Shuttle-Centaur

Centaur G and G-Prime

Illustration of Shuttle-Centaur G-Prime with Ulysses
Manufacturer	General Dynamics
Country of origin	United States
Centaur G-Prime
Height	9.3 m (31 ft)
Diameter	4.6 m (15 ft)
Empty mass	2,761 kg (6,088 lb)
Gross mass	22,800 kg (50,270 lb)
Powered by	2 x RL10-3-3A
Maximum thrust	73.40 kN (16,500 lbf) (per engine)
Specific impulse	446.4 s
Propellant	Liquid hydrogen / LOX
Centaur G
Height	6.1 m (20 ft)
Diameter	4.6 m (15 ft)
Empty mass	3,060 kg (6,750 lb)
Gross mass	16,928 kg (37,319 lb)
Powered by	2 x RL10-3-3B
Maximum thrust	66.80 kN (15,020 lbf) (per engine)
Specific impulse	440.4 s
Propellant	Liquid hydrogen / LOX

Shuttle-Centaur was a proposed Space Shuttle upper stage using the Centaur upper stage. Two variants were produced: Centaur G-Prime, which was planned to launch the Galileo and Ulysses robotic probes to JJupiter, and Centaur G, a shortened version planned for use with Department of Defense Milstar satellites and the Magellan Venus probe. The use of the powerful Centaur upper stage allowed heavier deep space probes, and for them to reach Jupiter sooner, thus saving on battery life and wear and tear to the spacecraft. Support for the project also came from the United States Air Force (USAF) and the National Reconnaissance Office), which asserted that its classified satellites required the power of Centaur. The USAF agreed to pay half the cost of Centaur G.

Both versions were cradled in the reusable Centaur integrated support system (CISS), an aluminum structure that handled communications between the Space Shuttle and the Centaur. The Space Shuttle Challenger and Space Shuttle Atlantis were modified to carry the CISS. The Centaur periodically vented hydrogen, be stored below −253 °C (−423 °F) to keep it from evaporating or boiling. Modifications were made to the Centaur and the Space Shuttle to permit venting, and to allow the fuel to be dumped in the event of an emergency.

After the Space Shuttle Challenger accident, and just months before the Shuttle-Centaur had been scheduled to fly, NASA concluded that it was far too risky to fly the Centaur on the Shuttle. The probes were ultimately launched using the much less powerful solid-fueled Inertial Upper Stage (IUS), with Galileo needing multiple gravitational assists from Venus and Earth to reach Jupiter. The USAF mated a variant of the Centaur G Prime upper stage with the Titan rocket to produce the Titan IV, which made its first flight in 1994. Over the next 18 years, Titan IV and Centaur G Prime would place eighteen military satellites in orbit.

Centaur was developed in the last 1950s and early 1960s as an upper stage rocket using liquid hydrogen as a fuel and liquid oxygen as an oxidiser.^[1] A rocket utilising liquid hydrogen as a rocket fuel can theoretically lift 40 per cent more payload per kilogram of liftoff weight than one with a conventional rocket fuel like kerosene. This was a particularly attractive prospect in the early days of the Space Race, but to use liquid hydrogen, rocket engineers had to first overcome enormous technological challenges. Liquid hydrogen is a cryogenic fuel, meaning that it assumes liquid form only at extremely low temperatures, and therefore must be stored below −253 °C (−423 °F) to keep it from evaporating or boiling. It must therefore be carefully insulated from sources of all sources of heat, particularly the rocket exhaust, atmospheric friction during flight through the atmosphere at high speeds, and the radiant heat of the Sun.^[2] The tiny molecules of hydrogen were capable of leaking through microscopic holes.^[3]

Designed and built by General Dynamics, Centaur was powered by twin Pratt & Whitney RL10 engines.^[4] It adopted radical weight-saving features pioneered by the Atlas rocket family: a monocoque steel shell that held its shape only when pressurized, and hydrogen and oxygen tanks separated by a common bulkhead. There was no internal bracing, and no insulation surrounding the propellants.^[5] The development of Centaur was dogged by technical difficulties, and in October 1962 NASA Headquarters transferred management of the project from NASA's Marshall Space Flight Center to its Lewis Research Center in Ohio. The technical problems were overcome, and Centaur ultimately outlived its competitors, and most of its critics too. The development of technology for handling liquid hydrogen proved crucial to winning the Space Race, for it paved the way for the use of liquid hydrogen in the upper stages of the Saturn V Moon rocket, albeit with the internal bracing that Wernher von Braun preferred, and later by the Space Shuttle.^[6]

Centaur upper stages were used in Atlas-Centaur rockets by the Surveyor program in the 1960s, which sent robotic spacecraft on the Moon,^[1] and in the late 1960s and 1970s by the Mariner missions to Mercury, Venus and Mars, the Pioneer 10 and Pioneer 11 probes to Jupiter and Saturn, and the Pioneer Venus project.^[7] In the 1970s, Centaur was placed atop the United States Air Force (USAF) Titan IIIE booster to create an even more powerful launch vehicle system. It was used for seven missions in the 1970s, including the Viking missions to Mars, Helios probes to the Sun, and the Voyager probes to Jupiter and the outer planets.^[8] By 1980, Centaur had an impressive record of 53 successful missions and two failures.^[9]

When the Titan IIIE-Centaur was rolled out for the first time in 1973, it was viewed as the last of the unmanned expendable launch vehicles. John Noble Wilford from The New York Times wrote that it was "expected to be the last new launching vehicle to be developed by the National Aeronautics and Space Administration until the advent of the reusable Space Shuttle which should be ready in 1978."^[10] Titan IIIE-Centaur was only used seven times. The future was seen to belong to reusable launch systems, and all expendable rockets would be phased out and replaced by the Space Shuttle. This augured badly for the projects to explore the Solar System with unmanned probes, which were coming under intense scrutiny by increasingly cost-conscious Congress.^[11]

The Space Shuttle was never intended to operate beyond low Earth orbit, but many satellites needed to be in higher orbits, particularly communications satellites, for which geostationary orbits were preferred. The original concept was called for a crewed space tug, which would be launched by a Saturn V. It would use a space station as a base, and be serviced and refueled by the Space Shuttle. Budget cutbacks in the early 1970s led to the termination of Saturn V production, and the abandoning of plans to build a space station, and the space tug became an upper stage, to be carried into space by the Space Shuttle. As a hedge against further cutbacks or technical difficulties, NASA also commissioned studies of reusable Agena and Centaur upper stages. The USAF was approached for assistance, and an agreement was reached on 11 July 1974 for the USAF to develop an Interim Upper Stage (IUS). A series of study contracts were let, resulting in a decision that the IUS would be an expendable solid-fuel upper stage. A call for bids was then issued, and the competition was won by Boeing in August 1976. The Interim Upper Stage was renamed the Inertial Upper Stage in December 1977.^[12] The Marshall Space Flight Center was designated the lead center for managing IUS work.^[13]

In April 1978 the quote for the development of the IUS was $263 million (equivalent to $965 million in 2023), but by December 1979 it was renegotiated for $430 million (equivalent to $1.46 billion in 2023).^[14] The main problem with the IUS was that it was not powerful enough to launch a payload to Jupiter without resorting to using a series of gravitational slingshot maneuvers around planets to garner additional speed, something most engineers regarded as inelegant, and which planetary scientists at NASA's Jet Propulsion Laboratory (JPL) disliked because it meant that the mission would take months or years longer to reach Jupiter.^[15][16] However, the IUS was constructed in a modular fashion, with two stages, a large one with 9,700 kilograms (21,400 lb) of propellant, and a smaller one with 2,700 kilograms (6,000 lb). This was sufficient for a most satellites. It could also be configured with two large stages to launch multiple satellites.^[12] A configuration with three stages, two large and one small, would be enough for a planetary mission, so NASA contracted for the development of a three-stage IUS.^[16]

Congress approved funding for the Jupiter Orbiter Probe on 12 July 1977.^[17] The following year the spacecraft was renamed Galileo after Galileo Galilei, the 17th-centry astronomer who had discovered the largest four of Jupiter's moons.^[18] What saved Galileo from cancellation was the intervention of the USAF. The JPL had considerable experience with autonomous spacecraft.^[19] This was a necessity for deep space probes, since a signal from Earth takes anything from 35 to 52 minutes to reach Jupiter.^[20] The USAF was interested in providing this capability for its satellites so that they would be able to determine their attitude using onboard systems rather than relying on ground stations, which were not "hardened" against nuclear attacks,^[21] and could take evasive action in the face of anti-satellite weapons.^[22] It was also interested in the manner in which the JPL was designing Galileo to withstand the intense radiation of the magnetosphere of Jupiter. On 6 February 1981 Strom Thurmond, the President pro tempore of the Senate, wrote directly to David Stockman, the Director of the Office of Management and Budget (OMB), arguing that Galileo was vital to the nation's defense.^[21] The Galileo project aimed for a launch window in January 1982, when the alignment of the planets was favorable to using Mars for a slingshot maneuver to reach Jupiter.^[23] It was also hoped that it would be able to make a flyby of asteroid 29 Amphitrite. It would be the fifth spacecraft to visit Jupiter, and the first to orbit it, and the probe would be the first to enter its atmosphere.^[24]

To enhance reliability and reduce costs, the Galileo project's engineers decided to switch from a pressurized atmospheric entry probe to a vented one. This added 100 kilograms (220 lb) to its weight. Another 165 kilograms (364 lb) was added in structural changes to improve reliability. This would require additional fuel in the IUS.^[25] But the three-stage IUS was itself overweight, by about 3,200 kilograms (7,000 lb).^[23] Lifting Galileo and the IUS would require the use of the special lightweight version of the Space Shuttle external tank, the Space Shuttle orbiter stripped of all non-essential equipment, and the Space Shuttle main engines (SSME) running at full power—109 percent of their rated power level.^[16] Running at this power level necessitated the development of a more elaborate engine cooling system.^[26] By late 1979, delays in the Space Shuttle program pushed the launch date for Galileo back to 1984. While a Mars slingshot was still possible in 1984, it would no longer be sufficient.^[27]

An alternative possibility considered was to launch Galileo using a Centaur upper stage atop a Titan IIIE, but this would have added at least $125 million (equivalent to $423 million in 2023) to the price of the $285 million (equivalent to $423 million in 2023) Galileo project because it would have required rebuilding the launch complex at Cape Canaveral.^[23] In retrospect, this would have been the best way forward, but this was not apparent in 1979,^[16] when there was a disdain for expendable launch vehicles. Moreover, Titan had been developed by, and was owned by, the USAF.^[28]

Another option was to use Centaur as a upper stage with the Space Shuttle. Shuttle-Centaur would require neither 109 percent power from the SSME, nor a slingshot maneuver to send the 2,000 kilograms (4,500 lb) to Jupiter.^[23] In 1979, NASA's Associate Administrator for Space Transportation Systems, John Yardley, directed the Lewis Research Center to determine the feasibility of integrating Centaur with the Space Shuttle. The engineers at Lewis concluded that it was both feasible and safe.^[29] A source inside NASA told The Washington Post journalist Thomas O'Toole that while it would cost money to modify Centaur so it could be carried on the Space Shuttle, "it would be worth it. The Centaur would give us such good rocket performance that we wouldn't even have to go in 1982, we could wait a year and still do the whole mission."^[23]

Although Galileo was the only American planetary mission scheduled, there was one other mission on the cards: the International Solar polar Mission. This was renamed Ulysses in 1984 after mythical hero who, according Dante, sought to explore "an uninhabited world behind the Sun".^[30] This was originally conceived in 1977 as a two-spacecraft mission, with NASA and the European Space Agency (ESA) each providing one, but the American spacecraft was cancelled in 1981. The ESA would therefore supply the sole spacecraft, with NASA's contribution being the power supply, launch vehicle, and tracking via the NASA Deep Space Network.^[31] The object of the mission was to gain an enhanced knowledge of the heliosphere by putting a satellite into a polar orbit around the Sun. Because Earth's orbit is inclined only 7.25 degrees to the Sun's equator, the solar poles cannot be observed from Earth.^[31] Scientists hoped to gain a greater understanding of the solar wind, the interplanetary magnetic field, cosmic rays and cosmic dust. It was never intended to make a close approach to the Sun; engineers joked that it would never be closer to the Sun than when it was sitting on the launch pad in Florida. In order to get into a solar polar orbit, the Ulysses probe would first have to travel out to Jupiter, and use a slingshot maneuver to leave the ecliptic plane. Thus, its initial destination was the same as that of Galileo.^[32]

NASA Administrator Robert A. Frosch stated in November 1979 that he was not favor of using Centaur, but Centaur found a champion in Congressman Edward P. Boland, who considered the IUS too underpowered for deep space missions, although he did not oppose its development for other purposes. He was impressed by Centaur's ability to put Galileo in Jupiter orbit with just two years' flight, and saw potential military applications for it as well. He chaired the House Intelligence Committee and the House Independent Agencies Appropriations Subcommittee of the House Appropriations Committee, and he got the Appropriations Committee to instruct NASA to use Centaur if weight problems with Galileo prompted a further postponement. Orders from a Congressional committee had no legal standing, so NASA was free to disregard it. Appearing before the Senate a few days later, Frosch was non-committal, saying only that NASA had the matter under consideration.^[33]

Instead, NASA decided to split Galileo into two separate spacecraft, an atmospheric probe and a Jupiter orbiter, with the orbiter launched in February 1984 and the probe following a month later. The orbiter would be in orbit around Jupiter when the probe arrived, allowing it to perform its role as a relay. Separating the two spacecraft was estimated to cost an additional $50 million (equivalent to $169 million in 2023),^[34] but NASA hoped to be able to recoup some of this through separate completive bidding on the two. The problem was that while the atmospheric probe was light enough to launch with the two-stage IUS, the Jupiter orbiter was too heavy to do so, even with a gravity assist from Mars, so the three-stage IUS was still required.^[27]

By late 1980, the price tag for the IUS had risen to $506 million (equivalent to $1.44 billion in 2023).^[12] The USAF could absorb this cost overrun (and indeed had anticipated that it might cost far more), but NASA was faced with a quote of $179 million (equivalent to $508 million in 2023) for the development of the three-stage version,^[16] which was $100 million (equivalent to $284 million in 2023) it had budgeted for.^[35] At a press conference on 15 January 1981, Frosch announced that NASA was withdrawing support for the three-stage IUS, and going with Centaur because "no other alternative upper stage is available on a reasonable schedule or with comparable costs."^[36]

Centaur provided two major advantages over the IUS. The main one was that it was far more powerful. The probe and orbiter could be recombined, and the probe could be delivered directly to Jupiter in two years' flight time.^[16][15] This was not merely a matter of impatience; longer travel times meant that components would age and the onboard power supply and propellant would be depleted. Some of the gravity assist options also meant flying closer to the Sun, which would induce thermal stresses.^[37] While more powerful, Centaur was also more gentle than the IUS. It generated its thrust more slowly, thereby minimizing the chance of damage to the payload. And unlike solid-fuel rockets which burned to completion once ignited, Centaur could be switched off and on again. This gave it flexibility, which increased the chances of a successful mission. The only concern was about safety; solid-fuel rockets were considered far safer than liquid-fuel ones, especially ones containing liquid hydrogen.^[16][15] NASA engineers estimated that additional safety features might take up to five years to develop and cost up to $100 million (equivalent to $284 million in 2023).^[35][34]

The decision to go with Centaur pleased the planetary scientists, and was welcomed by the communications industry, because it meant that larger satellites could be placed into geostationary orbits, where the Shuttle and IUS were limited to 3,000-kilogram (6,600 lb) payloads. NASA Headquarters liked it as an answer to the ESA's Ariane rocket family; by 1986, new versions of the Ariane under development were expected to be able to be able to lift payloads heavier than 3,000 kg into geostationary orbits, cutting NASA out of a lucrative segment of the satellite launch business. And the USAF, while disappointed with NASA's decision to drop the three-stage IUS, foresaw a need for USAF satellites to carry more propellant than hitherto in order to engage in avoidance maneuvers against anti-satellite weapons.^[22]

Two groups in particular were unhappy with the decision: Boeing and the Marshall Space Flight Center.^[38] Other aerospace companies were disappointed that NASA had decided to adapt the existing Centaur upper stage rather than develop a new high energy upper stage (HEUS) or the orbital transfer vehicle (OTV), as the space tug was now called. The OMB was not opposed to Centaur on any technical grounds, but it was a discretionary expense, and in the budget-cutting atmosphere of 1981, one that the OMB felt could be dropped for the fiscal year 1983 budget, which was submitted on Congress in February 1982. Gallileo was reconfigured for a 1985 launch using the two-stage IUS, which would take four years to get to Jupiter, and reduce the number of moons visited by half when it got there.^[39]

Senator Harrison Schmitt, the Chairman of the Senate Subcommittee on Science, Technology, and Space,^[22] and a former astronaut who walked on the Moon on Apollo 17,^[40] was vociferously opposed to the OMB decision. The House and Senate Appropriations Committees were also opposed, but Congressman Ronnie G. Flippo, whose electorate in Alabama encompassed the Marshall Space Flight Center, was the Chairman of the House Subcommittee on Science, Technology and Space, and he supported OMB decision. In July 1982, the proponents of Centaur added $140 million (equivalent to $374 million in 2023) to the Emergency Supplemental Appropriations Act, which was signed into law by President Ronald Reagan on 18 July 1982. In addition to the funding, it directed NASA and Boeing to cease work on the two stage IUS for Galileo.^[22]

Flippo fought this decision. He argued that Centaur was too expensive, as it cost $140 million in the current year with the whole Shuttle-Centaur project was estimated to cost around $634 million (equivalent to $1.69 billion in 2023); that it was of limited use, since it was only required for two deep space missions; and that it was a prime example of faulty procurement, because an important contract was being given to General Dynamics without any form of tender process (a common practice in the USAF). He enlisted the support of Congressman Don Fuqua, the Chairman of the House Committee on Science, Space, and Technology. Centaur was staunchly defended by Congressman Bill Lowery, whose San Diego electorate included General Dynamics.^[39]

On 15 September, Flippo moved an amendment to the 1983 NASA appropriations bill that would have forbidden further work on Centaur, but his position was undermined by Edward C. Aldridge Jr.,^[41] the Under Secretary of the Air Force (and Director of the National Reconnaissance Office),^[42], and NASA Administrator James M. Beggs who contended that contamination observed during early Space Shuttle flights, meant that more shielding would be required for classified Defense satellites, which would add more weight, and therefore require the power of Centaur. Aldridge and Beggs announced that they would soon conclude an agreement for the joint development of Shuttle-Centaur. Flippo's amendment was defeated by a vote of 316 to 77, clearing the way for the Shuttle-Centaur project.^[41]

The IUS made its first flight atop a Titan 34D in October 1982, when it placed two military satellites in geosynchronous orbit.^[14] It was then used on a Space Shuttle mission, STS-6 in April 1983, to deploy the first Tracking and Data Relay Satellite (TDRS-1),^[43] but the IUS's nozzle changed its position by one degree, resulting in the satellite being placed in the wrong orbit. It took two years to determine what had gone wrong, and how to prevent it happening again.^[44]

A third mission for Shuttle-Centaur appeared in the form of the Venus Radar Mapper, which was later renamed Magellan. The first mission integration panel meeting for this probe was held at the Lewis Research Center on 8 November 1983. Various Space Shuttle upper stages were considered, including the Orbital Sciences Corporation Transfer Orbit Stage (TOS), the Astrotech Corporation Delta Transfer Stage, and the Boeing IUS, but Centaur was chosen as the best option. Magellan was tentatively scheduled for launch in April 1988.^[45] The USAF adopted Shuttle-Centaur in 1984 for the launch of its Milstar satellites. These military communications satellites were hardened against interception, jamming and nuclear attack. Having the USAF on board had saved the project from cancellation, but dealing with them was not always easy. Telephone conversations with General Dynamics regarding the project had to be conducted over secure phone lines. The USAF also asked for design changes and performance enhancements. One such change was to allow the Milstar to have a direct connection with Centaur that would be separated using explosive bolts. This required testing to determine what the effects of this would be.^[45]

On 30 August 1982, a meeting of representatives of the NASA centers and Centaur contractors was held at General Dynamics in San Diego to discuss the requirements of the project. From this arose two new versions of Centaur: Centaur G and Centaur G Prime. The principal constraint was that they had to fit inside the Space Shuttle's 18-meter (60 ft) cargo bay. This restricted the width to 4.6 meters (15 ft). Centaur G was intended for USAF missions, specifically to place satellites into geostationary orbits, and the $269 million (equivalent to $719 million in 2023) to design and develop it were split 50-50 with the USAF. It was 6.1 meters (20 ft) long, allowing for large USAF payloads up to 12.2 meters (40 ft) long. Its dry weight was 3,060 kilograms (6,750 lb), and it weighed 16,928 kilograms (37,319 lb) fully loaded. Centaur G Prime was intended for deep space missions, and was 9.0 meters (29.5 ft) long, allowing it to carry more propellant, but restricting the length of the payload to 2.8 meters (9.3 ft). The dry weight of the Centaur G Prime was 2,761 kilograms (6,088 lb), and it weighed 22,800 kilograms (50,270 lb) fully loaded.

The two versions were very similar, with 80 percent of their components being the same. The Centaur G Prime stage had two RL10-3-3A engines, each with 73,400 newtons (16,500 lb_f) thrust, and a specific impulse of 446.4 seconds, with a 5:1 fuel ratio. The Centaur G stage had two RL10-3-3B engines, each with 66,700 newtons (15,000 lb_f) thrust, and specific impulse of 440.4 seconds, with a 6:1 fuel ratio. The engines were capable of multiple restarts after long periods of coasting in space, and had a hydraulic gimbal actuation system powered by the turbopump.^[46][47][48]

The Centaur G and G Prime avionics were the same as that the standard Centaur, and were still mounted in the forward equipment module. It used a 24-bit Teledyne Digital Computer Unit with 16 kilobytes of RAM to control guidance and navigation. It still used the same pressurized steel tank, but with some additional insulation including a two-layer foam blanket over the forward bulkhead and a three-layer radiation shield.^[46] Other changes included new forward and aft adapters; a new propellant fill, drain, and dump system; and an S band transmitter and RF system compatible with the tracking and data relay satellite system.^[49] Considerable effort was put into making the Centaur safe, with redundant components to overcome malfunctions, and a propellant draining, dumping and venting system so that the propellants could be dumped in case of emergency.^[50]

Both versions were cradled in the Centaur integrated support system (CISS), a 4.6 meters (15 ft) aluminum structure that handled communications between the Space Shuttle and the Centaur upper stage. It helped keep the number of modifications to the Space Shuttle to a minimum. When the cargo doors opened, the CISS would pivot 45 degrees into a ready position to launch Centaur. After twenty minutes, the Centaur would be launched by a set of twelve coil springs with a 10-centimeter (4 in) stroke known as the Super*Zip separation ring. The Centaur upper stage would then coast at a speed of 0.30 meters per second (1 ft/s) for 45 minutes before starting its main burn a safe distance from the Space Shuttle. For most missions, only a single burn was required. Once the burn was complete, the spacecraft would then separate from the Centaur upper stage, which could still maneuver to avoid striking the spacecraft, or the planet below.^[50][51]

All electrical connections between the Orbiter and the Centaur were routed through the CISS. Electrical power for the Centaur was provided by a 150-ampere-hour (540,000 C) silver zinc battery. Power for the CISS was provided by two 375-ampere-hour (1,350,000 C) batteries. Since the CISS was also plugged into the Orbiter, this provided two-failure redundancy.^[52] The Centaur G CISS weighed 2,947 kilograms (6,497 lb), and the Centaur G Prime CISS 2,961 kilograms (6,528 lb).^[48] The CISS was fully reusable for ten flights, and would be returned to Earth. The Space Shuttle Challenger and Space Shuttle Atlantis were modified to carry the CISS.^[50][49]

By June 1981, the Lewis Research Center had awarded four contracts for Centaur G Prime worth a total of $7,483,000 (equivalent to $20 million in 2023): General Dynamics was to develop the Centaur rockets; Teledyne, the computer and multiplexer s; Honeywell, the guidance and navigation systems; and Pratt & Whitney, the four RL10A-3-3A engines.^[53]

Christopher C. Kraft Jr., William R. Lucas, and Richard G. Smith, the directors of the Johnson Space Center, Marshall Space Flight Center and Kennedy Space Center respectively, did not like NASA Headquarters' decision to assign Shuttle-Centaur to the Lewis Research Center. In a letter to Alan M. Lovelace, the acting Administrator of NASA, in January 1981, they argued that management of the Shuttle-Centaur project should instead be assigned to the Marshall Space Flight Center, which they claimed had experience with cryogenic propellants, and more experience with the Space Shuttle, which they regarded as a complex system that only their three centers understood.^[54]

Engineers at the Lewis Research Center saw matters differently. The director of the Lewis Research Center, John F. McCarthy wrote to Lovelace in March, providing reasons why the Lewis Research Center was the best choice: it had led the project to evaluate the feasibility of mating the Space Shuttle with Centaur; its experience with Centaur was the greatest of all the NASA centers; it had managed the successful Titan-Centaur project; had experience with space probes with the Surveyor, Viking and Voyager projects; and it boasted a highly-skilled workforce where the average engineer had thirteen years of experience. In May 1981, Lovelace informed Lucas of his decision to have the Lewis Research Center manage the project.^[54] In November 1982, the Lewis Research Center signed a Memorandum of Agreement with the JPL on the Galileo project; JPL was responsible for the design and management of the mission, while the Lewis research Center had "all responsibilities necessary to integrate the Galileo spacecraft with Centaur and the Space Transportation System."^[55]

Morale at the Lewis Research Center had been low in the 1970s and early 1980s. The cancellation of the NERVA nuclear rocket engine caused a round of layoffs, and many of the older hands had elected to retire.^[56] Between 1971 and 1981, staff numbers fell from 4,200 to 2,690. In 1982, the staff became aware that the Reagan administration was considering closing the center, and they mounted a vigorous campaign to save it.^[57] McCarthy retired in July 1982, and Andrew J. Stofan became the director of the Lewis Research Center. He was an associate administrator at NASA Headquarters, whose involvement with Centaur dated back to 1962, and who had headed the Atlas-Centaur and Titan-Centaur Offices in the 1970s.^[58][59] Under Stofan, the Lewis Research Center budget steadily increased, from $133 million (equivalent to $342 million in 2023) to $159 million (equivalent to $395 million in 2023) and $188 million (equivalent to $452 million in 2023) in 1985. This permitted an increase in staff for the first time in 20 years, with 190 new engineers hired in 1983. ^[53]

William H. "Red" Robbins was appointed the head of the Shuttle-Center Project Office at the Lewis Research Center. Although most of his experience was with NERVA and this was his first experience with Centaur, he was an experienced project manager. He handled the project's administration and financial arrangements.^[60] Vernon Weyers was his deputy. Major William Files became the USAF deputy project manager. He brought with him six USAF officers who assumed key roles.^[61] Marty Winkler headed the Shuttle-Centaur program at General Dynamics.^[62] Steven V. Szabo, an old hand on Centaur who had worked on it since 1963, was head of the Lewis Research Center's Space Transportation Engineering Division. He was responsible for the technical side of the activities related to the integration of the Space Shuttle and Centaur, which included the propulsion, pressurization, structural, electrical, guidance, control and telemetry systems. Within the Shuttle-Centaur Project Office, Edwin Muckley was in charge of the Mission Integration Office, which was responsible for the payloads. Frank Spurlock managed trajectory mission design, and Joe Nieberding took charge of the Shuttle-Centaur group within the Space Transportation Engineering Division. Spurlock and Nieberding hired many young engineers, giving the Shuttle-Centaur project a mixture of youth and experience.^[60]

The Shuttle-Centaur Project had to be ready to launch in May 1986, which was just three years away. The cost of a delay was estimated at $50 million (equivalent to $129 million in 2023).^[62] Failure to meet the deadline meant waiting another year until the planets were properly aligned again.^[63] The project adopted a mission logo depicting a mythical centaur emerging from the Space Shuttle and firing an arrow into at the stasrs.^[62] Larry Ross, the Director of Space Flight Systems at the Lewis Research Center,^[64] had the logo emblazoned on project stationery and memorabilia link drink coasters and campaign buttons. A special Shuttle-Centaur project calendar was produced, with 28 months on it from January 1984 to April 1986. The cover sported the logo, with the project motto, co-opted from the movie Rocky: "Go for it!"^[62]

When it came to integrating Centaur with the Space Shuttle, there were two possible approaches: as an element or a payload. Elements were components of the Space Shuttle like the external tank and the sold rocket boosters; whereas a payload was something being carried into space like a satellite. The 1981 Memorandum of Agreement between the Johnson Space Center and the Lewis Research Center defined the Centaur as an element. At first, the engineers at the Lewis Research Center preferred to have it declared a payload, because time was short and this minimized the amount of interference in their work by Johnson. Centaur was therefore declared to be a payload in 1983. Payload status was originally conceived as being for inert pieces of cargo. The Johnson Space Center added additional requirements for Centaur. Complying with the requirements of this status resulted in a series of safety waivers. Both centers wanted to make the Centaur as safe as possible, but differed over trade offs were acceptable.^[65]

Two missions were scheduled: STS-61-F for Ulysses on 15 May 1986, and STS-61-G for Galileo on 20 May. Crews were assigned in May 1985. STS-61-F would be commanded by Frederick "Rick" Hauck, with Roy D. Bridges Jr. as the pilot, and mission specialists John M. Lounge and David C. Hilmers. STS-61-G would be commanded by David M. Walker, with James "Ox" Van Hoften and John M. Fabian as mission specialists.^[66][67] In September, Norman Thagard replaced Fabian.^[68] The four-person crews would be the smallest since STS-6 in April 1983. The missions would fly into a very low orbit, just 170 kilometers (110 mi), which was the best that the Space Shuttle could do with a fully fueled Centaur on board.^[69] In addition to being the STS-61-F commander, Rick Hauck was also the Shuttle-Centaur project officers at the Astronaut Office. Hauck and Walker attended key Shuttle-Centaur meetings, which was unusual for astronauts.^[70]

The main safety issue involved what would happen in the case of an aborted mission. If the Space Shuttle had to return to Earth with Centaur still on board, the center of gravity of the Shuttle would be further aft than on any previous mission. Centaur would periodically vent boiling hydrogen to maintain the proper pressure inside the Centaur. Deployment of the probes would occur just seven hours after launch. In an emergency, all the propellant could be drained in 250 seconds through valves on both sides of the Space Shuttle's fuselage, but their proximity to the main engines and the Orbital Maneuvering System was a concern for the astronauts, who feared fuel leaks and explosions.^[70][69]

At the Kennedy Space Center, preparations were made to launch the two missions. The two launches would only have a one-hour launch window, and there would be just six days between them. Because of this, two launch pads would be used: Launch Complex 39A for STS-61-G and Atlantis, and Launch Complex 39B for STS-61-F and Challenger. Launch Complex 39B had only recently been refurbished to handle the Space Shuttle. The first Centaur G Prime, SC-1, was rolled out from the General Dynamics factory in Kearny Mesa, San Diego, to a fanfare not seen since the days of Project Apollo. The theme music from Star Wars was played, a crowd of 300, mostly General Dynamics employees, was in attendance, and speeches were given by dignitaries.^[71][69][72]

SC-1 was then flown to the Kennedy Space Center, where it was mated with CISS-1, which had arrived two months before. SC-2 and CISS-2 followed in November. The USAF made its Shuttle Payload Integration Facility at the Cape Canaveral Air Force Station available in November and December so SC-1 and SC-2 could be processed at the same time. A problem was detected with the propellant level indicator in the oxygen tank in SC-1, which was promptly redesigned, fabricated, and installed. There was also a problem with the drain valves, which was found and corrected. Shuttle-Centaur was certified as flight ready by NASA Associate Administrator Jesse Moore.^[72]

The Johnson Space Center committed to lifting 29,000 kilograms (65,000 lb) but the engineers at Lewis Research Center were aware that the Space Shuttle was unlikely to be able to lift that amount. To compensate, the Lewis Research Center reduced the amount of propellant in the Centaur. This reduced the number of possible launch days to just six. Concerned that this was too few, Nieberding lobbied Moore to allow the engines to be run at 109 percent. Moore approved the request on the spot.^[73]

The astronauts were concerned about running the engines at 109 percent, and about what would happen in the event of an abort, a failure of the Space Shuttle main engines to put them into orbit. In that case, they would have to dump the Centaur's propellant and land. This was an extremely dangerous maneuver under any circumstance, and one that would in fact would never occur. Hauck and the Chief of the Astronaut Office, John Young, took their concerns to the Johnson Space Center Configuration Control Board, which ruled that the risk acceptable.^[74][75] Engineers at the Lewis Research Center, the JPL and General Dynamics dismissed the astronauts' concerns about liquid hydrogen, pointing out that the Space Shuttle was itself propelled by liquid hydrogen, and that at liftoff they had 25 times the Centaur's fuel in the Space Shuttle's external tank.^[76]

On 28 January 1986, Challenger lifted off on STS-51-L. 73 seconds into flight, a failure of the solid rocket booster created an explosion that tore apart the Space Shuttle and killed all seven crew members.^[77] The Space Shuttle Challenger disaster was America's worst space disaster up to that time.^[75]

On 20 February, Moore ordered the Galileo and Ulyssess missions postponed. Too many key personnel were involved in the analysis of the accident for the missions to proceed. The earliest they could be flown was in thirteen months. Engineers continued to performs tests, and the Galileo probe was moved to the Vertical Processing Facility at the Kennedy Space Center, where it was mated with the Centaur.^[78] Of the four safety reviews required of the Shuttle-Centaur missions, three had been completed, although some issues aring from the last two remained to be resolved. The final review was originally scheduled for late January. Some additional safety changes had been incorporated into the Centaur Gs being built for the USAF, but had not made it to SC-1 and SC-2 owing to the strict deadline. After the disaster, $75 million (equivalent to $177 million in 2023) was earmarked for Centaur safety improvements.^[63]

Although completely unrelated to the accident, Challenger had broken up immediately after throttling to 104 percent power. This contributed to an increased perception at the Johnson and Marshall Space Flight Centers that it was too risky to go to 109 percent. At the same time, the engineers at Lewis were aware that safety improvement to the Space Shuttle were likely, and that this could only add more weight. Without 109 percent power, it seemed unlikely that the Shuttle could lift Centaur.^[78]

In May a series of meetings was held with aerospace industry engineers at the Lewis Research Center in which the safety issues around Centaur were discussed, and it was concluded that Centaur was safe. At a meeting at NASA Headquarters on 22 May, though, Hauck argued that Centaur posed an unacceptable degree of risk. A review by the House Appropriations Committee chaired by Boland recommended that Shuttle-Centaur be canceled. On 19 June NASA Administrator James C. Fletcher terminated the project.^[79][80] Stop work orders went out to the contractors. Most work was completed by 30 September, with all work done by the end of the year.^[81] About $700 million (equivalent to $1.65 billion in 2023) had been spent on Shuttle-Centaur.^[82]

Galileo was not launched until 17 October 1989, using the IUS.^[83] It took six years to reach Jupiter instead of just two, as it had to fly by Venus and Earth twice to garner enough speed to reach Jupiter.^[84] When the JPL tried to use its high gain antenna, it was found to have been damaged, most likely from being shipped from the JPL in California to the Kennedy Space Center in Florida in 1985, then back again in 1986, and finally back to the Kennedy Space Center again in 1989.^[85] Ulysses had to wait even longer; it was launched using the IUS on 6 October 1990.^[31] The USAF mated the Centaur G Prime upper stage with the Titan booster to produce Titan IV, which made its first flight in 1994.^[86] Over the next 18 years, Titan IV with Centaur G Prime would place eighteen military satellites in orbit.^[87] In 1997 NASA used it to launch the Cassini–Huygens probe to Saturn.^[86]

A Centaur G Prime was on display at the U.S. Space & Rocket Center in Huntsville, Alabama, for many years. In 2016, the center wanted to move it to make way for a redesigned outdoor display, and it was transferred to NASA's Glenn Research Center. It was officially placed on outdoor display on 6 May 2016 after a ceremony attended by forty retired NASA and contractor staff who had worked on the rocket thirty years before, and by officials including Glenn Director Janet Kavandi and former Glenn Director Lawrence J. Ross.^[87][88]