Ok so we seem to need to find an energy source. How about fusion as opposed to fission? Any way to power WITHOUT gobs and gobs of reactor mass?
Fusion should be a possibility. We are at the equivalent stage of the Newcomen Steam Engine prior to James Watt's improvements in it.
Reactor mass should not be a factor in the design. What currently restricts us the ability to constrain such a volatile plasma so as to get it to combine in sufficient numbers to power the contraption. It wants to fly apart something fierce!
Sufficient magnetic field strength, combine with injected radio and electrical energy, should be able to be put together eventually in something not much larger than conventional internal combustion engines.
One of the most intriguing designs I've seen involved treating magnetic Fusion as if it were an amplification problem in electronics, and building a two-meter diameter "vacuum tube" with magnetic components replacing the electrical ones, and protons standing in for the electrons. Kinda makes you go, "Hmmm..."