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.[6] However, this was also the first Atlas-Centaur equipped with the uprated 165,000 lb (75,000 kg) 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.[1] 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 booster fuel prevalves. The low-pressure booster fuel ducting was found to have collapsed from a sudden loss of fuel flow, but had not ruptured. The investigation concluded that the fuel prevalves had only opened partially and the propellant flow was enough to push them shut, starving the booster engines of RP-1 and causing a LOX-rich shutdown. Engine start had proceeded normally and all booster systems functioned properly until the valve closed. Booster shutdown occurred at T+1.7 seconds and the vehicle impacted on the pad at T+2.8 seconds. Bench testing confirmed that there were several possible ways that the valve would only open partially, although the exact reason was not determined. This failure mode had never occurred in the 240 Atlas launches prior to AC-5 despite always having been possible.[citation needed]

Until a more permanent solution could be found, a temporary fix was made for Atlas-Agena vehicles by equipping the valve with a manual lock that would be enabled during the pre-launch countdown. A manual E-series sustainer prevalve was also installed as a precautionary measure in addition, an unrelated system malfunction in AC-5 was discovered when an examination of telemetry data found that a power failure had occurred in the guidance computer. As a temporary fix for Atlas-Centaur AC-6, 7, and 8, several unused components were removed from the computer in order to reduce system complexity and failure points.

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