On supersonic combustion and hypersonic propulsion

The advanced engine has been the core technology of the aviation industry for several decades. The air-breathing hypersonic propulsion is the top problem for future aerospace flight. The engine's performance depends on its energy conversion method and combustion mode, and its relevant theory is of fundamental and revealing significance. In this paper, the supersonic combustion is discussed first since it is the theoretical basis for the research and development of scramjet engines. Then, by reviewing related research progresses, three criteria of the air-breathing hypersonic ramjet propulsion are established. The first one can be used to determine the local subsonic or supersonic flow states of combustion products in supersonic reacting gas flows, revealing the mechanism of the upstream-traveling shock wave. The second one defines the critical Mach number for hypersonic ramjet operation for all the combustion modes, and is a necessary condition that needs to be considered in the engine design under the equivalent ratio combustion. The last one gives a critical wedge angle corresponding to the CJ oblique detonation, and its physical basis is the critical initiation state of detonation. Finally, the experimental research progress on the stationary oblique detonation ramjet (Sodramjet) engine is summarized, and its feasibility as a hypersonic engine for future aerospace flight is demonstrated.