Editing Propelling nozzle (section)

===Convergent-divergent (C-D) nozzle===
{{main|de Laval nozzle}}
Engines capable of supersonic flight have [[De Laval nozzle|convergent-divergent]] exhaust duct features to generate supersonic flow.  Rocket engines — the extreme case — owe their distinctive shape to the very high area ratios of their nozzles.

When the pressure ratio across a convergent nozzle exceeds a critical value, the flow [[Choked flow|chokes]], and thus the pressure of the exhaust exiting the engine exceeds the pressure of the surrounding air and cannot decrease via the conventional [[Venturi effect]]. This reduces the thrust producing efficiency of the nozzle by causing much of the expansion to take place downstream of the nozzle itself. Consequently, rocket engines and jet engines for supersonic flight incorporate a C-D nozzle which permits further expansion against the inside of the nozzle. However, unlike the [[rocket engine nozzles|''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 nozzle pressure ratio that occurs with speeds from subsonic to over Mach{{nbsp}}3.

Nonetheless, [[#With low area ratio|low area ratio nozzles]] have subsonic applications.