Two-Phase Thermal Switch for Lunar Lander and Rover Thermal Management

The 14-earth day long lunar night poses a significant challenge to the thermal management of future lunar landers and rovers. For these vehicles to operate for long durations on the lunar surface, the on-board electronics must be maintained above their survival temperatures during the lunar night. Thermal switches are among the passive thermal management technologies that may be utilized for helping lunar vehicles survive the lunar night while minimizing the use of electric power for survival heating. Thermal switches are designed to minimize heat transfer in the OFF condition, and to maximize it when in the ON condition. In this work, a passive thermal switch design with high turndown is developed for lunar landers and rovers. This thermal switch design utilizes a sealed flexible bellows containing a two-phase working fluid. The switching mechanism is passively actuated by the temperature of the heat source. At low temperatures, the vapor pressure within the bellows is low, and the bellows is not in contact with the heat sink, restricting heat transfer through the switch. At higher temperatures, the increased vapor pressure causes the bellows to expand and come in contact with the heat sink, allowing more effective heat transfer through the switch. The actuation temperature is determined by the balance of forces on the bellows. A coupled thermal-mechanical model has been developed to illustrate the dynamic performance of the two-phase thermal switch. Several thermal switch prototypes were fabricated and tested for concept demonstration and model validation, and trade studies for enhancing the ON/OFF conductance ratio were performed. Finally, a prototype thermal switch with high ON/OFF ratio was developed and tested for a small lunar rover application.