Thermal System Specification

Scott Miller
smiller@mail.utexas.edu
August 1995

The spacecraft thermal system maintains all of the other spacecraft systems within their required temperature limits. Each component of the spacecraft has a temperature range in which it functions properly, and it must be maintained within this range during all mission phases.

The spacecraft receives heat inputs from the sun, earth, and moon, as well as on-board electronic components and thruster firings. The heat output is radiation to space. In order to maintain the temperatures of the spacecraft components within their required limits, either passive or active thermal control systems can be used. Passive systems use space radiators connected to heat generating equipment to dissipate the heat. The temperature is maintained within the proper limits by the thermal balance of heat input and output. Paints can be used with the desired absorptivity and emissivity characteristics, as well as thermal insulators, in order to obtain the correct thermal balance.

If the temperature cannot be maintained within the required limits using a passive system, then an active system must be used. An active system controls the temperature in some way, such as with electric heaters to keep temperatures above the lower limits, or louvers to control the amount of heat dissipation from the radiators.

Estimated Temperature Ranges
Electronics0 to 40 C
Batteries0 to 15 C
Solar Arrays-160 to 60 C (survival)
0 to 20 C (operational)
Propellant7 to 35 C
Structure-45 to 65 C
Sensors-20 to 10 C
IMU-34 to 71 C
Oxygen Plant?

Mission Phases

Trans-Lunar Phase

During the trans-lunar phase of the mission, it is desirable from a thermal standpoint to spin the spacecraft slowly in order to give all areas of the spacecraft limited exposure time to both the sun and to cold space. Spinning the spacecraft will affect the design of the control and communications systems, and it will be necessary to do a trade-off study to determine if this is the best method.

The thermal group will need to determine if the temperature requirements during this phase of the mission can be maintained using passive thermal control, or if an active system will be necessary.

Lunar Surface (Day)

While on the lunar surface, it may be necessary to deploy a shield to protect the spacecraft from long term exposure to the sun. The shield can be composed of solar cells for power generation.

Since the oxygen plant needs high temperatures to operate, it will be necessary to determine if these temperatures can be achieved by direct solar heating, or if a solar dish will be needed to concentrate the solar energy.

Lunar Surface (Night)

It will be desirable to maintain the spacecraft during the lunar night in order to resume operation during the next day. In order to do so, electric heaters will be needed to keep the spacecraft components from freezing. In order to use minimum power, the components can be maintained at their minimum temperature for survival, which in many cases is lower than the minimum temperature required for operation.

Thermal and Other Subsystem Interactions

The thermal subsystem interacts with all of the other spacecraft systems, since each system has temperature requirements which must be maintained. It is an integral part of the structure, since the thermal conduction takes place through the structural members. In addition, the placement of each component within the structure will need to be done while considering the overall thermal system design. Also, the mass and power requirements of the thermal system will need to be defined.



Thermal System References

Collicott, Howard E. and Paul E. Bauer, eds., "Spacecraft Thermal Control, Design, and Operation", American Institute of Aeronautics and Astronautics, New York, NY, 1983.

Larson, Wiley J. and James R. Wertz, eds., "Space Mission Analysis and Design", Microcosm Inc., Torrance, California, 1992.

Lucas, John W., "Fundamentals of Spacecraft Thermal Design", The MIT Press, Cambridge, Massachusetts, 1972.



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