Rocket Engines
A rocket engine's is usually high due to the high combustion temperatures and pressures, and long nozzle employed. The value varies slightly with altitude due to atmospheric pressure on the outside of the nozzle/engine, but can be up to 70%. Most of the remainder is lost as heat energy in the exhaust.
Rocket engines have a slightly different propulsive efficiency than airbreathing jet engines as the lack of intake air changes the form of the equation. This also means that rockets are able to exceed their exhaust velocity. See diagram.
As with ducted jet engines, matching the exhaust speed and the vehicle speed gives optimum efficiency in principle. Although in practice rocket exhausts are high and typically fixed, this can be a useful theoretical consideration. Unlike ducted engines, rockets give thrust even when the two speeds are equal.
Rockets often also have an additional significant source of energy, because they are able to leverage the kinetic energy of their propellant with the Oberth effect. This is also a factor for air-breathing aircraft, but much smaller one due to their lower speed.
Read more about this topic: Propulsive Efficiency, Estimation
Famous quotes containing the words rocket and/or engines:
“A rocket is an experiment; a star is an observation.”
—José Bergamín (1895–1983)
“America is like one of those old-fashioned six-cylinder truck engines that can be missing two sparkplugs and have a broken flywheel and have a crankshaft that’s 5000 millimeters off fitting properly, and two bad ball-bearings, and still runs. We’re in that kind of situation. We can have substantial parts of the population committing suicide, and still run and look fairly good.”
—Thomas McGuane (b. 1939)