Mike Watson Dave McLain 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

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)

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

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: 29.0 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

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.

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

COB Thermal Thermal Results 10K 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. 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 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

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)

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

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 = 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

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

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

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

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)

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