Performance of air-breathing engines burning hydrogen fuel.
An air-breathing engine is an engine that takes in air from its surroundings in order to burn fuel. Examples include the ramjet, scramjet, turbojet, turbofan, and pulse-jet. These contrast with a rocket which carries its own oxidizer and thus can operate in space.
Some vehicles, such as space planes, may be fitted with both air-breathing and rocket engines for efficient operation both within and beyond the atmosphere.
The maximum velocity of air-breathing engines is limited to 1–3 km/s due to extreme temperature and dissociation of the exhaust gas; however, the maximum velocity of a hydrogen-breathing engine of the same design is about 4 times higher.