Editing Specific impulse (section)

{{Short description|Change in velocity per amount of fuel}}
{{Use dmy dates|date=March 2020}}

'''Specific impulse''' (usually abbreviated {{math|''I''<sub>sp</sub>}}) is a measure of how efficiently a [[reaction mass]] engine, such as a [[rocket engine|rocket]] using propellant or a [[jet engine]] using fuel, generates [[thrust]]. In general, this is a ratio of the ''[[Impulse (physics)|impulse]]'', i.e. change in momentum, ''per mass'' of propellant.  This is equivalent to "thrust per massflow". The resulting unit is equivalent to velocity, although it does not represent any physical velocity (see below); it is more properly thought of in terms of momentum per mass, since this represents a physical momentum and physical mass.

The practical meaning of the measurement varies with different types of engines. Car engines consume onboard fuel, breathe environmental air to burn the fuel, and react  (through the tires) against the ground beneath them. In this case, the only sensible interpretation is momentum per fuel burned. Chemical rocket engines, by contrast, carry aboard all of their combustion ingredients and reaction mass, so the only practical measure is momentum per reaction mass. Airplane engines are in the middle, as they only react against airflow through the engine, but some of this reaction mass (and combustion ingredients) is breathed rather than carried on board. As such, "specific impulse" could be taken to mean either "per reaction mass", as with a rocket, or "per fuel burned" as with cars. The latter is the traditional and common choice. In sum, specific impulse is not practically comparable between different types of engines.

In any case, specific impulse can be taken as a measure of efficiency. In cars and planes, it typically corresponds with fuel mileage; in rocketry, it corresponds to the achievable [[Delta-v|delta-''v'']],<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><ref name="ars20130414">{{cite web|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 |website=[[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> which is the typical way to measure changes between orbits.

[[Astronautics|Rocketry]] traditionally uses a "bizarre" choice of units: rather than speaking of momentum-per-mass, or velocity, the rocket industry typically converts units of velocity to units of time by dividing by a standard reference acceleration, that being [[standard gravity]] g{{sub|0}}. This is a historical result of competing units, [[Imperial_and_US_customary_measurement_systems|imperial units]] vs [[metric units]]. They shared a common unit of time (seconds) but not common units of distance or mass, so this conversion by reference to g{{sub|0}} became a standard way to make international comparisons. This choice of reference conversion is arbitrary and the resulting units of time have no physical meaning. The only physical quantities are the momentum change and the mass used to achieve it.