Thermodynamic Vent System Test in Low Earth Orbit Simulation

*A thermodynamic vent system installed in a cryogenic propellant tank was tested at small multipurpose research facility at NASA Glenn. The test was conducted to simulate the storage of cryogenic oxygen in low earth orbit. There is renewed interest in cryogenic oxygen storage for an advanced second generation Orbital Maneuvering System and Reaction Control Systems in a Low Earth Orbit as Cryogenic propellants are more energetic and environmentally friendly than current storable propellants. Cryogenic storage systems also have an advantage in reduced weight compared to super-critical tanks. Unfortunately, heat transfer or heat leak into these storage systems increases the tank pressure, limiting storage time. On earth, pressure is easily controlled by venting from the ullage space. In 0-g venting is more complicated as the location of vapor is unknown. Historically upper stages have used auxiliary thrusters to resettle the tank contents and fix the location of the ullage space in orbit. However, this incurs weight penalties and resettling may be required at inopportune times in the mission. An active thermodynamic vent system has been proposed for 0-g, which consists of a Joule-Thomson valve and heat exchanger coupled with the tank mixer-pump. The combination is used to extract thermal energy from the tank fluid, reducing temperature and ullage pressure. A thermodynamic vent system was designed for this test. Nitrogen was used as the test fluid as it has similar properties to oxygen, but is much safer to work with. The thermodynamic vent system was sized so that the mixer only operated a small fraction of the time. Initially the axial jet mixer used sub-cooled liquid to control pressure. After ullage pressure reached 21 psi and the mixed tank temperature had risen to above 80 Kelvin, fluid was vented. Pressure cycles were performed until cycle characteristics repeated and steady-state operation was demonstrated. Three test runs were conducted at tank fill levels of 97, 80 and 63 % fill. Each test was begun with a boil-off test to determine heat leak into the tank. The 80 and 63 % tank fills had time averaged vent rates very close to steady state boil-off rates. Thus, the thermodynamic vent system was as efficient as the traditional 1-G vent system for lower tank fills and the vent fluid was completely vaporized within the test tank.