I've wondered if it would be possible to build an efficient rocket engine that accelerates its reaction mass so hard that it gains an appreciable amount of rela

I've wondered if it would be possible to build an efficient rocket engine that accelerates its reaction mass so hard that it gains an appreciable amount of relativistic mass? The fundamental problem with interstellar travel is the amount of reaction mass you need to make the trip in a reasonable amount of time. The rocket equation is a harsh mistress. If you were accelerating each gram of propellant so hard that it gained a metric ton of relativistic mass you could bypass it, assuming you had some sort of lightweight yet near limitless power source.