Spacecraft Thermal Control

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1 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

2 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 2

3 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 11/14/

4 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 4

5 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 5

6 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 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 11/14/2012 6

7 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 11/14/2012 7

8 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 11/14/2012 8

9 Methods of Thermal Control (4/4) Shutters and louvers Actively pumped fluid loops Voyager TIROS/DMSP Conceptually identical to your automobile s system Air, water methanol, water/glycol, Freon, carbon tetrachloride, etc 11/14/2012 9

10 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 Q = κa dt dx where: Q = power (BTUs) A = area κ = thermal conductivity T = temperature, K x = linear distance over conduction path 11/14/

11 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 11/14/

12 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/m 2 )K 4 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/

13 ε and α of Real Surfaces 1.0 α 0.8 Black Nickel, Chromium, Copper Black Paint Polished Metal Aluminum Paint Silvered Teflon White Paint ε 11/14/

14 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 11/14/

15 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) Nodes Electronics packages Heaters Cooling devices Radiators 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 15

16 Spacecraft Energy Balance Q ss = σ s A s F s,s (T 4 s T 4 space) Q i Q sun =α s A I sun SC Q se =σ s A s F s,e (T 4 s T 4 e) Radiated To Earth Q er =aα s F s,se A s I sun EARTH 11/14/

17 Spacecraft Energy Balance Q sun +Q er + Q i = Q ss + Q se Q sun =α s A I sun = solar input to spacecraft Q er =aα s F s,se A s I sun = earth reflected solar input a=earth albedo ( ) α s =spacecraft surface absorptivity =spacecraft surface emissivity Ɛs Q i =internally generated power Q se =σ s A s F s,e (T 4 s T 4 e) net power radiated to earth Q ss = σ s A s F s,s (T 4 s T 4 space) net power radiated to space View Factors F s,s = fraction of radiant energy leaving SC that is intercepted by space F s,e = fraction of radiant energy leaving SC that is intercepted by earth 11/14/

18 Spacecraft Energy Balance Simplifying assumptions T space 0 and F s,s + F s,e =1 The energy balance equation becomes Ɛ s σa s T 4 s = Ɛ s σasf s,e T 4 e + Q sun +Q er + Q i This equation can be used to estimate the average spacecraft temperature 11/14/

19 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/

20 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 11/14/