Basic Study on Thermodynamic Vent System in Propulsion System for Future Spacecraft

In planned planetary explorations, cryogenic liquids such as liquid hydrogen (LH2), liquid oxygen (LOX), and liquefied natural gases (LNG) are used as fuel and oxidants in the propulsion systems of spacecraft. Such explorations require long-term storage for those cryogens, as well as heat insulation technology to protect the heat from the outside and pressure control technology to suppress rises in pressure due to evaporation gas in the propellant tank. However, current vent systems that discharge the evaporated gas to the outside of the spacecraft are suboptimal, for the propellant’s uncertain position in the tank when spacecraft operate in microgravity environments causes the significant loss of propellant during venting. In response, we examined a method using jet mixing in a thermodynamic vent system (TVS) that adjusts the tank pressure by cooling the inside of the tank and reducing boil-off gas. To that end, thermal and fluid analyses were conducted to design a test tank system before the ground verification test of the TVS using liquid nitrogen (LN2) as the simulated cryogen. To evaluate our results, we compared experimental and numerical results regarding the formation of thermal stratification. The experimentally performed verification of the TVS function revealed that jet mixing can lower the liquid’s temperature in the tank after the experimental apparatus was modified to supply a stable subcooled mixing jet.