Editing Jet engine (section)

===Comparison of types===
[[File:Gas turbine efficiency.png|thumb|Propulsive efficiency comparison for various gas turbine engine configurations]]

Propeller engines handle larger air mass flows, and give them smaller acceleration, than jet engines. Since the increase in air speed is small, at high flight speeds the thrust available to propeller-driven aeroplanes is small. However, at low speeds, these engines benefit from relatively high [[propulsive efficiency]].

On the other hand, turbojets accelerate a much smaller mass flow of intake air and burned fuel, but they then reject it at very high speed. When a [[de Laval nozzle]] is used to accelerate a hot engine exhaust, the outlet velocity may be locally [[Supersonic speed|supersonic]]. Turbojets are particularly suitable for aircraft travelling at very high speeds.

Turbofans have a mixed exhaust consisting of the bypass air and the hot combustion product gas from the core engine. The amount of air that bypasses the core engine compared to the amount flowing into the engine determines what is called a turbofan's bypass ratio (BPR).

While a turbojet engine uses all of the engine's output to produce thrust in the form of a hot high-velocity exhaust gas jet, a turbofan's cool low-velocity bypass air yields between 30% and 70% of the total thrust produced by a turbofan system.<ref>{{cite book|author=Federal Aviation Administration (FAA)|url=http://www.faa.gov/library/manuals/aircraft/airplane_handbook/media/FAA-H-8083-3B.pdf|title=FAA-H-8083-3B Airplane Flying Handbook Handbook|publisher=Federal Aviation Administration|year=2004|url-status=dead|archive-url=https://web.archive.org/web/20120921094453/http://www.faa.gov/library/manuals/aircraft/airplane_handbook/media/FAA-H-8083-3B.pdf|archive-date=2012-09-21}}</ref>

The net thrust ('''''F<sub>N</sub>''''') generated by a turbofan can also be expanded as:<ref>{{cite web|url=http://www.grc.nasa.gov/WWW/K-12/airplane/turbfan.html|title=Turbofan Thrust|access-date=2012-07-24|archive-url=https://web.archive.org/web/20101204031217/http://www.grc.nasa.gov/WWW/K-12/airplane/turbfan.html|archive-date=2010-12-04|url-status=dead}}</ref>

:<math>F_N = \dot{m}_e v_{he} - \dot{m}_o v_o + BPR\, (\dot{m}_c v_f)</math>

where:

{| border="0" cellpadding="2"
|-
|align="right"|'''''ṁ<sub>&thinsp;e</sub>'''''
|align="left"|= the mass rate of hot combustion exhaust flow from the core engine
|-
|align=right|'''''ṁ<sub>o</sub>'''''
|align=left|= the mass rate of total air flow entering the turbofan = '''''ṁ<sub>c</sub>''''' + '''''ṁ<sub>f</sub>'''''
|-
|align=right|'''''ṁ<sub>c</sub>'''''
|align=left|= the mass rate of intake air that flows to the core engine
|-
|align=right|'''''ṁ<sub>f</sub>'''''
|align=left|= the mass rate of intake air that bypasses the core engine
|-
|align=right|'''''v<sub>f</sub>'''''
|align=left|= the velocity of the air flow bypassed around the core engine
|-
|align=right|'''''v<sub>he</sub>'''''
|align=left|= the velocity of the hot exhaust gas from the core engine
|-
|align=right|'''''v<sub>o</sub>'''''
|align=left|= the velocity of the total air intake = the true airspeed of the aircraft
|-
|align=left|'''''BPR'''''
|align-right|= Bypass Ratio
|}

[[Rocket engine]]s have extremely high exhaust velocity and thus are best suited for high speeds ([[hypersonic]]) and great altitudes. At any given throttle, the thrust and efficiency of a rocket motor improves slightly with increasing altitude (because the back-pressure falls thus increasing net thrust at the nozzle exit plane), whereas with a turbojet (or turbofan) the falling density of the air entering the intake (and the hot gases leaving the nozzle) causes the net thrust to decrease with increasing altitude. Rocket engines are more efficient than even scramjets above roughly Mach 15.<ref>{{cite web |url=http://www.energy.kth.se/courses/4A1346/2ndLecture/KTH%20High%20Speed.pdf |title=Microsoft PowerPoint – KTHhigspeed08.ppt |access-date=2010-03-26 |archive-url=https://web.archive.org/web/20090929130946/http://www.energy.kth.se/courses/4A1346/2ndLecture/KTH%20High%20Speed.pdf |archive-date=2009-09-29 |url-status=dead }}</ref>