Thermodynamic Analysis of an Autogenous Pressurization System

Pressurized gas feed systems have been vital to spacecraft where a pump-fed design would prove too large, heavy, or complicated to be effective. This project investigates autogenous pressurization – a pressurized feed system where the intentional vaporization of liquid propellant to fill the ever-increasing ullage space with its own warm, low-density gas. Motivation for this research originates from the University of Tennessee Space Institute’s (UTSI) contract with Gloyer-Taylor Laboratories (GTL) to assist the development of an innovative pressurization system.

While other pressurization systems have been studied and used extensively, modeling of an autogenous system is less established. Despite the relative simplicity of pressure-fed systems, the plethora of heat and mass transfer mechanisms mentioned earlier complicate the modeling process. Therefore, the research presented in this thesis does not attempt to define a single, generalized model of autogenous pressurization. Instead, this research aims to develop boundary models of an autogenous pressurization system and compare them with a model of standard-practice external blowdown pressurization. The simulation results for these models indicate that an autogenous pressurization mechanism is desirable for reducing the internal volume required to pressurize methane and oxygen propellants, requiring as little as a seventh of the fluid volume of a comparable blowdown system; however, the pressurizing fluid mass for these propellants in an autogenous system will theoretically be greater than the mass of helium needed in a blowdown system – except at high pressurizing temperatures.

Additionally, this research employs heat transfer models to simulate the heating requirements of an autogenous system for a small-scale laboratory experiment. This experiment uses liquid nitrogen as a propellant and acts as a proof-of-concept means for heating and controlling cryogenic propellant flow for future work validating the autogenous boundary models. The results from the heating experiment indicate that the design program and its heat transfer models can predict within about 10% the heat rate required for the experiment despite non-ideal insulation, liquid quality concerns, and thermocouple tolerances. The experimental work also shows the importance of the latent heat of vaporization for autogenous systems – nearly half of the heating requirement goes into boiling the cryogenic liquid.