Side Loads in Sub-scale Dual Bell Nozzles

Abstract

The contour inflection of the dual bell nozzle forces the flow to a symmetrical and controlled separation in sea level mode. At a certain altitude, the transition to high altitude mode takes place: the flow attaches rapidly to the nozzle extension wall, down to the exit plane. During this transition, the separation point moves in the extension generating potential high side load peaks due to its asymmetrical position. A cold flow subscale test campaign has been conducted on three nozzle models at the German Aerospace Center to evaluate the generation of side loads in dual bell nozzles. The phenomenology is given for the different nozzle flow regimes. Both operating modes are related to very low side loads. Transition and retransition induce a strong short time peak. The phase of sneak transition, corresponding to a flow separation within the inflection region before the start of the actual transition, generates comparable side loads to separated conventional nozzles. The influence of the various geometrical parameters on flow behavior and side load generation was also investigated in this study. The extension length is shown to be the critical parameter for flow stability, transition duration and side load generation, leading to the necessity of a trade off for the optimization of the dual bell concept in rocket applications.