A Shock-Induced Combustion Ramjet Engine (abbreviated as Shcramjet; also called Oblique Detonation Wave Engine (ODWE), or simply referred to as Shock-Ramjet Engine) is a new concept in airbreathing ramjet engines, proposed to be used for hypersonic, as well as, single-stage-to-orbit (SSTO) propulsion applications. From the overall design point of view, the shcramjet engine is similar to a scramjet engine; however, unlike the diffusive mode of combustion in scramjet engine, a shcramjet engine combustion takes place across a very thin region of standing oblique shock and/or detonation waves stabilized over a wedge, blunt body etc. Since combustion in a shcramjet engine is confined to very short region across the igniting wave, the combustor length in a shcramjet can be significantly shorter than the scramjet that requires a lengthy combustor for complete fuel-air mixing and combustion. Hence, compared to scramjet, the shcramjet has a reduced overall weight (which directly leads to substantial reduction in overall vehicle weight of a hypersonic waverider design) and higher efficiency. Also, the shcramjet is believed to have a better overall propulsive performance than the scramjet at higher Mach numbers, especially above Mach 12. Recent researches have stipulated that airbreathing engines such as scramjets and shcramjets provide a more efficient high-speed vehicle propulsion system than rocket engines. These potential advantages of shcramjets have recently attracted substantial efforts into their research all around the world.
Basic Principles
The shcramjet engine geometry is extremely simple and closely similar to scramjet, the only major difference being the combustor design. The engine includes a supersonic inlet followed by a combustion chamber and a nozzle, respectively. The design principle for a shcramjet inlet is similar to a scramjet, in which the whole nose structure of waverider vehicle is used as inlet. Combustion in a shcramjet can occur in two different modes, shock-induced combustion or detonation combustion, depending on the strength of inducing shock at any Mach number. If ignition occurs far enough downstream that the ensuing combustion process does not influence the preceding shock, the combustion is said to be shock-induced. However, for extremely fast reactions, ignition occurs close to the preceding shock wave and the combustion process couples with the shock wave and forms a detonation wave. Therefore, detonation wave ramjet, or oblique detonation wave engine, is only a particular case of shcramjet.
Though shock and detonation waves are related to high total pressure loss during combustion, the theoretical total pressure loss associated with shcramjet combustor approaches that of the scramjet engine at increasing Mach numbers. This fact, together with simpler engine geometry with concomitant increase in component efficiencies, results in possible superior predicted performance of the shcramjet compared to scramjet at flight Mach numbers beyond 12.