Editing Jet engine (section)

==Other types of jet propulsion==
===Rocket===
{{Main article|Rocket engine}}
[[File:Rocket thrust.svg|thumb|right|Rocket engine propulsion]]
The rocket engine uses the same basic physical principles of thrust as a form of [[reaction engine]],<ref>[https://www.collinsdictionary.com/dictionary/english/reaction-engine Reaction engine] definition, Collins online dictionary: ''"an engine, such as a jet or rocket engine, that ejects gas at high velocity and develops its thrust from the ensuing reaction"'' (UK), or ''"an engine, as a jet or rocket engine, that generates thrust by the reaction to an ejected stream of hot [[exhaust gas]]es, ions, etc."'' (US) (retrieved 28 June 2018)</ref> but is distinct from the jet engine in that it does not require atmospheric air to provide oxygen; the rocket carries all components of the reaction mass. However some definitions treat it as a form of [[jet propulsion]].<ref>[https://www.collinsdictionary.com/dictionary/english/jet-propulsion Jet propulsion], Collins online dictionary definition. (retrieved 1 July 2018)</ref>

Because rockets do not breathe air, this allows them to operate at arbitrary altitudes and in space.<ref>AC Kermode; ''Mechanics of Flight'', 8th Edition, Pitman 1972, pp. 128–31.</ref>

This type of engine is used for launching satellites, [[space exploration]] and crewed access, and permitted [[landing on the Moon]] in 1969.

Rocket engines are used for high altitude flights, or anywhere where very high accelerations are needed since rocket engines themselves have a very high [[thrust-to-weight ratio]].

However, the high exhaust speed and the heavier, oxidizer-rich propellant results in far more propellant use than turbofans. Even so, at extremely high speeds they become energy-efficient.

An approximate equation for the net thrust of a rocket engine is:

:<math>F_N = \dot m\, g_0\, I_\text{sp,vac} - A_e\, p \;</math>
Where <math>F_N</math> is the net thrust, <math>I_\text{sp,vac}</math> is the [[specific impulse]], <math>g_0</math> is a [[standard gravity]], <math>\dot m</math> is the propellant flow in kg/s, <math>A_e</math> is the cross-sectional area at the exit of the exhaust nozzle, and <math>p</math> is the atmospheric pressure.

{| class="wikitable"
|-
! Type
! Description
! Advantages
! Disadvantages
|-
![[Rocket]]
|Carries all propellants and oxidants on board, emits jet for propulsion<ref>{{cite web|url=http://www.grc.nasa.gov/WWW/K-12/airplane/rockth.html |title=Rocket Thrust Equation |publisher=Grc.nasa.gov |date=2008-07-11 |access-date=2010-03-26}}</ref>
|Very few moving parts. Mach 0 to Mach 25+; efficient at very high speed (> Mach 5.0 or so). Thrust/weight ratio over 100. No complex air inlet. High compression ratio. Very high-speed ([[hypersonic]]) exhaust. Good cost/thrust ratio. Fairly easy to test. Works in a vacuum; indeed, works best outside the atmosphere, which is kinder on vehicle structure at high speed. Fairly small surface area to keep cool, and no turbine in hot exhaust stream. Very high-temperature combustion and high expansion-ratio nozzle gives very high efficiency, at very high speeds.
|Needs lots of propellant. Very low [[specific impulse]] – typically 100–450 seconds. Extreme thermal stresses of combustion chamber can make reuse harder. Typically requires carrying oxidizer on-board which increases risks. Extraordinarily noisy.
|}

===Hybrid===
Combined-cycle engines simultaneously use two or more different principles of jet propulsion.
{| class="wikitable"
|-
!'''Type'''
!'''Description'''
!'''Advantages'''
!'''Disadvantages'''
|-
![[Air turborocket|Turborocket]]
|A turbojet where an additional [[oxidizer]] such as [[oxygen]] is added to the airstream to increase maximum altitude
|Very close to existing designs, operates in very high altitude, wide range of altitude and airspeed
|Airspeed limited to same range as turbojet engine, carrying oxidizer like [[LOX]] can be dangerous. Much heavier than simple rockets.
|-
![[Air-augmented rocket]]
|Essentially a ramjet where intake air is compressed and burnt with the exhaust from a rocket
|Mach 0 to Mach 4.5+ (can also run exoatmospheric), good efficiency at Mach 2 to 4
|Similar efficiency to rockets at low speed or exoatmospheric, inlet difficulties, a relatively undeveloped and unexplored type, cooling difficulties, very noisy, thrust/weight ratio is similar to ramjets.
|-
![[Precooled jet engine|Precooled jets]] / [[Liquid air cycle engine|LACE]]
|Intake air is chilled to very low temperatures at inlet in a heat exchanger before passing through a ramjet and/or turbojet and/or rocket engine.
|Easily tested on ground. Very high thrust/weight ratios are possible (~14) together with good fuel efficiency over a wide range of airspeeds, Mach 0–5.5+; this combination of efficiencies may permit launching to orbit, single stage, or very rapid, very long distance intercontinental travel.
|Exists only at the lab prototyping stage. Examples include [[RB545]], [[Reaction Engines SABRE]], [[ATREX]]. Requires liquid hydrogen fuel which has very low density and requires heavily insulated tankage.
|}

===Water jet===
{{Main article|Pump-jet}}
A water jet, or pump-jet, is a marine propulsion system that uses a jet of water. The mechanical arrangement may be a [[ducted propeller]] with nozzle, or a [[centrifugal compressor]] and nozzle. The pump-jet must be driven by a separate engine such as a [[Diesel engine|Diesel]] or [[gas turbine]].

[[File:Pump jet.PNG|thumb|right|A pump jet schematic.]]
{| class="wikitable"
|-
!'''Type'''
!'''Description'''
!'''Advantages'''
!'''Disadvantages'''
|-
![[Pump-jet|Water jet]]
|For propelling [[water rocket]]s and [[jetboat]]s; squirts water out the back through a nozzle
|In boats, can run in shallow water, high acceleration, no risk of engine overload (unlike propellers), less noise and vibration, highly maneuverable at all boat speeds, high speed efficiency, less vulnerable to damage from debris, very reliable, more load flexibility, less harmful to wildlife
|Can be less efficient than a propeller at low speed, more expensive, higher weight in boat due to entrained water, will not perform well if boat is heavier than the jet is sized for
|}