[Spaceships] Homebrew: Alternate Chemical Rocket Fuel

[Agemegos]

Originally Posted by DaltonS

Actually, the equation is international. Isp is measured in seconds

Isp (specific impulse) is not measured in seconds in the SI, only in US customary units.

I is impulse, measured in Newton-seconds, a.k.a. kg-m-s^-1. "Specific" means "per unit mass", so you divide by kg and get specific impulse (Newton-seconds per kilogram) in metres per second. Or in US Customary units, impulse is in pounds-force seconds (or poundal-seconds) and to get specific impulse you divide by pounds-mass (or slugs). You only get seconds if you divide pounds-force by pounds-mass and get a dimensionless number instead of an acceleration. You can't even do that in the Système International. Specific impulse works out identical to the effective exhaust speed.