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Thermal Control Subsystem. Objective: To maintain all the components of the satellite within their respective temperature limits during all modes of operation. Guide to Our Journey. Formulate Objective Identify the enemy Gear for Battle Plan of Attack Rally Cry.
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Thermal Control Subsystem • Objective: • To maintain all the components of the satellite within their respective temperature limits during all modes of operation.
Guide to Our Journey • Formulate Objective • Identify the enemy • Gear for Battle • Plan of Attack • Rally Cry
Sources of Thermal Energy • Solar Radiation • Albedo • Earth emitted infrared • heat generated by onboard equipment.
Methods of Control • Passive Control relies on conductive and radiative heat paths and relies on no electrical or mechanical input. • Active Control relies on pumps, thermostats, moving parts, and rely on electrical power.
Radiator Device with a large surface area used to radiate heat into space. Sizing depends on heat loads and temperature requirements. This is the primary system of heat rejection on spacecraft. Passive Control Components
Phase Change Devices Absorbs thermal energy by changing from a solid to a liquid. Useful for electrical equipment that experiences short power spikes. Disadvantaged by inability to absorb heat upon melting. A common type consists of some sort of wax in an aluminum container, resting between the heat dissipating device and a cold plate. Thermal Control Coating Coatings such as white/black, or gold or silver foil, have special radiation properties which govern heat transfer through applied surface. It is desirable to have a highly emissive and minimally absorbing surface to reject as much heat into space as possible without collecting much. Very efficient and light weight, but its performance will degrade over its lifetime. Passive Control Components
Passive Control Components • Multi-Layer Insulation • Closely spaced layer of aluminized Kapton or Mylar alternated with a course netted material. • Reduces the amount of heat flow per unit area between two boundary surfaces and prevents a large heat influx. • Sensors and payloads can be wrapped with MLI, to thermally isolate them and reduce thermal control requirements. • Thermal Doublers • Effectively a heat sink, a highly conductive material is placed in thermal contact with a material. • Prevents severe cooling/heating. • Commonly used to control temperature of electrical equipment that is subject to cyclical power dissipation. • Also can be used to spread heat out over radiator surfaces.
Cold Plates Used for mounting heat dissipating equipment. In an active system there are fluid passages within the plate itself. This fluid is pumped to a radiator where it dumps its absorbed heat and begins the cycle again In a passive system, the cold plate is usually combined with the radiator. Passive/Active Control Components
Heat Pipe Heat dissipated by evaporation and condensation. Thermal energy absorbed by liquid contained within the pipe. Liquid is turned into gas where it travels to the other end of the pipe, where it releases the energy to a radiator upon condensing back to a liquid. Pipe is usually made of aluminum and the liquid is usually ammonia. Heat pipes provide a highly conductive heat path and extremely high heat transfer rates. Active Control Components
Louvers Mounted on the surface of the radiator, they act like Venetian blinds that can be opened and closed. Different thermal coatings are applied to the blades and the base plate . The absorptive and emmissive thermo-optical properties of the radiator can be varied by opening and closing the blades by use of actuators. Offer a controlled rate of heat transfer, but can result in high temperatures if pointed toward sun. Second Surface Mirrors are more cutting edge and have all but replaced louvers in industry. Instead of louvers with different coatings, mirrors act to reflect incident radiation while radiating out internal energy Active Control Components
Thermal Switch Provides a direct conduction path between a heat source and the equipment mounting plate when the switch is closed. Electrical Heater Controlled by a thermostat, and used to heat cold regions of the satellite. Heat is generated by running current through a resistor. Generally used for fine temperature control and for short periods of time. Active Control Components
Begin with Passive System, adding components as needed Use active system if there is only a few degrees of tolerance in the required temperature or if several kilowatts are to be dissipated Design is typically modeled for the coldest case Typical industrial results are for the thermal subsystem to make up 3-4% of the overall weight and cost of the satellite. Design Process
Conclusion • Teamwork is essential to survival • Ideas must be communicated
References • http://www.esa.int/est/prod/prod0661.htm • http://www.srh.noaa.gov/lch/prep/heat.jpg • http://engineering.dartmouth.edu/~dartsat/albedo.jpg • http://www.tsgc.utexas.edu/archive/subsystems/thermal.pdf • Barter, Neville J. “Space Data” Northrup Grumman Space Technology, 5th ed. (2002) • Wertz, James R. and Larson, Wiley J. “Space Mission Analysis and Design”, Microcosm Press, 3rd ed. (1999)
