Skip_this_post @ 85 said:

“For interplanetary travel, Ions are the way to go.”

This is certainly true and I’ve been following the ion propelled Dawn mission with close attention, refer to:

http://dawn.jpl.nasa.gov/mission/index.asp

The big problem with ion propulsion is the power supply. Dawn uses big and heavy solar arrays for its power supply that restrict the vehicle to operating within the inner solar system. Years ago I was involved in the Jupiter Icy Moons Orbiter (JIMO) project. JIMO was a nuclear powered, ion propelled spacecraft that was intended to travel to the Jupiter system. In our enthusiasm for the project, there was a brief period of time where we actually thought JIMO might happen. Unfortunately JIMO died after we got into the nasty details. A big problem was the thermal cycle for the reactor. Ideally we would have used a Brayton cycle but there were just too many moving parts (the power supper must be reliable for decades). The alternative was a thermoelectric system based upon direct conversion of heat to electricity using solid state components. The reliability was extreme for thermoelectric but thermal efficiency was dreadful. Bad thermal efficiency means significant heat rejection which means hauling around a huge radiator. JIMO ended up becoming a huge radiator where the nuclear reactor, ion propulsion system and payload where almost insignificant after thoughts. There’s the old story that arm chair generals worry about tactics while real generals worry about logistics. Arm chair rocket scientists worry about propulsion while real rocket scientists worry about heat rejection.