Nozzles can be described as ''convergent'' (narrowing down from a wide diameter to a smaller diameter in the direction of the flow) or ''divergent'' (expanding from a smaller diameter to a larger one). A [[de Laval nozzle]] has a convergent section followed by a divergent section and is often called a convergent-divergent nozzle.
Convergent nozzles accelerate fluids. If the nozzle pressure ratio is high enough the flow will reach sonic velocity at the narrowest point (i.e. the ''nozzle throat''). In this situation, the nozzle is said to be ''choked''.
Increasing the nozzle pressure ratio further will not increase the throat [[Mach number]] beyond unity. Downstream (i.e. external to the nozzle) the flow is free to expand to supersonic velocities.
Divergent nozzles slow fluids, if the flow is subsonic, but accelerate sonic or supersonic fluids.
[[de laval nozzle|Convergent-divergent nozzles]] can therefore accelerate fluids that have choked in the convergent section to supersonic speeds. This CD process is more efficient than allowing a convergent nozzle to expand supersonically externally.
Since exhaust velocity has to exceed airspeed, supersonic aircraft also very typically use a con-di nozzle despite the weight and cost penalties. Supersonic jet engines, like those employed in [[fighter]]s and [[SST]] aircraft (e.g. Concorde), indeed have relatively high nozzle pressure ratios. Because subsonic jet engines require low exhaust velocities, they require only subsonic exhaust and thus have modest nozzle pressure ratios and employ simple convergent nozzles.
[[Rocket motor]]s use convergent-divergent nozzles, to maximise thrust and exhaust velocity and thus extremely high nozzle pressure ratios are employed.