1. LAT Thermal Systems Analysis
Gamma-ray Large Area Space Telescope
LAT Thermal Systems Analysis
Jeff Wang
LMCO
LAT Thermal Engineer

2. Agenda Introduction Design trade analyses performed and results
Thermal systems overview
Thermal parameters
Requirements and interfaces
Analysis parameters, environments, and case definitions
Analysis update
Hot- and cold-cases analyses
Survival-case analysis
Other non-design case analyses
Failure-case analyses
Thermal Control System Design
Summary and Further Work

3. LAT Thermal Systems Overview
Radiators
Two panels, parallel to the LAT XZ-plane
Size per panel: m x 1.56 m = 2.84 m2
Aluminum honeycomb structure
Heat Pipe design
Constant-conductance heat pipes on Grid Box
Ammonia working fluid
Extruded aluminum, with axial groove casings
Heat pipes
Variable-conductance Heat Pipes
6 VCHP’s per Radiator panel
Provides feedback control of grid temperature
Top Flange Heat Pipes (not shown)
Isothermalize grid structure
X-LAT Heat Pipes
Remove waste heat from electronics
Connect radiators for load-sharing
Downspout Heat Pipes
Transport waste heat from grid to Radiators
MLI thermal shielding surrounding ACD, Grid Box, Electronics
Down Spout Heat Pipes connect Grid to Radiators
On-Orbit Thermal Environment and LAT Process Power
Survival
Cold
Hot
Units
Earth IR
208
265
W/m2
Earth Albedo
0.25
0.40
Solar Flux
1286
1419
LAT Process Power
535
612 W
X-LAT Heat Pipes shunt electronics power to Radiators
Active VCHP control allows for variable Radiator area to maintain constant interface temp to LAT
LAT Thermal Overview

4. 3-D ISO OF RADIATOR, X-LAT, DSHP’S FROM RADIATOR TALK

5. LAT Thermal System Schematic Diagram
LAT Thermal Schematic Diagram

6. Internal Thermal Design Changes Since Delta-PDR
The following design changes have been incorporated in the CDR thermal model
Added high emissivity black paint to TKR sidewalls
Lowers peak TKR temperature by radiatively coupling modules together
Raises ACD survival temperature and lowers TKR hot-case peak temperature by improving radiative coupling between the two
Connected TKR to Grid with 4 heat straps/module
Increases temperature gradient across the thermal joint
Improves thermal joint reliability compared to Delta-PDR thermal gasket design
Replaced outer ACD MLI blanket layer with germanium black kapton (FOSR before)
Preferred by subsystem, since MLI is unsupported
Marginally raises survival case temperatures
Increased total LAT power (w/o reservoirs) to 612 W (was 602W)
Total is still within the 650 W allocation
CAL and TKR power increased 26 W
Electronics power dropped 18 W
ACD power increased 3 W
Net effect is to raise hot-case peak temperatures for the TKR and CAL
Added S-bend to VCHP transport section
Results in net drop in survival heater power needs
Reduces survival-case heat leak out of Grid
Increases anti-freeze radiator heater power
Improves flexibility for better compliance at integration
Increases transport capacity requirement on VCHP’s

7. LAT Thermal Interface Design Changes Since Delta-PDR
The following interface changes have been incorporated in the CDR thermal model
Increased Radiator area to 2.78 m2 but decreased efficiency by shortening it
Modified Radiator aspect ratio at request of Spectrum to accommodate solar arrays
This change results in slightly higher LAT hot-case temperatures
Finalized Radiator cut-outs
Added cut-outs for solar array launch locks
Increased size of cut-out for solar array mast

8. Trade Studies Since Delta-PDR
Solar Array interface for survival/cold cases
Delta-PDR total survival grid + anti freeze heater power calculated to be 171 watts (28.0 watts reservoirs)  191 W Total
Using the Spectrum PDR Solar Array, survival heater power increased to 244 W (28 W for reservoirs)
With no solar array, total survival heater power increased to 330 watts
Conclusion: using the Spectrum Astro PDR solar array in the LAT cold- and survival-case models was agreed as reasonable
Reservoir size reduction
Desire to maximize radiator area and temperature margins
Used Delta-PDR model to assure that smaller reservoir could totally close heat pipes for survival and provide adequate cold case control
Reduced size provides more condenser length
Conclusion: reduce reservoir size from Delta PDR volume of 288 cc to 75 cc. This produces a net gain of 100 mm in condenser length

9. Thermal Systems Peer Review RFA Status
RFA 13-Stowed Case
Limiting LAT component –VCHP Reservoirs if heaters not activated

10. Thermal Systems Peer Review RFA Status
RFA-14 Heater Flight sizing
RFA-15 With all YS-90 Tracker sidewalls, peak tracker temperature is
RFA-16 ACD limits
RFA-21Backup flight heater for anti-freezeheaters: not necessary
RFA-22 Maximum Tracker temperature with .03 MLI e* is
RFA-25 Correlation of flight thermistors at unit level
RFA-30 AO Effects on Germanium Black Kapton-See paper on AO from International SAMPE Technical conference, November 1996.
UPDATE

11. Driving Thermal Design Requirements
UPDATE

12. Thermal Model Details: LAT Dissipated Power
Dissipated power values are pulled directly from the LAT power budget held by the LAT System Engineer
All power allocations and geographical distribution is under CCB control
UPDATE
LAT Dissipated Power Values
Source: LAT-TD “A Summary of LAT Dissipated Power for Use in Thermal Design”, 13 Mar 2003

13. Thermal Model Details: Electronics Box Dissipated Power
UPDATE
LAT Dissipated Power Distribution in Special Electronics Boxes
Source: LAT-TD “A Summary of LAT Dissipated Power for Use in Thermal Design”, 13 Mar 2003

14. Environmental Temperature Limits

15. Verification Test Temperatures
UPDATE

16. LAT Thermal Math Model and Status
TSS Model-Calculates radks and heat rates
XXX Surfaces
YYY Active Nodes
Sinda Model
Submodels
ACD CDR model
Detailed TKR model
Reduced Cal model
Detailed Grid model
X-LAT and Electronics model updated
Bus model includes solar arrays and SV
IRD array for hot case
Cold case/survival uses Spectrum Astro PDR solar array
Detailed radiator and heat pipes
ZZZ nodes total
Heat pipe logic in VCHPs to predict gas front
Added VCHP heater control logic
Logic will be part of SIU control of thermal system
Model status: the model is mature, and interfaces understood. The electronics thermal model interface is the one deficiency, and is being worked on.

17. Thermal Model Details: Thermal Interfaces
Thermal interfaces to the Spacecraft
All specified in LAT IRD (433-IRD-0001) except cold-/survival-case solar array definition, which has been arrived at by mutual agreement between Spectrum, LAT, and the GLAST PO
Environmental parameters
PDR and Delta-PDR analysis shows that Beta = 0, pointed-mode is the LAT hot-case
Solar loading is per the LAT IRD
Sky-survey attitude and “noon roll” is based on an assumed slew rate of 9 degrees/min, max
Thermal design case parameters are tabulated on the following chart
SC-LAT Thermal Interface Parameters

18. Thermal Model Details: Design Case Details
LAT Thermal Case Description
Source: LAT-TD “LAT Thermal Design Parameters Summary”, 19 Mar 2003

19. UPDATE quote all predicts including 5C margin
Results Summary
Hot-Case peak temperatures predicts
Tracker
Predict: 24 oC max
Operating Limit: 30 oC
Calorimeter:
Predict: 16 oC max
Operating Limit: 25 oC
Electronics
Predict: 28 oC max
Operating Limit: 45 oC
These are “raw predicts” and do not include 5 oC uncertainty
UPDATE quote all predicts including 5C margin
Temperature Predicts for LAT Subsystems

20. Temperature Predicts and Margins to Operating Limit
Temperature predicts show that all subsystem components carry greater than 5 oC margin to their operating limit
Minimum margin of 6 oC is for the center TKR module
UPDATE
Temperature Predicts for LAT Subsystems

21. Hot Case TKR Peak Temperature Gradient
Peak temperature gradient is along the heat transfer path to the top of a center TKR module
Key temperature gradients
Up TKR wall: 5.7 deg C
TKR—Grid thermal joint: 4.0 deg C
Top of Grid—DSHP at VCHP: ~7.6 deg C
UPDATE
TKR Maximum Temperature Gradient in the LAT

22. Hot Case Environmental Orbit Loads
Hot Case Orbit: Beta 0, +Z Zenith, +X Sun Pointing
sun
UPDATE
Environmental Load on Radiators for Hot-Case Orbit

23. Hot Operational Orbit Average Qmap
Hot Case QMAP
Hot Case QMAP
2072 W to space
Instrument Power
2008 W orbital heating
612 W
46 W solar array heating
UPDATE
64 W orbital heating
91 W to space
24 W from bus
235 W orbital heating
252 W orbital heating
83.5 W solar array heating
84 W solar array heating
30 W from bus
30 W from bus
652 W to space
648 W to space
3.9 W to space
3.9 W to space Z
Orbital heating
Radiated to space
Bus heating
VCHP reservoir
Hot Operational Orbit Average Qmap Y

24. Predicted LAT Temperatures for Hot-Case Orbit
Hot Case Temperatures
UPDATE
Predicted LAT Temperatures for Hot-Case Orbit

25. Hot Case Tracker Temperature
UPDATE
Predicted TKR Temperature Showing Analysis Predict is Stabilizing Toward an Aymptote

26. Hot Case Radiator Temperatures
UPDATE

27. Hot Case with “Real” PDR Solar Arrays
UPDATE

28. Environmental Load on Radiators for Survival-Case Orbit
sun
Survival Orientation: +X Sun Pointing
UPDATE
Environmental Load on Radiators for Survival-Case Orbit

29. Survival Orbit Average Qmap
Survival Case QMAP
Survival Case QMAP
1569 W to space
Make-up Heaters
1529 W orbital heating
61W
22 W solar array heating
UPDATE
44 W orbital heating
63 W to space
13 W from bus
131 W orbital heating
130 W orbital heating
12 W from bus
12 W from bus
39 W solar array heating
38 W solar array heating
45.5 W heater power
45.5 W heater power
258 W to space
259 W to space Z
9.9 W to space
10.0 W to space
Orbital heating
Radiated to space
Bus heating
VCHP reservoir
Anti-freeze heaters
VCHP reservoir heaters
22 W heater power
23 W heater power Y
Survival Orbit Average Qmap

30. Survival Temperatures
UPDATE

31. Survival Case Temperatures
UPDATE
Predicted LAT Temperatures for Survival-Case Orbit

32. Survival Case Radiator Temperatures
UPDATE
Predicted Radiator Temperatures for Survival-Case Orbit

33. UPDATE Survival Heater Power Survival heater power (orbit average)
Grid make-up heaters 61 W
VCHP anti-freeze heaters 91 W
X-LAT Plate heaters 0 W
Total heater power 152 W
Allocation: 220 Watts
Heater power margin: +68 W (45% margin)
UPDATE

34. VCHP Reservoir Heater Power
Reservoir Heater Size
3.5 27V = 42 W for 12 (100% duty cycle)
Survival minimum required power = 1.5 W/reservoir
Heaters sized at > 200% of required minimum
Reservoir Duty Cycles
Hot Case: 0% and 0 W
Cold Case: ~ 30%  13 W orbit-averaged power
Survival: 100%  42 W orbit-averaged power (heaters locked on while LAT is off)
UPDATE

35. Cold Case Temperatures
UPDATE
Predicted Temperatures for Cold-Case Orbit

36. Cold Case Radiator Temperatures
UPDATE
Predicted Radiator Temperatures for Cold-Case Orbit

37. LAT Failure Analyses—Hot-Case
Summary of Hot-Case Failure Analyses

38. LAT Failure Analyses—Cold/Survival Cases
Summary of Cold-/Survival-Case Failure Analyses

39. Thermal Failure Analysis Results Summary
TABLE OF PEAK TEMPS AND TEMP CHANGES FOR FAILURE CASES RUN
UPDATE

40. Thermal Failure Analysis Results Summary
UPDATE

41. LAT Thermal Analysis Summary
UPDATE

42. Integration and Test Flow
UPDATE
LAT Integration and Test Flow

43. LAT Thermal Balance/Thermal-Vacuum Tests
Test goals
Thermal-Balance
Verify that the LAT thermal control system is properly sized to keep maximum temperatures within mission limits, while demonstrating at least 30% control margin
Validate the LAT thermal control system control algorithms
Verify that the VCHP control effectively closes the radiator to when the LAT is off
Validate the LAT thermal model by correlating predicted and measured temperatures
Thermal-Vacuum
Verify the LAT’s ability to survive proto-qualification temperature levels at both the high and low end
Test for workmanship on hardware such as wiring harnesses, MLI, and cable support and strain-reliefs which will not have been fully verified at the subsystem level
Demonstrate that the LAT meets performance goals at temperature
Provide stable test environment to complete LAT surveys, as detailed in LAT-MD-00895, “LAT Instrument Survey Plan”
Configuration
The LAT instrument will be fully integrated but the SC solar arrays will not be installed
The LAT will be powered on and off during testing per the test procedure
The LAT will be oriented with the Z-axis parallel to the ground to allow all heat pipes to operate and the +X axis facing up
All MLI blanketing will be in its flight configuration for the duration of the 2 tests
The LAT will NOT be reconfigured after the thermal-balance test

44. LAT Thermal Balance/Thermal-Vacuum Tests (cont)
Instrumentation
Thermocouples and RTD’s will be used to instrument the LAT and test chamber
LAT flight housekeeping instrumentation includes many thermistors and RTD’s. These will also be used for monitoring temperatures within the LAT
Specialized test equipment requirements
Chamber pressure of < 1 x 10-5 Torr
Chamber cold wall temperature of –180 oC to provide a cold sink for accumulation of contaminants
Thermally controlled surfaces in the chamber
5 plates for ACD surfaces, each individually controlled
2 plates for the radiators(one for each side), each individually controlled
1 plate to simulate the bus, controlling the environment to the X-LAT Plate and the back of each radiator
Heat exchangers mounted on the +/– X sides of the LAT Grid, to increase ramp rate during transitions
LAT heat pipes will be leveled to within 0.2 degrees
20 oC/hr max ramp rate
Facility capable of holding LAT stable to < 2 oC/hr rate of change (TBR)
Test profile
Dwell at high and low temps for 12 hours, min
Comprehensive Performance Tests conducted at select plateaus
Perform at ambient, during cold and hot soaks, and at return to ambient
Limited Performance Tests during transitions and plateaus
Check operating modes and monitor units for problems or intermittent operation

45. LAT Thermal Balance/Thermal-Vacuum Test Profile
LAT Thermal-Vacuum Test Profile
Source: LAT-MD , “LAT Thermal-Vacuum Test Plan,” March 2003

46. LAT Thermal Verification Test Temperatures
Thermal Test Levels
Test strategy
Drive components to PFQ limit for LAT, defined in the MAR as Operating limit +/- 10 oC, min
Minimum test margins
5 C margin from Operating to AT level
5 C margin from AT to LAT PFQ level
UPDATE
LAT Thermal Verification Test Temperatures
Source: LAT-SS “LAT Environmental Specification,” March 2003

47. UPDATE Summary Summary
We are working towards completing and using a fully integrated thermal model of the CDR design for generating temperature predicts for CDR
The Radiator thermal design has been changed to incorporate modifications to the spacecraft interface
Predicts show that we meet all operating limits, with adequate margin, when using the IRD solar arrays
UPDATE