Engineering Model for Self-Pressurizing Saturated-N2O-Propellant Feed Systems

Self-pressurizing propellant feed systems, due to their overall simplicity, offer attractive options for propulsion system designers. These systems inherently exhibit transient properties as the tank-fluid mass bleeds down, and modeling the two-phase blow-down or evacuation process using first-principle physics is an extremely complex and time-intensive problem. As an alternative, a simple engineering model is developed for self-pressurized saturated-propellant feed systems. The model assumes an adiabatic expansion for which, during evacuation, the current tank-fluid entropy and the fluid entropy that has escaped through the feed system equate to the initial tank-fluid entropy. Mass flow through the outlet orifice is modeled as a two-phase nonhomogeneous nonequilibrium process. The evacuated entropy is subtracted from the total entropy at the state of the fluid leaving the tank, then the new state with the existing mass and entropy is calculated using two-phase thermodynamic property tables. In this manner, the tank-fluid state is calculated continuously during the evacuation. This propellant feed-system evacuation model is developed using saturation fluid properties for nitrous oxide, but it is readily extensible to other saturated propellants. Model results are compared with data derived from cold-flow tests conducted on a small-scale hybrid rocket motor. The comparisons show excellent agreement.