<P> Even though overall energy efficiency η o (\ displaystyle \ eta _ (o)) is: </P> <Dl> <Dd> η o = η p η t h (\ displaystyle \ eta _ (o) = \ eta _ (p) \ eta _ (th)) </Dd> </Dl> <Dd> η o = η p η t h (\ displaystyle \ eta _ (o) = \ eta _ (p) \ eta _ (th)) </Dd> <P> for all jet engines the propulsive efficiency is highest as the exhaust jet velocity gets closer to the vehicle speed as this gives the smallest residual kinetic energy . For an airbreathing engine an exhaust velocity equal to the vehicle velocity, or a η p (\ displaystyle \ eta _ (p)) equal to one, gives zero thrust with no net momentum change . The formula for air - breathing engines moving at speed v (\ displaystyle v) with an exhaust velocity v e (\ displaystyle v_ (e)), and neglecting fuel flow, is: </P>

How many rpms does a jet engine spin