[american institute of aeronautics and astronautics 42nd aiaa/asme/sae/asee joint propulsion...

42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit AIAA 2006-4990 9-12 July 2006, Sacramento, California

American Institute of Aeronautics and Astronautics

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Taking the Cost Out of MIL-DTL-23659D & MIL-HDBK-1512 Qualified Initiators

Abrar A. Tirmizi*

Special Devices, Inc., Moorpark, California 93021

The paper discusses design, qualification testing and product specifications for lower cost, high reliability initiators, qualified to MIL-DTL-23659D and MIL-HDBK-1512 (USAF) standards. These two pin, single bridge initiators and Micro Gas Generators (MGG) can trace their origin in the automotive safety systems with a proven track record. The devices are produced using high degree of automation to lower costs without sacrificing quality or reliability. In certain applications they offer significant cost savings to the end user while offering proven reliability, fast response time, safety, consistency, repeatability, quick turn around time and lead free initiators. The paper also discusses limitation of these devices in current applications as well as looks at further development required in order to make these more adaptable to existing applications. For new applications they have been readily adapted to lower the overall program costs while meeting performance and reliability requirements. These devices are not only suitable for various military applications but also offer great potential for the commercial aviation & maritime Industries.

Nomenclature BW = bridgewire BWR = bridgewire resistance ESD = electro static discharge LAT = lot acceptance testing µS = microsecond ms = millisecond mV = millivolts TT = Thermal Transient TTFL = time to first light

I. Introduction ver the last 45 years, Special Devices, Inc. (SDI) has been known as producer of high quality pyrotechnic devices. SDI started out as a special effects company for the movie industry in the 1950’s and entered the

aerospace pyrotechnic device business in the 1960’s. In the 1980’s SDI entered the highly competitive automotive airbag initiator market. Since then SDI has

produced over 650 million initiators for this industry, with no recalls, making these one of the most reliable initiators in the market today. The units are produced on automated production lines in high volume and are known for their firing reliability, safety, consistency and ruggedness.

In 2003, SDI realized the need for a high volume, high reliability COTS type initiator for military applications and decided to base the design of such initiators on the automotive initiators. SDI harnessed the expertise gained from designing and producing high volume, high reliability automotive initiators for this application. The initiators were put through the qualification testing where they met or exceeded the performance requirements per MIL-DTL-23659D and MIL-HDBK-1512 (testing in progress).

In order to address this requirement SDI created a family of initiators, SDI part number 192430-XX for these two pin initiators, which has 13 active configurations (dash numbers) all of which can be produced on automatic equipment. In order to simplify manufacturing and keep the costs down, internal assembly parts such as header, output can, insulator cup etc. were standardized wherever possible. A common ignition train is used for all 13 configurations.

* Senior Project Engineer, Research & Development, 14370 White Sage Road, Moorpark, Ca. 93021 (Member)

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42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit9 - 12 July 2006, Sacramento, California

AIAA 2006-4990

Copyright © 2006 by Abrar A. Tirmizi and Special Devices, Inc. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.



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The primary difference between these configurations was the quantity and/or the type of energetic material used for output powder and the consolidation pressure for the output load. Total output charge ranging from 35-mg to 310-mg of various energetic materials (ZPP, THPP, BKNO3 etc.) is readily available. The total charge of the initiators may be changed by changing the amount of output charge consolidated.

In 2004, SDI completed the qualification testing and received approval for these militarized automotive initiator from NAVSEA, Indian Head. Since then the initiator has been tested successfully in several applications and offers significant cost savings and quick turn around time to the military while offering proven reliability, fast response time, safety, repeatability and lead free initiators.

II. Initiator Description

The ignition element consists of a single one-Ohm Bridge welded between a glass-sealed pin and a stainless steel header body. The second pin of the ignition element is welded to the header body. Both initiator pins are gold plated to provide excellent corrosion resistance and low contact resistance. A stainless steel charge sleeve is welded to the header body and provides a protective cavity for the ignition charge. 35-mg of ZPP is consolidated charge into this cavity over the bridge wire to form the ignition train. The output charge is consolidated to a design pressure in a stainless steel output can to form the output charge. The loaded output can is welded to the header assembly providing hermetic sealing of the entire ignition train. The hermeticity is achieved by the glass sealed pin to the header and the weld between the header and output can. An insulator cup, made from engineering thermoplastic is placed around the output can-header sub-assembly. The sub assembly is molded with glass filled engineering thermoplastic such that the only conductive surfaces on the initiators are the two initiator pins. Everything else is electrically isolated from the outside. This is a case grounded design that does not rely on spark gap for ESD protection and offers superior ESD performance.

Figure 2. Cross Sectional View of the Militarized Automotive Initiator

Figure 1. Initiator Assembly



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Configurations other than the one shown in figures 1 & 2 are also possible depending on application. For Example, the thermoplastic bi-conical seal may be molded in a different configuration and/or a metal flange may be added either during the final molding process or the bi-conical seal may be crimped into another metal housing that may be welded in place to the customer device.

III. Initiator Specifications Table 1. Design Specifications for the high volume Initiator.

Design Parameter: Specification:

1 Bridge Wire Resistance: 0.90 Ω to 1.10 Ω at full operating temperature range. 2 NO Fire Current 1.00 Amps, 300 second min. pulse with 99.99 reliability at 95% confidence

level. 3 All Fire Current 2.50 Amps, 50-msec max. pulse with 99.99 reliability at 95% confidence level. 4 Operating Temp. Range -80°F to +225°F. 5 Function Time (TTFL): Less than 5 ms with a 3.50 Amperes constant current pulse. 6 Insulation Resistance: >10 MΩ @ 500 VDC for 120 seconds between shorted pins & molded body. 7 ESD: Meets 500 pF capacitor charged to 25 kV, with 5000Ω or 500Ω series resistor

ESD pulse, pin to pin and shorted pin to initiator body. 8 Hermeticity: Better than 10-6 cc/second leak of helium/air with 1 atmosphere pressure

differential 9 Pressure Capability: Capable of withstanding 25,000 psi min. peak pressure for 2 msec at ambient

temperature range from -40°F to +230°F during deployment with optimum installation configuration.

10 Initiator Materials: Ignition Element: All stainless steel welded construction. Final Assembly: Glass filled Engineering Thermoplastic body & thermoplastic insulator cup

11 Primary Ignition Charge: 35-mg ZPP minimum (Common ignition charge for all dash nos.), no output charge

12 Output Ignition Charge: 275-mg ZPP (192430 -11), other output charge sizes & energetic materials available

Since the entire family of these initiators use a common ignition train, the function time, All-Fire, No-Fire

parameters are common among all 13 configurations. The figure below shows average function time for eight groups of initiators. Each group of initiators was fired with a fixed firing current and time to first light was measured and recorded. Both MIL-STD-23659D and MIL-HDBK-1512 require 5.0 Amperes constant current pulse for firing.

Figure 3. Potential Design Variations with Molded in Flange and Initiator Crimped into a Flange.



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TTFL Vs Firing Current

0.00

5.00

10.00

15.00

20.00

25.00

0.00 1.00 2.00 3.00 4.00 5.00 6.00

Firing Current (Amperes)

Tim

e to

Fir

st L

ight

TT

FL (m

sec)

Ave TTFL

St. Dev.

Figure 4. Firing Current Vs Function Time.

The firing reliability was estimated using the Bruceton methods during the development phase on 192430-2. Test

parts were fired at -80°F for All-fire Bruceton and at +225°F for the No-fire Bruceton. Firing reliability was calculated at 95% level of confidence. The results are shown in table 2. These numbers were later validated during the qualification testing on 192430-11 initiators. Table 2. All-Fire and No-Fire Ballistics Reliability Data.

Constant Current All-Fire and No-Fire Brucetons Militarized Automotive Initiators - P/N: 192430-2

Estimated Firing Reliability estimated

at 95% level of confidence

All-Fire current (Bruceton method)

at -80°°°°F Constant Current pulse for 50 msec.

No-Fire current (Bruceton method)

at +225°°°°F Constant Current pulse for 300 sec.

Mean 1.455 Amps 1.241 Amps

St dev 0.021 Amps 0.017 Amps

.99 1.526 Amps 1.182 Amps

.999 1.549 Amps 1.164 Amps

.999999 1.598 Amps 1.123 Amps

Effect of exposure to 1 amp / 1 watt for 300 seconds to the initiator performance was another concern that was addressed during the development phase. Automotive initiators are not required to meet this requirement and no historical data was available even close to this type of exposure. Minimal effects of this exposure were found during development or qualification testing, well within performance requirements of the initiators as shown below.



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Table 3. Effect of 1 ampere constant current pulse exposure for 300 seconds on Initiator performance.

Militarized Automotive Initiators - P/N: 192430-2 Bridge Wire Resistance before & after

1.0 amp constant current exposure for 300 second at +225°F

S/N:

BWR at +72°F before

exposure

BWR at +72°F (1 hour after

exposure)

BWR at +72°C (16 hours after

exposure)

Time to first light (TTFL)

(µs)

VIRGIN PARTS

- Time To

First Light TTFL (µµµµs)

31 1.009 1.059 1.084 700 575

32 1.018 1.041 1.058 720 548

33 1.024 1.037 1.034 640 530

34 1.029 1.032 1.026 640 536

35 1.011 1.025 1.05 700 530

36 1.013 1.032 1.037 660 560

37 1.018 1.039 1.051 720 548

38 1.006 1.028 1.049 660 540

39 1.004 1.023 1.018 640 548

40 0.987 1.028 1.012 740 540

Mean 1.012 1.034 1.042 682 546

St. dev. 0.012 0.01 0.021 38.2 13.9

Max 1.029 1.059 1.084 740 575

Min 0.987 1.023 1.012 640 530 Automotive airbag initiators have significantly different operating and environmental requirements. In order to

meet the requirements of MIL-DTL-23659D and MIL-HDBK-1512 the majority of work was done in developing the ignition train, especially to address the 300 second, 1 amp / 1 watt, No-Fire requirement and to address higher shock and vibration environment. Six sigma tools such as DOE and COV were used extensively to come up with a design that can satisfy these stringent requirements while still meeting the goal of using existing in-house automation and minimizing new tooling or components.

IV. Manufacturing All automotive initiators are produced using high degree of automation to lower costs while providing superior

quality, reliability and repeatability. This includes welding bridge wire to header assembly, installing and welding the charge sleeve, weighing and dispensing of ignition and output powders, ignition and output powder consolidation, welding of the loaded output cans to the header assembly, Radiflo leak detection, over molding to complete the assembly, as well as Final Electrical Testing and X-ray inspection.

Fig 5: BW welding & Measurement Fig. 6: Ignition Powder Loading Fig. 7: Output Powder Loading



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Fig 8: Molding Machine, Final Assy: Fig. 9: Final Electrical Testing: Fig.10: X-Ray Inspection:

In order to thrive in the automotive market place understanding process control and variation are vital to becoming a market leader. By quantifying effects of process variation and taking corrective actions where necessary, initiator performance can be made highly consistent, predictable and reliable. Thermal Transient tests and real time digital X-ray inspection are designed to root out problematic initiators “during” the production process. Automation is designed to look for and identify potential problems with initiators even before Lot Acceptance Testing. All initiators manufactured are required to pass these tests before reaching the LAT at the end of production process. This minimizes complete lot rejection during LAT and offers overall cost savings.

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40 45 50 55 60 65

Test

Seq

uenc

e

Militarized Automotive InitiatorThermal Transient Study

770 mA / 0.10 ms / 10 ms Test Pulse

Thermal Transient Response (mV)

Loaded Parts (90 mg ZPP)Average TT = 5.71 mV

Stdev = 0.300 mVMean + 4.5s = 7.06 mVMean - 4.5s = 4.36 mV

BW Temp = 23°C

Unloaded PartsAverage TT = 42.03 mV

Stdev = 7.56 mVMean + 4.5s = 76.07 mV

Mean - 4.5s = 8.0 mVBW Temp = 170°C

Figure 11. Thermal Transient Response (In process testing using automation).

A great deal of effort goes into understanding the processes employed and their effects on initiator performance and production yields. The following chart is an example of such efforts and represents pressure curves from three groups of initiators that were created to investigate the effects of varying consolidation pressure applied to the output charge and the resultant free volume between ignition and output charges on the ballistic performance of initiators. It was verified that manufacturing process variations and the resultant distance between ignition and output charges have no significant effect on the ballistic performance of these initiators.



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Figure 12: Affects of Output Powder consolidation on Ballistics performance

V. Cost Savings

SDI uses Lean and Six Sigma tools through out its operations to improve its efficiency. These tools are extensively used to increased safety and quality, reduced scrap rates, improved throughput, reduced costs and help it remain the industry leader. The benefits of this to the customers are demonstrated 100% on-time delivery, single digit PPM, consistent and reliable product and competitive pricing.

Automation, Lean Manufacturing, and Six Sigma methodology has resulted in significant cost savings without compromising quality, performance, or deliveries and benefits both SDI and its customers.

VI. Qualification Testing All parts intended to be used for the qualification testing were built in one lot on the existing production lines.

Parts were randomly selected from the build lot and serialized after welding ignition elements and helium leak testing including spares. The spares were used for setting up test equipment/fixtures. During the standard production process some of the tests are done only on a sample basis. Also, not all the test results are recorded and none of the test data is traceable to individual initiators during the standard production build process. Additional test data that was required to be traced to individual initiators was generated and recorded after the units had gone through the standard production build cycle. The first helium leak test was performed on all initiators including spares before molding and before serializing the initiators. All tests conducted on these serialized initiators were recorded and are traceable to the individual initiators through out the testing. Various tests were run in parallel to each other where ever the MIL standards permitted it and equipment and personnel were available. Some of the tests were conducted at SDI, Moorpark, California and others were conducted at an outside test facility.

Helium Leak testing is a requirement for the qualification process in order to test the seal integrity of the ignition train. However, plastics have a tendency to absorb helium and cause a false leakage reading during the test. In order to avoid this, only un-molded initiators went through helium leak test through out the tests. All initiators, including spares went through helium leak test prior to molding. Separate groups of initiators were built without molding the thermoplastic wherever leakage test was required after environments. The pressurized chamber helium leak test was



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only done on the initiators in un-molded condition to establish the seal integrity of the ignition train for all the initiators that were tested

Since the bi-conical seal is prone to damage/deformation if the initiators are over torqued or not installed correctly in fixtures, special attention was paid to multiple installations and removals from shock and vibration fixtures during the course of testing.

VII. Future Developments

The 192430 family is a derivative of automotive airbag initiators which are designed to be installed in airbag inflators and mate with automotive type connectors. This method has proven it self to be a reliable method of installation for automotive safety systems over many years. For new applications that are still under development and will be going through a qualification test program, the use and installation method for these type of initiators is relatively straight forward. As a matter of fact these initiators are already incorporated in several new military applications. However, for existing military applications that use a conventional 4-pin interface per MS3116 and 3/8-24 thread for installation, some sort of adaptation of this initiator is required. Using the 192430 family of initiator in its present form will require changing the existing connectors and installation method on the hardware which is not always possible and is highly undesirable. A more desirable option is to incorporate the 192430 initiator into a package which is readily installable into an existing application. However, the192430 family of initiators is too large to fit in the 3/8-24 threaded portion of the conventional aerospace initiator and the only place to install it is in hex part of the initiator. This is feasible but it will increase the overall length of the 4-pin initiator that may not be acceptable for some applications. Please see figures 13 & 14.

Another option is to develop a new family of initiators based on high volume industrial type initiators that can fit into the 3/8-24 threaded portion of the conventional initiator and qualify it to meet the requirements of MIL-DTL-23659D and MIL-HDBK-1512. However, this discussion is outside the scope of this paper.

For existing applications where the threaded portion of the existing initiators is 7/16-20 or larger threads, it is still possible to incorporate the 192430 family of initiators in the threads and come up with an assembly of identical size to the existing one. All these adaptations still need to be developed and tested and qualified to meet the requirements of MIL standards as well as the application.

VIII. Conclusions • Special Devices has successfully redesigned its automotive airbag initiators to meet the requirements of

MIL-DTL-23659D and MIL-HDBK-1512 and demonstrated it by putting these initiators through both of these qualification test programs.

• These initiators have already been installed in military hardware that successfully completed their

qualification testing for the specific application.

• For new applications they have been readily adapted to lower the overall program costs while meeting performance and reliability requirements.

• These devices are not only suitable for certain military applications but also offer great potential for the

Commercial Aviation & Maritime Industries. A variety of output charge sizes and energetic materials can be tailored to suit the intended applications.

• In order to gain the full benefits of these types of initiators further development is required to make these

more readily adaptable to existing military hardware.

Figure 14: Adaptation In to Hex

Figure 13: 192430 and 4-Pin Initiator

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