Water Phase Change Heat Exchanger System Level Analysis for Low Lunar Orbit

In low Lunar orbit (LLO) the thermal environment is cyclic – extremely cold in the eclipse and warmer than room temperature near the subsolar point. Phase change material heat exchangers (PCM HXs) are the best option for long term missions in these environments. The Orion spacecraft will use n pentadecane wax PCM HX for its envisioned mission to low Lunar orbit.

Using water as a phase change material is attractive because its higher heat of fusion and greater density result in a lighter, more compact PCM HX. To assess the use of a water PCM HX for a human spacecraft in a circular LLO, a system level analysis was performed for the Orion spacecraft. Three cases were evaluated: a) A direct replacement of the wax PCM HX on the internal thermal control loop with a water PCM HX (including control modifications), b) a water PCM HX on the internal thermal control loop with a reduced radiator return setpoint temperature, c) locating the water PCM HX on the external loop with a controlled bypass.

The model showed that the water PCM HX could not be used as a drop-in replacement for the wax PCM HX. It did not freeze fully during the eclipse owing to its lower freezing point. Even in the most advantageous location – on the external loop - at least 15% more radiator area than the Orion baseline was required to obtain equivalent performance. The study shows that, although water PCM HXs are attractive at a component level, system level effects mean that they are not a good choice for LLO.