After a strenuous flight test program, Atlas-Centaur went on to launch several crucial spaceflight missions for the United States, including Surveyor 1, and Pioneer 10/11.

Originally, Centaur was conceived of as a purely experimental project to develop an experience for larger, more powerful rocket stages so as not to distract Convair's focus on the all-important SM-65 Atlas missile program.

Technical problems caused the vehicle to sit on the launch pad for seven months, the most serious being leakage of liquid hydrogen through the intermediate bulkhead separating the propellant tanks combined with numerous lesser maladies with the guidance and propulsion systems.

It was unclear what had caused the failure at first, as tracking camera footage merely showed a large white cloud enveloping the booster followed by the explosion of the entire launch vehicle.

Beginning at T+44 seconds, the pneumatic system responded by venting propellant to reduce pressure levels, but eventually, they exceeded the LH2 tank's structural strength.

At T+54 seconds, the Centaur experienced total structural breakup and loss of telemetry, the LOX tank rupturing and producing an explosion as it mixed with the hydrogen cloud.

The panel had been meant to jettison at 49 miles (80 km) up when the air was thinner, but the mechanism holding it in place was designed inadequately, leading to premature separation.

The insulation panels had already been suspected during Centaur development of being a potential problem area, and the possibility of an LH2 tank rupture was considered as a failure scenario.

A Congressional investigation in June 1962 called the overall management of the Centaur program "weak", and Wernher von Braun recommended that it be cancelled in favor of a Saturn I with an Agena upper stage for planetary missions.

NASA transferred Centaur development from MSFC to the Lewis Research Center in Ohio where a team headed by Abe Silverstein worked to correct the insulation panel problems and various other design flaws.

[1] In November 1962, President Kennedy suggested cancelling Centaur entirely, but was talked out of it on the grounds that the experience gained with liquid hydrogen rocket engines was vital to the success of the Apollo program.

In addition, von Braun now proposed the Saturn-Agena be ruled out for cost reasons – Saturn was too expensive to justify as a launch vehicle for small uncrewed probes, and Agena was causing concerns to both the Air Force and NASA about its reliability.

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 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.

[6] 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.

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.

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.

[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.

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.

[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.

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.

[8] The failure of AC-5 resulted in another Congressional investigation, again headed by Rep. Joseph Karth, who argued that $600 million of taxpayer money had been spent on Centaur so far with little to show for it and that Convair was taking advantage of being the sole supplier of the Atlas-Centaur vehicle.

The committee proposed that NASA consider alternate choices for the planetary probe program, such as Titan IIIC, or outsource the manufacture of Centaur to other contractors.

The payload fairing and satellite were stripped from the booster, followed by the Atlas exploding as the thrust section fire touched off the propellant tanks at T+60 seconds.

NASA and U.S. Air Force officials, already busy investigating the launch failure of a Delta booster three weeks earlier (OTS-1), dredged the Atlas's engines from the ocean floor and sent them to Convair for examination.

It was concluded that a gas generator leak caused by improper brazing of a pipe led to overheating and fire in the thrust section of the Atlas.

The pipe also suffered corrosion from six years of sitting in a warehouse in the salty air along the Florida coast and the damage was in an area not visible during a preflight examination.

In the aftermath of the accident, NASA inspected their inventory of Atlas vehicles and found several more improperly brazed pipes which needed replacement.

It was discovered that the last command issued was a signal to gimbal the booster engines hard to right, caused apparently by lightning induced electromagnetic pulse altering a single word in the guidance program.