Theoretical Investigation of the Hypergolic Ignition in a Hybrid Rocket Motor

For some liquid propellant-based systems, ignition is achieved using hypergolic propellants, i.e., propellants that ignite upon contact. Hypergolic ignition is intrinsic to the propellants used, and with it, numerous advantages such as short ignition delay times, high efficiency, and high reliability. Adopting hypergolic ignition for hybrid rocket systems could allow all the additional advantages that hypergolic liquid systems possess. The hypergolic ignition of non-hypergolic propellants can be achieved by embedding within the solid grain a sufficient quantity of reactive additives that are hypergolic with the liquid oxidizer. If the reaction generates enough heat, at a high rate, leading to ignition.

The present study deals with the theoretical modeling of the hypergolic ignition of a solid fuel and a liquid oxidizer by means of reactive particles, as in hybrid rockets. First, a novel numerical model has been developed to analyze the ignition process by predicting ignition delay times. The model is used to analyze the influence of multiple parameters on ignition delay times. Second, a theoretical model has been developed for predicting the ignition delay time and the temperature rise behavior of general hypergolic configurations, through a simplified linearized approach based on heat diffusion and geometry.

The numerical model shows that the vaporization of the liquid oxidizer is the dominant mass transfer mechanism for the hybrid rocket configuration. It is found that the range of oxidizer-to-fuel ratios required for ignition varies with additive loading. Also, the optimal oxidizer-to-fuel ratio leading to the shortest ignition delay time is mostly a function of the additive loading. Pressure, propellant initial temperature, and oxidizer concentration, have a major influence on ignition delay times. The competition between the heat released and the heat lost by the system is shown to control the ignition capabilities.

The analytical model results show that ignition is favored by increasing the droplet/particle size, the initial heat released, and by decreasing the thermal conductivity. The results of this research allow a better understanding of hypergolic ignition processes in hybrid motors, while accurately predicting ignition delay times. To date, no models involving the complexity of hypergolic ignition for hybrid rocket systems are known to exist.