Experimental and numerical study on reinforcement mechanism of embedded rocket on back pressure resistance of RBCC inlet at starting stage

Favorable back pressure resistance of an RBCC inlet is vital for its smooth start and stable operation in ejector-to-ramjet mode transition. As a core component of the RBCC engine, the embedded rocket can be fully used to enhance the back pressure resistance of the inlet at the starting stage. The reinforcement mechanism of an embedded rocket on the back pressure resistance of the RBCC inlet is studied through experimental and numerical methods. An RBCC inlet model fully concerning the combustion pressure in the RBCC combustor, and the expansion and aerodynamic compression of the embedded rocket jet is established, whilst a facility nozzle is deployed to replace the compression section of the inlet in the ground direct-connect tests. The results show that three typical rocket operation stages may exist impacted by both the rocket jet and the back pressure, in terms of ‘Subcritical Rocket Operation’, ‘Critical Rocket Operation’, and ‘Supercritical Rocket Operation’. Meanwhile, two different patterns of flow separation caused by the synergy of the rocket jet and back pressure are observed. In the ‘Subcritical Rocket Operation’ stage, the two separation regions are strongly influenced by the back pressure. Nevertheless, the locations of the separations and the related shock train move downstream synchronously as the rocket pressure increases. In the ‘Supercritical Rocket Operation’ stage, the increasing rocket pressure drives the cowl wall separation downstream gradually, and however, it pushes the shock train moving upstream in the isolator resulting from the aerodynamic compression of the rocket jet in the mainstream. And in the ‘Critical Rocket Operation’ stage, an optimum status is found to exist such that the shock train stands most downstream from the physical throat, at which the RBCC inlet performs at the best back pressure resistance. Therefore, an appropriate embedded rocket operation can effectively reinforce the back pressure resistance of the RBCC inlet. Since the embedded rocket is operated at relatively high pressure, the under-expanded rocket jet will compress the mainstream and form a new aerodynamic throat in the flow passage, which can: 1) increase the real internal contraction ratio of the RBCC inlet to improve the compression efficiency of the incoming flow, and 2) enlarge the aerodynamic expansion ratio of the downstream flow passage. Further, the appropriate embedded rocket operation can also effectively assist the RBCC inlet to start at a certain Mach number, but with higher combustion pressure in the ejector mode.