Well, no one has yet mentioned NTR's or gas core reactors so I'll bring those up. Those are the kind of rockets we should put on for lunar missions. We could get two to three more times thrust with them than a standard chemical rocket engine. And the technology is very practical, we have already built and tested NTR's and they work extremely well. We were about to do an in flight test when Nixon cut the head off of the program. Too bad, they worked well and would be perfect for planetary exploration. It wouldn't be that hard to resurrect that project, we should, if we have good technology that works than why not make use of it?<br /><br />"Ask not what your country can do for you, ask what you can do for your country." John F. Kennedy