Propelling

Main article: Propelling nozzle
A jet exhaust produces a net thrust from the energy obtained from combusting fuel which is added to the inducted air. This hot air is passed through a high speed nozzle, a propelling nozzle which enormously increases its kinetic energy.
For a given mass flow, greater thrust is obtained with a higher exhaust velocity, but the best energy efficiency is obtained when the exhaust speed is well matched with the airspeed. However, no jet aircraft can maintain velocity while exceeding its exhaust jet speed, due to momentum considerations. Supersonic jet engines, like those employed in fighters and sst aircraft (e.g. concorde), need high exhaust speeds. Therefore supersonic aircraft very typically use a CD nozzle despite weight and cost penalties. Subsonic jet engines employ relatively low, subsonic, exhaust velocities. They thus employ simple convergent nozzles. In addition,bypass nozzles are employed giving even lower speeds.
Roket Motors use convergent-divergent nozzles with very large area ratios so as to maximise thrust and exhaust velocity and thus extremely high nozzle pressure ratios are employed. Mass flow is at a premium since all the propulsive mass is carried with vehicle, and very high exhaust speeds are desirable.