Well, for a space drive it's really attractive because combining the fuel and propellant lets you get around a lot of worries regarding things melting. The big

Well, for a space drive it's really attractive because combining the fuel and propellant lets you get around a lot of worries regarding things melting. The big limiting factor around conventional nuclear thermal drives is that the fuel wants to melt before it gets hot enough to give you the sort of ISPs you really want. You can have pure fission designs where your fuel is mixed with or only in contact with your propellant like the gas core[1] or the nuclear saltwater[2] rockets but those are in constant danger of exploding. [1]http://en.wikipedia.org/wiki/Gas_core_reactor_rocket http://en.wikipedia.org/wiki/Gas_core_reactor_rocket [2]http://en.wikipedia.org/wiki/Nuclear_salt-water_rocket http://en.wikipedia.org/wiki/Nuclear_salt-water_rocket