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.
The Centaur upper stage was initially a proposal by the United States Air Force (USAF) which was accepted by the Advanced Research Products Agency (ARPA, who would later go on to be renamed as DARPA in 1972) in August 1958 as a way to respond to the USSR's successful Sputnik 1 mission the year before by ramping up their launch capabilities.
The proposal, studied by the USAF with General Dynamics/Convair had the intention to develop a new high energy rocket stage to launch heavy payloads in the shortest possible time.
The slender 3.05m diameter would also have made the A-1 a structurally marginal configuration.
The success of the Centaur engine program did encourage the addition of liquid hydrogen to the upper stages of the Saturn launch vehicle.