Phase Change Material Heat Sink Flight Experiment Results

A flight experiment was conducted on the International Space Station (ISS) to prove out operation of a microgravity compatible paraffin wax phase change material (PCM) heat sink. A PCM heat sink can help to reduce the overall mass and volume of future exploration spacecraft thermal control systems (TCS). Vehicles such as the Orion Multipurpose Crew Vehicle can use PCM heat sinks to temporarily store thermal energy during mission phases where the radiators are unavailable or too warm to reject the heat as it’s generated, or sublimating water would require significant expendable mass. The experiment was conducted specifically to prove out a heat sink design that incorporates a novel phase management approach to prevent high pressures and structural deformation that often occur with PCM heat sinks undergoing cyclic operation. The PCM heat sink test article was incorporated into an ISS double EXPRESS rack, where it underwent performance testing and acceptance testing at NASA Johnson Space Center. The experiment was delivered to the ISS on the SpaceX 9 mission in the summer of 2016. It was successfully installed into the ISS and run remotely for several months to exercise the PCM heat sink. Freeze and thaw cycles were conducted with a range of coolant flow rates and heater powers to characterize the performance of the technology with regard to heat storage and wax pressure management. Heat storage performance met the objectives of the tests, but the novel phase management approach had mixed results. Wax cavity pressures remained low in some tests, but not others.