propulsive efficiency
The propulsive efficiency determines how much of the kinetic energy of the exhaust jet is useful for propelling a vehicle.
F is the thrust, u the absolute vehicle velocity, c the effective rocket exhaust velocity with respect to the vehicle, w the propellant weight flow rate, and Flp the propulsive efficiency. The propulsive efficiency is a maximum when the forward vehicle velocity is exactly equal to the exhaust velocity. Then the residual kinetic energy and the absolute velocity of the jet are zero and the exhaust gases stand still in space.
