal. The photon energy that strikes the crystal causes electrons within the crystal to be placed in an excited state. These electrons quickly drop back to their original energy state, in order to maintain their energy balance within the system as a whole. The “motion” of the electrons as they lower energy states

Crystal Krypton Reflective Cavity

106

causes photons to be released from within the crystal. These photons create the laser beam used for welding.

Now that a beam of light has been created, it needs to be focused and directed to the work piece. There are a variety of methods to perform the focusing, and most commonly it is done by using two main lenses and a series of focus lenses. The light emitted from the crystal does not come out in one direction, but rather from both sides of the crystal. The dual beam makes it necessary to reflect one portion of the beam back into the other half of the beam. This process allows for a better efficiency within the laser.

Figure 95 - Partially Reflective Lens and Shutter Control

Figure 96 - Rear Focus Lens

Beyond the actual laser, the support equipment required to operate a laser effectively, is rather involved. In particular, on the Raytheon S550 unit the laser head requires a water to water heat exchanger. There is such a great amount of heat being generated by the krypton cathode lamps that the heat needs to be regulated and dissipated effectively. This poses a critical problem within the laser. In order to cool the laser, a working fluid must be nearly perfect optical transparent. This allows the lamps to maintain a constant temperature and still emit the incoherent light that activates the Nd: YAG crystal that produces the laser beam. An example of a good working fluid for a typical water to water heat exchanger would be glycol; however, due to the optical constraints, de-ionized water is the best option. The de-ionized water

107

is maintained at a safe level by use of the heat exchanger which uses a constant flow of tap water to keep the de-ionized water cool.

Figure 97 - Water to Water Heat Exchanger

GENERAL PRINCIPLES OF LASER WELDING

The concept of a laser weld is very similar to a fusion weld created by a torch. The overall general intent is to super-heat the two parts at the joint above the materials’ melting points and then to force the molten material to flow together into one uniform welded piece. However, a laser is much more focused than a torch. The typical diameter of a laser beam is on the order of tenths of millimeters. This creates a very small, localized area of incredibly intense heat. The beam is so minute that it could create a weld about the diameter of the tip of a needle. Precision and accuracy are the two driving advantages of using a laser welding machine.

The laser beam then creates what is known as a key hole (which penetrates the material). The key hole causes internal refraction to occur within the weld and increases the effectiveness by lowering the amount of light that is reflected off the material.

108

Figure 98 - Weld Geometry

In addition to the keyhole effect created by the laser as it burns in to the material, the material that is burned off creates a plasma cloud around the point of the weld. The plasma cloud is extremely efficient at absorbing incident light and acts to increase the effectiveness of the laser. Add an inert shield gas to prevent weld sputter to occur, and the laser weld practically enhances and protects itself from negative effects caused by the ambient environment.

THEORY OF LASER WELDING

Material Properties

As with any form of welding, the material to be welded is critical to the weld’s ability to bond the two materials together. In the consideration of laser welding it is critical to understand the materials being welded. As stated previously, a laser is an intense beam of light. Light can easily be reflected, thus dissipating the energy it carries. It is important to understand the absorptivity of the material to be welded. In order to calculate the absorptivity the Bramson Equation is utilized. The equation relates the electrical conductivity as a function of temperature to the wavelength of light to be imparted on the surface.

For example, a typical absorptivity of aluminum using the Bramson Equation is 2-3%. The reflective loss is rather significant. Therefore, in the aluminum case, additional measures must be implored to minimize

Eλ T( ) 0.365ρ r T( )

λ

1

2

2

3

ρ r T( )

λ

⋅− 0.006ρ r T( )

λ

3

2

⋅+

Figure 99: Bramson Equation

Shielding Gas

Weld

Materia

Keyhole

Weld Direction

Melted Material

Laser Beam Plasma

Cloud

109

these losses, such as an absorbent powder or an anodized film. The surface finish of any material greatly affects the absorptivity of the laser.

It is important to note that there are alternative ways to circumvent the problems associated with absorptivity issues. The laser will function properly at the desired power level once the keyhole has been created. A simple test to create the initial key hole is to pulse the laser very briefly at a higher than normal operating intensity. This will usually overcome the absorptivity issues discussed above and allow the operator to make the weld as desired.

Perhaps the most intriguing ability of laser welding lies in the lasers potential to weld practically any metal. The laser creates a fusion weld with the two metals with no filler rod, as previously stated. Due to this nature, exotic metals can now be welded. Traditional methods were either unable to perform the weld or required too much complexity/difficulty thus nullified the practicality.

The laser’s exotic capability is also a feature of the laser beam’s energy density with the beam. The beam is capable of producing heat that is comparable and/or higher than that of the surface of the sun. No known material is capable of surviving such heat. This means that almost any metal, that will go molten before vaporizing, is capable of being laser welded.

Material Geometry

While lasers can be used to produce a wide variety of welds, perhaps the most unique ability is to weld on miniscule parts with great precision. In terms of the energy density the beam imparts on the material, there is a large potential for fine tuning on a laser welder. The agility of the laser allows for extremely thin materials to be welded together, as shown below.

Figure 100 - 16 Gage Stainless Steel Sheet Metal Weld

By pulsing the beam of the laser, a more controlled amount of energy can be directed to the weld. This technique allows the operator to greatly affect the weld penetration depth. In the use of the Raytheon S550, the following charts dictate how changing the pulse rate and pulse width change the laser output to the work piece.

Table 2 - Maximum Pulse Rate

Leading Edge PFN Tail Settings

110

(ms) 0 ms 1 ms 2 ms 3 ms 4 ms 5 ms 6 ms

0.3 200 Hz

100 Hz

60 Hz

60 Hz

45 Hz

45 Hz

30 Hz

0.5 200 Hz

100 Hz

60 Hz

60 Hz

45 Hz

45 Hz

30 Hz

0.9 200 Hz

100 Hz

60 Hz

60 Hz

45 Hz

45 Hz

30 Hz

1 200 Hz

100 Hz

60 Hz

60 Hz

45 Hz

45 Hz

30 Hz

1.1 150 Hz

86 Hz

55 Hz

55 Hz

42 Hz

42 Hz

29 Hz

1.4 150 Hz

86 Hz

55 Hz

55 Hz

42 Hz

42 Hz

29 Hz

Table 1-1: Maximum Pulse Rate

Through the understanding of pulse times of the laser, it is possible to estimate the weld penetration depth and correctly size the required power input to the weld, which permits proper weld penetration depth. Understanding the weld depth for a given set-up allows one to correctly size the laser to function on whatever size metal is needed to be welded. Adequate penetration depth in a weld is a driving feature required to properly manufacture a structurally sound weld.

Figure 101 - Weld Properties

Through the understanding of weld geometric properties and the understanding of how to obtain them, it is possible to see how a laser welder is capable of welding complex geometries. The parameters of are all internal to the laser. This allows for computer numerical control to be used in laser welding. Also with the addition of fiber optic weld tips, complex welds can be made even easier.

Table 3 - Rated Maximum Pulse Energy

Total Pulse Width (ms)

Maximum Pulse

Energy (J)

1 6.3 2 12.6 3 19 4 25.3 5 31.6 6 37.9 7 44.2

7.4 50.5

111

EMPIRICAL STUDY OF LASER WELDING

The theory behind a laser welder is very detailed and extremely precise, but there is still a great deal of “art” required to use a laser welder. The net operation of a laser requires a great deal of variability in order to operate the laser at peak efficiency. The best way to make a proper weld is to experiment with a variety of settings in order to get the desired weld penetration, weld width, weld strength, etc.

After using the Raytheon S550 and consulting the manual, calculating parameters theoretically and obtaining specimens, the results demonstrated that no matter how much theory was involved; the best welds were obtained through an iterative trial and error methodology. There are so many parameters on the laser to adjust that it is extremely difficult to account for all factors and compute the exact settings to weld with theoretically.

In the particular study of thin plate welds, it was observed through experiment that the best way to make quality welds was to perform most everything wrong. When making a weld, it usually requires that the laser to be focused directly on the surface of the metal and then pulsed to a given setting to accommodate the material thickness, feed rate, and desired pulse width of the weld. When welding thin material, it is extremely difficult to control the power intensity of the laser on the surface. It is possible to power the laser extremely low (which is detrimental to the operating life of the krypton cathode lamps) and to adjust the pulse and feed rate to weld thin material. However, even after making these adjustments the laser typically will melt right through the material. The easiest thing to do in order to overcome this problem is to un-focus the laser on the material. For the thin stainless steel cylinder shown below, the beam was unfocused by 0.150”. It is important that the focal point be slightly below the metal pieces. By understanding how the convex lenses focus the beam, it is easy to understand that raising or lowering the piece will make a much wider laser beam. The beam is much wider above and below the focal point; however, below the focal point the beam is still at too high of an energy intensity. By un-focusing the beam above the focal point, the laser’s intensity on the part is lower, but this still allows the krypton cathode lamps to operate at a safe level (safe to the lamps operating life). When using this technique it is extremely important that the correct shielding gas is used in order to prevent weld splatter from hitting the lens. There is no exact science to how much the beam needs to be unfocused, and it is again a trial and error process.

Figure 102 - 0.020" Cylinder Laser Welded Butt Joint

112

INDUSTRIAL USE OF LASERS

Laser welding technology has been around for a long time, but the use in industry is still rather new. Initially, there was no need to use lasers over traditional welding methods. As technology has progressed and the manufacturing market fuses in with the “high tech” fields, laser welding is making a strong presence.

Figure 103 - Industrial CNC Laser

In large scale welding functions the use of lasers is extremely beneficial in robotic welding. The size of the torch required to produce the amount of energy that comes out of a laser beam can be contained in a fiber optic cable. This factor allows for welds, which are usually extremely difficult to reach with traditional welders, to be easily performed using a laser welder.

Laser welding in the high-tech field allows for a level of precision that was previously unattainable. Delicate electronics can now be fused together using a laser welder with extreme precision. Laser welding technology is helping make electronics smaller and much more precise. The high power required for larger materials is not required at this small level, and the laser unit can be contained in a much smaller unit, which makes the laser welder at this level much easier to use.

Even in the obscure fields, such as the jewelry business, laser welders have made an appearance. The precision along with the ability to weld virtually any metal have made the use of laser welding a valuable tool to the industry.

113

SAFETY

It is important to understand the inherent danger of any welding process, especially for a laser welder. Laser welders operate in a wavelength of light that is invisible to the human eye, but is extremely dangerous to the eye. For the Raytheon S550, the wavelength of light utilized is 1060 nm. This wavelength of light will pass through most common welding shields and can severely damage the eye. Specially designed optical protection gear must be worn at all times when operating a laser welder.

The laser beam is capable of producing an extremely intense heat. Combine this heat with the fact that the beam is invincible, makes the laser extremely hazardous. If a person were to place there hand under the laser beam, there would be extreme burning of the appendage. Be safe.

The last safety precaution has less to do with the laser beam itself, but rather with the power supply that powers the laser. Typical lasers use up to 480V of potential. The systems are often water cooled, and the combination of high voltage and water should always make the operator attentive of what is occurring at all times.

Refer to the American National Standard for the Safe Use of Lasers for further instruction on the safe use of laser welding.

CONCLUSION

Welding is an important metal joining process. In order to properly create a weld, it is important to keep in mind of the material being welded, the size of the weld and the functionality expected of the weld. The use of laser welding allows for complex welds to be created when the weld used to be extremely difficult or even impossible.

ACKNOWLEDGMENTS

I would like to thank Greg Jackson and the entire staff of Alpha Engineering and Design for all of their time, assistance with the assembly and for the donated machine time of the Raytheon SS550 Precision Laser Welder.

NOMENCLATURE

F = static force

Sy = yield strength

Ssy = yield stress in shear

A = area

E = absorptivity

ρr = electrical resistivity

d = spot size of the laser

f = focal length of the lens

Θ = full angle of beam divergence

P = input power to the laser

114

k = thermal diffusivity

t = pulse time

λ = wavelength of light

T = temperature

REFERENCES

ASM Handobook; Volume 6 Welding, Brazing, and Soldering. United States of America, 1993.

Bakish, R.. Proceeding of the Conference The Laser vs. Electron Beam in Welding, Cutting and Surface Treatment State of the Art 1989. Englewood, NJ: Bakish Materials Corporation, 1989.

Cary, Howard B.. Modern Welding Technology. Columbus, OH: Prentice Hall, 2002.

Charschan, Sidney S.. American National Standard for the Safe Use of Lasers. Orlando, FL: Laser Institute of America, 1986.

Fox Valley Metal-Tech. “Specialty Services”. http://www.fvmt.com/images/cnclaser1.jpg (Accessed November 7, 2006)

Giachino, Joseph W. and Week, William. Welding Skills and Practices. Alsip, IL: American Technical Publishers Inc., 1976.

Griffin, Ivan H. and Roden, Edward M.. Welding Processes. New York, NY: Delmar Publishers, Inc., 1970.

Griffing, Len. Welding Handbook; 6th

Edition. New York, NY: American Welding Society, 1971.

Stinchcomb, Craig. Welding Technology Today; Principles and Practices. Englewood Cliffs, NJ: Prentice Hall, 1989.

Stuzale, Andrzej. Theory of Thermomechanical Processes in Welding. Dordrecht, Netherlands: Springer, 2005.

Technical Manual SS550/SS525 Series Precision Laser Welder/Driller. Quincy, MA: Raytheon Company, 1989.

Weisman, Charlotte. Welding Handbook; 7th

Edition Volume 1. Miami, FL: American Welding Society, 1976.

fast starting hollow cathode test pallet · flow to the tether, through the power supply, and back...

Documents