If your rocket is traveling at maximum velocity, it means it can't go any faster. Velocity is quickness of motion or action.
A synonym is celerity; a simpler word is speed. In physics, velocity specifically refers to the measurement of the rate and direction of change in position of an object. It is a vector quantity that specifies both the speed of a body and its direction of motion. The noun velocity descends from Middle English velocite, from Old French, from Latin vēlōcitās, from vēlōx "fast."