Jet engines breathe external air for both combustion and by-pass, and therefore have a much higher specific impulse than rocket engines. In an atmospheric context, specific impulse can include the contribution to impulse provided by the mass of external air that is accelerated by the engine in some way, such as by an internal turbofan or heating by fuel combustion participation then thrust expansion or by external propeller. In the case of a rocket, this means less propellant needed for a given delta-v, so that the vehicle attached to the engine can more efficiently gain altitude and velocity. For engines whose reaction mass is only the fuel they carry, specific impulse is exactly proportional to exhaust gas velocity.Ī propulsion system with a higher specific impulse uses the mass of the propellant more efficiently. Specific impulse (usually abbreviated I sp) is a measure of how efficiently a reaction mass engine (a rocket using propellant or a jet engine using fuel) creates thrust.
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