Atlas-Centaur

Atlas-Centaur

An Atlas-Centaur launching Surveyor 1
Function	Expendable launch system
Manufacturer	Convair General Dynamics
Country of origin	United States
Launch history
Status	Retired
Launch sites	LC-36, Cape Canaveral SLC-3E, Vandenberg
Total launches	197
Successes	181
Failures	13
Partial failures	3
First flight	May 8, 1962
Last flight	August 31, 2004

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 the Cape Canaveral Air Force Station in 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 (LH2) and liquid oxygen (LOX) as propellants. 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 an 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. Originally, Centaur was conceived of as a purely experimental project to develop experience for larger, more powerful rocket stages and not given a definitive mission, so as not to distract Convair's focus on the all-important Atlas program. Although originally under ARPA supervision, Centaur was transferred to NASA in July 1959, eleven months after the program's inception. However, the Air Force retained ultimate supervision in part because they intended to use Centaur for an ambitious 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.

Convair developed a specially-enhanced version of the Atlas D vehicle for mating with Centaur stages; the Atlas was equipped with an uprated propulsion system, 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 and Whitney. There were considerable difficulties integrating the two vehicles, especially because Atlas-Centaur would be almost 30% longer than an Atlas ICBM and there were doubts as to its aerodynamic stability in flight.

Wernher Von Braun's team at the Army Ballistic Missile Agency (ABMA) did not care for either Centaur or Atlas, having a dislike for their balloon structure and the former's hydrogen propellants, as Von Braun had had negative experience with experiments in hydrogen propellants back in Germany (also, the 1937 Hindenburg Disaster primarily occurred due to the use of hydrogen as an inflation gas). He preferred the use of a Saturn-Agena combo for planetary probes and gave minimal support to Centaur. This and lack of funds caused the project to drag on far longer than intended. Under original timetables, Centaur was to make its first flight in January 1961, but months slipped by. In April, NASA Lunar and Planetary Programs director Oran Nicks suggested that it might be necessary to use Atlas-Agena for Mariner instead. By September, there was no launch date anywhere in sight while program costs rose higher and higher. In April 1962, a month before Centaur's first test launch, it came out that the rocket stage's lift capacity was about 400 pounds less than anticipated, which meant that Surveyor could not carry as many experiments as originally intended.

The first Atlas-Centaur, Vehicle F-1 (Atlas 104D and Centaur 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. Flight plans amounted to little more than a single burn with a partially fueled Centaur, just to verify that the launch vehicle combination would actually fly. However, technical problems and other delays caused it to sit there for seven months, the most serious being leakage of liquid hydrogen through the intermediate bulkhead separating the propellant tanks, and numerous lesser maladies with the guidance and propulsion systems. The mission at last got under way on May 8, 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.

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 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 booster. However, analysis of telemetry data and closer examination of the launch films quickly confirmed the Centaur as the source of trouble and exonerated the Atlas.

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. The pneumatic system responded by excessive venting of propellant to reduce pressure levels, but eventually they exceeded the LH2 tank's structural strength. It then ruptured, causing flying debris to also rupture the Atlas's LOX tank, followed by complete launch vehicle self-destruction. The panel had been meant to jettison at 50 miles (80 kilometers) 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 was 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.

The Congressional committee was headed by Representative Joseph Karth (D-Minnesota) who expressed his concern that Centaur was a useless project and an ever-growing money sink. Although the failure of AC-1 was probably inevitable, the American public still felt the sting of Soviet space successes and the endless NASA planetary probe failures. Werner Von Braun thought Centaur was poorly supervised, in part because it had begun as simply a little test project and not a full-fledged program. He also reiterated his disdain for the Centaur's balloon tanks.

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.^[1]

The movie Koyaanisqatsi (1983) shows footage of Atlas-Centaur 1's launch, including the tracking cameras following the Atlas's sustainer engine down to impact with the ocean.

Despite questions over the value of Surveyor, NASA considered the effort vital to a manned lunar landing, as nobody could be certain that the Moon didn't contain vast pits of quicksand that could swallow up spacecraft and astronauts. 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, Werner Von Braun's proposed Saturn-Agena was ruled out for cost reasons--Saturn was much too expensive to justify as a launch vehicle for small unmanned probes.

Abe Silverstein set a timetable of eight Atlas-Centaur test missions, slated to finish by the end of 1964, and only then would a Surveyor mission fly. In addition, Lewis took over management of Agena, which was proving to be a reliability nightmare in both Air Force and NASA programs. Centaur was upgraded to a high priority project because of its direct relation to Apollo. Silverstein also decided on a direct ascent trajectory for Surveyor because it required less fuel and mission complexity, a move made possible by the lucky coincidence that the Moon in the mid-1960s was at a point in its orbit that occurred every 18 years where direct ascent was possible. However, some JPL officials worried that Surveyor might end up landing on the nighttime side of the Moon, most likely causing a complete mission failure.

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 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 and performed a single burn to geosynchronous orbit, where it remains today. Because the problem of jettisoning the insulation panels had still not been solved, Abe Silverstein ordered them simply bolted to the side of the Centaur. Vibration data obtained during the flight also proved that the panels would have failed in a manner similar to AC-1 had they not been bolted down. The ultimate fix to the panel problem added more weight to Centaur, further dropping its payload capacity.

AC-3 was launched on June 30, 1964. Insulation panel and payload shroud jettison were performed for the first time, and the rocket carried a Surveyor mockup. Although the boost phase went entirely according to plan, a shaft in a hydraulic pump acutator failed during the Centaur burn, causing engine gimbaling to fail. The Centaur tumbled and lost thrust. It reached orbit, but could not be restarted, however the small ullage rockets were successfully tested. AC-4 (December 11) reached orbit successfully, but 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. Venting liquid hydrogen caused the Centaur to tumble out of control.

The fifth flight on March 2, 1965 was only intended to carry out a single burn of the Centaur, and program officials felt confident that this simple mission could be performed with no difficulties. In addition, the booster carried the operational model of the MA-5 engines with uprated thrust for the first time. Instead, AC-5 proved a complete disaster as the Atlas's fuel prevalves accidentally closed one second after liftoff, cutting off thrust to the booster engines. The sustainer engine by itself could not lift the 150 ton rocket and it fell back onto LC-36A in the biggest pad explosion yet 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; the damage to LC-36A was not as severe as it looked and repairs were largely completed in three months.^[1] The accident marked the first failure of an Atlas 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.

Postflight investigation found that the fuel prevalves had only opened partially and the propellant flow force 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 prevalves closed. Bench testing confirmed that there were several possible ways that the prevalves would only open partially, and in this case, it appeared to be the result of an incorrectly-installed pressure transducer. This failure mode had never occurred in the 240 Atlas launches prior to AC-5 although it had always been possible. To prevent a recurrence, the prevalves were equipped with a lock that would be enabled during the prelaunch countdown.

The AC-5 disaster brought about another Congressional investigation headed by Rep. Joseph Karth who charged that $600 million of taxpayer money had been spent on Centaur so far with little to show for it other than three failed missions and an incinerated launch pad. The committee argued that Convair, as the sole supplier of the Atlas-Centaur vehicle, were taking advantage of this situation to charge exorbitant prices for the booster (unlike Atlas-Agena where a different contractor built the upper stage), and that NASA should consider alternate choices for the planetary probe program such as Titan IIIC, and indeed Martin were eager to jump on this proposal and rack up some additional Titan business. Congress also proposed outsourcing manufacture of Centaur to other contractors with the hope that they could do a better job than Convair. However, NASA representatives argued that this was impossible since no other aerospace company had the experience or technical capability to manufacture the Centaur's balloon tanks.

After LC-36B was hastily brought online, AC-6 launched on August 11 and was entirely successful. Although Centaur appeared flight-ready, Surveyor was still taking a long time to develop. Vehicles AC-7 and AC-10 were designated for the first Surveyor missions, with AC-8 to carry out one more test, which took place on April 7, 1966. The Centaur's ullage motors failed yet again because they did not have enough propellant for the mission, another overlooked design flaw.

Beginning with AC-13 (Surveyor 5), Atlas-Centaur vehicles switched to the standardized SLV-3 Atlas core.

Operational launches

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

AC-33, launched on February 20, 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. 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 September 29, 1977 on AC-43. Shortly after liftoff, abnormal temperatures were detected in the Atlas's thrust section and continued to rise as the booster ascended. A visible thrust section fire could be seen starting at T+33 seconds and sustainer hydraulic pressure was lost at T+55 seconds, causing total loss of vehicle control. 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. The Centaur flew free until being destructed by the Range Safety Officer a few seconds later. 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 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 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 March 26, 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 May 8, 1962 and ended with the last Atlas III (Centaur), AC-206, launched on February 3, 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-designed and built RD-180 twin-chamber 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^[2] 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.^[3]

Variants

References