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CRaTER Thermal Analysis

CRaTER Thermal Analysis. Huade Tan 6/27/05. Contents. System Overview Requirements Inputs and Assumptions Power Dissipations Lunar Orbit Current Model Results Exterior instrument temperatures Orbital temperature ranges Performance Predictions Conclusions. System Overview.

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CRaTER Thermal Analysis

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  1. CRaTER Thermal Analysis Huade Tan 6/27/05

  2. Contents • System Overview • Requirements • Inputs and Assumptions • Power Dissipations • Lunar Orbit • Current Model • Results • Exterior instrument temperatures • Orbital temperature ranges • Performance Predictions • Conclusions

  3. System Overview • Current Thermal System Requirements • Temperature Margin Philosophy • Hard/Survival limits define the range in which the instrument will not receive damage or permanent performance degredation • Qualification Limits are defined as the range of temperatures 10 degrees C wider than the flight predict limits • Flight Design limits define the range given by the current best estimates including margins of uncertainty in the given analysis. These limits must be within 10 degrees C of the hard limits. • Current Best Estimate ranges are determined by current state of testing and analysis • Requirement Exceedances • Current design does not exceed the given thermal requirements.

  4. Inputs • Power Dissipations in the E-box • 200 mW distributed evenly throughout analog PCB • 2.1 W distributed evenly throughout digital PCB • Two power supplies, 1.2W and 0.9W mounted on digital PCB with a conductive resistance of Copper in a vacuum at 30 C • Power Dissipations in the telescope • 300 mW distributed evenly through three PCB’s, evenly stacked • Conduction characteristics modeled as wedge clamps along the sides of each board to the telescope housing.

  5. Current Instrument Schematic

  6. MLI and Optical Bench • MLI outer layer optical properties: • Effective emittance: e* for MLI assumed to be .005 and .03 between best and worst cases. • CBE optical bench temperature margins between 16 and –19 C. • Modeled optical bench temperature margins between 35 and –30 C hot and cold.

  7. Orbit • The current model is generated based on a basic Beta zero orbit at an altitude of 122.1 km. • This orbit was chosen in order to generate an orbital period of 7200 seconds. • Reducing the orbit to 50 km will shorten the orbital period and reduce the amplitude of resultant temperature fluctuations. • At a Beta angle of zero, the model simulates the worst case scenario where the instrument cycles from one temperature extreme to the other twice every period. • The total heat absorbed by the instrument through the given orbit is computed by the Radcad Monte Carlo method. • The model assumes a contact resistance of the mounting feet to LRO to be .5 W/cm2C. Radiation to the LRO is assumed to be through 15 layer MLI

  8. Environmental Parameters • Orbital Heat Rate Factors: • Infrared Lunar Emissions are modeled after the temperature of the lunar surface. Lunar surface temperatures are modeled after the characteristic Lambertian surface having a subsolar temperature of 400 K and a shadow temperature of 100 K. • Surface temperatures across the bright side varies as a function of Tsubsolarcos1/4θ where θ is the angle measured from the orbital position to local noon. Brightness Temperatures of the Lunar Surface: The Clementine Long-Wave Infrared Global Data Set. Lawson SL and Jakosky BM.

  9. Current Instrument Model • The reference coordinate system shown here is used to describe the exterior surfaces in the following slides • Where: • Xmax = left • Xmin = right • Ymax = front • Ymin = rear • Zmax = top • Zmin = bottom

  10. Results: Instrument

  11. Instrument Exterior Temperatures (hot case)

  12. Mean Orbital Temperatures (hot case)

  13. Instrument Exterior Temperatures (cold case)

  14. Mean Orbital Temperatures (cold case)

  15. Transient Results Summary • Current best estimates for CRaTER is primarily dependant upon the temperature margins given for the optical bench. • Instrument Interface temperatures vary +7 to –3 degrees C from the optical bench temperature between extremes of hot and cold. • Nine degrees C maximum temperature difference in instrument from mounting interface at the top cover (hot case). May consider an MLI outer layer with a lower absorbptivity.

  16. Summary and Conclusions • Current Best Estimate: • Instrument interface temperature: 35 C  1 C Hot & -30C  • Maximum instrument temperature exceeds no more that 2.6 degrees C from the interface temperature during orbit. • Uncertainties and Modeling Improvements: • Temperature dependence of material properties: Given a temperature fluctuation of a few degrees C through a beta 0 orbit, the temperature dependence of thermal properties can safely be neglected. • Incorporating TEPs into the thermal model • Finalizing mounting interface resistance to and relative view factors (to space) from the LRO • Incorporating actual circuitry details on the PCBs • Fine tuning MLI optical characteristics

  17. Backup Slides

  18. Inputs • Thermal and Physical properties: • Optical Properties:

  19. Assumptions • Material properties: • Thermophysical properties of Al-6061 obtained from Matweb databases • Optical properties of Aluminum obtained from Cooling Techniques for Electronic Equipment: Second Edition • MLI assumptions: • Currently modeled using bulk properties • PCB assumptions: • 2 ground and power layers (80% fill), 4 signal layers (20% fill), 1 mm thick • Properties determined at www.frigprim.com/online/cond_pcb.html • TEP assumptions: • Currently not modeled

  20. Assumptions • Conductive Resistances: • Between PCB and Aluminum assumed to be characteristic of copper in vacuum at 30 C referred to in Heat Transfer. Holman, J.P • Within the Ebox assumed to be characteristic conduction of Al-6061 (assuming that the ebox is constructed out of a single block of aluminum) • Internal Radiation: • View factors of internal surfaces determined by Radcad using radk ray trace method • Emissivity factors calculated assuming either infinite parallel planes or general case for two surfaces from dissipating surfaces to interior walls. • Heat Flow to the Space Craft: • Assuming interface properties at 20 degrees C • Contact resistance of mounting feet to LRO assumed to be 20 W/cm2C • Radiation conduction to the LRO through 15 layer MLI

  21. Heat Rates Absorbed Over One Orbit

  22. Instrument Heat Losses (hot case)

  23. Instrument Heat Losses (cold case)

  24. Cold Case Orbit (bright to dark)

  25. Current Telescope Model Note: the circular apertures on the top and bottom sides of the scope are insulated with a single layer of 3 mil black kapton

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