Influence of a Conical Axial Injector on Hybrid Rocket Performance

This paper analyzes the results obtained from a series of static firings of a lab-scaled hybrid rocket in which gaseous oxygen was supplied into axial-symmetric polyethylene cylindrical grains through two different injector configurations: an axial conical subsonic nozzle and a radial injector. The axial injector is interesting because of its relatively easy design, the higher regression rates, and the noticeably stable motor operation. To exploit its qualities, not only the assessment of the regression rate but rather the entire behavior of the motor is required. For the investigated set of operating conditions, the instantaneous regression rates exhibit a time dependence caused by the impinging jet zone dynamics, while the average regression rates are higher and less mass flux dependent than those achieved with the radial injection motor and expected from the classical turbulent-boundary-layer diffusion- limited theory. A comparison to the data from the radial injector was further drawn in terms of combustion efficiency and fuel regression uniformity. Concerning combustion stability, some observations are made. The radial injector, at the same mass flux and pressure, produces lower regression rates, high pressure oscillations, and worse combustion efficiency, but more uniform fuel consumption.