Review and Evaluation of Models for Self-Pressurizing Propellant Tank Dynamics

Nitrous oxide is a popular oxidizer choice for many in the hybrid rocket community due to its ease of handling, storability at room temperature, and high vapor pressure. This high vapor pressure can be used as the pressurant, making it what is referred to as a selfpressurizing propellant. Unfortunately no comprehensive models exist for the dynamics of a self-pressurizing propellant tank and thus the modeling of system performance of nitrous oxide hybrids is difficult. In this paper, three existing models (equilibrium, Zilliac & Karabeyoglu, Casalino & Pastrone) of the system are described, evaluated, and compared against a variety of experimental data. The Zilliac & Karabeyoglu model performs the best in 5 of the 6 test cases but is limited by its complexity and its requirement of an empirical factor that is system-dependent. The equilibrium model does not capture the initial transient behavior and often over-predicts the pressure but is the simplest model and requires properties at saturation only. The Casalino & Pastrone model is a compromise between the two but does not perform well here, possibly due to differences in sources for thermodynamic properties.