A Rip engine was a device that allowed for time travel. As Jack put it, it was "a crude method of time travel." Rip engines tore a hole in the fabric of space/time.
The Ninth Doctor intended to take Rose and Jack to the planet Kegron Pluva but instead arrived in 2005 Bromley to investigate a time distortion. There, he encountered Das, a Neanderthal who was transported from 29,185 BC to 2005 by a rip engine. It prevented him from travelling in time again. (PROSE: Only Human)