(47a) Cryogenic Propellant Transfer Line Chilldown Experiments in 1-g using Low Thermally Conductive Coatings and Pulse Flow

Cryogenic fluids are used throughout the medical, food, aeronautical, and spaceflight industries. NASA and the aerospace community maintains a strong desire to develop cryogenic fluid management technology to enable future manned and unmanned missions beyond Low Earth Orbit (LEO). Cryogenic propellants are advantageous over traditional storable propellants like hydrazine due to relative improvement on safety and environmental concerns (storable propellants are toxic) and higher specific performance. However, cryogens exist as gases at standard temperature and pressure and are thus difficult to store and difficult to transfer as single-phase liquids due to a 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 propellants 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 have been transferred in space, the mass-efficient transfer of cryogenic propellants in a reduced or microgravity environment has never been demonstrated to date. NASA and universities are currently investigating mass-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 the chilldown and transfer process.

Cryogenic transfer line experiments are ongoing to test the performance enhancement of two improvements: (1) using thermal coatings on the inside of the transfer line and (2) pulse flow to speed up the chilldown process. First, 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. Second, using pulse flow has the potential to reduce the total amount of propellant consumption. Here, the line inlet valve is cycled while the line exit valve remains open such that liquid only flows when the valve is open, creating liquid pulses. The relative length of time that the valve is open is determined by the duty cycle, which is the ratio of valve open time to the period. The advantage of pulse flow is that complete liquid vaporization is more likely because the sensible and latent energy of the fluid is utilized more effectively, potentially leading to higher transfer efficiencies.

This presentation will cover details of recent 1-g liquid nitrogen transfer line chilldown tests performed at the University of Florida using these two performance enhancements. Results are compared between bare tubes with continuous flow and coated tubes with pulse flow to assess the performance benefit in terms of chilldown time and chilldown mass. Based on ground test results, savings in excess of 70% reduction in chilldown mass are achievable using the combination of thermal coatings and pulse flow over the base case of a bare tube with continuous flow in a 1-g environment.

This work is funded through the Reduced Gravity Cryogenic Transfer Project under the Technology Demonstration Mission program under the Space Technology Mission Directorate at NASA.