The recent launch of the Atlas V with its Centaur upper stage was just the latest in a long series of such flights stretching back over half a century. Although Atlas V flights seem routine today, the Atlas-Centaur had a very troubled start owing to the novel nature of its cryogenic hydrogen fuel. After reaching a low point 50 years ago with a spectacular on-pad explosion, the Atlas-Centaur program struggled back from the brink to launch America’s first successful lunar lander, Surveyor 1, in 1966 (see “Surveyor 1: America’s First Lunar Landing“). Today’s successes would not have been possible without the work of thousands of dedicated engineers, scientists and technicians a half century ago who struggled to create new technology in the early years of the Space Age.

Origins

Rocket pioneers like Konstantin Tsiolkovsky and Robert Goddard recognized over a century ago that hydrogen would be the ideal fuel for a chemical rocket because of the large amount of energy it released during combustion and the low molecular weight of its exhaust products. When combined with liquid oxygen (LOX), liquid hydrogen generates a third again more thrust than an equal mass of most other liquid rocket fuels. Unfortunately, liquid hydrogen’s low temperature of -253° C (just 20° above absolute zero), its low density (which requires large fuel tanks), the small size of its molecules (which allows it to pass through the tiniest microscopic hole) and other properties presented a range of difficult engineering challenges that had to be resolved before a practical hydrogen-fueled rocket could be developed. While there were some experiments with this technology performed in the years following World War II, most development work during this era focused on more “conventional” fuels like alcohol, kerosene or hydrazine.

While work with liquid hydrogen propulsion slowly advanced with no near-term application, all that changed with the launch of the Soviet Sputnik 1 satellite in October 1957. In the wake of this historic event, the US made every effort to close the perceived gap with the Soviet’s lead in rocket and satellite technology. Building on the work with hydrogen propulsion over the past decade, General Dynamics submitted a proposal to the Department of Defense (DoD) in August 1958 to combine a modified version of the Atlas ICBM they were developing for the USAF (see “The First Atlas Test Fights“) with a new liquid hydrogen-fueled upper stage to serve as the basis of an advanced satellite launch vehicle. By the end of that month, the DoD’s Advanced Research Project Agency (ARPA) issued an order for General Dynamics to develop their proposed rocket under the direction of the USAF using engines being developed by Pratt & Whitney and the Atlas-Centaur was born.

With the founding of NASA in October 1958, most of ARPA’s non-military space projects were transferred to the new civilian agency. After much resistance from the USAF, control of the development of Centaur was officially transferred to NASA on July 1, 1959 with the USAF relegated to an advisory role. NASA initially planned to use the Atlas-Centaur to launch heavy scientific satellites into Earth orbit, their one-ton Mariner probes to Venus and Mars as well as eventually send their proposed Surveyor lander to the Moon. Plans were also developed to use the Centaur as the third stage of the Saturn heavy lift launch vehicle being developed by a team led by famed rocket pioneer Wernher von Braun at what would become NASA’s Marshall Space Flight Center (MSFC). Because of this, NASA headquarters gave MSFC responsibility for the Centaur development in July 1960.

The 3-meter in diameter Atlas-Centaur consisted of two stages connected by an interstage adapter. The first stage was a modified Atlas D ICBM which used special grade of kerosene known as RP-1 and LOX as propellants for its three main engines and pair of vernier engines. With a total length of 20 meters including the interstage adapter, the modifications to the Atlas included a redesigned forward section to accommodate the Centaur which was substantially wider and heavier than upper stages used earlier with the Atlas. The MA-3 propulsion system built by Rocketdyne for the Atlas D was also replaced with a higher performance MA-5 which generated 1,730 kilonewtons of thrust at lift off. The MA-5 consisted of a pair of uprated booster engines generating 733 kilonewtons each which were jettisoned about two and a half minutes after launch when they were no longer needed as well as a sustainer engine which was ignited on the pad with the boosters and would consume the remainder of the propellant. This innovative “stage and a half” design greatly improved the performance of the Atlas.

The new Centaur second stage had a total length of 14 meters including its nose shroud. It used a pair of Pratt & Whitney RL-10 engines that generated 67 kilonewtons of thrust each. Because the types of orbits for NASA’s proposed missions required a restart capability, Centaur included an attitude control system and ullage jets powered by hydrogen peroxide to keep the stage oriented properly and its propellants settled in their tanks during the weightless coast between engine burns. In order to minimize the boil off the cryogenic liquid hydrogen fuel, Centaur was fitted with insulation panels which were jettisoned after the rocket had left Earth’s atmosphere and were no longer needed to help improve the payload capability.

Because of a low budget and insufficient resources initially dedicated to the Centaur development effort, progress towards the first test launch of the Atlas-Centaur originally scheduled for January 1961 was slow at best. In order to speed the development of the Centaur, it borrowed heavily from Atlas missile technology and even used its tooling whenever possible. The situation was not helped by the open animosity between von Braun and the developers of the Atlas and Centaur at General Dynamics who employed radically new engineering in their rockets. In order to minimize the empty mass of the rocket, for example, Atlas did not employ the traditional rocket design philosophy espoused by the more conservative von Braun where the propellant tanks and exterior shell were attached to an internal framework. Instead, both the Atlas and Centaur used the same thin stainless steel structure to act an both the outer shell and propellant tanks with internal pressure providing the rigidity needed to keep it from collapsing. Von Braun felt that this and other innovative design approaches for the Centaur were too risky and advocated cancelling the project.

The First Test Flights

With the lack of support at MSFC and insufficient resources, the Atlas-Centaur development proceeded slowly towards its first test flight which was finally ready for launch 15 months later than originally scheduled. Atlas-Centaur 1 lifted off from the new Launch Complex 36A (LC-36A) at Cape Canaveral on May 8, 1962 on a simple single-burn test flight of the Centaur which was carrying only a partial propellant load. All was going well until the ascending vehicle exploded 54 seconds after launch near the point of maximum dynamic pressure. The initial investigation placed the blame on the insulation panels which were believed to have ripped off and ruptured Centaur’s hydrogen tank. Tests performed five years later instead suggested that the real culprit was differential expansion between the fiberglass nose fairing and the stainless steel fuel tank which caused the forward ring to peel off. Either way, the Atlas-Centaur was still a far way off from becoming operational.

The delays and problems involved in the development of the Atlas-Centaur had a ripple effect on NASA’s plans. The original one-ton Mariner spacecraft to Venus and eventually Mars were scrapped in favor of a lighter design based on the Ranger lunar spacecraft which could be launched using the less capable Atlas-Agena (see “The Prototype That Conquered the Solar System“). Development of other payloads that would use the Atlas-Centaur were also deferred or altered. The USAF, for a variety of practical and political reasons, lost interest in the program and officially pulled out on January 1, 1962 preferring to use the Titan III family of rockets based on their Titan II ICBM to meet military launch needs (see “50 Years Ago Today: The First Titan III Launch”). Finally on October 8, 1962, NASA headquarters transferred responsibility of Centaur from MSFC to Lewis Research Center (today known as Glenn Research Center) in Ohio which had already been sponsoring research into hydrogen propulsion for the past eight years. Responsibility for the RL-10 stayed at MSFC since these same engines were also used in the second stage of their Saturn I (see “The Coolest Rocket Ever”).

With more funding and resources to devote towards an extensive testing program and renewed support from Lewis, work began to resolve Centaur’s problems. Finally on November 23, 1963, Atlas-Centaur 2 was launched from Cape Canaveral with the modest goal of a single burn of the Centaur to achieve Earth orbit following separation from the Atlas. This time the Centaur successfully fired for 380 seconds to reach a 547 by 1691-kilometer orbit gathering data on jettisoning its new insulation panels and nose cone. This was the first time that a hydrogen-fueled rocket engine had been fired in space. The next test flight, Atlas-Centaur 3, was launched seven months later on June 30, 1964. Its mission was to gather data on its redesigned insulation panels on its way into orbit as well as perform tests in preparation for an attempted restart of the RL-10 engines on a future test flight. While it successfully jettisoned its insulation panels and nose cone, a freak drive shaft failure in the hydraulic system caused the Centaur to spin out of control prematurely ending the flight.

After an investigation and further modifications, Atlas-Centaur 4 was prepared with the objective of testing Centaur’s restart capability. For this flight, the rocket carried a 939-kilogram mass model of the Surveyor unmanned lunar lander which would be placed into a simulated trajectory towards the Moon after an in-orbit restart of Centaur’s engines. Atlas-Centaur 4 was launched on December 11, 1964 and successfully reached its low 165 by 178-kilometer Earth parking orbit as planned. Unfortunately, venting from the Centaur caused it to tumble unexpectedly while coasting preventing its RL-10 engines from restarting as planned. The Centaur still had problems to be resolved before it could carry a real payload.

The Atlas-Centaur 5 Mission

As issues surrounding the Centaur’s untested restart capability were being addressed, the decision was made that the first flights of NASA’s unmanned Surveyor lunar lander mission would use a direct ascent trajectory to the Moon which would not require a Centaur restart. Such an ascent profile is less than ideal since it typically limits launch window options and can affect launch vehicle payload performance, but the early Surveyor flights would use light-weight spacecraft that could be launched sooner without having to wait for Centaur’s restart problems to be solved. Testing this direct ascent launch profile was the primary objective of Atlas-Centaur 5 mission.

Atlas-Centaur 5 had a total height of 34 meters and a fully-fueled launch mass of 136 metric tons. The Atlas first stage on this mission was the first in the Atlas-Centaur test flight series to use the upgraded MA-5 power plant. The Centaur was fitted with modified, lighter-weight insulation panels with a mass of 570 kilograms that would be jettisoned during ascent followed by the 840-kilogram nose fairing shortly afterwards. The rocket was heavily instrumented and carried a telemetry system designed for a minimum lifetime of six hours.

For this test flight, Atlas-Centaur 5 carried a dynamic model of the Surveyor spacecraft meant to simulate the actual hardware’s mass, inertia and vibration properties. The model, designated SD1, consisted of a frame supplied by Surveyor’s prime contractor, Hughes Aircraft, and a simulated Surveyor retrorocket built by General Dynamics. Inert masses were hung in key locations on the frame to simulate the mass properties of actual equipment on Surveyor. The lander’s mast assembly with its antenna and solar panels were similar to those of the real Surveyor as was the simulated landing gear. Surveyor SD1 was heavily instrumented with temperature sensors, accelerometer, microphones, position sensors and strain gauges designed to monitor the model’s behavior and environment. The total mass of the Surveyor SD1 was 640 kilograms compared to the 945-kilogram mass of expected of the first actual spacecraft.

For the Atlas-Centaur 5 mission, Surveyor SD1 was to be placed directly into an extended 160 by 800,000 kilometer orbit meant to simulate a trajectory to the Moon. After the Surveyor SD1 separated from the Centaur, the spent stage would turn and vent its residual propellant to deflect its trajectory away from Surveyor. Mounted on top of the Surveyor’s mast was an omnidirectional antenna used by an S-band transponder for post-separation tracking by NASA’s Deep Space Network for up to 20 hours to monitor the position of the spacecraft in its extended orbit.

Atlas-Centaur 5 lifted off from LC-36A on March 2, 1965 after a nearly flawless countdown. Unfortunately only two seconds after launch and barely a meter and a half off the pad, a fuel prevalve in the MA-5 boosters of Atlas 156D unexpectedly snapped closed and the engines shutdown. The rocket fell back to the pad and about a hundred tons of RP-1, liquid hydrogen and LOX ignited creating a spectacular explosion – probably the largest the Cape had seen to that point in its history. The explosion completely destroyed Atlas-Centaur 5 and severely damaged LC-36A which required a year’s worth of repair work before it could be used again. In the aftermath of the launch accident, efforts turned towards getting LC-36B operational to support Atlas-Centaur test flights. LC-36B was nearly complete at the time of the accident and was in a “caretaker” status after work on the pad had been suspended eight months earlier in a money-saving move.

Finally on August 11, 1965, Atlas-Centaur 6 lifted off from the just-completed LC-36B and successfully placed its 953-kilogram Surveyor SD2 dynamic model into a 166 by 815,085-kilometer orbit after a single 435-second burn of the Centaur. The use of the Atlas-Centaur for a single-burn, direct ascent trajectory to the Moon for the first Surveyor flights was now cleared. As work continued towards a successful test of Centaur’s in-orbit restart capability in 1966, the loss of Atlas-Centaur 5 did have a practical application in the end. A detailed analysis of the explosion and its aftermath helped to verify models of the results from an on-pad explosion of NASA’s Saturn V which also used RP-1, liquid hydrogen and LOX but had a launch mass over twenty times greater than the Atlas-Centaur. This work helped to ensure the safety of ground personnel and the Apollo astronauts. In the mean time, the final push was made to get Surveyor on the Moon for the first lunar landing.

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Related Video

Here is a short film showing the launch of Atlas-Centaur 5 and the unfortunate aftermath.

And here is a documentary film by the NASA and Convair/General Dynamics reporting on activities in the Centaur program during the second half of 1965.

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Related Reading

“Riding Piggyback on an ICBM”, Drew Ex Machina, January 21, 2015 [Post]

“Vintage Micro: The Talking Atlas”, Drew Ex Machina, December 18, 2014 [Post]

“The First Atlas Test Fights”, Drew Ex Machina, June 11, 2015 [Post]

“Surveyor 1: America’s First Lunar Landing”, Drew Ex Machina, May 30, 2016 [Post]

General References

Virginia P. Dawson and Mark D. Bowles, Taming Liquid Hydrogen: The Centaur Upper Stage Rocket 1958-2002, NASA SP-2004-4230, 2004

J.D. Hunley, US Space-Launch Vehicle Technology: Viking to Space Shuttle, University Press of Florida, 2008

“NASA’s Fifth Centaur Flight to test Surveyor Dynamic Model”, NASA Press Release 65-65, March 1, 1965