Boeing just patented a... novel engine idea.
Basically use lasers to generate micro fusion reactions.
This generates thrust out the back. And on the front the high energy neutrons excite a uranium coating a thrust plate. Said plate then heats up coolant which is run through turbines... which power the lasers.
So it's kind of like a lighter version of the Direct Air Cycle design for the old Aircraft Nuclear Propulsion program.
Oh... and the ANP programs were at least workable. The whole, laser fusion thing... I don't think there's an idea on a way to have the system be able to keep itself powered with the given fuel.