Combustion Instability Mechanism in Hybrid Rocket Motors with Diaphragm

A diaphragm can be installed inside a hybrid motor to enhance the fuel regression rate and combustion efficiency in hybrid rockets. However, significant pressure oscillations have been observed in such motors. This paper describes an experimental investigation of the mechanism that triggers the instability in the presence of a diaphragm. Lab-scale firing tests under various experimental conditions and combustion visualization tests are conducted to identify the instability mechanism. The excitation source of the pressure oscillations is fouimpingement of vortices on the rigid rear diaphragm, a phenomenon known as the hole-tone instability. Large pressure oscillations occur in hybrid motors with diaphragms due to the resonance between the natural frequency of the hybrid motor system and the excitation frequency of the hole-tone instability. Finally, a new grain design without a diaphragm, named the “stepped grain” configuration, is proposed to mitigate the excitation source of the instability. The regression rate of this stepped grain design is noticeably higher than that of a cylindrical single-port grain configuration under stable combustion.