Propulsive efficiency

In aerospace engineering, concerning aircraft, rocket and spacecraft design, overall propulsion system efficiency ${\displaystyle \eta }$ is the efficiency with which the energy contained in a vehicle's fuel is converted into kinetic energy of the vehicle, to accelerate it, or to replace losses due to aerodynamic drag or gravity. Mathematically, it is represented as ${\displaystyle \eta =\eta _{\mathrm {c} }\eta _{\mathrm {p} }}$,^[1] where ${\displaystyle \eta _{\mathrm {c} }}$ is the cycle efficiency and ${\displaystyle \eta _{\mathrm {p} }}$ is the propulsive efficiency.

The cycle efficiency is expressed as the percentage of the heat energy in the fuel that is converted to mechanical energy in the engine, and the propulsive efficiency is expressed as the proportion of the mechanical energy actually used to propel the aircraft. The propulsive efficiency is always less than one, because conservation of momentum requires that the exhaust have some of the kinetic energy, and the propulsive mechanism (whether propeller, jet exhaust, or ducted fan) is never perfectly efficient. It is greatly dependent on exhaust expulsion velocity and airspeed.