Variable Conductance Heat Pipe Radiator for Lunar Fission Power Systems

Nuclear power systems for long-term Lunar and Martian missions present many challenges to thermal management systems, such as variable thermal loads, large temperature swings between day and night, and freezing of the working fluid. The radiator to reject the waste heat must be sized for the maximum power at the highest sink temperature, but is then oversized for other conditions, such as the Lunar/Martian night, or periods when the power to be rejected is low. A Variable Conductance Heat Pipe (VCHP) radiator can passively accommodate changing thermal loads and environments. A variable conductance thermosyphon radiator was developed and tested, with heat supplied by a single-phase pumped loop. The radiator is designed to operate in the 370 to 400 K temperature range, which is above the operating range of standard aluminum/ammonia radiator panels. To operate at these higher temperatures, the radiator has a titanium heat exchanger, titanium/water thermosyphons, and graphite fiber reinforced composite radiator panels. The radiator is capable of: 1) Accommodating changes in power and sink temperature, 2) Successfully starting up from an initially frozen state with excess water frozen in an arbitrary location, and 3) Shutting down, freezing, and then successfully restarting. A low mass design was developed that accommodates the coefficient of thermal expansion (CTE) mismatch between the titanium heat exchanger and the graphite radiator panel. A full-scale VCHP radiator was fabricated and tested in order to demonstrate functionally at a larger, more representative size and determine maximum heat rejection.