Manuel Frey

Restricted Shock Separation in Rocket Nozzles

In overexpanded rocket nozzles the e ow separates from the nozzle wall at a certain pressure ratio of wall pressure to ambient pressure. Flow separation and its theoretical prediction have been the subject of several experimental and theoretical studies in the past decades. Two distinctive e ow separation phenomena, the freeshock and restricted-shock separation, were observed in experiments with nozzles. Both phenomena are discussed in detail, and the system of recompression shocks and expansion waves is described. For the free-shock case three different shock structures in theplume can occur, namely the regular shock ree ection, the Mach disk, or a cap-like shock pattern. Theappearanceofthesedifferentplumepatternsis discussed. Theseshock structuresareconserved for the full-e owing, but overexpanded, nozzle. Numerical results obtained for existing rocket nozzles, e.g., Space ShuttleMain EngineorVulcain, show a qualitativegood agreement with experimental photographs.Furthermore, an explanation for the appearance of restricted shock separation, which has been widely unknown up to now, is given, analyzing why and under what conditions it occurs. The type of nozzle contour strongly ine uences this form of e ow separation, and restricted shock separation also occursin full-scale, thrust-optimized rocket nozzles. Based on the results established for e ow separation, an outlook on the generation of side loads is given.

Side-Load Phenomena in Highly Overexpanded Rocket Nozzles

The operation of rocket engines in the overexpanded mode, that is, with the ambient pressure considerably higher than the nozzle exit wall pressure, can result in dangerous lateral loads acting on the nozzle. These loads occur as the boundary layer separates from the nozzle wall and the pressure distribution deviates from its usual axisymmetric shape. Different aerodynamic or even coupled aerodynamic/structural mechanic reasons can cause an asymmetric pressure distribution. A number of subscale tests have been performed, and three potential origins of side loads were observed and investigated, namely, the pressure fluctuations in the separation and recirculation zone due to the unsteadiness of the separation location, the transition of separation pattern between free-shock separation and restricted-shock separation, and aeroelastic coupling, which indeed cannot cause but do amply existing side loads to significant levels. All three mechanisms are described in detail, and methods are presented to calculate their magnitude and pressure ratio at which they occur.

Critical Assessment of Dual-Bell Nozzles

A critical assessment of dual-bell nozzles is given in this paper. The principal flow field development in dual-bell 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 flow 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 flow transition in dual-bell nozzles. Finally, new ideas are presented to minimize the duration of the critical flow 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).