Role of injection in hybrid rockets regression rate behavior

A series of firing tests was carried out to investigate the influence of the oxidizer injection on the solid fuel regression rate behavior in a hybrid rocket engine. For this purpose, a conical subsonic nozzle as the injector of the gaseous oxidizer was selected to generate nonuniform conditions at the entrance of the fuel port. Gaseous oxygen and polyethylene fuel cylindrical grains were used. When the oxygen was fed by this kind of injector, the fuel regression in the region of the oxygen impingement on the grain's surface was increased several times, which led to irregular fuel consumption with the characteristic afterburn port shape typical of solid fuel ramjets having a rearward-facing step at the air inlet. The local instantaneous regression rates, measured by means of the ultrasound pulse-echo technique, showed regression rate-time profiles strongly dependent on the geometric configuration and helped to explain the complex regression phenomenon deriving from the impingement zone displacement during the time. Empirical correlations for the prediction of the average regression rate were developed taking into account the influence of mass flux, pressure, and port diameter. Finally, a nondimensional semi-empirical correlation involving the effect of the blowing number yielded improved accuracy.