Investigation of Feed System Coupled Low Frequency Combustion Instabilities in Hybrid Rockets

TLDR: In this paper, the transient combustion theory has been extended to hybrid rockets using liquid oxidizers with feed systems characterized by finite response times, and transfer functions for the feed coupled system with and with out flow isolation elements have been developed.

Abstract: In this paper, the transient combustion theory has been extended to hybrid rockets using liquid oxidizers with feed systems characterized by finite response times. Models for generic hybrid feed systems have been developed and these have been coupled to the combustion chamber dynamics with lag times introduced to model the delays associated with oxidizer vaporization and fuel gasification processes. This study has been limited to a simplified behavior for the combustion chamber that only includes the filling/emptying dynamics. The set of Ordinary Differential Equations that represent the system behavior have been linearized and nondimensionalized. Using the technique of Laplace Transformation, transfer functions for the feed coupled system with and with out flow isolation elements have been developed. The model has been used to investigate the stability behavior of the feed coupled system and to develop stability criteria in terms of the practical operational parameters for liquid fed hybrid motors. The oxidizer vaporization delay is treated as an input parameter which can be adjusted to match the observed oscillation frequency to the model prediction. As a practical application of the model, the estimated vaporization delay can be used to evaluate the atomization characteristics of injectors and pre-combustion chamber designs for hybrid rocket motors.