Combined‐Cycle Engines

AbstractA combined‐cycle engine has two or more operating modes with a single flowpath. The motivation behind a combined‐cycle engine is to extend the engine's operating envelope and attain higher average performance. Several combined‐cycle engines and a combined engine system are presented. The rocket‐based combined‐cycle (RBCC) engine is a combination of a rocket engine and a ramjet engine. RBCC has wide operating range and structural simplicity, since there are no rotating parts. The drawback is a relatively lower specific impulse in the subsonic and transonic range. RBCC is suitable for the single‐stage‐to‐orbit (SSTO) vehicle due to its operability in low ambient pressure conditions and in space. The liquid air cycle engine (LACE) liquefies air using cryogenic propellants to reduce the onboard oxygen requirement. The advantages of LACE are reduced engine weight and wide operating conditions. However, owing to the low liquefaction efficiency of air, LACE suffers from low specific impulse and high fuel consumption. The turbo‐ramjet (TRE) and the air‐turbo‐rocket (ATR) engines employ turbocompressors to compress the airflow like in turbojet engines. Despite the advantage of increased specific impulse, both engines have low thrust to weight ratios. The dual‐mode scramjet engine like the RBCC has no moving parts. The combustion is subsonic in the ramjet mode and supersonic in the scramjet mode. The combination engine system, which mounts several engines on a vehicle, extends the operating range of the vehicle by operating appropriate engines according to the flight conditions.