(440g) Performance Benefit of Thermal Coatings for Future in-Space Cryogenic Propellant Transfer Systems

Cryogenic liquids are used throughout the medical, food, and spaceflight industries. NASA maintains a strong desire to develop cryogenic fluid management technology to enable future manned and unmanned missions beyond Low Earth Orbit (LEO). Cryogenic propellants offer benefits over traditional storable propellants like hydrazine, such as a relative improvement on safety and environmental concerns (storable propellants are toxic) and higher specific impulse. However, cryogenic propellants which exist as gases at room temperature are difficult to store as liquids and are difficult to transfer as single phase flow due to the high propensity to boil. Perhaps the most prolific use of cryogenic fluids is in the proposed fuel depots. A depot is defined as an Earth-orbiting propellant storage vessel that will be used to store liquid oxygen and liquid hydrogen in LEO indefinitely to refuel spacecraft. A depot will enable long duration missions because a higher percentage of the spacecraft mass can be used for payload or for larger engines, and the vehicle can achieve higher velocities once outside the gravity well of Earth.

While storable propellants are routinely transferred in space, the transfer of cryogenic propellants in a reduced or microgravity environment has never been demonstrated to date. NASA is currently investigating efficient methods with which to transfer cryogenic propellant in reduced gravity environments, particularly for cryogenic fuel depots, upper stages, and Lunar or Martian ascent or descent stages. Efficient cryogenic fluid transfer methods will reduce the transfer time or amount of propellant consumed for chilldown of transfer line hardware and tanks. Most importantly, it will ensure successful engine restart or fill of a customer receiver tank (depot). Before cryogenic liquid can flow between depot storage tank and customer receiver tank, the transfer line and associated hardware must be chilled down or “quenched” from 300K to temperatures below the fluid saturation temperature. The most direct, repeatable, and reliable method to remove heat is to use the cryogen itself to quench the transfer system. Due to the low normal boiling point of cryogens, phase change, complex flow patterns, two-phase flow boiling, and high heat transfer are inevitable during the chilldown process. Due to the cost to launch and store propellant in space, it is desired to use the least amount of propellant as possible during chilldown.

Cryogenic transfer line experiments are ongoing to test the performance enhancement of using thermal coatings on the inside of the transfer line to speed up the chilldown process. The line is coated with a thin (< 100 microns) low thermally conductive material that acts as an insulation barrier between warm metal tube and cold fluid. Because the fluid “sees” a colder temperature, the coated surface temperature chills down very quickly without chilling down the entire metal mass. The lower surface temperature earlier on in chilldown implies that liquid will stay in contact with the coating over a longer time. For an uncoated tube, most of the chilldown time is spent in film boiling where a vapor blanket exists between the warm metal and cold fluid. For coated tubes, the presence of the thermal coating allows the surface temperature to reach the Leidenfrost point sooner, leading to a higher chilldown efficiency because nucleate boiling and single phase liquid heat transfer are far more efficient at removing heat than film boiling.

This presentation will cover details of recent 1-g liquid nitrogen transfer line chilldown tests performed at the University of Florida. Results are compared between bare tubes and coated tubes to assess the performance benefit in terms of chilldown time and chilldown mass. Testing also investigates the effect of coating thickness on the chilldown efficiency to determine if there exists an optimal coating thickness that minimizes the chilldown time. Based on ground test results, savings in excess of 40% reduction in chilldown time are achievable using the thermal coatings.

This work is funded through the Game Changing Technology Development project under the Space Technology Mission Directorate at NASA.