l23thermalcontro

l23thermalcontro - Spacecraft Thermal Control Systems Col....

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1 Spacecraft Thermal Control Systems Col. John E. Keesee Lesson Objectives: 1. The student will understand thermal control processes 2. The student will be able to calculate thermal balances and equilibrium temperatures 3. The student will be able to size and select thermal control systems.
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2 Outline • Purpose of thermal control systems • Review of heat transfer fundamentals • Space system thermal analysis – Equations –Mode ls – Analysis programs
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3 Purposes of Thermal Control • To control the operating temperature environment of spacecraft systems – Most systems become less reliable when operated outside their design operating environment – Propellant freezes – Thermal cycling damage – Instrument/antenna/camera alignment – Instrument requirements for very cold temperatures • Example operating temperatures – SMAD Table 11-43
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4 Temperature Requirements • Operating temperature ranges • Switch-on temperatures • Non-operating temperature ranges • Temperature stability • Temperature uniformity
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5 Typical Spacecraft Design Temperatures -100 to 125 -100 to 125 Solar panels -20 to 70 0 to 50 Momentum wheels -35 to 35 -35 to 0 Solid-state particle detectors -269 to 35 -269 to –173 IR detectors 0 to 35 10 to 20 Batteries -20 to 70 0 to 40 Analog electronics -20 to 70 0 to 50 Digital electronics Survival Temperature (C) Operating Temperature (C) Component/ System
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6 Review of Heat Transfer Fundamentals • Convection – heat transfer via flowing fluids • Conduction – heat transfer within materials other than flowing fluids • Radiation – heat transfer via electromagnetic waves
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7 Convection • h = heat transfer coefficient • Important to spacecraft during launch after fairing separation • Convective heat transfer is used in some pumped-liquid thermal control systems, especially in manned spacecraft T A h q =
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8 Conduction • Rectangular • Cylindrical • Spherical • k is the thermal conductivity ) ( ) ( 4 ) / ln( ) ( 2 ) ( 2 1 2 1 2 1 i o o i i o R R T T R kR q D D T T L k q T T x kA q = = = π
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9 Radiation ε =emissivity at the wavelength mix corresponding to temperature T σ =Stefan-Bolzmann’s constant = 5.670 x 10 -8 W/m 2 -K 4 T is temperature in Kelvin 4 T q εσ = Primary energy transfer mechanism for spacecraft. Most spacecraft have large radiators to rid themselves of heat. q is the heat transfer per unit area and T is the surface temperature.
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10 Planck’s Equation λ= wavelength h=Planck’s constant c=speed of light k=Bolzmann’s constant 1 1 2 / 5 2 = T k ch b e hc E λ π At any temperature above absolute zero, all materials emit thermal (blackbody) radiation. For a perfect blackbody, the rate of total energy emission
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l23thermalcontro - Spacecraft Thermal Control Systems Col....

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