Regarding his underplaying of fuel mass, check out the rocket equation to get an idea of how hard it is to achieve the delta-v required for various maneuvers (e

Regarding his underplaying of fuel mass, check out the rocket equation to get an idea of how hard it is to achieve the delta-v required for various maneuvers (e.g. suborbital, orbital, escape): http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation Essentially, your fuel mass requirements increase exponentially as you need more delta-v. It's very much harder to get into orbit than to stay sub-orbital. And you will need a significant amount of fuel left over if you wish to orbit or land at your destination. You can mitigate that with atmospheric aerobraking, but that's not possible on, say, the Moon.