Testing and evaluation of a double-tube hybrid rocket motor

This experimental study investigates the solid fuel regression rate and performance of a hybrid rocket using a double-tube configuration. The paper analyzes the results of a series of static firings of a laboratory-scale hybrid rocket motor with two coaxial cylindrical fuel grains of high-density polyethylene. The gaseous oxygen is injected into the combustion chamber using two different injector types: a double-tube configuration and a conventional axial showerhead injector, which is used as reference. In the double-tube design tested, all the oxidizer is injected through a coaxial inner tube with injector holes distributed along the motor longitudinal axis. Moreover, the inner fuel grain, which is supported by the inner tube, allows the oxidizer jets to enter the combustion chamber at a given injection angle. In this case, the gaseous oxygen is injected counter-flowing generating as a consequence strong recirculation zones that improve significantly the species mixing. The experimental test firings reveal that, for the same oxidizer mass flux rate, the double-tube configuration achieves a regression rate over twice faster than the conventional axial showerhead injector. In addition, the double-tube configuration performed with very stable motor operation and smoother pressure traces than the conventional axial showerhead injector. However, the characteristic flow field of the double-tube configuration provokes a higher unevenness in the fuel consumption.