Conventional rocket designs rely on "Equal and opposite forces" exerted between the ship and propellents in the engine exhaust. Because the exhaust emissions are relatively low mass compared to the ship, the momentum is balanced through a relatively high velocity. But high velocity means that the exhaust emissions take most of the kinetic energy from the engine, and relatively little kinetic energy is taken by the ship. If the exhaust emits mass m at velocity v, then a ship is of mass M and gains velocity =mv/M. Looking at how the energy is distributed: Total energy in the exhaust emission: E = (1/2) mv^2 + (1/2) M (mv/M)^2 = (1/2) mv^2 + (1/2) m^2 v^2 /M = (1/2)mv^2 . [M+m]/M Proportion of energy taken by the exhaust: = M/(m+M) = Most. Proportion of energy taken by the ship: = m/(m+M) = Hardly any. Is there any way we can imagine accelerating a ship without using a conventional rocket engine design? For example, What if we fired a highly collimated beam of accelerated particles at the ship from a planetary station, to push it along? What if the ship fired a particle beam out of the back of the ship, but the beam swung around a star and returned to the ship. So the ship gains the propellent back, AND double momentum. The energy liberated on recapture of that energetic particle beam could also be fed back into the fired beam. Any other thoughts? And nobody mention warp engines.