Editing Specific impulse (section)

==General considerations==
Specific impulse should not be confused with [[energy efficiency (physics)|energy efficiency]], which can decrease as specific impulse increases, since propulsion systems that give high specific impulse require high energy to do so.<ref>{{cite web |url=http://www.geoffreylandis.com/laser_ion_pres.htp |title=Laser-powered Interstellar Probe (Presentation) |access-date=2013-11-16 |url-status=dead |archive-url=https://web.archive.org/web/20131002200923/http://www.geoffreylandis.com/laser_ion_pres.htp |archive-date=2 October 2013}}</ref>

Specific impulse should not be confused with total [[thrust]]. Thrust is the force supplied by the engine and depends on the propellant mass flow through the engine. Specific impulse measures the thrust ''per'' propellant mass flow. Thrust and specific impulse are related by the design and propellants of the engine in question, but this relationship is tenuous: in most cases, high thrust and high specific impulse are mutually exclusive engineering goals. For example, [[liquid rocket propellant#Hydrogen|LH<sub>2</sub>/LO{{sub|2}}]] bipropellant produces higher {{math|''I''<sub>sp</sub>}} (due to higher chemical energy and lower exhaust molecular mass) but lower thrust than [[RP-1]]/[[Liquid oxygen|LO{{sub|2}}]] (due to higher density and propellant flow). In many cases, propulsion systems with very high specific impulse—some [[ion thruster]]s reach 25x-35x better {{math|''I''<sub>sp</sub>}} than chemical engines—produce correspondingly low thrust.<ref name="exploreMarsnow">{{cite web|url=http://www.exploremarsnow.org/MissionOverview.html|title=Mission Overview|publisher=exploreMarsnow|access-date=23 December 2009}}</ref> 

When calculating specific impulse, only propellant carried with the vehicle before use is counted, in the standard interpretation. This usage best corresponds to the cost of operating the vehicle. For a chemical rocket, unlike a plane or car, the propellant mass therefore would include both fuel and [[oxidizer]]. For any vehicle, optimising for specific impulse is generally not the same as optimising for total performance or total cost. In rocketry, a heavier engine with a higher specific impulse may not be as effective in gaining altitude, distance, or velocity as a lighter engine with a lower specific impulse, especially if the latter engine possesses a higher [[thrust-to-weight ratio]]. This is a significant reason for most rocket designs having multiple stages. The first stage can optimised for high thrust to effectively fight [[gravity loss|gravity drag]] and air drag, while the later stages operating strictly in orbit and [[Rocket_engine_nozzle#Vacuum_use|in vacuum]] can be much easier optimised for higher specific impulse, especially for high delta-v orbits.

===Propellant quantity units===
The amount of propellant could be defined either in units of [[mass]] or [[weight]]. If mass is used, specific impulse is an [[Impulse (physics)|impulse]] per unit of mass, which [[dimensional analysis]] shows to be equivalent to units of speed; this interpretation is commonly labeled the ''effective exhaust velocity''. If a force-based unit system is used, impulse is divided by propellant weight (weight is a measure of force), resulting in units of time. The problem with weight, as a measure of quantity, is that it  depends on the acceleration applied to the propellant, which is arbitrary with no relation to the design of the engine. Historically, [[standard gravity]] was the reference conversion between weight and mass. But since technology has progressed to the point that we can measure Earth gravity's variation across the surface, and where such differences can cause differences in practical engineering projects (not to mention science projects on other solar bodies), modern science and engineering focus on mass as the measure of quantity, so as to remove the acceleration dependence. As such, measuring specific impulse by propellant mass gives it the same meaning for a car at sea level, an airplane at cruising altitude, or a [[Ingenuity (helicopter)|helicopter on Mars]].

No matter the choice of mass or weight, the resulting quotient of "velocity" or "time" has no physical meaning. Due to various losses in real engines, the actual exhaust velocity is different from the I{{sub|sp}} "velocity" (and for cars there isn't even a sensible definition of "actual exhaust velocity"). Rather, the specific impulse is just that: a physical momentum from a physical quantity of propellant (be that in mass or weight).

The particular habit in rocketry of measuring I{{sub|sp}} in [[seconds]] results from the above historical circumstances. Since metric and imperial units had in common only the unit of time, this was the most convenient way to make international comparisons. However, the choice of reference acceleration conversion, (g{{sub|0}}) is arbitrary, and as above, the interpretation in terms of time or speed has no physical meaning.