Star Smashers of the Galaxy Rangers by Harry Harrison 1973 (First time in paperback!)
(...back cover...bolding the important bits)
The Prank...
...That Became Inter-galactic Mayhem
Chuck and Jerry, a couple of fun-loving students at State, happen upon a faster-than-light space drive and smuggle it into the football team's plane. They, together with pert Sally, crusty Pop and lovable Old John view with horror a practical joke gone awry as the plane screams off to Titan, a frozen moon of Saturn. But that's only the beginning. When lovable Old John's true and awful identity becomes known, a wild battle across the Universe and through centuries ensues, catapulting friends and deadly foes into the midst of a yarn spun from the grandest tradition of the classic "space opera".