Editing Jet engine (section)

===Propelling nozzle===
{{Main article|Propelling nozzle}}
A propelling nozzle produces a high velocity exhaust [[jet (fluid)|jet]]. Propelling nozzles turn internal and pressure energy into high velocity kinetic energy.<ref>Jet Propulsion for Aerospace Applications Second Edition 1964, Hesse and Mumford, Pitman Publishing Corporation, {{LCCN|6418757}}, p. 48</ref> The total pressure and temperature don't change through the nozzle but their static values drop as the gas speeds up.

The velocity of the air entering the nozzle is low, about Mach 0.4, a prerequisite for minimizing pressure losses in the duct leading to the nozzle. The temperature entering the nozzle may be as low as sea level ambient for a fan nozzle in the cold air at cruise altitudes. It may be as high as the 1000 [[Kelvin]] exhaust gas temperature for a supersonic afterburning engine or 2200&nbsp;K with [[afterburner]] lit.<ref>"Jet Propulsion" Nicholas Cumpsty 1997, Cambridge University Press, {{ISBN|0-521-59674-2}}, p.  197</ref> The pressure entering the nozzle may vary from 1.5 times the pressure outside the nozzle, for a single stage fan, to 30 times for the fastest manned aircraft at Mach 3+.<ref>{{Cite web|url=https://www.enginehistory.org/Convention/convention1.shtml|title=AEHS Conventions 1|website=www.enginehistory.org}}</ref>

Convergent nozzles are only able to accelerate the gas up to local sonic (Mach 1) conditions. To reach high flight speeds, even greater exhaust velocities are required, and so a [[convergent-divergent nozzle]] is needed on high-speed aircraft.<ref>{{Cite book|url=https://arc.aiaa.org/doi/abs/10.2514/6.2004-3923|title=40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit|first1=Eric|last1=Gamble|first2=Dwain|last2=Terrell|first3=Richard|last3=DeFrancesco|chapter=Nozzle Selection and Design Criteria|year=2004|publisher=American Institute of Aeronautics and Astronautics|doi=10.2514/6.2004-3923|isbn=978-1-62410-037-6}}</ref>

The engine thrust is highest if the static pressure of the gas reaches the ambient value as it leaves the nozzle. This only happens if the nozzle exit area is the correct value for the nozzle pressure ratio (npr). Since the npr changes with engine thrust setting and flight speed this is seldom the case. Also at supersonic speeds the divergent area is less than required to give complete internal expansion to ambient pressure as a trade-off with external body drag. Whitford<ref>Design For Air Combat" Ray Whitford Jane's Publishing Company Ltd. 1987, {{ISBN|0-7106-0426-2}}, p. 203</ref> gives the F-16 as an example. Other underexpanded examples were the XB-70 and SR-71.

The nozzle size, together with the area of the turbine nozzles, determines the operating pressure of the compressor.<ref>"Jet Propulsion" Nicholas Cumpsty 1997, Cambridge University Press, {{ISBN|0-521-59674-2}}, p. 141</ref>