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> 9.81ms^-2 there is not any actual acceleration, just an agreed conversion factor.

I'd look it up but my textbooks[1] are in boxes, but the units everyone uses are wrong. Actually inappropriately simplified.

[1] I have one and exactly one textbook that has the full units for gc.

Isp often defined as "thrust per unit of mass flow". That is, how many Newtons of thrust engine gets from each kilogram per second of propellant spent. Since N = kg * m / s^2 , and mass flow is in kg / s , Isp becomes (kg * m * s) / (s^2 * kg) = m/s , i.e. Isp has units of speed. In vacuum Isp is equal to the speed of gases flying from the engine.

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