For a highly relativistic rocket powered by an external laser such that the thrust is 2*Power/c, the efficiency is dismal at the start when v is low yet approaches 100% as v approaches c. I take efficiency to mean the ability to convert the energy of the beam into usable kinetic energy. Does the momentum kick actually change as the ship gains speed? Is 2P/c just the value when v=0? Conceptually, it's hard to imagine that since most energy is reflected at the start and most energy is converted at the end, that the acceleration is fixed. Yet it is isn't it? Thanks.