The AC-4 flight (Atlas 146D and Centaur #4) was launched on December 11, 1964 at 14:25:02 GMT with a payload of 2993 kg (that carried a mass model of the Surveyor spacecraft). It performed propulsion and stage separation tests, following two scrubbed attempts due to bad weather. The guidance system was operated closed-loop for the first time and an attempt was made to recover the payload shroud, which was equipped with a balloon designed to release green marker dye into the ocean. The shroud was sighted by recovery crews but sank into the ocean and could not be located. The Atlas phase of the flight and the initial phase of Centaur flight were uneventful. The mission went awry when the Centaur could not be restarted due to an ill-conceived design modification — the ullage rockets were reduced in size to save weight, however, they proved insufficient to settle the propellants in the tanks. Subsequently, venting hydrogen caused the vehicle to tumble out of control. After ten orbits, the Centaur reentered over the South Pacific, on December 12, 1964
