Thermodynamic Analysis of a Cryogenic Upper Stage Supercritical Tank with Active Pressurization

Abstract

In a study published by the author in 2004, a cryogenic upper stage powered by a pressure-fed engine and without an active pressurization system was found to impose heating requirement so severe that the concept was not considered feasible. In this study, the same cryogenic upper stage is allowed to have helium pressurization in addition to heating. Although a pressurization system is still needed, this system will be more reliable than a pump-fed system, since it does not require turbopumps. A thermodynamic analysis of the supercritical hydrogen and oxygen tanks, however, indicates that such a system requires a considerable amount of helium to prevent substantial decays of the tank pressure and flow rate. This helium will dilute the hydrogen and oxygen and decrease the flammability limit of the mixture. Mixture ratio variation during burn will also degrade the engine performance. These challenges make a pressure-fed system with simultaneous heating and active pressurization an unviable concept for a cryogenic upper stage.