Studied by Marshall Space Flight Center in 1968, the Saturn V-Centaur booster would have been used for deep space missions if it had flown. It consisted of an ordinary Saturn V launch vehicle, except that the Apollo spacecraft would be replaced with a Centaur upper stage (known as the S-V in the plans), as a high-energy liquid-fueled fourth stage, which would provide a 30% performance improvement over Saturn V-A/Saturn INT-20.[1] This combination never flew.