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Topsfield Engineering Service, Inc.
JWST Testing Issues – Thermal & Structural
William Bell, Frank Kudirka, & Paul-W. Young
Topsfield Engineering Service, Inc.
Topsfield Engineering Service, Inc.
Slide 2
Purpose
This study explores JWST thermal and structural testing issues and possible
solutions, as presented to NASA in June 2004
Topsfield Engineering Service, Inc.
Slide 3
Summary Facility Goals
Thermal Design
Helium Refrigeration
Structural Design
Vibration Isolation
Clean Room Considerations
Topsfield Engineering Service, Inc.
Slide 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
Topsfield Engineering Service, Inc.
Slide 5
Proposed TestFacility
Vacuum Chamber
80 K Shroud
20 K Shroud or Dewar
Test Article
Ties
Helium Refrigerator Flow Paths
Thermal Desktop Model
Topsfield Engineering Service, Inc.
Slide 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
Topsfield Engineering Service, Inc.
Slide 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
Topsfield Engineering Service, Inc.
Slide 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 ChamberFor 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
Topsfield Engineering Service, Inc.
Slide 9
Test Article Heat Load Distribution
Case
Radiative Load - watts
Conductive Load - watts
1 1000 0
4 200 800
6 50 950
7 25 975
9 5 995
Thermal Model CasesMethod No
Description
Vacuum - torr
Test Article
Chamber
1A He Shroud & N2 Shroud - no MLI 1e-5 1e-5
1B He Shroud & MLI only - 2” blanket 1e-5 1e-5
1C He Shroud & MLI only - 2” blanket 0.01 0.01
2A He Dewar & N2 Shroud - no MLI 0.10 1e-5
2B He Dewar & N2 Shroud - no MLI 1.00 1e-5
4A He Shroud & N2 Shroud - 1” MLI both Shrouds 0.01 0.01
4B He Shroud & N2 Shroud - 2” MLI both Shrouds 1.00 1.00
MLI Model thermal conductivity vs. pressure
Pressure - torr k - watts/in K
1e-5 8.382e-7
0.01 6.604e-6
1 0.000127
Topsfield Engineering Service, Inc.
Slide 10
Thermal Analysis CasesCHAMBER
METHOD 1A
80K GN2 SHROUD
20K Helium SHROUD
TA
10-5 Torr
TA
CHAMBER
METHODS 2A & 2B
80K GN2 SHROUD
20K Helium dewar - pressure tight
10-5 Torr
2A 0.1 Torr2B 1.0 Torr
CHAMBER
Note: Both shrouds as tight as possible
80K GN2 SHROUD
20K He SHROUD
TA
4A 0.01 Torr4B 1.0 Torr
MLI BLANKETS
4A: 1” thick4B: 2” thick
METHODS 4A & 4B
CHAMBER
METHOD 1B
20K He SHROUD
TA
10-5 Torr
2” MLI BLANKET
CHAMBER
METHOD 1C
20K He SHROUD
TA
10-2 Torr
2” MLI BLANKET
Topsfield Engineering Service, Inc.
Slide 11
Thermal Model Construction
Nodes: 33 Linear Conductors: 46 Radiation Conductors: 58 Lumps: 3 Paths: 2 Ties: 7
Topsfield Engineering Service, Inc.
Slide 12
Thermal Model Surface Finish/Emissivity
LN2 Shroud
He ShroudChamber Wall
Test Structure
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
1
7
23
54
6
Topsfield Engineering Service, Inc.
Slide 13
Flow Regime Definition
Knudsen NumberKn = /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
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Slide 14
Chamber Flow Regimes
Pressure Temperature
inchesKn
Flow Regime
torr K - - -
1 30 0.0004 3.6e-5 Continuum
0.1 30 0.004 0.00036 Continuum
0.01 30 0.04 0.0036 Continuum
10-6 30 426 35.5 Molecular
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Slide 15
Thermal Desktop Capability
Molecular Conduction
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Slide 16
Thermal Desktop Capability
Natural Convection
Caution
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Slide 17
Method 1 Results
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Slide 18
Method 2 Results
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Slide 19
Method 4 Results
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Slide 20
Thermal Analysis Results
Method CaseTime to reach Steady State
days
Test Article Steady State Temperature -
K
Meets Test Article Cool down
Temperature and Time goals
Cold Steady State Helium Gas flow rate grams/sec
Cold Steady State Heat Rate to Helium gas
watts1A 1 8.3 72 NO 40 11671A 4 8.3 49 NO 40 11671A 9 8.3 29 YES 40 11671B 1 8.3 73 NO 44 12931B 4 8.3 47 NO 44 12931B 9 8.3 29 YES 44 12931C 1 10.8 58 NO 109 32561C 4 10.0 35 YES 109 3256
1C 9 10.0 25 YES 109 3256
2A 1 7.9 40 NO 62 17872A 4 7.9 26 YES 62 17872A 9 7.9 22 YES 62 17872B 1 7.9 29 YES 62 17872B 4 7.9 24 YES 62 17872B 9 7.9 22 YES 62 17874A 1 8.3 88 NO 137 39874A 4 8.3 50 NO 137 39874A 9 8.3 25 YES 137 39874B 1 5 33 YES 194 56254B 4 5 28 YES 194 56254B 9 5 24 YES 194 5625
Topsfield Engineering Service, Inc.
Slide 21
Helium Refrigeration Helium Plant PFD
COMPRESSOR
LN2 Supply
GAS STORAGE
FROM SHROUD/STRUCTURETO SHROUD/STRUCTURE
80K – 300K
< 80K
EXPANDERS
He Plant Size is based on analysis results shown on Slide 18
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Slide 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
Topsfield Engineering Service, Inc.
Slide 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
Topsfield Engineering Service, Inc.
Slide 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
Topsfield Engineering Service, Inc.
Slide 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
Topsfield Engineering Service, Inc.
Slide 26
Support Concept
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Slide 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
Topsfield Engineering Service, Inc.
Slide 28
Class 10,000Clean Room
20 K Shroud w/plenum
80 K Shroud
Access Platform
Test Support & Article
Note that top shroud panels are removed for visibility
Clean Air Flow InClean Air Flow Out
Topsfield Engineering Service, Inc.
Slide 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
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