Shuttle-Centaur was a version of the Centaurupper stage rocket designed to be carried aloft inside the Space Shuttle and used to launch satellites into high Earth orbits or probes into deep space. Two variants were developed: Centaur G-Prime, which was planned to launch the Galileo and Ulysses robotic probes to Jupiter, and Centaur G, a shortened version planned for use with United States Department of DefenseMilstar satellites and the MagellanVenus probe. The powerful Centaur upper stage allowed for heavier deep space probes, and for them to reach Jupiter sooner, prolonging the operational life of the spacecraft. However, neither variant ever flew on a Shuttle. Support for the project 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 design and development costs of Centaur G, and the National Aeronautics and Space Administration (NASA) paid the other half.

Proposed Space Shuttle upper stage

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 66.70 kN (14,990 lbf) (per engine)
Specific impulse 440.4 s
Propellant Liquid hydrogen / LOX

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. All Centaur rockets periodically vented hydrogen, which needs to be stored below −253 °C (−423 °F) to keep it from boiling. Two Shuttle-Centaur missions were scheduled, with one-hour launch windows six days apart, so two separate spacecraft and launch pads were required. The Space Shuttles Challenger and Atlantis were modified to carry the CISS. The Space Shuttle main engines would have been run at 109 percent of the original design thrust. The payloads needed to be deployed on the first day in orbit, so the missions would be flown by four-person crews composed of astronauts who had already flown in space and were known to not suffer from space adaptation syndrome. The first Centaur G-Prime was rolled out from the General Dynamics factory in Kearny Mesa, San Diego, on 13 August 1985.

Just months before the Shuttle-Centaur was scheduled to fly, the Challenger disaster occurred, and the project was canceled. The Galileo and Ulysses probes were ultimately launched using the much less powerful solid-fueledInertial Upper Stage (IUS), 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 its 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 placed eighteen military satellites in orbit.

. . . Shuttle-Centaur . . .

Centaur was an upper stage rocket that used liquid hydrogen as fuel and liquid oxygen as an oxidizer. It was developed by General Dynamics in the late 1950s and early 1960s and powered by twin Pratt & WhitneyRL10 engines.[1][2] Rockets utilizing liquid hydrogen as fuel theoretically can lift 40 percent more payload per kilogram of liftoff weight than rockets burning kerosene, but the challenges of using liquid hydrogen required new technology to be developed. Liquid hydrogen is a cryogenic fuel, meaning that it condenses at extremely low temperatures, and must be stored below −253 °C (−423 °F) to keep it from boiling. Thus, insulation from all sources of heat, including the rocket exhaust, the relatively warm liquid oxygen, aerodynamic heating, and the radiant heat of the Sun, was required.[3]

A Centaur rocket during assembly at General Dynamics in 1962. Its development pioneered the use of liquid hydrogen as a rocket fuel.

Fuel could be lost through microscopic holes that only hydrogen could leak through, but sealing the fuel tank created another problem.[4] Even when insulated, heat leaks could cause the temperature to rise and boil the hydrogen; pressure in the tank can then build up and rupture it unless proper venting is provided, but too much venting will cause the loss of excessive amounts of fuel.[5] These challenges dogged the development of Centaur with technical difficulties, such as fuel leaking through the welds, and the shrinking of the metal bulkhead when coming into sudden contact with the cryogenic temperatures of liquid hydrogen.[6] Further complicating matters was the explosion of an RL10 on an engine test stand during a demonstration for United States Air Force (USAF) and National Air and Space Administration (NASA) officials.[6]

The project’s management was transferred from NASA’s Marshall Space Flight Center in Huntsville, Alabama, to its Lewis Research Center in Ohio in October 1962, and Abe Silverstein, a strong advocate of liquid hydrogen, took charge.[7] He insisted on a thorough testing regime, which both identified problems and suggested solutions to them.[8] The technical problems of the Centaur project were gradually overcome. The design notably included the weight-saving features pioneered by the Atlas rocket family: a monocoque steel shell that held its shape only when pressurized, hydrogen and oxygen tanks separated by a common bulkhead, and no internal bracing or insulation surrounding the propellant tanks.[9] The technology for handling liquid hydrogen in Centaur was also used the S-II and S-IVB upper stages of the Saturn V rocket, and later by the Space Shuttle external tank and Space Shuttle main engines (SSME).[7] Throughout the 1960s and 1970s, Centaur was used as the upper stage of Atlas-Centaur launch vehicles, which helped launch seven Surveyor missions,[2] five Mariner missions, and the Pioneer 10 and 11 probes.[10] In the 1970s, Centaur was also placed atop the USAF’s Titan III booster to create the Titan IIIE launch vehicle, which was used to launch the Viking, Helios, and Voyager missions.[11] By 1980, Centaur upper stages had flown 55 times, failing only twice.[12]

. . . Shuttle-Centaur . . .

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. . . Shuttle-Centaur . . .