The lunar environment poses significant challenges for heat rejection. As the atmospheric density is negligible, thermal radiation is primarily used for this task. During lunar day, the radiative sink temperature can reach 380 K at the equator while dropping to 40 K at night or in polar craters. While the maximum temperature can be mitigated by radiator design and location, any long-term exploration system must survive the lunar night. Loop Heat Pipes (LHPs) are good choices for low gravity environments because they can passively transport kilowatts of thermal energy over several meters. However, the standard LHP has no way to stop flow during normal operation. As a result, for low sink temperatures, there is a risk that an LHP could bring component temperatures below their minimum survival temperature. An option to mitigate these low temperatures is to include electric heaters that rely on batteries for power. But considering the length and low temperatures of the lunar night, even a well-insulated system will require significant battery mass to prevent damage to sensitive components. To overcome this challenge, Advanced Cooling Technologies, Inc. (ACT) is developing a low cost, mass, and volume Thermal Control Valve (TCV) to passively shut down heat rejection from a LHP once the evaporator temperature reaches a user-specified value. As part of a NASA SBIR Sequential Phase 2 project, ACT has developed multiple TCV prototypes and tested them as part of a full-scale LHP system in both laboratory and vacuum environments. The work discussed here focuses on the TCV concepts, LHP test data, and potential applications.
Ellis et al. (2024) studied this question.