Two-Phase Thermal Switch for Spacecraft Passive Thermal Management

Future manned and unmanned spacecraft will venture far beyond the relatively benign environment of low Earth orbit. The combination of extreme environments and high turndown requirements present a significant challenge for spacecraft thermal control systems. Thermal switches are among the thermal control devices that will be required to dissipate a wide range of heat loads in widely varying environments. A novel two-phase passive thermal switch technology has been developed and demonstrated. This technology uses the condensing vapor of a saturated two-phase working fluid to both transfer the heat and provide the contact pressure for the heat transfer surfaces of the switch. The switching mechanism is passively triggered by the temperature of the heat source. In addition to the On/Off capability of a thermal switch, the technology serves as a variable thermal link while in the On condition to maintain a heat source set point temperature. This set point temperature is determined by the design of the switch. In this paper, the principles of operation of the two-phase thermal switch are presented. A prototype switch was built and tested over a range of conditions. The set point temperature was determined for a range of enclosure gas counter pressures, and the maintenance of a heat source set point temperature is demonstrated. The performance of the unoptimized prototype switch is characterized and shown to have a nominal On thermal conductance of 0.7 W/K and an On/Off conductance ratio of 20.