1. Mike Watson Dave McLain
Cooled Optical Bench (COB) for EMAS Mechanical Design Review Nov. 15, 2010
Mike Watson
Dave McLain

2. Introduction System layout Mechanical properties Thermal performance
Structural performance
Vacuum operations
Open issues

3. System Layout Vacuum Gage Preamp Electronics Box
Alvatec Barium Tube Getter
Vacuum Port/Valve
Vacuum Housing
LN2 Feedthrus
P2 Beamsplitter
SunPower GT Cryocooler
50pin Connector (x3)

4. System Layout
Radiation Shield (gold plated on outside, black on inside)
Optical Bench
Thermal Links
LN2 Heat Exchanger
G10 Mounts

5. COB Mechanical Mechanical Requirements Total Mass: 29.0 lb
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: lb
Vacuum 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

6. 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.

7. COB Thermal Thermal model System Level Results FEMAP/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 Results
Unacceptable 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

8. COB Thermal Thermal Results 10K gradient throughout radiation shield
1-2K gradient on optical bench
40mK gradient on optical tube

9. 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.
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.

10. COB Cooldown Cooldown conditions Results
300W 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
Results
System <80K at ~40min
30 minute period without cooling warms the system ~10K.
Note: ~3hrs to cooldown without LN2

11. COB Structural Structural 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 loads
The 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)

12. COB Structural Design 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 analysis

13. COB Structural Structural model Model Inputs: Model Results
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 Results
MOS against yielding = (3.4 if only 1atm)
MOS against ultimate = (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

14. COB Structural – Lenses
Lens Assemblies
Titanium 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

15. COB Structural – ZnSe Window
Diameter = 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

16. COB Vacuum Vacuum Requirements
Mechanical measurement DELIVERABLE documenting demonstrated leak rate consistent with 30 mTorr 3 months after pump down
Vacuum Housing
Vacuum Gage
Alvatec Barium Tube Getter
Vacuum Service Port/Valve

17. COB Vacuum Vacuum 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 sequences
Alvatec barium getter capacity (per gram of barium)

18. COB- Open Issues Mass overage Cryocooler heat sinking
Changing heat sink path
Providing alternate means of cooling during cooldown and during ground operations
Forced gas heat exchanger
Cryocooler vibration
60Hz input mounted directly to vacuum shell
No modes of concern near 60Hz in COB
Potential resonance inputs