On the Role of Vortex Shedding in Hybrid Rockets Combustion Instability

This paper is concerned with an investigation into the stability behavior of a hybrid rocket where gaseous oxygen is fed with a conical axial nozzle or a radial injector. Attention is paid to the influence of vortex shedding, in both pre- and post-combustion chamber, on the combustion stability. By comparing pressure spectra, it was found that the axial and radial injectors caused completely stable and unstable motor operations, respectively. This has been attributed to the fluid-dynamics and unsteady heat release in the forward plenum. In particular, the unstable combustion in the radial-flow injector motor is characterized by low frequency pressure oscillations, around 10-20 Hz. The low frequency pressure oscillations were always accompanied by acoustic modes. In some cases, the pressure oscillations amplitude abruptly increases reaching peak-to-peak amplitude close to 70% of the mean chamber pressure, which is somewhat unusual for hybrids. Vortex shedding in the aft-mixing chamber is considered as the triggering event for this behaviour