The thermal control subsystem's purpose is to maintain all parts of the satellite within specified temperature ranges. On Earth, three different modes of heat transfer operate to remove heat from or add heat to a system: (1) conduction (heat transfer through a solid); (2) convection (heat transfer through a fluid); and (3) radiation (heat transfer through a vacuum).
7.1.1 Continuity (Olsen - 1/95)
7.1.1.1 Functional Tour (Olsen - 1/95)
On Earth convection tends to be the dominate mode of heat transfer with radiation often not significant. However, in space, the primary mode of heat transfer is radiation. In fact, convection rarely exists, and, when it does, it exists only as an internal mode to the satellite which redistributes the heat but cannot remove or add any. All heat removal from or heat addtion to a satellite in space must be ultimately done through radiation. A balance between the different modes of heat transfer must be done to determine the temperature of the satellite (and each portion of the satellite).
Many think that because the temperature of space is near absolute zero (usually taken around 5 K), the problem is to keep things warm. This is rarely the case because there is enough energy emitted by the sun to make the satellite too hot. When the satellite is in eclipse (shadowed from the sun), if this eclipse is for a relatively long period, the satellite can get too cold.
Thermal control subsystems can be passive, semi-passive, or active. Passive systems usually have no moving parts or heaters and rely instead on paints, mirrors, multilayer insulation (MLI), phase-change devices, and radiators. Semi-passive systems add simple temperature-activated controls to close or open conductive paths. Heat pipes and louvers are thought of as semi-passive. Active systems include heaters and mechanical refrigerators.
For the SQUIRT program, the goal is to control the temperature of each satellite passively and primarily through the use of surface coatings and MLI. The temperature of each satellite will be monitored by temperature sensors placed in key locations throughout the satellite.
7.1.1.3 References (Olsen - 1/95)
Any undergraduate heat transfer text will have adequate background for understanding the heat transfer modes. For specifics related to satellites, look at Space Mission Analysis and Design by James R. Wertz and Wiley J. Larson and Space Vehicle esign by Michael D. Griffin and James R. French.
7.1.1.4 Suppliers (Olsen - 1/95)
No suppliers have been researched as of August 8, 1994.
7.1.1.5 Terminology (Olsen - 1/95)
Conductivity, k (W/m/K): a measure of the heat transfer capability of a solid. For aluminum, 221 W/m/K is an average value. Q = k*A*dT/dx where Q is the heat flux, A is the cross-sectional area and T is the temperature.
Convective coefficient, h (W/m^2/K): a measure of the heat transfer capability of a fluid. dQ = h*A*dT.
Stefan-Boltzmann constant, sigma (5.67e-8 W/m^2/K^4) used in the radiant heat transfer equation Q = sigma*epsilon*A*T^4
Emissivity, epsilon: a unitless number between 0 and 1 which defines how well a surface emits radiation, also called emittance. This value can be different for each wavelength. The tabulated values are usually averaged over the infrared wavelengths. A perfect emitter is called a black body and has a value of 1.
Absorptivity, alpha: also called absorptance, basically the same as emissivity, however, the tabulated values are averaged over the solar spectrum. The value represents how well a surface absorbs radiation.
Reflectivity, rho: also called reflectance. A measure of how well a surface reflects radiation. Tabulated values are also averaged in general. A value of 1 is a perfect reflector.
Transmissivity, tau: also called transmittance. A measure of how well radiation passes through a material. Tabulated values are also averaged in general. A value of 1 is a perfect transmitter.
The previous 3 values alpha (or epsilon), rho, and tau must add up to 1 for any given wavelength.
Heat capacity, c (J/kg/K or W*sec/kg/K): how much heat a material can store. Used in unsteady heat transfer analysis Q=m*c*dT/dt where m is the mass of the material, T is the temperature, and t is time.
7.1.2 Acquisition Reports (Olsen - 1/95)
Nothing has been ordered or donated as of August 8, 1994.
7.1.3 Baseline Trade-off (Olsen - 1/95)
The major trade has already been done: the thermal control will be done passively. This was because it will consume no power, it requires little mass, and it is much less expensive. The other options are described briefly at the beginning of this section. The study of exactly what kinds of coatings and insulations and where they will be placed is still ongoing.
7.1.4 Hardware Modification Plan (Olsen - 1/95)
For this subsystem, hardware modification will consist of painting various components, requiring a specific surface finish on the metallic structural components, adding radiators to the external surface, adding conductive paths, or adding insulation. Other modification may include the accomodation of temperature sensors. None of this as of August 8, 1994 has been specifically decided.
7.1.5 Electronic Modification Plan (Olsen - 1/95)
No electronic modification should be necessary.
7.1.6 Software Modification Plan (Olsen - 1/95)
The CPU will be required to record the temperature sensor readings.