varied during non-afterburning or dry thrust conditions. Typically the nozzle is fully open for starting and at idle. It may then close down as the thrust lever is advanced reaching its minimum area before or at the Military or maximum dry thrust setting. Two examples of this control are the General Electric J-79 and the Klimov RD-33 in the MIG-29. Reasons for varying the nozzle area are explained in section: Nozzle area control during dry operation. Principal geometries Convergent nozzle Convergent nozzles are used on many jet engines. If the nozzle pressure ratio is above the critical value (about 1.8:1) a convergent nozzle will choke, resulting in some of the expansion to atmospheric pressure taking place downstream of the throat (i.e., smallest flow area), in the jet wake. Although jet momentum still produces much of the gross thrust, the imbalance between the throat static pressure and atmospheric pressure still generates some (pressure) thrust. Divergent nozzle The supersonic speed of the air flowing into a scramjet allows the use of a simple diverging nozzle Convergent-divergent (C-D) nozzle Engines capable of supersonic flight have 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 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 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 Mach3. Nonetheless, low area ratio nozzles have subsonic applications. Types of nozzle Fixed-area nozzle Non-afterburning subsonic engines have nozzles of a fixed size because the changes in engine performance with altitude and subsonic flight speeds are acceptable with a fixed nozzle. This is not the case at supersonic speeds as described for Concorde below. With low area ratio At the other extreme, some