Ok, so I'm a little sketchy on where I heard it originally, bu I think I was watching some sort of space show that said that hydrogen and oxygen mix together to propel a rocket but they capture some of the emitted water vapor and turn it back into water for other uses. I can't find specific reference to this recycling on the internet, but liquid hydrogen is a commonly used rocket propellant.

http://en.wikipedia.org/wiki/Liquid_hydrogen