Spacecraft Thermal ControlOBJECTIVE: Maintain the temperature of all spacecraft components within

appropriate limits over the mission lifetime, subject to a givenrange of environmental conditions and operating modes.

D. B. Kanipe 01/30/2014

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Thermal ControlTwo classesPassive (preferred when possible)

SunshadesCooling finsSpecialty paints and coatings Insulating blanketsHeat pipesGeometry

Active (when passive control is insufficient or unsuitable)) Pumped fluid loopsAdjustable louvers or shuttersRadiatorsOperational work arounds

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Heat Transfer Mechanisms Radiation Direct solar radiative Solar radiation reflected from nearby planets Thermal energy radiated from nearby planets Radiation from the spacecraft to deep space Conduction Primarily controls the flow of energy between

different parts of the spacecraft itself Convection Relatively unimportant in space

vehicle design

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Spacecraft Thermal Environment (1/2)

Design concerns in planetary orbit Variation of eclipse time as orbit precesses Variation of solar intensity with the seasons Reflected solar energy from the planet Orbital altitude Albedo Orbit inclination

Concerns in interplanetary flight Variation of sun’s intensity with distance Effect of destination planet

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Spacecraft Thermal Environment (2/2) Operational activities Free molecular flow On/off switching of onboard equipment Thruster firings (chemical propellant)

Propellant tank and/or line cooling Local heating near thruster

Expenditure of propellant Reduces spacecraft thermal mass Changes transient thermal response

Effects of time in space Surface characteristics change from exposure

Ultraviolet light Atomic oxygen (oxygen atom)Micrometeoroid and orbital debris impact (MMOD)

Affects absorptivity and emissivity Anomalous events; i.e., must include margin in the design

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Methods of Thermal Control (1/6)

Passive thermal control Geometry Design with thermal control in mind

Insulation blankets Multi-layer design (usually)

Aluminized Mylar layered with sheets of nylon or Dacron mesh External coatings (fiberglass, Dacron, etc)

Sun shields As simple as polished or gold plated aluminum Silvered Teflon

Acts as a second surface mirror Silver coating provides good visible light reflectivity Teflon provides high infrared emissivity

Glass mirror is thermally more efficient, but heavy

Methods of Thermal Control (2/6)

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Cooling fins Dissipate large amounts of heat, or Dissipate smaller amounts of heat at low temperatures Large numbers of fins:

May be difficult to obtain adequate view factor Larger fins have limited effectiveness

Heat pipe (two-phase flow) Tube with a wick and partially filled with fluid (anhydrous ammonia) Tube conducts heat from a hot spot to a cold spot

Fluid evaporates at hot end (gas) Condenses at cold end (liquid) Capillary action of the wick draws fluid back to hot end

Conducts heat as long as temperature differential exists Issues

Wick can dry out at the hot endWick can freeze at the cold end 0 g function difficult to simulate 50% margin customary

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Methods of Thermal Control (3/6)

Active Thermal Control HeatersSimple Control from the ground, autonomously, or both

Mechanically pumped two-phase heat pumpLiquid loopsCan be massive for space appsWater is preferred for internal, pressurized modulesCharacteristics of liquid for external loops

Specific heat high Dynamic viscosity low Boiling point highMust not freeze

Shuttle: Freon-21ISS: ammonia

Methods of Thermal Control (4/6)

Heat pumps Increase temperature of a radiatorWhy do that?Heat radiated from a surface is proportional to the fourth power of

its temperature Small increase in radiator temperature rate of heat dissipation

PenaltyMass increase Power increase ∴ if possible, increase radiator size

Applications Cool communication components: increase S/N Refrigeration for food or biological samples

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Methods of Thermal Control (5/6)Refrigeration

+4°C to -20°C (Food storage) Thermoelectric cooling (Peltier effect) Two-phase heat pump (Rankine cycle) Domestic refrigerator Problems with 0g

-80°C (Long duration storage of bio samples) Single phase gas cycle Brayton cycle ISS and Hubble Space Telescope

-193°C (Earth observation) GEO: carefully designed radiators LEO: mechanical coolers (Stirling cycle)

< -269°C Of interest to astronomers

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Methods of Thermal Control (6/6)

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Shutters and louvers

TIROS/DMSPVoyager

Heat Transfer Mechanisms (1/2)

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Conduction Usually the primary heat transfer mechanism within a spacecraft Lack of convection: must provide adequate conduction paths

Material selection important Un-welded joints: poor thermal conductors

Conduction pads Thermal grease Metal loaded epoxy

High thermal conductivity high electrical conductivity Situations requiring high thermal conductivity and electrical

isolation can be challenging Beryllium oxide (BeO)

High thermal conductivity Excellent insulator Dust is highly toxic

Fourier’s Law Q = power (BTUs)A = area

where: κ = thermal conductivityT = temperature, °Kx = linear distance over

conduction path

−=

dxdTAQ κ

Radiative Heating Transport of energy by electromagnetic waves Typically, the only practical means of heat

transfer between a spacecraft and its environment

Heat flux from a surface varies as the fourth power of its temperatureMay create configuration issues

Frequencies of interest for thermal transport: 200nm < frequency < 200µm Between middle ultraviolet and far infrared

Heat Transfer Mechanisms (2/2)

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Stefan-Boltzman lawT = surface temperatureA = surface area

= emissivity (1 for a BB)= Stefan-Boltzman constant= 5.67 X 10-3 (w/m2)K4

Blackbody Idealization: neither reflects nor transmits incident energy Perfect absorber at all wavelengths and angles; α = 1 Emits the maximum possible energy at all wavelengths

and angles for a given temperature (ε = 1) Radiative energy is a function of temperature only Need to know absorptivity and emissivity of real

substances for design trades 2/15/2016

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Emissivity (ε) and Absorptivity (α)

4ATQ εσ=

ε and α of Real Surfaces

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α1.0

0.8

0.6

0.4

0.2

0 0.2 0.4 0.6 0.8 1.0

Black Nickel,Chromium, Copper Black

Paint

Polished Metal

AluminumPaint

Silvered Teflon

WhitePaint

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Radiative Heat Transfer Between Surfaces

Need to be able to compute radiative heat transfer between parts of the spacecraft and its surroundings

Every surface radiates to and receives radiation from all other surfaces within its hemispherical field of view

Typically, requires a numerical solution

Spacecraft Thermal Modeling & Analysis

Lumped Mass Approximation Simplest analytical thermal model Each node represents a thermal mass Each node is connected to other nodes by thermal resistances Must identify heat sources and sinks (internal and external) Electronics packages Heaters Cooling devices Radiators

Nodes Major pieces of structure

Thermal resistancesModel the conductive and radiative links (including joints) Emissivity and absorptivity of the surfaces

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Spacecraft Energy Balance

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SC

Qi

Qss= σsAsFs,s(T4s – T4

space)

Qsun=αsA Isun

Qer=aαsFs,seAsIsun

Radiated To

Earth

Qse=σsAsFs,e(T4s – T4

e)

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Qsun +Qer + Qi = Qss + Qse

Qsun=αsA Isun = solar input to spacecraftQer=aαsFs,seAsIsun = earth reflected solar inputa = earth albedo (0.07-0.85)αs= spacecraft surface absorptivityƐs= spacecraft surface emissivity

Qi= internally generated powerQse=σsAsFs,e(T4

s – T4e) net power radiated to earth

Qss= σsAsFs,s(T4s – T4

space) net power radiated to space

Fs,s= fraction of radiant energy leaving SCView that is intercepted by spaceFactors Fs,e= fraction of radiant energy leaving SC

that is intercepted by earth

Spacecraft Energy Balance

Spacecraft Energy Balance

Simplifying assumptionsTspace ≈ 0 andFs,s + Fs,e =1

The energy balance equation becomesƐsσAsT4

s = ƐsσAsFs,eT4e + Qsun +Qer + Qi

This equation can be used to estimate the “average” spacecraft temperature

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Thermal Analysis Tools SINDA (early version called CINDA) Finite difference analysis Create a nodal mesh, or grid Apply desired boundary conditions

FLUINT Developed for internal one dimensional flows such as

pumped fluid loops Restricted to low-speed, incompressible flow of one

viscous fluidSINDA versions that contain FLUINT are available

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AccuracyThermal analysis is typically not as accurate as other

disciplinesDuring early design phases, the thermal system should

be able to handle a heat load at least 50% > than analytically predicted

Over the course of the project – as more is learned – the final margin may be as low as 20%

Large margins may mean that the system is over-designed

But, the consequences of under-design can be catastrophic

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Mission Profile1

- Launch- Maneuvers- Orbits- Cruise Periods

Locations & Orientations(Distance & Direction to Heat Source)- Sun- Planet Reflection- Planet IR 2

TemperatureRequirements- Average Limits- Local Limits- Gradients

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Electrical Power- Components- Duty Cycles

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Select SimplifiedGeometry

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INFORMATION

ANALYSISSelect Surface FinishProperties, (α & ε)

Compute All AverageIncident Heat Fluxes

Compute RadiantDissipation Capability

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Compute AbsorbedHeat Fluxes

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Requirements10

Compute Temperatures for Heat Balance9

Change Areas and FinishesDesign

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