Editing Atlas-Centaur (section)

== Research and development (R/D) flights ==
=== First flight ===
[[File:Atlas 104D Technicians Readying Booster 104D for Erection; AMR Complex 36. Date- 10-31-1961 (21061503084) (2).jpg|thumb|Technicians readying booster 104D for erection]]
[[File:Atlas-LV3C Centaur-A (AC-1).jpg|thumb|Atlas-LV3C Centaur-A (AC-1)]]
In October 1961, the first Atlas-Centaur (Vehicle Flight-1: Atlas 104D and Centaur F-1) arrived at [[Cape Canaveral Space Force Station|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 ====
[[File:Atlas-Centaur launch.jpg|thumb|Unsuccessful maiden flight of the Atlas-Centaur AC-1]]
The vehicle was launched at 2:49 PM EST (18:49 [[Greenwich Mean Time|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 [[Aurora 7|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 [[SM-65D Atlas|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&nbsp;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 [[RM-81 Agena|Agena upper stage]] for planetary missions. 

In addition, the production Centaur stage had less lift capacity than originally planned, leading to [[DARPA|ARPA]] cancelling Project ADVENT. NASA transferred Centaur development from MSFC to the [[Glenn Research Center|Lewis Research Center]] in [[Ohio]] where a team headed by [[Abe Silverstein]] worked to correct the insulation panel problems and various other design flaws.<ref name=":0">{{cite web |url=http://www.spacelaunchreport.com/aclv3cb.html|title=Atlas Centaur LV-3C Development|last=Kyle|first=Ed|date=May 28, 2005|website=spacelaunchreport.com|publisher=Space Launch Report|access-date=April 15, 2016 | archive-url = https://web.archive.org/web/20100821064737/http://www.spacelaunchreport.com/aclv3cb.html | archive-date= August 21, 2010 |url-status = usurped}}</ref>

In November 1962, [[John F. Kennedy|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.{{citation_needed|date=July 2019}}

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.<ref>{{cite web|title=An Historical Meeting on Spaceflight: Background and Analysis|url=https://history.nasa.gov/JFK-Webbconv/pages/backgnd.html
|quote=Although Centaur was intended to launch robotic probes to the Moon and outer planets, Webb argued that NASA would also gain vital experience with liquid hydrogen by building Centaur and would be able to apply this experience toward Apollo.
|publisher=NASA|access-date=February 25, 2023}}</ref>

Meanwhile, the [[Department of Defence|Department of Defense]] (DoD) had settled on the [[Titan (rocket family)|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.{{citation_needed|date=July 2019}}

=== Second flight ===
[[File:Atlas-LV3C Centaur-B (Centaur-AC 2).jpg|thumb|Atlas-LV3C Centaur-B (AC-2)]]
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&nbsp;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&nbsp;kg and instrumented with 907&nbsp;kg of sensors, equipment, and telemetry.<ref name="Display">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1963-047A|title=Display: Atlas-Centaur 2 1963-047A|publisher=NASA|date=5 April 2021|access-date=9 April 2021}} {{PD-notice}}</ref>

=== Third flight ===
[[File:Atlas Negative Collection Image (21675576522).jpg|thumb|Erection of Atlas 135D (AC-3), Pad 36A]]
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&nbsp;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 [[Atlantic Ocean|South Atlantic]]. Postflight investigation traced the malfunction to a failure of the Centaur-2 engine hydraulic gimbal actuator.<ref>"Postflight Evaluation Report, Atlas-Centaur AC-3, NASA Lewis Research Center" {{PD-notice}}</ref>

=== 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&nbsp;kg (that carried a mass model of the [[Surveyor program|Surveyor spacecraft]]). It performed propulsion and stage separation tests,<ref name="Display2">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1964-082A|title=Display: Atlas-Centaur 4 1964-082A|publisher=NASA|date=5 April 2021|access-date=9 April 2021}} {{PD-notice}}</ref> 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 motor|ullage rocket]]s 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 [[Pacific Ocean|South Pacific]], on 12 December 1964.<ref>"Postflight Evaluation Report, Atlas-Centaur AC-4, NASA Lewis Research Center" {{PD-notice}}</ref>

=== Fifth flight ===
[[File:Explosion-du-vol-AC-5-.jpg|thumb|The spectacular explosion of AC-5 in March 1965 left Pad 34A a mass of blackened rubble.]]
The AC-5 flight (Atlas 156D) on 2 March 1965 at 13:25 GMT from [[Cape Canaveral Space Force Station|Cape Kennedy]] in a highly elliptical orbit, with a payload (Surveyor SD-1) of 951&nbsp;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&nbsp;km that simulated a lunar transfer trajectory.

==== Failure ====
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 [[Liquid hydrogen|LH2]] load going off in a huge fireball for the biggest on-pad explosion yet seen at Cape Canaveral.<ref>{{cite web|title=Beyond Earth, A Chronicle of Deep Space Exploration, 1958–2016|first=Asif A.|last=Siddiqi|url=https://www.nasa.gov/sites/default/files/atoms/files/beyond-earth-tagged.pdf|publisher=NASA|access-date=November 5, 2022}}</ref> This was also the first Atlas-Centaur equipped with the uprated {{cvt|165000|lb}} thrust [[MA-5 (rocket stage)|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 [[Cape Canaveral Launch Complex 36|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&nbsp;°C.<ref name=":0"/> 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 was the last on-pad explosion at Cape Canaveral until 2016 ([[List of Falcon 9 and Falcon Heavy launches#Loss of AMOS-6 on the launch pad|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 [[Liquid oxygen|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|date=July 2019}}

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. 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.<ref>{{cite web|title=Postflight Evaluation of Atlas-Centaur AC-5 (Launched March 2, 1965) |url=https://ntrs.nasa.gov/api/citations/19750077283/downloads/19750077283.pdf|publisher=NASA|pages=23–24|access-date=February 25, 2023}}</ref>

==== Investigation ====
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. NASA representatives argued that this was impossible as no other aerospace company had the experience or technical capability to manufacture the Centaur's balloon tanks.{{citation_needed|date=July 2019}}