Cooled Optical Bench (COB) for EMAS Mechanical Design Review

Nov. 15, 2010

Mike Watson

Dave McLain

Introduction

System layout

Mechanical properties

Thermal performance

Structural performance

Vacuum operations

Open issues

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System Layout

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SunPower GT Cryocooler

Vacuum Housing

LN2 Feedthrus

Preamp Electronics Box

Vacuum Gage Alvatec Barium

Tube Getter

Vacuum Port/Valve

P2 Beamsplitter50pin Connector

(x3)

System Layout

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LN2 Heat Exchanger

Radiation Shield (gold plated on outside, black on inside)

Optical BenchThermal Links

G10 Mounts

COB MechanicalMechanical Requirements

Mechanical measurement DELIVERABLE documenting weight less than 25 pounds fully integrated and flight ready with support electronics and other structures required operationally.

Mechanical drawing DELIVERABLE documenting that Vacuum package measures less than given dimensions in section A and support electronics measure less than 5.5”x7”x7” or can be placed more than 36” away

Volume has been redefined through model/hardware exchange

Total Mass: 29.0 lbVacuum box 8.8lb

Cryocooler 7.3lb

Optical bench 2.2lb

Port 4 lens assy 1.5lb

Radiation shield 1.0lb

Internal thermal links 0.8lb

Cryocooler thermal link 1.0lb

Vacuum acc. 1.0lb

Grating assy 0.3lb

Cables 1.0lb

Misc 4.1lb

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COB Thermal Thermal Requirements

An opto-mechanical design for a vacuum cooled optical bench.

The cold stage must be cooled by a mechanical sterling cycle cooler with appropriate vibration mitigation.

The FPAs must be cooled to 77 degrees K or less (65 degrees desired), +/-10mK (+/-3 mK desired).

Updated to cooling entire optical system and FPAs to 77K controlled to within +/-100mK by cryocooler controller.

The cold stage must include a shroud that is temperature controlled at a level that reduces and stabilizes the background seen by the detector array outside the imager optics.

The system must be able to reach operational temperature in 15 minutes and a stable steady state temperature within 45 minutes. The system may use a liquid nitrogen reservoir to help speed cooldown times but may not rely on it. The system must be capable of reaching steady-state operating temperatures with just the mechanical cooler when cryogens are not available.

The system must be able to dissipate heat conductively due to the lack of convective cooling in the low pressure environment.

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COB Thermal

Thermal modelFEMAP/SINDA/NEVADA

Internal bench cooled to 80K by cryocooler or by LN2 heat exchanger

External vacuum housing bolted to mounting plate at baselined to operate around -10C

System under vacuum

No MLI blanketing, gold plated radiation shield

Radiation between radiation shield / vacuum skin / internal components is included

~300mW from window applied to radiation shield

3x G-10 legs to optical bench

3x G-10 legs to radiation shroud

300W of cryocooler power sunk to baseplate

System Level ResultsUnacceptable gradients in baseplate and lower baseplate

Investigating methods of sinking heat to lower structure

Modeled this way for worst case conditions on COB

Could be as low as 200W depending upon steady state heat loads on cryocooler

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COB Thermal

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Thermal Results10K gradient throughout radiation shield

1-2K gradient on optical bench

40mK gradient on optical tube

COB Cryocooler Heat Budget

Operational heat budget:G-10 Legs for optical bench: 1.2W

G-10 legs for radiation shield: 1.2W

Radiation load on radiation shield: 2.5W

Radiation load through window: .3W

Wiring heat sink: 2.5W

Plumbing lines: .3W

Total: 8W

At this cooling load, the cryocooler will sink ~160W to the baseplate during steady state operation.

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During cooldown, cryocooler will need to sink up to 320 watts. Conductance links designed to carry steady state loads will not suffice during cooldown, allowing cryocooler to overheat (>80C). Conductance links large enough to carry the cooldown load will incur a weight penalty (1-2 lbs). Either way, we are investigating adding a high temperature automatic shutdown to the cryocooler.

COB Cooldown

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Cooldown conditions300W from cyrocooler still applied to mounting plate

System heat still removed through lower structure

LN2 channel activated

LN2 and cryocooler secured for 30 minutes at the 60 minute mark

ResultsSystem <80K at ~40min

30 minute period without cooling warms the system ~10K.

Note: ~3hrs to cooldown without LN2

COB StructuralStructural Requirements

An opto-mechanical design for a vacuum cooled optical bench.Adequate means of mitigation against vibration generated by the mechanical cooler must be provided. The bench that holds the cold optics, shroud and FPAs must be attached to the mechanical cold head through vibration isolating flexible thermal links. The COB must maintain operability and alignment after undergoing a shake test described by this curve. The vacuum package of the COB must remain sealed and stable through the altitude and pressure cycle of a typical 8 hour mission.In general, structures designed to withstand pressurization shall be designed to an ultimate pressure of 2*P with a 50% safety margin, in addition to acceleration and aerodynamic loadsThe instrument must withstand design limit-loads without deformation or failure

ER-2 acceleration design loads:Below 50,000 feet

Nx = ±1.2 g's (longitudinal)

Ny = ±1.0 g's (lateral)

Nz = +5.0, -2.5 g's (vertical)

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COB StructuralDesign loads:

To determine maximum acceleration inputs into the COB we need to compare:

+5.0g acceleration design load

COB accelerations experienced under random vibration input

Random vibration input applied on non-COB side of vibration isolators

N5220-H Barry Isolator• ~20Hz natural frequency when loaded to

40# per• Natural frequency lower in transverse

directions (~15Hz)• Transmissibility at resonance: 4

Because of the relatively low natural frequency of the isolators (compared to COB), the COB with undergo rigid body motion.

Multiply transmissibility curve with random vibration input (ASD) to get frequency response curve.

Integrate, then square root frequency response curve to get Grms acceleration.

Results: Acceleration = .96grms

Even with 3sigma multiplier, this is less than the 5g design load input

Note: this result correlates with Mile’s equation approximation

Use ER-2 acceleration design load for structural analysis12

COB StructuralStructural model

FEMAP/NASTRAN

SDOF calculations for random vibration inputs vs. vibration isolators

Static loads analysis

Modal analysis

Model Inputs:Constrained at vacuum shell bolted flanges

Loaded with +5gs vertical, 1.2gs lateral and longitudinal

2atm of differential pressure across vacuum boundary

Safety factor for yield = 1.5

Safety factor for ultimate = 1.5

Model ResultsMOS against yielding = 1.2 (3.4 if only 1atm)

MOS against ultimate = 1.4 (3.8 if only 1atm)

First overall mode = 224Hz (mounting isolators)

First mode involving optical elements = 302Hz

Fatigue life for vacuum chamber >10^7 cycles

Bolted joint analysis for preload/CTE/seperation yet to be done

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COB Structural – Lenses

Lens AssembliesTitanium housing and spacers

Spring loading in axial and radial directions

Springs designed to maintain preload against a 5g acceleration (1.8 safety factor)

Relatively long springs allow for more precise loading

Resistant to assembly CTE stress effects

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COB Structural – ZnSe Window

ZnSe WindowDiameter = 44mm

Thickness = 3mm

O-ring sealed

O-ring groove designed such that window will not bottom out onto aluminum given 1atm differential pressure

Safety factor calculations

Strength ZnSe = 95Mpa

1.125 stress multiplier for unclamped condition

1atm of pressure

Safety factor = 4

MOS = 2.5

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COB Vacuum

Vacuum RequirementsMechanical measurement DELIVERABLE documenting demonstrated leak rate consistent with 30 mTorr 3 months after pump down

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Vacuum Housing

Vacuum Gage Alvatec Barium

Tube Getter

Vacuum Service Port/Valve

COB VacuumVacuum Design

O-ring designs from Parker’s O-Ring Vacuum Sealing Handbook 5705B

Using Butyl or Viton

30% compression ratio

Largest diameter o-rings as possible

O-Ring groove surface finish improvements

No MLI blanketing

Alvatec barium tube getter (loaded with 3g of barium)

No regeneration available

Regenerative getters are too large

Heat activated- indium seal melted to initiate pumping

Disposable

~90$ per with 3 week delivery (from Europe +65$ shipping per batch)

Standard swagelok fitting

Does not pump noble gases

~30% capacity used for N2 & O2 after 90 days

Installed convectron gage

1*10^-4 – 1000 Torr range

Vacuum service port

Removable operator

System testing

Helium leakcheck

System bakeout – temperature/duration not yet determined

Pressure rise sequences17

Alvatec barium getter capacity (per gram of barium)

COB- Open Issues

Mass overage

Cryocooler heat sinkingChanging heat sink path

Providing alternate means of cooling during cooldown and during ground operations

Forced gas heat exchanger

Cryocooler vibration60Hz input mounted directly to vacuum shell

No modes of concern near 60Hz in COB

Potential resonance inputs

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