The Atlas-Centaur was a United States expendable launch vehicle derived from the SM-65 Atlas D missile. The vehicle featured a Centaur upper stage, the first such stage to use high-performance liquid hydrogen as fuel. Launches were conducted from Launch Complex 36 at the Cape Canaveral Air Force Station (CCAFS) in Florida. 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. The vehicle would be continuously developed and improved into the 1990s, with the last direct descendant being the highly successful Atlas II.
Early development Convair, the manufacturer of the Atlas, developed the Centaur upper stage specifically for that booster, sharing its pressure-stabilized tank structure.
Technical
Centaur was the first rocket stage to utilize liquid hydrogen (LH2) and liquid oxygen (LOX) as propellants. Despite boasting high performance, LH2 had to be chilled to extremely low temperatures (lower than LOX) and its low density meant that large fuel tanks were needed.
The first attempt at using an LH2/LOX-fueled engine was the U.S. Air Force's top-secret Lockheed CL-400 Suntan reconnaissance aircraft program in the mid-1950s. The progress made during the aborted venture was picked up by Convair and others for rocket stage use. 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.
Convair developed a specially-enhanced version of the Atlas D vehicle for mating with Centaur stages; the Atlas was equipped with an uprated booster section, the MA-5, which had twin turbopumps on each booster engine, and the structure reinforced for the large upper stage, along with elongated fuel tanks. Centaur development was made somewhat difficult by the insistence on modifying Atlas components rather than developing totally new ones. This was done for time and budget reasons and because it allowed the Centaur to be manufactured on the existing Atlas assembly line at Convair. The engines were manufactured by Pratt & Whitney.
Program Originally under ARPA supervision, Centaur was transferred to NASA in July 1959, eleven months after the program's inception. The Air Force retained overall supervision in part because they intended to use Centaur to launch a network of military communications satellites known as ADVENT. A constellation of ten satellites would provide round-the-clock instant communications for the three main branches of the US military. The first three would be launched on an Atlas-Agena, then the remainder on Centaur. ADVENT never got off the drawing board, but Centaur quickly found a use for several NASA planetary probe projects, namely Mariner and Surveyor.
An initial lack of funds caused the project to take far longer than intended. Under original timetables, Centaur was to make its first flight in January 1961.
Research and development (R/D) flights First flight
In October 1961, the first Atlas-Centaur (Vehicle Flight-1: Atlas 104D and Centaur F-1) arrived at Cape Canaveral and was erected at the newly completed and specifically built LC-36A. 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.
Failure
The vehicle was launched at 2:49 PM EST (18:49 GMT) on 8 May 1962, with the intention of performing a single burn with a partially fueled Centaur. Slightly under a minute into the launch, the Centaur stage ruptured and disintegrated, taking the Atlas with it in a matter of seconds. 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. Initial assumptions were that Atlas had suffered a LOX tank failure, either from a pressurization problem, rupture of the tank from flying debris, or structural bending/aerodynamic issues caused by the unproven Atlas-Centaur combination, and indeed there had been several previous occurrences of these failure modes on Atlas launches. Scott Carpenter's Mercury flight was only days away, and if the failure were caused by the Atlas, it could mean significant delays for that mission, which used a similar Atlas D derived Atlas LV-3B booster. However, analysis of telemetry data and closer examination of the launch films quickly confirmed the Centaur as the source of trouble.
The failure was determined to be caused by an insulation panel that ripped off the Centaur during ascent, resulting in a surge in tank pressure when the LH2 overheated. 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. Two seconds later, flying debris ruptured the Atlas's LOX tank followed by complete destruction of the launch vehicle. 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. Testing was suspended while efforts were made to correct the Centaur's design flaws.
Investigation 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.
In addition, the production Centaur stage had less lift capacity than originally planned, leading to ARPA cancelling Project ADVENT. 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.
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.
Eight Atlas-Centaur test missions were scheduled to be completed by the end of 1964, followed by the first Surveyor program launch. Centaur was upgraded to a high priority project because of this direct relation to Apollo.
Meanwhile, the Department of Defense (DoD) had settled on the Titan family for its heavy-lift launching needs and so the Atlas-Centaur would remain a civilian launch vehicle used by NASA to fly scientific and commercial payloads. A conflict between the Air Force, who had primary oversight of the Atlas, and NASA also existed as the Centaur stage required various modifications to the basic Atlas. By 1962, the Air Force had considered the Atlas fully developed and operational and was against any further significant changes to it which might potentially jeopardize the ICBM program. The dispute was ultimately resolved by NASA agreeing to purchase standard Atlas D vehicles which could be custom-modified for Centaur launches. However, when the Atlas ICBM program ended in 1965, Convair replaced all of the earlier variants with a standardized booster for all space launches.
Second flight
More than a year later, on 27 November 1963 at 19:03:23 GMT, AC-2 (Atlas 126D and Centaur stage #2) took place five days after President Kennedy's assassination. The redesigned Centaur stage functioned without any problems and performed a single burn to geostationary transfer orbit (GTO) (orbit of 474 x 1586 km, inclination of 30.4° and period of 105.8 minutes), where it remained as of 2021. The insulation panels were permanently attached to the stage as the jettison problem had still not been solved. Vibration data proved that the panels would have prematurely detached had they not been bolted down. The ultimate fix to the panel problem added more dry mass to Centaur, further dropping its payload capacity. This Atlas-Centaur 2 launch vehicle was used for performance and structural integrity tests. It carried a payload of 4621 kg and instrumented with 907 kg of sensors, equipment, and telemetry.
Third flight
The AC-3 flight (Atlas 135D and Centaur #3) was launched on 30 June 1964 at 14:04:22 GMT with a payload of 4815 kg. Atlas performance was close to nominal with the sustainer running slightly LOX-rich for the first 70 seconds of flight and the trajectory being more lofted than expected. Insulation panel and payload shroud jettisons were tested for the first time. Following Centaur staging and engine start, the number two (C-2) engine began to lose roll control. The C-1 engine was able to compensate for a time, but the Centaur eventually lost control and began tumbling. Premature engine shutdown due to propellant starvation occurred at T+496 seconds, and the Centaur impacted in the South Atlantic. Postflight investigation traced the malfunction to a failure of the Centaur-2 engine hydraulic gimbal actuator."Postflight Evaluation Report, Atlas-Centaur AC-3, NASA Lewis Research Center"
Fourth flight The AC-4 flight (Atlas 146D and Centaur #4) was launched on 11 December 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 12 December 1964."Postflight Evaluation Report, Atlas-Centaur AC-4, NASA Lewis Research Center"
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