Editing Specific impulse (section)

==Examples==
{{main list|Spacecraft propulsion#Table of methods}}
{{Thrust engine efficiency}}
{{Specific impulse examples}}

An example of a specific impulse measured in time is 453&nbsp;seconds, which is equivalent to an [[effective exhaust velocity]] of {{cvt|4.440|km/s|ft/s}}, for the [[RS-25]] engines when operating in a vacuum.<ref>{{Cite web|url=http://www.astronautix.com/engines/ssme.htm|title=SSME|website=www.astronautix.com|url-status=dead|archive-url=https://web.archive.org/web/20160303190701/http://www.astronautix.com/engines/ssme.htm|archive-date=March 3, 2016}}{{cbignore|bot=medic}}</ref> An air-breathing jet engine typically has a much larger specific impulse than a rocket; for example a [[turbofan]] jet engine may have a specific impulse of 6,000&nbsp;seconds or more at sea level whereas a rocket would be between 200 and 400&nbsp;seconds.<ref>{{Cite web|url=http://web.mit.edu/16.unified/www/SPRING/propulsion/notes/node85.html|title=11.6 Performance of Jet Engines|website=web.mit.edu}}</ref>

An air-breathing engine is thus much more propellant efficient than a rocket engine, because the air serves as reaction mass and oxidizer for combustion which does not have to be carried as propellant, and the actual exhaust speed is much lower, so the kinetic energy the exhaust carries away is lower and thus the jet engine uses far less energy to generate thrust.<ref>{{cite web|last=Dunn|first=Bruce P.|date=2001|title=Dunn's readme|url=http://www.dunnspace.com/isp.htm|url-status=dead|archive-url=https://web.archive.org/web/20131020061623/http://www.dunnspace.com/isp.htm|archive-date=20 October 2013|access-date=2014-07-12}}</ref> While the ''actual'' exhaust velocity is lower for air-breathing engines, the ''effective'' exhaust velocity is very high for jet engines. This is because the effective exhaust velocity calculation assumes that the carried propellant is providing all the reaction mass and all the thrust. Hence effective exhaust velocity is not physically meaningful for air-breathing engines; nevertheless, it is useful for comparison with other types of engines.<ref>{{Cite web|url=https://www.britannica.com/technology/effective-exhaust-velocity|title=Effective exhaust velocity &#124; engineering|website=Encyclopedia Britannica}}</ref>

The highest specific impulse for a chemical propellant ever test-fired in a rocket engine was {{convert|542|isp}} with a [[Tripropellant rocket|tripropellant]] of [[lithium]], [[fluorine]], and [[hydrogen]]. However, this combination is impractical. Lithium and fluorine are both extremely corrosive, lithium ignites on contact with air, fluorine ignites on contact with most fuels, and hydrogen, while not hypergolic, is an explosive hazard. Fluorine and the hydrogen fluoride (HF) in the exhaust are very toxic, which damages the environment, makes work around the launch pad difficult, and makes getting a launch license that much more difficult. The rocket exhaust is also ionized, which would interfere with radio communication with the rocket.<ref>{{Cite web|url=https://space.stackexchange.com/questions/19852/where-is-the-lithium-fluorine-hydrogen-tripropellant-currently|title=fuel - Where is the Lithium-Fluorine-Hydrogen tripropellant currently?|website=Space Exploration Stack Exchange}}</ref><ref>{{Cite book|chapter-url=https://dx.doi.org/10.2514/6.1968-618|doi = 10.2514/6.1968-618|chapter = Investigation of the lithium-fluorine-hydrogen tripropellant system|title = 4th Propulsion Joint Specialist Conference|year = 1968|last1 = Arbit|first1 = H.|last2 = Clapp|first2 = S.|last3 = Nagai|first3 = C.}}</ref><ref>ARBIT, H. A., CLAPP, S. D., NAGAI, C. K., [https://archive.org/details/nasa_techdoc_19700018655 Lithium-fluorine-hydrogen propellant investigation Final report] NASA, 1 May 1970.</ref>

[[Nuclear thermal rocket]] engines differ from conventional rocket engines in that energy is supplied to the propellants by an external nuclear heat source instead of the [[heat of combustion]].<ref>{{Cite web |url=http://trajectory.grc.nasa.gov/projects/ntp/index.shtml |title=Space Propulsion and Mission Analysis Office |access-date=20 July 2011 |archive-date=12 April 2011 |archive-url=https://web.archive.org/web/20110412093255/http://trajectory.grc.nasa.gov/projects/ntp/index.shtml |url-status=dead }}</ref> The nuclear rocket typically operates by passing liquid hydrogen gas through an operating nuclear reactor. Testing in the 1960s yielded specific impulses of about 850 seconds (8,340&nbsp;m/s), about twice that of the Space Shuttle engines.<ref>{{Citation|last=National Aeronautics and Space Administration|title=Nuclear Propulsion in Space|date=5 January 2017 |url=https://www.youtube.com/watch?v=eDNX65d-FBY |archive-url=https://ghostarchive.org/varchive/youtube/20211211/eDNX65d-FBY| archive-date=2021-12-11 |url-status=live|language=en|access-date=2021-02-24}}{{cbignore}}</ref>

A variety of other rocket propulsion methods, such as [[ion thruster]]s, give much higher specific impulse but with much lower thrust; for example the [[Hall-effect thruster]] on the [[SMART-1]] satellite has a specific impulse of {{cvt|1640|isp}} but a maximum thrust of only {{cvt|68|mN|lbf}}.<ref>{{Cite web |url=http://www.mendeley.com/research/characterization-of-a-high-specific-impulse-xenon-hall-effect-thruster/ |title=Characterization of a High Specific Impulse Xenon Hall Effect Thruster &#124; Mendeley |access-date=20 July 2011 |archive-date=24 March 2012 |archive-url=https://web.archive.org/web/20120324114628/http://www.mendeley.com/research/characterization-of-a-high-specific-impulse-xenon-hall-effect-thruster/ |url-status=dead }}</ref> The [[variable specific impulse magnetoplasma rocket]] (VASIMR) engine currently in development will theoretically yield {{cvt|20|to|300|km/s|ft/s}}, and a maximum thrust of {{cvt|5.7|N|lbf}}.<ref>{{Cite web|last=Ad Astra|date=November 23, 2010|title=VASIMR® VX-200 MEETS FULL POWER EFFICIENCY MILESTONE|url=http://www.adastrarocket.com/AdAstra%20Release%2023Nov2010final.pdf|url-status=dead|access-date=23 June 2014|archive-date=30 October 2012|archive-url=https://web.archive.org/web/20121030193000/http://www.adastrarocket.com/AdAstra%20Release%2023Nov2010final.pdf}}</ref>