Topsfield Engineering Service, Inc. JWST Testing Issues – Thermal & Structural William Bell, Frank...

Topsfield Engineering Service, Inc.

JWST Testing Issues – Thermal & Structural

William Bell, Frank Kudirka, & Paul-W. Young



Slide 2

Purpose

This study explores JWST thermal and structural testing issues and possible

solutions, as presented to NASA in June 2004


Slide 3

Summary Facility Goals

Thermal Design

Helium Refrigeration

Structural Design

Vibration Isolation

Clean Room Considerations


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


Slide 5

Proposed TestFacility

Vacuum Chamber

80 K Shroud

20 K Shroud or Dewar

Test Article

Ties

Helium Refrigerator Flow Paths

Thermal Desktop Model


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


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


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


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


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


Slide 11

Thermal Model Construction

Nodes: 33 Linear Conductors: 46 Radiation Conductors: 58 Lumps: 3 Paths: 2 Ties: 7


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


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


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


Slide 15

Thermal Desktop Capability

Molecular Conduction


Slide 16

Thermal Desktop Capability

Natural Convection

Caution


Slide 17

Method 1 Results


Slide 18

Method 2 Results


Slide 19

Method 4 Results


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


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


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


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


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


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


Slide 26

Support Concept


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


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


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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