Restricted Shock Separation in Rocket Nozzles

Two distinctive flow separation phenomena, the free-shock 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 the plume can occur, namely, the regular shock reflection, the Mach disk, or a cap-like shock pattern. The appearance of these different plume patterns is discussed. These shock structures are conserved for the full-flowing, but overexpanded, nozzle. Numerical results obtained for existing rocket nozzles, e.g., Space Shuttle Main Engine or Vulcain, show qualitative good agreement with experimental photographs. An explanation for the appearance of restricted shock separation is given, with an analysis of why and under what conditions it occurs. The type of nozzle contour strongly influences this form of flow separation, and restricted shock separation also occurs in full-scale, thrust-optimized rocket nozzles. On the basis of the results established for flow separation, an outlook on the generation of side loads is given. (Author)