Editing Specific impulse (section)

===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.