Dual-bell nozzle for space launchers with fluidic control of transition

The dual-bell nozzle is a promising altitude adaptive nozzle concept for improving the performance of existing space launchers as well as future reusable launch vehicles. It is characterized by an inflection in the wall geometry which links a base nozzle with a nozzle extension. Two working modes are therefore attained: a low-altitude mode in which the expansion is limited to the first bell and a high-altitude mode in which full expansion is achieved. Even though this capability makes it possible to reduce the non-adaptation losses in engines working from sea level to almost vacuum conditions, the natural transition between the two modes usually takes place prematurely during the ascent trajectory and destructive side-loads may arise. This results in a limitation on the performance gain. In this work, a dual-bell nozzle design capable of upgrading the parallel-staged Ariane 5 launcher is studied. Fluidic control is investigated as a possible solution to limit premature transition and reduce side-loads. The control is obtained by means of a secondary injection in the proximity of the inflection point: different injection strategies are investigated. Reynolds-averaged Navier–Stokes equations at steady state conditions are solved with an in-house CFD tool, to investigate the flow field and determine the required secondary mass flow rate and the range of nozzle pressure ratio for which the control is effective.