I'd really love an explanation of what specific impulse is. I've looked it up a few times but the units seconds^-1 confuses me, what does this represent?
I think the unit is just one of those weird dimensional equivalences that pop up from time to time. E.g. fuel efficiency in cars is measured as distance / volume (of fuel consumed), so it's dimensionally equivalent to area^-2. But we don't use this because "1 mile per gallon" makes a lot more sense than "42.5 cm^-2".
