Thermal Control Valve for Lunar Night Survival

NASA's upcoming long duration missions on the Moon require addressing the issue of equipment survival during the Lunar night. The primary power source for near-term Lunar surface science missions is a combination of solar photovoltaic arrays and batteries. A thermal control system that rejects daytime heat efficiently and conserves energy through the night is essential to keep the payloads, batteries, and other critical components at suitable temperatures. Conventional loop heat pipes (LHPs) provide very efficient heat transfer from electronics to spacecraft radiators but require continuously 2-3 W of power to shut down and minimize heat transfer through the night. This can increase battery mass substantially if applied for the entire Lunar night. A passive thermal control valve (TCV) able to direct the vapor in the LHP either to the condenser or to the compensation chamber was previously tested. It was found able to maintain the evaporator temperature within the desired range, while the condenser temperature varied widely. On/off TCVs, much less expensive, simpler, and lighter than currently used proportional valves were designed and proved able to maintain a constant minimal temperature of the evaporator over wide variations of the heat load and condenser temperature. Several different designs are demonstrated for the TCV. A LHP containing a TCV was integrated into a hybrid thermal control system being developed at Marshall Space Flight Center (MSFC) to be utilized for various surface assets that must survive in extreme lunar environments. Versions of the on/off TCVs are developed for smaller heat loads in space applications where weight and size are at a premium. Results are presented showing the feasibility of the concept.