Comment Re:Hard to imagine Project Orion? (Score 2) 41
There's actually a fantastic refinement of Project Orion, called the Nuclear Salt Water Rocket. (see: wikipedia article, also this article)
Basically -- the idea is to have a mix of water and U235 mixed to a level _just below criticality_ stored as your fuel. You then inject it at high velocity into your rocket exhaust where it reaches critical density; and ignites. Giving you a "constant flow" nuclear explosion (i.e. not intermittent bursts like Orion). It's usable today, using todays technology.
The only major disadvantage is you obviously can't use it in Earths atmosphere -- you do need to haul the rocket up to orbit (and point it away from earth) before you ignite it.