Subspace Survivors by E.E. Smith

The _Procyon_ bored on through space, at one unchanging gravity of acceleration. It may not seem, at first glance, that one gravity would result in any very high velocity; but when it is maintained steadily for days and weeks and months, it builds up to a very respectable speed. Nor was there any question of power, for the _Procyon_'s atomics did not drive the ship, but merely energized the "Chaytors" -- the Chaytor Effect engines that tapped the energy of the expanding universe itself.

Thus, in less than six months, the _Procyon_ had attained a velocity almost half that of light. At the estimated mid-point of the flight the spaceship, still at one gravity of drive, was turned end-for-end; so that for the ensuing five-and-a-fraction months she would be slowing down.