Design criterion of the dual-bell nozzle contour

In this study, we performed a CFD analysis to determine the optimum dual-bell nozzle contour. Especially, we investigated the effect of the deflection angle at the wall inflection on the separation point transition. Based on our present analysis, we found that the deflection angle at the wall inflection should be larger than the angle determined by a simple Prandtl-Meyer expansion. Also, the time to accomplish the separation point transition from the wall inflection to the extension nozzle exit is estimated to be less than 10 ms when our design criterion of the dual-bell nozzle contour is applied to the booster engine of H-2A launch vehicle. This time interval is considered to be essentially instant from the practical point of view of the safe nozzle operation.