Atlas-Centaur

The Atlas-Centaur was an American expendable launch system derived from the SM-65 Atlas D missile. Launches were conducted from Launch Complex 36 at Cape Canaveral Air Force Station, Florida.

Early development and testing
Convair, the manufacturer of the Atlas, developed the Centaur upper stage specifically for that booster, sharing its inflated balloon skin. It was also the first production rocket stage to utilize liquid hydrogen and liquid oxygen as propellant. Despite high performance, LH2 nonetheless had problems because it had to be chilled at extremely low temperatures (lower than LOX) and its light molecular density meant that large fuel tanks were needed.

The first attempt at using a LH2/LOX-fueled engine was the Air Force's top-secret Lockheed CL-400 Suntan reconnaissance aircraft program in the mid-1950s, but it was judged too unsafe, expensive, and impractical for that purpose. However, the progress made during the aborted venture was picked up by Convair and others for rocket stage use.

Convair developed a specially-enhanced version of the Atlas D vehicle for mating with Centaur stages; the Atlas's engines were upgraded 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 develop 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 first Atlas-Centaur, Vehicle F-1, arrived at Cape Canaveral in October 1961 and was erected at the newly completed LC-36A, a pad built specifically for A/C flights. However, technical problems and other delays caused it to sit there for 7 months. The mission at last got under way on 8 May 1962. All went well until about T+53 seconds when the Centaur stage ruptured and disintegrated, taking the Atlas with it in a matter of seconds.

The failure was determined to be caused by an insulation panel that ripped off the Centaur during ascent, causing the LH2 tank to overheat and eventually explode. As the liquid hydrogen spilled down the side of the launch vehicle, it ignited on contact with the Atlas's engine exhaust, leading to the complete destruction of the launch vehicle. The panel had been meant to jettison at 50 miles up when the air was thinner, but the mechanism holding it in place was designed inadequately and led to premature separation. The insulation panels had already been suspected during Centaur development of being a potential problem area and the possibility of a LH2 tank rupture considered as a failure scenario. Testing was suspended while efforts were made to correct the Centaur's design flaws. A Congressional investigation in June 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 Center in Ohio where a team headed by Abe Silverstein worked to correct the insulation panel problems and various other design flaws.

Meanwhile, the Department of Defense 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. This also was partially connected to conflict between the Air Force, who had primary oversight of the Atlas, and NASA because the Centaur stage required various modifications to the basic Atlas design. 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 was ended in 1965, Convair simply replaced all of the earlier variants with a standardized booster for all space launches.

More than a year later, the second test took place a few days after President Kennedy's assassination. The redesigned Centaur stage functioned without any problems, reaching a low Earth orbit, firing its engine again, and moving to a geosynchronous orbit.

The third flight (30 June 1964) which carried a dummy Surveyor probe was a partial success. The Centaur reached orbit, but could not be restarted due to a fuel leak. Flight four (11 December 1964) was completely successful.

The 5th flight on 2 March 1965 proved a complete disaster as the Atlas's fuel prevalves accidentally snapped shut one second after liftoff, causing the booster to fall back onto LC-36A in the biggest pad explosion ever seen at Cape Canaveral. As a result, NASA was forced to finish work on LC-36B, constructed as a backup pad, but abandoned when it was 90% completed (fortunately, the damage to LC-36A was not as severe as it looked and repairs were largely completed in three months). The accident was found to be the result of an improperly installed pressure transducer that sent a signal to close the prevalves. It also marked the first failure of an Atlas in a space launch since Midas 8 in May 1963, a new record at the time of 26 consecutive flights with only malfunctions of the upper stages or payload.

Ten more Atlas-Centaur failures occurred, but most of them were high altitude events that occurred late in the launch.

AC-33, launched on 20 February 1975, was carrying an Intelsat-4 communications satellite when the flight went awry around the two-minute mark. During booster engine separation, a swivel lanyard designed to pull out an electrical plug supplying power to the booster section failed to detach, causing a voltage spike that reset the Atlas's guidance computer. Control of the booster started to fail and Range Safety sent the destruct signal at around 403 seconds into launch. Investigation showed that the lanyard was not only inadequately designed, it was an off-the-shelf component designed for marine equipment and not rockets or aircraft. Atlas vehicles had been flying with them for years without anyone noticing or considering this potential failure mode. Fortunately, the backup Intelsat was launched successfully on AC-35 the following May.

Two years later, another attempted launch of an Intelsat communications satellite took place 30 September 1977 on AC-43. Shortly after liftoff, abnormal temperatures were detected in the Atlas's engine compartment and continued to rise as the booster ascended. At T+33 seconds, a fire broke out, causing control of the vehicle to gradually fail. At T+55 seconds, the sustainer engine shut down and the Atlas pitched over, causing the entire launch vehicle to disintegrate from structural stress. The payload fairing and satellite were stripped from the booster, followed by the Atlas breaking in half and exploding. The Centaur was ejected and flew free until Range Safety issued the destruct command at T+60 seconds. NASA and Air Force officials, already busy investigating the launch failure of a Delta booster three weeks earlier, 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 boattail 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 preflight examination. The Atlas used on this flight had been delivered to the Cape in 1971 and kept in storage since then, an unusually long time. In the aftermath of the accident, NASA inspected their inventory of Atlas vehicles and found several more improperly brazed pipes which needed replacement.

The second involved the launch of a Navy FLTSATCOM satellite on 26 March 1987. Weather conditions were extremely poor that day, with thick clouds and heavy precipitation. Although engineers objected to launching, the NASA program directors gave the go-ahead anyway. The Atlas disappeared into the cloud cover shortly after liftoff and was struck by a lightning bolt around 38 seconds into launch. Control of the booster started to fail and it broke apart from structural loads at T+50 seconds. Range Safety sent the destruct command, but there was no evidence that the booster ever received it. Debris rained out of the clouds onto the pad area. This accident was the culmination in the string of disasters that had befallen the US space program since the Titan 34D failure of August 1985 and caused significant reappraisals of weather guidelines at Cape Canaveral.

Most of the debris from AC-67 landed on the shoreline or in shallow water just off of it and was easily recovered. A section of the payload fairing was found to have multiple small holes burned in it due to repeated lightning strikes. The key piece of evidence was the Atlas's flight computer, which was recovered intact and examined. It was discovered that the last command issued was a signal to gimbal the booster engines hard to right, caused apparently by a lightning bolt altering a single word in the guidance program.

With the retirement of the Agena stage in 1978, all Atlases flown from that point onward were paired with Centaurs except for a few military flights involving decommissioned Atlas E/F missiles.

Originally designed and built by Convair Division of General Dynamics in San Diego, California, production of Atlas Centaurs at Convair ended in 1995 but was resumed at Lockheed-Martin in Colorado. The list of Atlas Centaur ID numbers began with AC-1 launched on 8 May 1962 and ended with the last Atlas III (Centaur), AC-206, launched on 3 February 2005.

The Rocketdyne-powered Atlas-Centaur was sometimes referred to as a 21/2 stage launch vehicle because the Atlas first stage (in most cases) jettisoned the twin-thrust-chamber booster engine prior to the completion of the first stage burn. Atlas Centaurs with a Rocketdyne-powered first stage were used for 167 launches between 1962 and 2004 by which time they had been superseded by Atlas Vs with a new first stage powered by a much more powerful Russian RD-180 engine. (Atlas Vs are not generally referred to as "Atlas Centaurs" and do not share the AC- serial numbers of the original Atlas Centaurs that had the Rocketdyne-powered first stages.)

Initially, a modified Atlas D, designated LV-3C, was used as the first stage This was quickly replaced by SLV-3C, and later the SLV-3D, both derived from the standard Atlas SLV-3 rocket. Two spaceflights, with the Pioneer 10 and Pioneer 11 space probes to Jupiter, Saturn and exiting the Solar System, used a spin-stabilized "Star-37E" solid propellant final stage weighing 2473 pounds and contributing 8000 mph to the velocities of the spacecraft.