Cryogenic Propulsion With Zero Boil-Off Storage Applied To Outer Planetary Exploration

§This paper describes a study of the potential application of cryogenic propulsion systems using liquid oxygen and hydrogen propellants to planetary spacecraft. A conceptual liquid oxygen/ liquid hydrogen propulsion stage designs were developed. Mission-level assessments of the impacts of adopting cryogenic propulsion with the latest in zero boil off cryogenic storage techniques were made. Results are presented for the three missions studied: Titan Explorer, Mars Sample Return Earth Return Vehicle, and Comet Nucleus Sample Return. Thermal analysis results show that it is possible to store LOX/LH 2 at reasonable tank pressures using only passive radiation cooling when the field of view of propellant tanks can be kept clear of warm planetary bodies. This situation is typical of interplanetary cruise, spacecraft orbiting bodies with low effective blackbody temperatures, and spacecraft with very short stay times near the target planet, asteroid, or comet. Passive storage was accomplished using a combination of sun shades, spacecraft configuration considerations, spacecraft pointing constraints, and the low conductance Passive Orbit Displacement Strut (PODS). Actively cooled designs use cryocoolers and mechanically pumped fluid loops to reject heat from the propellant tanks. From the results of the mission studies performed in this study, it appears that the applicability of cryogenic propulsion (specifically pump-fed LOX / LH 2 systems) is limited for missions in the Discovery and New Frontiers class or for unmanned exploration of Mars. Significant benefits were found for missions that share the following two characteristics: 1) Very large 8V requirements (over 3000 m/s) and 2) No requirement to store the propellants for an extended period in a low orbit about a planetary body. The large dry mass fraction of the cryogenic system, as conceived, reduced or outweighed the benefits from high Isp performance for missions with lower delta-V or which required active cooling.