crater thermal analysis

Cosmic RAy Telescope for the Effects of Radiation

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

• 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.

Hard (Survival) 50 C -40 COperational 35 C -30 CQualification 45 C -40 CFlight Design 50 C -37 CCBE (Based on bench margins) 45 C -32 C


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.


Current Instrument Schematic


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.

Coating LocationAbsorptance

S

Emittance

H

Absorptance

S

Emittance

H

Kapton 3mil 0.45 0.80 0.51 0.76Black Kapton 3 mil 0.91 0.81 0.93 0.78Germanium Black Kapton 0.49 0.81 0.51 0.78

Silver Teflon (5 mil)3,4 MLI Blanket 0.08 0.78 0.11 0.73

Silver Teflon (10 mil)4 MLI Blanket 0.09 0.87 0.13 0.83

Cold Case Hot Case


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 amplitudeof resultant temperature fluctuations.

• At a Beta angle of zero, the model simulatesthe worst case scenario where the instrumentcycles from one temperature extreme to theother 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 resistanceof the mounting feet to LRO to be .5 W/cm2C.Radiation to the LRO is assumed to be through15 layer MLI


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.

Hot Case Cold CaseSolar Constant 1420 W/m2 1280 W/m2Albedo Factor 0.13 0.06Infrared Emission --- ---


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


Results: Instrument


Instrument Exterior Temperatures (hot case)


Mean Orbital Temperatures (hot case)

290

295

300

305

310

315

320

325

330

0 1000 2000 3000 4000 5000 6000 7000 8000

time (s)

W

interface

scope zmin

mean pcb

max pcb

mean zmax

zmax


Instrument Exterior Temperatures (cold case)


Mean Orbital Temperatures (cold case)

235

240

245

250

255

260

265

270

275

0 1000 2000 3000 4000 5000 6000 7000 8000

time (s)

Tem

pera

ture

(K)

interface

scope zmin

mean pcb

max pcb

mean zmax

zmax


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.

Hot Case Max Operating Temperature [optical bench at 35C]

Cold Case Min Operating Temperature [optical bench at -30 C]

instrument interface 42 -33pcb's 44.5 -32nadir 51 -35scope 44.5 -36


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


Backup Slides


Inputs

Material k (W/m/K) Cp (J/kg/K) rho (kg/m 3̂) e*Aluminum 6061 180 869 2700 0.8PCB 59.8 1003 2819 0.73mil Black Kapton Film 0 0 0 0.81MLI 0 0 0 0.05

Material a eAluminum 6061 0.1 0.025PCB -- --3mil Black Kapton Film 0.91 0.81

• Thermal and Physical properties:

• Optical Properties:


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


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


Heat Rates Absorbed Over One Orbit


Instrument Heat Losses (hot case)

-60

-40

-20

0

20

40

60

80

0 1000 2000 3000 4000 5000 6000 7000 8000

time (s)

W

to space

to space

interface

to space

nadir

zenith


-120

-100

-80

-60

-40

-20

0

20

40

0 1000 2000 3000 4000 5000 6000 7000 8000

Time (s)

W

to space

to space

interface

to space

nadir

zenith

Instrument Heat Losses (cold case)


Cold Case Orbit (bright to dark)


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

crater thermal analysis

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