Critical assessment of dual-bell nozzles

A critical assessment of dual-bell nozzles is given in this paper. The principal flowfield development in dual-hell nozzles, as well as design aspects for the contour of the base nozzle, the wall inflection, and the nozzle extension are discussed. Special regard is focused on the transition behavior from sea Level to altitude operation and its dependence on the contour type used for the nozzle extension. Parametric numerical simulations of the flowfield development were performed to quantify the different loss effects. It is shown that the additional performance losses caused by the dual-bell nozzle contour are surprisingly low. An analytical derivation of the how transients from the separated to the fully attached flow is presented. The necessity of further experimental investigations on dual-bell nozzles is emphasized, which will lead to a better understanding of the now transition in dual-bell nozzles. Finally, new ideas are presented to minimize the duration of the critical how transition by varying the thrust chamber pressure on system level, to ensure a sudden and controlled jump of the separation point from the wall inflection (sea-level operation) to the exit plane (altitude operation).