According to Wikipedia: http://en.wikipedia.org/wiki/Propulsive_efficiency the propulsive efficiency of a rocket is at its peak when the rocket is travelling at the same speed as it is ejecting its exhaust. I understand that, but the graph (and equation) on that Wikipedia page (and elsewhere on the internet) show the efficiency to be 0 when the rocket is as rest. I don't understand - how can the rocket ever take off without infinite energy if the efficiency is 0? It doesn't intuitively feel right either - if I'm stood on a skateboard at rest and I throw a rock off the skateboard, I'd expect the skateboard to move (assuming the rock was heavy enough and I threw it hard enough). I'd really appreciate any help that explained what I'm misunderstanding.