I just thought of a new twist on the nuclear powered rocket concept. That is an engine that uses nuclear reactor heat to blast indigenous gases out the back for thrust. Indigenous gases being acquired from wherever in the Solar System a vehicle happens to be -- up until now presumably stored in advance of flight.
But, when I saw a Wired magazine article on line about a computer simulated flight around Mars derived from overhead satellite maps (I think; did not get around to reading the article yet), it occurred to me that the nuclear powered vehicle could conceivably scoop in enough gas in Mars atmosphere -- 1% the density of Earth's -- to act like a jet; would not have to store indigenous gas in advance -- cutting weight and making range virtually infinite.
The lower the atmospheric density the less would be the trade off in performance for a larger scoop so it could be very doable. Mar's atmosphere requiring a hovering type design -- too thin for a winged version -- could make for further compatibility with a giant scoop design.