Combustion performance of a novel hybrid rocket fuel grain with a nested helical structure

Abstract

The combustion performance of a novel fuel grain having a nested helical structure was experimentally investigated using a laboratory-scale hybrid rocket engine. This grain comprised a paraffin-based fuel embedded in an acrylonitrile-butadiene-styrene (ABS) substrate that provided a helical structural framework. The helical structure of the grain was maintained throughout the combustion process due to the much lower regression rate of ABS compared with that of the paraffin-based fuel. Using oxygen as the oxidizer at mass flow rates of 7–30 g/s, firing tests were conducted to assess combustion performance parameters of the novel fuel grain, including ignition characteristics, pressure oscillations, regression rate, and combustion efficiency. Pure paraffin-based fuel grains were also tested as a baseline fuel and compared. The novel fuel grain exhibited rapid, reliable ignition with stable combustion pressures. Analysis of pressure fluctuations by fast Fourier transform showed peaks at approximately 62, 130 and 320 Hz, which are consistent with the characteristics of a pure paraffin-based fuel grain. It is highly likely that the nested helical structure did not introduce additional combustion oscillation mechanisms into the hybrid rocket engine. Significant improvements in regression rate were obtained using this novel grain. The regression rate for the novel fuel grain is approximately 20% higher than that of the paraffin-based fuel at an oxidizer mass flow rate of 30 g/s, and the rate of the regression rate rise was higher than that of the pure paraffin-based fuel as the oxidizer mass flow rate increases. Moreover, the nested helical structure was also found to improve combustion efficiency. A tentative explanation of all improvements was proposed, resorting to the exacerbated turbulence and the strengthened heat transfer.