1964-12-11 USA Atlas-Centaur 4

An Atlas-Centaur 4 launch cover from "SpaceCraft", postmarked at Cape Canaveral with a hand cancel on December 11, 1964.

The AC-4 flight (Atlas 146D and Centaur #4) launched on December 11, 1964, at 14:25:02 GMT, carrying a 2,993 kg mass model of the Surveyor spacecraft. It aimed to test propulsion and stage separation, following two weather-related scrubs. The flight marked the first closed-loop operation of the guidance system and included an attempt to recover the payload shroud, which had a balloon designed to release green marker dye into the ocean. Although the shroud was sighted, it sank before recovery.

The Atlas and initial Centaur stages performed as expected, but the mission failed when Centaur couldn't restart due to an ill-advised modification that reduced the size of the ullage rockets to save weight. This reduction proved insufficient to settle the propellants, causing venting hydrogen to throw the vehicle into a tumble. After ten orbits, Centaur reentered Earth's atmosphere over the South Pacific on December 12, 1964.