Nitrogen flow boiling and chilldown experiments in microgravity using pulse flow and low-thermally conductive coatings.

The enabling of in-space cryogenic engines and cryogenic fuel depots for future manned and robotic space exploration missions begins with technology development of advanced cryogenic fluid management systems upstream in the propellant feed system. Before single-phase liquid can flow to the engine or customer spacecraft receiver tank, the connecting transfer line must first be chilled down to cryogenic temperatures. The most direct and simplest method to quench the line is to use the cold propellant itself. When a cryogenic fluid is introduced into a warm transfer system, two-phase flow quenching ensues. While boiling is well known to be a highly efficient mode of heat transfer, previous work has shown this efficiency is lowered in reduced gravity. Due to the projected cost of launching and storing cryogens in space, it is desired to perform this chilldown process using the least amount of propellant possible, especially given the desire for reusable systems and thus multiple transfers. This paper presents an assessment of two revolutionary new performance enhancements that reduce the amount of propellant consumed during chilldown while in a microgravity environment. Twenty-eight cryogenic transfer line chilldown experiments were performed onboard four parabolic flights to examine the independent as well as combined effect of using low thermally conductive coatings and pulse flow on the chilldown process. Across a range of Reynolds numbers, results show the combination significantly enhances performance in microgravity, with a reduction in consumed mass up to 75% relative to continuous flow for a bare transfer line.