Two-Stage Refrigeration for Subcooling Liquid Hydrogen and Oxygen as Densified Propellants

Abstract

The thermodynamic design on two-stage refrigeration cycles is performed as one of several feasibility studies for the densified propellant technologies funded by the NASA Glenn Research Center. The refrigeration is required to increase the density of liquid oxygen and liquid hydrogen by subcooling to 65 K and 15 K, respectively, and to reduce the gross lift-off weight of a launch vehicle by up to 20%. The objective of this study is to seek the most efficient and economic two-stage refrigeration cycle, which satisfies the specific cooling requirements at the two temperature levels so that both densified propellants can be supplied simultaneously on a scheduled launch countdown. Recuperative cycles such as Claude and reverse Brayton refrigeration can be modified for subcooling liquid oxygen and liquid hydrogen in a manner commonly used for large capacity flows at lower temperatures. It is proposed to use a hybrid or cascade cycle, combining recuperative heat exchangers, expander and Joule-Thomson (J-T) valve at 65 K and 15 K. A variety of two-stage cycles derived from J-T, reverse Brayton and Claude cycles are examined in this paper. The essential features and characteristics of selected hybrid two-stage cycles are reported through a rigorous thermodynamic analysis. Among the examined cycles, the two-stage reverse Brayton helium refrigeration system shows a very suitable possibility for cryogenic propellant densification technology.