Spacecraft Thermal Control OBJECTIVE: Maintain the temperature of all spacecraft components within appropriate limits over the mission lifetime, subject to a given range of environmental conditions and operating modes

D. B. Kanipe March 20, 2012

Thermal Control  Two classes  Passive (preferred when possible)  Sunshades  Cooling fins  Specialty paints and coatings  Insulating blankets  Heat pipes  Geometry

 Active (when passive control is insufficient)  Pumped fluid loops  Adjustable louvers or shutters  Radiators  Operational work arounds 11/14/2012

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Heat Transfer Mechanisms Radiation Radiative heat transfer dominates in 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 11/14/2012

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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 response

 Effects of time in space  Surface characteristics change from exposure

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

 Affects absorptivity and emissivity

 Anomalous events  Must include margin in the design 11/14/2012

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Methods of Thermal Control (1/4)  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   

 11/14/2012

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 6

Methods of Thermal Control (2/4)  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

 Tube with a wick and partially filled with fluid (ammonia)  Tube conducts heat from a hot spot to a cold spot  Fluid evaporates at hot end  Condenses at cold end  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 end  Wick can freeze at the cold end  0 g function difficult to simulate  50% margin customary

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Methods of Thermal Control (3/4)  Active Thermal Control  Heaters

 Ground control, autonomous, or both

 Cooling

 Thermoelectric (Peltier) cooling  Villaumier refrigerator  Cryostat  Expansion of a high pressure gas through an orifice  Two stage cryostats can get very low temperatures  Cryogenic  Expand supercritical Helium (stored at 4.2° K)  Can get down to 1.6° K  Infrared telescopes  Limited lifetime

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Methods of Thermal Control (4/4)  Shutters and louvers

Voyager

TIROS/DMSP

 Actively pumped fluid loops  Conceptually identical to your automobile’s system  Air, water methanol, water/glycol, Freon, carbon tetrachloride, etc 11/14/2012

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Heat Transfer Mechanisms (1/2)  Conduction

 Usually the primary heat transfer mechanism within a spacecraft  Lack of convection: provide adequate conduction paths  Material selection important  Un-welded joints  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

 dT  Q = −κA  dx   11/14/2012

where:

Q = power (BTUs) A = area κ = thermal conductivity T = temperature, °K x = linear distance over conduction path

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Heat Transfer Mechanisms (2/2)  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 temperature  May create configuration issues  Frequencies of interest for thermal transport:  200nm < frequency < 200µm  Between middle ultraviolet and far infrared

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Emissivity (ε) and Absorptivity (α)  Stefan-Boltzman law

Q = εσAT 4

T = surface temperature A = 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 11/14/2012

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ε and α of Real Surfaces 1.0

α Black Nickel, Chromium, Copper

0.8

Black Paint

0.6

0.4 Aluminum Paint

0.2 Polished Metal

0 11/14/2012

0.2

White Paint

Silvered Teflon

0.4

0.6

0.8

1.0

ε

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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

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Spacecraft Thermal Modeling and 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  Tanks  Electronic units  Thermal resistances  Model the conductive and radiative links  Conductivity of the joints 11/14/2012  Emissivity and absorptivity of the surfaces

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Spacecraft Energy Balance Qss= σsAsFs,s(T4s – T4space)

Qi

Qsun=αsA Isun

SC

Qer=aαsFs,seAsIsun

Qse=σsAsFs,e(T4s – T4e) Radiated To Earth

EARTH

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Spacecraft Energy Balance Qsun +Qer + Qi = Qss + Qse Qsun=αsA Isun = solar input to spacecraft Qer=aαsFs,seAsIsun = earth reflected solar input a=earth albedo (0.07-0.85) αs=spacecraft surface absorptivity Ɛs=spacecraft surface emissivity Qi=internally generated power Qse=σsAsFs,e(T4s – T4e) net power radiated to earth Qss= σsAsFs,s(T4s – T4space) net power radiated to space View Factors 11/14/2012

Fs,s= fraction of radiant energy leaving SC that is intercepted by space Fs,e= fraction of radiant energy leaving SC that is intercepted by earth 17

Spacecraft Energy Balance  Simplifying assumptions  Tspace ≈ 0 and  Fs,s + Fs,e =1

 The energy balance equation becomes

ƐsσAsT4s = 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 fluid  SINDA versions that contain FLUINT are available 11/14/2012

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Accuracy  Thermal analysis is typically not as accurate as other 

  

disciplines During 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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