Jet propulsion

Source: Wikipedia, the free encyclopedia.
The jet engine of a Boeing 787 Dreamliner.
A pump-jet on a ferry.

Jet propulsion is 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 engines or animals when thrust is generated by a fast moving 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.[1]

In animals, the most efficient jets are pulsed, rather than continuous,[2] at least when the Reynolds number is greater than 6.[3]

Specific impulse

Main article: Specific impulse

Specific impulse (usually abbreviated Isp) is a measure of how effectively a

pound-force) is used instead, then specific impulse has units of time (seconds). Multiplying flow rate by the standard gravity (g0) converts specific impulse from the mass basis to the weight basis.[5]

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.[4][6] 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

specific fuel consumption
(SFC) by the relationship Isp = 1/(go·SFC) for SFC in kg/(N·s) and Isp = 3600/SFC for SFC in lb/(lbf·hr).

Thrust

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

where Ve is the effective exhaust velocity and is the propellant flow rate.

Types of reaction engine

Main article: 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 article: Jet engine

A jet engine is a

Pulse jets
also omit the compressors and turbines but can generate static thrust and have limited maximum speed.

Rocket engine

Main article: Rocket engine

The rocket is capable of

nuclear rocket, heats an inert propellant (such as liquid hydrogen) by forcing it through a nuclear reactor
.

Plasma engine

Main article: Plasma propulsion engine

Plasma thrusters accelerate a

electromagnetic
means.

Pump-jet

Main article: 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 article: Aquatic locomotion § Jet propulsion

Cephalopods such as squid use jet propulsion for rapid escape from predators; they use other mechanisms for slow swimming. The jet is produced by ejecting water through a 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.[1] Sea hares (gastropod molluscs) employ a similar method, but without the sophisticated neurological machinery of cephalopods they navigate somewhat more clumsily.[1]

Some

teleost fish have also developed jet propulsion, passing water through the gills to supplement fin-driven motion.[7]
: 201 

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.[8]

Scallops and

salps),[11][12] and some jellyfish[13][14][15] also employ jet propulsion. The most efficient jet-propelled organisms are the salps,[11] which use an order of magnitude less energy (per kilogram per metre) than squid.[16]

See also

References

  1. ^
    S2CID 85088231
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  2. PMID 20709925
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  3. PMID 16048770
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  4. ^ a b "What is specific impulse?". Qualitative Reasoning Group. Retrieved 22 December 2009.
  5. ^ a b Benson, Tom (11 July 2008). "Specific impulse". NASA. Archived from the original on 24 January 2010. Retrieved 22 December 2009.
  6. ^ Hutchinson, Lee (14 April 2013). "New F-1B rocket engine upgrades Apollo-era design with 1.8M lbs of thrust". Ars Technica. Retrieved 15 April 2013. 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.'
  7. ISBN 978-0-226-31573-7
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  8. S2CID 45066664
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  9. ISBN 9789048132980
    .
  10. S2CID 84754313
    .
  11. ^
    doi:10.1111/j.1469-7998.1983.tb05071.x
    .
  12. doi:10.1016/0022-0981(84)90059-5
    .
  13. doi:10.1242/jeb.134.1.313
    .
  14. doi:10.1242/jeb.134.1.333
    .
  15. doi:10.1242/jeb.134.1.347
    .
  16. doi:10.1139/z90-111
    .
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