Editing Specific impulse (section)

===Actual exhaust speed versus effective exhaust speed===

When an engine is run within the atmosphere, the exhaust velocity is reduced by atmospheric pressure, in turn reducing specific impulse. This is a reduction in the effective exhaust velocity, versus the actual exhaust velocity achieved in vacuum conditions. In the case of [[gas-generator cycle]] rocket engines, more than one exhaust gas stream is present as [[turbopump]] exhaust gas exits through a separate nozzle. Calculating the effective exhaust velocity requires averaging the two mass flows as well as accounting for any atmospheric pressure.<ref>{{Cite web |title=Rocket Thrust Equations |url=https://www.grc.nasa.gov/WWW/k-12/airplane/rktthsum.html |archive-url=http://web.archive.org/web/20241109090800/https://www.grc.nasa.gov/www/k-12/airplane/rktthsum.html |archive-date=2024-11-09 |access-date=2024-12-11 |website=www.grc.nasa.gov}}</ref>

For air-breathing jet engines, particularly [[turbofan]]s, the actual exhaust velocity and the effective exhaust velocity are different by orders of magnitude. This happens for several reasons. First, a good deal of additional momentum is obtained by using air as reaction mass, such that combustion products in the exhaust have more mass than the burned fuel. Next, inert gases in the atmosphere absorb heat from combustion, and through the resulting expansion provide additional thrust. Lastly, for turbofans and other designs there is even more thrust created by pushing against intake air which never sees combustion directly. These all combine to allow a better match between the airspeed and the exhaust speed, which saves energy/propellant and enormously increases the ''effective'' exhaust velocity while reducing the ''actual'' exhaust velocity.<ref>{{Cite journal |date=2023-01-23 |title=Research on Efficient Heat Transfer for Air Breathing Electric Propulsion |url=https://doi.org/10.2514/6.2023-0450.vid |access-date=2024-12-11 |website=doi.org|doi=10.2514/6.2023-0450.vid |url-access=subscription }}</ref> Again, this is because the mass of the air is not counted in the specific impulse calculation, thus attributing ''all'' of the thrust momentum to the mass of the fuel component of the exhaust, and omitting the reaction mass, inert gas, and effect of driven fans on overall engine efficiency from consideration. 

Essentially, the momentum of engine exhaust includes a lot more than just fuel, but specific impulse calculation ignores everything but the fuel. Even though the ''effective'' exhaust velocity for an air-breathing engine seems nonsensical in the context of actual exhaust velocity, this is still useful for comparing absolute [[fuel efficiency]] of different engines.