Convergent-divergent Nozzles
Engines capable of supersonic flight have convergent-divergent duct features to generate supersonic flow. Rocket engines — the extreme case — owe their distinctive shape to the their very high area ratios of their nozzles.
The high nozzle pressure rations of convergent nozzles often cause the pressure of exhaust exiting the engine to exceed the pressure of the surrounding air and thereby reduce efficiency by causing much of the expansion to take place downstream of the nozzle itself. Consequently, some engines (notably, rockets) incorporate a convergent-divergent nozzle, which, by causing more of the exhaust to expand against the inside of the nozzle. However, unlike the fixed convergent-divergent nozzle used on a conventional rocket motor, those on turbojet engines must have heavy and expensive variable geometry to cope with the great variation in the nozzle pressure ratio that engine throttling creates.
Read more about this topic: Propelling Nozzle