Editing Jet propulsion

{{Short description|Thrust produced by ejecting a jet of fluid}}
[[File:Rolls-Royce Trent 1000 jet engine.jpg|thumb|The jet engine of a [[Boeing 787 Dreamliner]].]]
[[File:Pump-jet on NatchanWorld 02.JPG|thumb|A [[pump-jet]] on a ferry.]]
'''Jet propulsion''' is the [[propulsion]] of an object in one direction, produced by ejecting a [[jet (fluid)|jet]] of [[fluid]] in the opposite direction. By [[Newton's third law]], the moving body is propelled in the opposite direction to the jet. [[Reaction engine]]s operating on the principle of jet propulsion include the [[jet engine]] used for [[aircraft propulsion]], the [[pump-jet]] used for [[marine propulsion]], and the [[rocket engine]] and [[plasma thruster]] used for [[spacecraft propulsion]]. Underwater jet propulsion is also used by several marine animals, including [[Cephalopod|cephalopods]] and [[salp]]s, with the [[flying squid]] even displaying the only known instance of jet-powered aerial flight in the animal kingdom.

==Physics==
Jet propulsion is produced by some [[reaction engine]]s or animals when thrust is generated by a fast moving [[jet (fluid)|jet]] of [[fluid]] in accordance with [[Newton's laws of motion]]. It is most effective when the [[Reynolds number]] is high—that is, the object being propelled is relatively large and passing through a low-viscosity medium.<ref name='Packard1972'>{{Cite journal| first1 = A.| title = Cephalopods and Fish: the Limits of Convergence| journal = Biological Reviews| volume = 47| issue = 2| last1 = Packard| pages = 241–307| year = 1972| doi = 10.1111/j.1469-185X.1972.tb00975.x| s2cid = 85088231}}</ref>

In animals, the most efficient jets are pulsed, rather than continuous,<ref name="ref_d">{{Cite journal | last1 = Sutherland | first1 = K. R. | last2 = Madin | first2 = L. P. | doi = 10.1242/jeb.041962 | title = Comparative jet wake structure and swimming performance of salps | journal = Journal of Experimental Biology | volume = 213 | issue = Pt 17 | pages = 2967–75 | year = 2010 | pmid =   20709925| url = https://authors.library.caltech.edu/19738/1/Sutherland2010p11195J_Exp_Biol.pdf | doi-access = free }}</ref> at least when the Reynolds number is greater than 6.<ref name="ref_e">{{Cite journal | last1 = Dabiri | first1 = J. O. | last2 = Gharib | first2 = M. | doi = 10.1098/rspb.2005.3109 | title = The role of optimal vortex formation in biological fluid transport | journal = Proceedings of the Royal Society B: Biological Sciences | volume = 272 | issue = 1572 | pages = 1557–1560 | year = 2005 | pmid =  16048770| pmc = 1559837}}</ref>

===Specific impulse===
{{Main|Specific impulse}}
Specific impulse (usually abbreviated ''I''<sub>sp</sub>) is a measure of how effectively a [[rocket engine|rocket]] uses propellant or [[jet engine]] uses fuel. By definition, it is the [[impulse (physics)|total impulse]] (or change in [[momentum]]) delivered per unit of [[propellant]] consumed<ref name="QRG1">{{cite web|url=http://www.qrg.northwestern.edu/projects/vss/docs/propulsion/3-what-is-specific-impulse.html|title=What is specific impulse?|publisher=Qualitative Reasoning Group|access-date=22 December 2009|archive-date=4 July 2016|archive-url=https://web.archive.org/web/20160704233223/http://www.qrg.northwestern.edu/projects/vss/docs/Propulsion/3-what-is-specific-impulse.html|url-status=dead}}</ref> and is [[dimensional analysis|dimensionally equivalent]] to the generated [[thrust]] divided by the propellant [[mass flow rate]] or weight flow rate.<ref name="SINasa"/> If [[mass]] ([[kilogram]], [[pound-mass]], or [[slug (unit)|slug]]) is used as the unit of propellant, then specific impulse has units of [[velocity]]. If weight ([[newton (unit)|newton]] or [[pound-force]]) is used instead, then specific impulse has units of time (seconds).  Multiplying flow rate by the standard gravity ([[standard gravity|''g''<sub>0</sub>]]) converts specific impulse from the mass basis to the weight basis.<ref name="SINasa">{{cite web|url=http://www.grc.nasa.gov/WWW/K-12/airplane/specimp.html|title=Specific impulse|last=Benson|first=Tom|date=11 July 2008|publisher=[[NASA]]|access-date=22 December 2009|archive-url=https://web.archive.org/web/20100124223955/http://www.grc.nasa.gov/WWW/K-12/airplane/specimp.html|archive-date=24 January 2010|url-status=dead}}</ref>

A propulsion system with a higher specific impulse uses the mass of the propellant more effectively in creating forward thrust and, in the case of a rocket, less propellant needed for a given [[delta-v]], per the [[Tsiolkovsky rocket equation]].<ref name="QRG1" /><ref name="ars20130414">{{cite news|last=Hutchinson|first=Lee |title=New F-1B rocket engine upgrades Apollo-era design with 1.8M lbs of thrust |url=https://arstechnica.com/science/2013/04/new-f-1b-rocket-engine-upgrades-apollo-era-deisgn-with-1-8m-lbs-of-thrust/ |access-date=15 April 2013 |publisher=[[Ars Technica]] |date=14 April 2013 |quote=The measure of a rocket's fuel effectiveness is called its specific impulse (abbreviated as 'ISP'—or more properly Isp).... 'Mass specific impulse...describes the thrust-producing effectiveness of a chemical reaction and it is most easily thought of as the amount of thrust force produced by each pound (mass) of fuel and oxidizer propellant burned in a unit of time. It is kind of like a measure of miles per gallon (mpg) for rockets.'}}</ref> In rockets, this means the engine is more effective at gaining altitude, distance, and velocity. This effectiveness is less important in jet engines that employ wings and use outside air for combustion and carry payloads that are much heavier than the propellant.

Specific impulse includes the contribution to impulse provided by external air that has been used for combustion and is exhausted with the spent propellant. Jet engines use outside air, and therefore have a much higher specific impulse than rocket engines. The specific impulse in terms of propellant mass spent has units of distance per time, which is an artificial velocity called the "effective exhaust velocity". This is higher than the ''actual'' exhaust velocity because the mass of the combustion air is not being accounted for. Actual and effective exhaust velocity are the same in rocket engines not utilizing air.

Specific impulse is inversely proportional to [[Thrust specific fuel consumption|specific fuel consumption]] (SFC) by the relationship ''I''<sub>sp</sub> = 1/(''g<sub>o</sub>''·SFC) for SFC in kg/(N·s) and ''I''<sub>sp</sub> = 3600/SFC for SFC in lb/(lbf·hr).

===Thrust===

From the definition of specific impulse thrust in SI units is:

:<math>F = \dot m V_e</math>

where V{{sub|e}} is the effective exhaust velocity and <math>\dot m</math> is the propellant flow rate.

==Types of reaction engine==
{{Main|Reaction engine}}
Reaction engines produce thrust by expelling solid or fluid [[reaction mass]]; jet propulsion applies only to engines which use fluid reaction mass.

===Jet engine===
{{Main|Jet engine}}
A jet engine is a [[reaction engine]] which uses ambient air as the working fluid and converts it to a hot, high-pressure gas which is expanded through one or more [[nozzle]]s. Technically, most jet engines are [[Gas turbine|gas turbines]], working on the [[Brayton cycle|Brayton Cycle]]. Two types of jet engines, the [[turbojet]] and [[turbofan]], employ [[axial-flow compressor|axial-flow]] or [[centrifugal compressor]]s to raise the pressure before [[combustor|combustion]] and [[turbines]] to drive the compression. [[Ramjet]]s operate only at high flight speeds because they omit the compressors and turbines, depending instead on the [[dynamic pressure]] generated by the high speed (known as ram compression). [[Pulse jet engine|Pulse jets]] also omit the compressors and turbines but can generate static thrust and have limited maximum speed.

===Rocket engine===
{{Main|Rocket engine}}
The rocket is capable of [[spaceflight|operating in the vacuum of space]] because it is dependent on the vehicle carrying its own [[oxidizer]] instead of using the oxygen in the air, or in the case of a [[nuclear rocket]], heats an inert propellant (such as liquid [[hydrogen]]) by forcing it through a [[nuclear reactor]].

===Plasma engine===
{{Main|Plasma propulsion engine}}
Plasma thrusters accelerate a [[plasma (physics)|plasma]] by [[electromagnetic]] means.

===Pump-jet===
{{Main|Pump-jet}}
The pump-jet, used for [[marine propulsion]], uses water as the working fluid, pressurized by a [[ducted propeller]], [[centrifugal pump]], or a combination of the two.

==Jet-propelled animals==
{{Main|Aquatic locomotion#Jet propulsion}}

[[Cephalopod]]s such as squid use jet propulsion for rapid [[Anti-predator adaptation#Escape|escape from predators]]; they use other mechanisms for slow swimming. The jet is produced by ejecting water through a [[Siphon (mollusc)|siphon]], which typically narrows to a small opening to produce the maximum exhalent velocity. The water passes through the gills prior to exhalation, fulfilling the dual purpose of respiration and locomotion.<ref name='Packard1972'/> 
[[File:Swimming behaviour of Notarchus punctatus.png|thumb|175px|Illustration of [[Notarchus|Notarchus punctatus]] swimming motion <ref name=":0">{{Cite book |last=Martin |first=Rainer |title=On the swimming behaviour and biology of ''Notarchus punctatus'' Phillipi (Gastropoda, Opisthobranchia) |year=1966 |series=Pubblicazioni della Stazione Zoologica di Napoli |volume=35 |pages=61–75}}</ref>]]
Sea hares ([[Gastropoda]], [[Opisthobranchia]]) employ a similar method, but without the sophisticated neurological machinery of cephalopods and the absence of fins to steer their movements, they navigate somewhat more clumsily.<ref name=":0" />

The swimming cycle has two phases: (A) the propulsive phase, where the animal ejects a jet of water to move, and (B) the rolling phase, where it performs a somersault to reset its position.<ref name=":0" />

A study by Mazzarelli (1893) suggests that [[Dromia]] crabs may be predators of [[Notarchus]]. In a confined tank with little sea water, two sea hares suffocated after being tightly held by the crab for hours, despite showing no visible injuries upon dissection. It is unclear if this behavior occurs in the wild, but Notarchus may escape simply by swimming if space allows.<ref name=":0" /><ref>{{Cite journal |last=Mazzarelli |first=G. |date=1893 |title=Monografia delle Aplysiidae del Golfo di Napoli |url=https://gdz.sub.uni-goettingen.de/id/PPN641030126 |journal=Memorie della Società Italiana delle Scienze |volume=9 |pages=1–222}}</ref>

Some [[teleost fish]] have also developed jet propulsion, passing water through the gills to supplement fin-driven motion.<ref name="Hanken1993">{{cite book |last=Wake |first=M.H. |title=Craniology: Getting a Head: The Skull |publisher=[[University of Chicago Press]] |year=1993 |isbn=978-0-226-31573-7 |editor-last=Hanken |editor-first=James |volume=3 |at=p. 201 |chapter=The Skull as a Locomotor Organ |doi=10.1126/science.263.5154.1779 |editor-last2=Hall |editor-first2=Brian K.}}</ref><ref>{{Cite book |last=Lindsey |first=C. C. |title=Form, Function, and Locomotory Habits in Fish |publisher=Academic Press |year=1978 |isbn=978-0-12-350407-4 |editor-last=Hoar |series=Fish Physiology |volume=7 |pages=1–100 |doi=10.1016/S1546-5098(08)60163-6 |issn=1546-5098 |editor-last2=Randall}}</ref>

In some [[dragonfly]] larvae, jet propulsion is achieved by the expulsion of water from a specialised cavity through the anus. Given the small size of the organism, a great speed is achieved.<ref name='Mill2004'>{{Cite journal| first1 = P. J.| first2 = R. S. | title = Jet-propulsion in anisopteran dragonfly larvae| last1 = Mill | journal = Journal of Comparative Physiology| volume = 97| issue = 4 | pages = 329–338 | year = 1975 | doi = 10.1007/BF00631969| last2 =  Pickard | s2cid = 45066664 }}</ref>

Scallops and [[cardiids]],<ref>{{Nautilus|chapter=32. Locomotion of ''Nautilus''|author=Chamberlain Jr, John A.}}</ref> [[siphonophore]]s,<ref name="ref_" >{{Cite journal | last1 = Bone | first1 = Q. | last2 = Trueman | first2 = E. R. | doi = 10.1017/S0025315400057271 | title = Jet propulsion of the calycophoran siphonophores ''Chelophyes'' and ''Abylopsis'' | journal = Journal of the Marine Biological Association of the United Kingdom | volume = 62 | issue = 2 | pages = 263–276 | year = 2009 | s2cid = 84754313 }}</ref> tunicates (such as [[salps]]),<ref name="ref_a" >{{Cite journal | last1 = Bone | first1 = Q. | last2 = Trueman | first2 = E. R. | doi = 10.1111/j.1469-7998.1983.tb05071.x | title = Jet propulsion in salps (Tunicata: Thaliacea) | journal = Journal of Zoology | volume = 201 | issue = 4 | pages = 481–506 | year = 2009 }}</ref><ref name="ref_b" >{{Cite journal | last1 = Bone | first1 = Q. | last2 = Trueman | first2 = E. | doi = 10.1016/0022-0981(84)90059-5 | title = Jet propulsion in Doliolum (Tunicata: Thaliacea) | journal = Journal of Experimental Marine Biology and Ecology | volume = 76 | issue = 2 | pages = 105–118 | year = 1984 }}</ref> and [[Polyorchis|some jellyfish]]<ref>{{cite journal |last1=Demont |first1=M. Edwin |last2=Gosline |first2=John M. |date=January 1, 1988 |title=Mechanics of Jet Propulsion in the Hydromedusan Jellyfish, ''Polyorchis Pexicillatus'': I. Mechanical Properties of the Locomotor Structure |url=http://jeb.biologists.org/cgi/content/abstract/134/1/313 |journal=J. Exp. Biol. |volume=134 |issue=134 |pages=313–332 |bibcode=1988JExpB.134..313D |doi=10.1242/jeb.134.1.313}}</ref><ref>{{cite journal |last1=Demont |first1=M. Edwin |last2=Gosline |first2=John M. |date=January 1, 1988 |title=Mechanics of Jet Propulsion in the Hydromedusan Jellyfish, ''Polyorchis Pexicillatus'': II. Energetics of the Jet Cycle |url=http://jeb.biologists.org/cgi/content/abstract/134/1/333 |journal=J. Exp. Biol. |volume=134 |issue=134 |pages=333–345 |bibcode=1988JExpB.134..333D |doi=10.1242/jeb.134.1.333}}</ref><ref>{{cite journal |last1=Demont |first1=M. Edwin |last2=Gosline |first2=John M. |date=January 1, 1988 |title=Mechanics of Jet Propulsion in the Hydromedusan Jellyfish, ''Polyorchis Pexicillatus'': III. A Natural Resonating Bell; The Presence and Importance of a Resonant Phenomenon in the Locomotor Structure |url=http://jeb.biologists.org/cgi/content/abstract/134/1/347 |journal=J. Exp. Biol. |volume=134 |issue=134 |pages=347–361 |doi=10.1242/jeb.134.1.347}}</ref> also employ jet propulsion.  The most efficient jet-propelled organisms are the salps,<ref name="ref_a" /> which use an order of magnitude less energy (per kilogram per metre) than squid.<ref name="ref_c">{{Cite journal | last1 = Madin | first1 = L. P. | title = Aspects of jet propulsion in salps | journal = Canadian Journal of Zoology | volume = 68 | issue = 4 | pages = 765–777 | year = 1990 | doi = 10.1139/z90-111 }}</ref>
==See also==
*[[Jet aircraft]]

==References==
{{Reflist|colwidth=35em}}

{{Authority control}}

[[Category:Energy conversion]]
[[Category:Aquatic locomotion]]
[[Category:Propulsion]]