Literature survey for a first choice of a fuel-oxidiser couple for hybrid propulsion based on kinetic justifications

Hybrid rocket propulsion presents an increasing demand because of safety and cost reasons. Numerous works cover the main related areas (regression rate, oxidiser injection, grain design and additives in solid reducer). Nevertheless, the use of detailed kinetic studies, either experimentally or numerically, remains scarce. In particular, the auto-ignition and pyrolysis delays, the nature of chemical species impacting the diffusion flame and the rate of heat release are some of the important parameters. They are rarely considered when choosing a reducer–oxidiser couple. Understanding these chemical parameters could help choosing, or later improving, the solid reducers which present low regression speeds (limiting step of hybrid engine due to low combustible gas flow rate). Since detailed chemistry is still a foreign field for hybrid rocket studies, a wide review is first presented to determine how existing chemical works can be of interest for this propulsion technique. Mechanical and thermal characteristics are also considered in addition to combustion ones. Analytical estimation of the requirements to be met by the reducer–oxidiser couple is proposed (combustion heat release and associated heat fluxes, endothermic pyrolysis effect, chemical induction delays). Moreover, the choice of a reducer depends on the operating conditions under which it is used. For this reason, some equilibrium calculations are also provided to investigate the pressure and temperature effects. Amongst sixteen solid reducers, only three are finally selected in a first step (high density polyethylene – HDPE, polymethylmethacrylate – PMMA, hydroxyl terminated polybutadiene – HTPB). A similar work for oxidisers is conducted amongst thirteen compounds and three of them are selected (hydrogen peroxide – H2O2, nitrous oxide – N2O, pure oxygen – O2).

► The coupling between fuel pyrolysis and combustion is analysed transiently. ► The importance of chemical kinetics in hybrid rocket applications is demonstrated. ► The regression rate is shown to depend on the heating rate. ► The heating rate is proved to be of major importance in the production of species. ► A choice of a oxidiser/reducer couple is proposed on the basis of kinetic results.