A continuous stream of air flows through the air-breathing engine. The air is compressed, mixed with fuel, ignited and expelled as the exhaust gas. Thrust produced by a typical air-breathing engine is about eight times greater than its weight. Turbopump-fed liquid rocket engines are much better in this respect; some of them have thrust-to-weight ratio of over 100. On the other hand, the air-breathing engine vehicles are do not carry oxidizer because they utilize ambient air as the oxidizer. The maximum velocity of the air-breathing engine vehicle is limited to 2 km/s due to extreme temperature and dissociation of the exhaust gas.
Turboengines designed for the maximum velocity of less than 1 km/s are light-weight, durable, and have a high specific impulse.
Ramjets can operate at a higher velocity than turboengines because a smaller area of the engine is exposed to the extreme heat. Fixed geometry ramjet is better for cruising at constant speed than accelerating because its specific impulse varies dramatically with velocity.
The air-breathing engines can be used instead of the expensive first stage of the chemical rocket launcher. The idea is called airplane-rocket relay.
The maximum velocity of a hydrogen-breathing engine is about 4 times higher than the maximum velocity of an air-breathing engine of the same design. Practicable hydrogen-breathing engines are slower because a burning mixture of hydrogen and oxygen is used instead of pure hydrogen. It is possible to fly the engine in a long tunnel cut in antarctic ice sheet and filled with hydrogen (ice gun). Another option is to fly the engine in a disposable tinfoil balloon filled with very cold hydrogen gas (ramjet in balloon).
AIAA Air Breathing Propulsion Technical Committee.
An engine which combines rocket and airbreathing components into a single propulsion unit is called rocket-based combined-cycle (RBCC) engine.
Detlef Manski, Christoph Goertz, Hagen-D. Sabnick, James R. Hulka, B. David Goracke, and Daniel J. H. Levack, "Cycles for Earth-to-Orbit Propulsion," Journal of Propulsion and Power, Vol. 4, No. 5, September-October 1998, pp. 588-604.
R. Daines and C. Segal, "Combined Rocket and Airbreathing Propulsion Systems for Space-Launch Applications," Journal of Propulsion and Power, Vol. 4, No. 5, September-October 1998, pp. 605-612.
F. Oi, J. Wang, and C.-P. Chen, "Preliminary Analysis of an Airbreathing and Rocket Combined-Cycle Engine," Journal of Propulsion and Power, Vol. 4, No. 5, September-October 1998, pp. 613-619.
Vladimir V. Balepin, Glenn W. Liston, and Raymond H. Moszée, "Combined Cycle Engines with Inlet Air Conditioning," AIAA-2002-5148.