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 12 December 1964. Fifth flight The AC-5 flight (Atlas 156D) on 2 March 1965 at 13:25 GMT from Cape Kennedy in a highly elliptical orbit, with a payload (Surveyor SD-1) of 951 kg, was only intended to carry out a single burn of the Centaur C, and program officials felt confident. This mission was designed to rehearse a complete Centaur upper stage burn in support of the Surveyor lunar lander program. On a nominal mission, the Centaur would boost its payload on a direct ascent trajectory to the Moon. On this test flight, NASA planned to deliver the payload, a non-functional dynamic model known as SD-1, into an orbit of 167 × 926.625 km that simulated a lunar transfer trajectory. Failure The flight quickly ended in disaster as the Atlas's booster engines shut down after a few feet of vehicle rise and the rocket fell back onto LC-36A and exploded, the Centaur's LH2 load going off in a huge fireball for the biggest on-pad explosion yet seen at Cape Canaveral. However, this was also the first Atlas-Centaur equipped with the uprated thrust MA-5 booster engines after the previous testing on two Atlas-Agena flights. The damage to LC-36A was not as severe as it looked and repairs were largely completed in three months, but the completion of LC-36B was also accelerated. Most damages were thermal rather than structural, and the upper portion of the umbilical tower, which was in the center of the LH2 blast, had been subjected to temperatures of 3315 °C. The accident marked the first failure of an Atlas booster in a space launch since Midas 8 in June 1963, a new record at the time of 26 consecutive flights with only malfunctions of the upper stages or payload. This would be the last on-pad explosion at Cape Canaveral until 2016 (SpaceX Falcon 9 pre-flight mishap). Post-flight investigation examined several possible reasons for the booster engine shutdown, including an accidental closure of the booster fuel staging disconnect valve, an open fuel fill/drain valve, or an accidental BECO signal. These failure modes were quickly ruled out and attention quickly centered on closure of the