JWST Testing Issues – Thermal & Structural

1. JWST Testing Issues – Thermal & Structural William Bell, Frank Kudirka, & Paul-W. Young Topsfield Engineering Service, Inc.

2. Purpose This study explores JWST thermal and structural testing issues and possible solutions, as presented to NASA in June 2004

3. Summary • Facility Goals • Thermal Design • Helium Refrigeration • Structural Design • Vibration Isolation • Clean Room Considerations

4. Testing Facility Goals • Provide for controlled cool-down, soak, and warm-up • Be capable of taking the Test Assembly from 300 K to 35 K and steady state within 10 days • Hold to a set point temperature, within ± 1 K, during steady state conditions • Vibration isolation

5. Proposed TestFacility Vacuum Chamber 20 K Shroud or Dewar Test Article 80 K Shroud Thermal Desktop Model Ties Helium Refrigerator Flow Paths

6. Proposed Test Facility – Weights - lbs • Chamber 740,000 • N2 Shroud 101,000 • He Shroud 45,000 • Outer Structure 60,000 • Inner Structure 190,000 • Test Article 8,000

7. Thermal Control Considerations • Evaluate different options for Cool down • Radiative Heat Transfer • Natural Convective Heat Transfer • Sealed 20 K Shroud (dewar) at 1 torr • Unsealed 20 K Shroud with operation at 0.01 torr • Forced Convection Heat Transfer • In shroud tubing • Tracing tubing on structure • Mass Flow Heat Transfer • Within Shroud and Tracing tubing • Direct contact spraying • Evaluate options for thermal insulation • MLI Blankets • Minimize temperature difference - <2 C across structure

8. Cool down Methods • Method 1 - Radiation only • Method 2 - Radiation and natural convection in the 20 K Dewar at a pressure above the Chamber Pressure • Method 4 - Radiation and natural convection in the entire Chamber For each above Method, the 80 K shroud is cooled down at a rate consistent with the 20 K shroud (dewar) The Test Support structure and the 20 K shroud (dewar) are cooled by forced convection flow of Helium gas from the 5 kW refrigerator

10. CHAMBER 20K Helium SHROUD 80K GN2 SHROUD TA 10-5 Torr METHOD 1A CHAMBER CHAMBER 20K He SHROUD 20K He SHROUD TA TA 2” MLI BLANKET 2” MLI BLANKET 10-5 Torr 10-2 Torr METHOD 1C METHOD 1B CHAMBER CHAMBER 2A 0.1 Torr 2B 1.0 Torr MLI BLANKETS 20K Helium dewar - pressure tight 4A: 1” thick 4B: 2” thick 20K He SHROUD 80K GN2 SHROUD 80K GN2 SHROUD TA 4A 0.01 Torr 4B 1.0 Torr 10-5 Torr METHODS 2A & 2B Note: Both shrouds as tight as possible Thermal Analysis Cases TA METHODS 4A & 4B

11. Thermal Model Construction • Nodes: 33 • Linear Conductors: 46 • Radiation Conductors: 58 • Lumps: 3 • Paths: 2 • Ties: 7

12. LN2 Shroud Chamber Wall He Shroud Item Description Surface Emissivity i/o 1. SPF Chamber Inner Bare Aluminum 0.10 2. LN2 Shroud Outer Bare Aluminum 0.10 3. LN2 Shroud Inner Z307 0.87 4. He Shroud Outer Bare Aluminum 0.10 5. He Shroud Inner Z307 0.87 6. Test Article/Structure SS304L/Z307 0.15/.7 7. SPF Chamber Outer Bare Aluminum 0.10 7 2 3 6 1 4 5 Test Structure Thermal Model Surface Finish/Emissivity

13. Flow Regime Definition • Knudsen Number • Kn = /p, where  is the mean free path and p is the characteristic dimension. • Continuum flow – Kn < 0.01 • Transition flow – 0.01 < Kn < 0.3 • Molecular flow – Kn > 0.3

14. Chamber Flow Regimes

15. Molecular Conduction Thermal Desktop Capability

16. Thermal Desktop Capability Natural Convection Caution

17. Method 1 Results

18. Method 2 Results

19. Method 4 Results

21. Helium Plant PFD GAS STORAGE COMPRESSOR 80K – 300K LN2 Supply < 80K EXPANDERS TO SHROUD/STRUCTURE FROM SHROUD/STRUCTURE Helium Refrigeration He Plant Size is based on analysis results shown on Slide 18

22. Structural Design Considerations • 80 K and 20 K shroud support • 6 stainless steel columns in corners • Must allow for 1” of radial shrinkage • Not connected to test structure • Columns could bend if long enough or could be placed on rollers • A thermal break is required - G-10 block sandwiched between flanges • 20 K shroud hung off 80 K shroud

23. Structural Design Considerations • 20 K Dewar • Clamshell design • 3 stainless steel columns in corners • Rollers at base to move unit around and allow radial shrinkage • 80 K shroud hung off 20 K Dewar

24. 20 K Shroud Design • 20 K Shroud to be “Pressure Tight” • Design Pressure inside shroud 1 to 10 torr • 20 K Shroud to be “Flow Tight” • Pressure in entire chamber 1x 10-2 torr

25. Structural Design Considerations • Test Support Material Selection • Differential material strain - Al and SS are virtually the same below 25 K • Al shrinks 37% more than SS from ambient to about 20 K • Al can be made as stiff as SS by making the beams deeper, the equivalent beam in Al will weigh 41% as much as SS • Al must be heat treated after welding to recover its strength • Welded Al or Welded SS may have different properties than un-welded, small differences below 20 K • Different alloying materials in different heats of either Al or SS could result in slightly different properties. Again small effect below 20 K • Micro-yield stress in Al is lower than SS, but so is the modulus. Allowable temperature rise in a restrained beam is almost equal between the 2 materials • Al is 10 times more conductive than SS, therefore, easier to isolate thermally SS columns than Al columns. Both, however, need thermal breaks • Because of structure size consideration and the heat treat requirement of Al, SS is recommended over Al

26. Support Concept

27. Vibration Design Considerations • Minimize, or eliminate, any vibration transmission to the Test Assembly • To avoid subjecting the test assembly to random or non-repeating load (s), of a magnitude that would affect optical test stability • Support of Test Fixture on “hard points” until after cool-down

28. Test Support & Article 20 K Shroud w/plenum Access Platform Clean Air Flow In Clean Air Flow Out 80 K Shroud Note that top shroud panels are removed for visibility Class 10,000Clean Room

29. Acknowledgements • The Study that led to the development of this presentation was accomplished under a contract with Crawford Consulting Services, Inc. for the NASA Plumbrook Facility Team