Side Loads in Dual Bell Nozzles, Part I: Phenomenology

Kurzfassung

DLR conducted tests to study the side load characteristics in cold flow subscale dual bell nozzles. The side loads are related to different nozzle flow regimes. The most important are strong short time peaks induced by transition and retransition from sea level to high altitude mode and vice versa. In sea level and high altitude mode the dual bell creates less side loads than a comparable conventional nozzle. An additional flow regime could be confirmed, addressed as sneak transition. Contrary to the accepted doctrine, the flow separation in sea level mode will not be continuously fixed at the contour inflection. Before the actual transition to high altitude mode takes place the flow separates within the inflection region. As a result a distinct phase of side loads comparable to separated conventional nozzles is given. A procedure to reduce the regime of sneak transition is proposed.