Thermal–Combustion Coupled Instability in Hybrid Rockets with Fuel Blowing

Thermal–combustion coupled instability creates one of the natural frequencies in hybrid rocket systems and is commonly observed as the dominant hybrid oscillation frequency. This type of instability is caused by the coupling between the thermal lag in the solid fuel and the combustion transients in the boundary layer, and it is unique to hybrid rocket systems. This study investigates the nature of thermal–combustion coupled instability, considering various test variables with a laboratory-scale hybrid rocket motor. The main parameters affecting the thermal–combustion coupled instability frequency are the fuel length and the freestream velocity in the fuel port, whereas the chamber pressure and the oxidizer mass flow rate have no significant direct effect. A new thermal–combustion coupled model is proposed that can account for changes in flow variables caused by the blowing by considering the blowing mass from the solid fuel surface. The new model’s thermal–combustion coupled instability frequencies are in good agreement with the experimental data over a wide range of test conditions.