Transient Thrust Model for a Small Scale Self-Pressured Hybrid Rocket Motor

Abstract

Self-pressurizing hybrid rocket motors, due to their inherent safety and simplicity, offer attractive options for space system designers. This paper presents a numerical performance model for self-pressurized hybrid systems where an existing enthalpy-based, longitudinally averaged fuel regression model is combined with an entropy-based model of a self-pressurized oxidizer feed system. Combustion product characteristics are calculated and presented as a function of propellant mixture ratio using the NASA –developed Chemical Equilibrium with Applications program. The regression model relies on an enthalpy balance between fuel ablation heat and convective heat transfer from the combustion flame zone to the fuel surface. Convective heat transfer is related to surface skin friction using the Colburn analogy for non-unity Prandtl numbers. The oxidizer evacuation model is developed for nitrous oxide using an entropy-based expansion for saturated gas-liquid mixtures. Because this model is based on fundamental physics instead of motor-specific correlations, it is applicable to many different hybrid rocket configurations. A small-scale nitrous oxide and hydroxyl-terminated polybutadiene hybrid motor is tested to validate the simulation and highlight the areas where the simulation may be improved. The regression model calculates longitudinally averaged fuel regression rate and is shown to accurately predict chamber pressure, thrust, and specific impulse.