The Yoyodyne Propulsion Systems propulsion technology was utilized in DY-500-class starships in the early-2120s and allowed them to serve as early Earth colony ships. (TNG: "Up The Long Ladder" , production art) The propulsion system was also used in the late-23rd century on Whorfin-class starships utilized by El-Aurians. The YPS pulse fusion propulsion system allowed the transports to travel at a maximum warp speed of warp factor 4. (Star Trek Generations production art)