I suspect Cripplecreek is referring to nuclear-pulse propulsion drives. Some of the more optimistic calculations seem to indicate a craft propelled by such an engine could make a noticeable percentage of the speed of light. And its one of the only proposed engines that is at least theoretically capable of being constructed with modern technology. Emphasis on “theoretically” - I think that a lot of the proposals for NPP spacecraft are extraordinarily optimistic.
No matter what, the energy requirement set my newton don't go away See post #95 All of that is just science ficion. It boils down to "You can't get there from here."