Onboard Cryogenic Liquid-Propellant Subcooler Based on Thermodynamic Vent for Upper-Stage Propulsion System

Abstract

Subcooling of cryogenic liquid propellants offers significant advantages for launch vehicles. Liquid subcooling can reduce the extent of cavitation generation in turbopumps, which provides advantages for upper-stage engine reignition in orbit. Additionally, liquid-propellant subcooling densifies the liquid, which can reduce the propellant tank volume and increase launch capability when it is applied during the propellant-filling phase on the ground. Here, a cryogenic propellant subcooling system derived from thermodynamic vent system (TVS) concepts is proposed. In contrast to prior approaches with TVS, the proposed method only focuses on the liquid subcooling and only requires the installation of a heat exchanger at the tank bottom, making it easily applicable to conventional upper stages and enhancing launch capability. First, ground tests were conducted using liquid nitrogen. We introduced energy efficiency to evaluate the subcooler performance, and an optimum point was found for the coolant supply pressure and Joule–Thomson orifice diameter. To investigate the advantages of the proposed method, a theoretical model for the subcooling process before engine reignition in orbit was developed based on experimental observation. The model assuming liquid hydrogen demonstrated an improved weight penalty in the proposed method compared to the conventional vent/pressurization method. A study that quantitatively addresses the enhancement of launch capability through TVS-induced liquid subcooling is unprecedented, paving the way for a new cryogenic propulsion system.