Studied by Marshall Space Flight Center in 1968, the Saturn V-Centaur booster would have been used for deep space missions if it had actually flown. It consisted of an ordinary Saturn V launch vehicle, except that the Apollo spacecraft would be replaced with a Centaur (known as the S-V in the plans) as a high-energy liquid-fueled fourth stage. This combination never flew.
Lowther, Scott, Saturn: Development, Details, Derivatives and Descendants (work in progress). Available chapters may be ordered directly from Scott Lowther at web site indicated. Accessed at: http://www.webcreations.com/ptm.