Atlas C was a prototype of the Atlas missile. The Atlas C test program began with the successful flight of Missile 3C on December 23, 1958. All systems performed well and the extended booster burn time was carried out with no ill effects. It was the final development version of the Atlas rocket, prior to the operational Atlas D. It was originally planned to be used as the first stage of the Atlas-Able rocket, but following an explosion during a static test on 24 September 1959, this was abandoned in favor of the Atlas D.
(Reference from SM-65C Atlas)
