Propellant tank pressurization modeling for a hybrid rocket

Hybrid rockets are currently being examined as a simpler and safer alternative to solid and liquid rocket propulsion. A basic hybrid rocket oxidizer delivery system utilizes the self-pressurizing nature of a liquid oxidizer at ambient temperatures in conjunction with a non-condensable pressurant to provide a high oxidizer tank pressure that drives liquid oxidizer flow to the combustion chamber. In this study, the oxidizer fluid is nitrous oxide since it produces high vapor pressures at ambient temperatures, and helium is the pressurant. The goal of this thesis is to model the pressure draining history of the oxidizer tank to within +/- 5% accuracy.;Previous studies of the self-pressurization processes in a propellant tank have focused on long-term cryogenic storage applications where the primary concern is heat leaking into the tank, causing unwanted pressure increases that result in propellant loss through necessary venting. Results of these cryogenic studies have been used to justify the inclusion of heat and mass transfer resistances in propellant tank models in order to be sufficiently accurate. However, these cryogenic studies were performed under conditions of very low vapor pressures and temperatures, and it is not clear that the conclusions drawn from such studies are valid for self-pressurization under ambient temperature conditions. Thus, the validity of simpler thermodynamically-based models has not been considered for self-pressurizing, draining tanks under high pressure conditions.;In this study, two models are developed, both assuming thermodynamic equilibrium states at every point in time throughout draining. The first model assumes the P-V-T behavior of the nitrous oxide/helium mixture follows the ideal gas law; the second model assumes that the mixture adheres to the non-ideal Peng-Robinson equation-of-state. Both models are compared to experimental data from pure nitrous oxide draining tests, published in G. Zilliac & M. Karabeyoglu (Modeling of Propellant Tank Pressurization, AIAA 2005-3549, 41st AIAA/ASME/ASEE Joint Propulsion Conference). Sensitivity studies have been performed in order to determine the effect of experimental error on the time-dependent flow from the tank and to assess the behavior of the models near physical extremes, such as high or low initial nitrous oxide fill-levels. Theoretical draining histories for the Peregrine hybrid sounding rocket (a joint effort between NASA Ames Research Center and Stanford University), soon to be launched from NASA Wallops Flight Facility, have also been examined.;A variety of comparisons with available experimental data, theoretical sensitivity studies, and theoretical launch data demonstrates that the non-ideal draining model provides favorable agreement. The additional complexity introduced by a non-ideal equation-of-state is necessary due to the high pressures encountered in the tank during draining. Sensitivity studies reveal that small deviations in the initial temperature, initial fill-level, or the discharge coefficient can produce nearly 5% errors in liquid drain time, demonstrating the need for accurately measured model inputs and suggesting sources of error in experimental design. It is found that despite the highly nonlinear nature of the draining process, the liquid flow rate from the tank remains reasonably constant, which is a highly desirable characteristic of a rocket oxidizer delivery system. The study provides a proposal for a new set of experiments necessary to assess and refine the theoretical models to allow for design and scale-up of hybrid rocket oxidizer delivery systems.