Long duration human exploration missions to the Moon will require active thermal control systems which have not previously been used in space. The relatively short duration Apollo missions were able to use expendable resources (water boiler) to handle the moderate heat rejection requirement. Future NASA missions to the Moon will require higher heat loads to be rejected for long periods of time near the lunar equator. This will include heat rejection during lunar noon when direct radiation heat transfer to the surrounding environment is impossible because the radiator views the hot lunar surface.
The two technologies which are most promising for long term lunar base thermal control are heat pumps and radiator shades. Heat pumps enable heat rejection to space at the hottest part of the lunar day by raising the radiator temperature above the environment temperature. Conversely, radiator shades block some of the heat striking the radiator from the lunar surface, thus reducing the thermal environment temperature below the radiator temperature.
Recent trade-off studies at the Johnson Space Center have focused development efforts on the most promising heat pump and radiator shade technologies. Since these technologies are in the early stages of development and many parameters used in the study are not well defined, a parametric study was done to test the sensitivity to each assumption. The primary comparison factor in these studies was total system mass, with power requirements included in the form of a mass penalty for power.
Heat pump technologies considered were thermally driven heat pumps such as metal hydride, complex compound, absorption and zeolite. Also considered were electrically driven Stirling and vapor compression heat pumps. The electrically driven vapor compression cycle was the leading candidate due primarily to the low efficiencies of the thermally driven cycles and to the relatively “cheap” electrical power available during the lunar day when the heat pump is required.
Radiator shade concepts considered included step shaped, V-shaped and parabolic (or catenary) shades and ground covers. Of these, the parabolic (or catenary) shade appears to hold the most promise for moderate temperature heat rejection from a lunar base near the equator. The parabolic shade system may be optimized by reducing the shade size in relation to the radiator.
A further trade study compared the masses of heat pump and radiator shade systems. Although the radiator shade system was lighter in the base case considered, the masses of both systems were found to be very sensitive to several of the input parameters. A recommendation is made to continue development of both radiator shade and heat pump technologies.