Shuttle-Centaur
Shuttle-Centaur was a version of the Centaur upper 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 Defense Milstar satellites and the Magellan Venus 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 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 paid the other half.
Both versions were cradled in the reusable Centaur integrated support system, an aluminum structure that handled communications between the Space Shuttle and the Centaur. All Centaur stages periodically vented hydrogen, which needs to be stored below 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 and were modified to carry the CISS. The Space Shuttle main engines would have been run at 109 percent of the rated power level. 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-fueled Inertial Upper Stage, 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.
Background
Centaur
is 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 & Whitney RL10 engines. 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 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.Fuel could be lost through microscopic holes that only hydrogen could leak through, but sealing the fuel tank created another problem. 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. 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. Further complicating matters was the explosion of an RL10 on an engine test stand during a demonstration for United States Air Force and National Air and Space Administration officials.
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. He insisted on a thorough testing regime, which both identified problems and suggested solutions to them. 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. 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.
Throughout the 1960s and 1970s, Centaur was used as the upper stage of Atlas-Centaur launch vehicles, which helped launch seven Surveyor missions, five Mariner missions, and the Pioneer 10 and 11 probes. 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. By 1980, Centaur upper stages had flown 55 times, failing only twice.
Space Shuttle upper stages
The 1972 decision to develop the Space Shuttle augured badly for the projects to explore the Solar System with robotic probes, which were coming under intense scrutiny by an increasingly cost-conscious Nixon administration and United States Congress. The Space Shuttle was never intended to operate beyond low Earth orbit, but many satellites needed to be higher, particularly communications satellites, for which geostationary orbits were preferred. The Space Shuttle concept originally 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 led to the decision to terminate Saturn V production in 1970 and the abandoning of plans to build a space station. 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.With funding tight, NASA sought to offload Space Shuttle-related projects onto other organizations. NASA Deputy Administrator George Low met with Malcolm R. Currie, the Director of Defense Research and Engineering, in September 1973, and reached an informal agreement that the USAF would develop an interim upper stage for the Space Shuttle, to be used for launching satellites in higher orbits pending the development of the space tug. After some debate, Pentagon officials agreed to commit to the IUS on 11 July 1974. The Secretary of Defense, James R. Schlesinger, confirmed the decision when he met with NASA Administrator James C. Fletcher and Low four days later. 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 IUS was renamed the Inertial Upper Stage in December 1977. The Marshall Space Flight Center was designated the lead center for managing IUS work.
In April 1978, the quote for the development of the IUS was $263 million, but by December 1979 it was renegotiated for $430 million. The main drawback of the IUS was that it was not powerful enough to launch a payload to Jupiter without resorting to gravitational slingshot maneuvers around other planets to garner more speed, something most engineers regarded as inelegant, and which planetary scientists at NASA's Jet Propulsion Laboratory disliked because it meant that the mission would take months or years longer to reach Jupiter. The IUS was constructed in a modular fashion, with two stages: a large one with of propellant and a smaller one with, which was sufficient for most satellites. It could also be configured with two large stages to launch multiple satellites. The USAF asked NASA to develop a configuration with three stages, two large and one small, that could be used for a planetary mission like Galileo. NASA contracted with Boeing for its development.
Deep space probes
Congress approved funding for the Jupiter Orbiter Probe on 12 July 1977. The following year the spacecraft was renamed Galileo after Galileo Galilei, the 17th-century astronomer who had discovered the largest four of Jupiter's moons, now known as the Galilean moons. During the early 1980s, Galileo struggled with both technical and funding difficulties, and the Office of Management and Budget targeted NASA for budget cuts. The intervention of the USAF saved Galileo from cancellation. It was interested in the development of autonomous spacecraft like Galileo that could take evasive action in the face of anti-satellite weapons, and in the manner in which the JPL was designing Galileo to withstand the intense radiation of the magnetosphere of Jupiter, which had application in surviving nearby nuclear detonations. The Galileo project aimed for a launch window in January 1982 when the alignment of the planets would be favorable to using Mars for a slingshot maneuver to reach Jupiter. Galileo would be the fifth spacecraft to visit Jupiter, and the first to orbit it, while the probe it carried would be the first to enter its atmosphere. In December 1984, Galileo project manager John R. Casani proposed that Galileo make a flyby of asteroid 29 Amphitrite while en route. It would be the first time a US space mission visited an asteroid. NASA Administrator James M. Beggs endorsed the proposal as a secondary objective for Galileo.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 to its weight, and another was added in structural changes to improve reliability, all of which would require extra fuel in the IUS. But the three-stage IUS was itself overweight, by about against its design specifications. 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 SSME running at full power—109 percent of their rated power level. This necessitated the development of a more elaborate engine cooling system. By late 1979, delays in the Space Shuttle program pushed the launch date for Galileo back to 1984, when the planets would no longer be aligned so that a Mars slingshot would be sufficient to reach Jupiter.
An alternative to the IUS was to use Centaur as an upper stage with the Space Shuttle. Shuttle-Centaur would require neither 109 percent power from the SSME, nor a slingshot maneuver to send the to Jupiter. 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. A source inside NASA told The Washington Post journalist Thomas O'Toole that the cost of modifying Centaur so it could be carried on the Space Shuttle would be worth it, as the performance benefit of Centaur would mean that Galileo was no longer tied to a 1982 launch window.
A third possibility considered was to launch Galileo using a Centaur upper stage atop a Titan IIIE, but this would have required rebuilding the launch complex at Cape Canaveral, which would have added at least $125 million to the cost of the $285 million Galileo project. Beggs insisted that expendable launch vehicles were obsolete and that any money spent on them would only undermine the Space Shuttle's cost-effectiveness. Moreover, Titan had been developed by and was owned and controlled by, the USAF, and its use would mean that NASA would have to work closely with the USAF, something that NASA management hoped to avoid as much as possible. While NASA and the USAF collaborated and depended on each other to some extent, they were also rivals, and NASA resisted attempts by United States Department of Defense to manage the space program. On 13 November 1981, President Ronald Reagan issued National Security Decision Directive Number 8, which directed that the Space Shuttle would be the primary launch system for all military and civil government missions, but Edward C. Aldridge Jr., the Under Secretary of the Air Force doubted that NASA could meet its target of twenty-four Space Shuttle launches a year; he thought that twelve was more likely, and given that only the newest two orbiters, and could lift his largest payloads, there might not be enough Space Shuttle flights. Reagan was persuaded to revise his policy to permit a mixed fleet of ELVs and Space Shuttles, and the USAF ordered ten Titan IV rockets in 1984. NASA historian T. A. Heppenheimer noted that in retrospect, "it was a mistake not to go with the Titan IIIE-Centaur", given the delays and higher costs ultimately involved in using the Shuttle, but this was not apparent in 1984.
Although Galileo was the only American planetary mission scheduled, there was another mission in preparation: the International Solar Polar Mission, which was renamed Ulysses in 1984. It was originally conceived in 1977 as a two-spacecraft mission, NASA and the European Space Agency each providing one spacecraft, but the American one was canceled in 1981, and NASA's contribution was limited to the power supply, launch vehicle, and tracking via the NASA Deep Space Network. 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. Scientists hoped to gain a greater understanding of the solar wind, the interplanetary magnetic field, cosmic rays and cosmic dust. The Ulysses probe had the same initial destination as Galileo, as it would first have to travel out to Jupiter and then use a slingshot maneuver to leave the ecliptic plane and enter a solar polar orbit.
Another mission for Shuttle-Centaur subsequently appeared in the form of the Venus Radar Mapper, later renamed Magellan. The first mission integration panel meeting for this probe was held at the Lewis Research Center on 8 November 1983. Several Space Shuttle upper stages were considered, including the Orbital Sciences Corporation Transfer Orbit Stage, the Astrotech Corporation Delta Transfer Stage, and the Boeing IUS, but the meeting chose Centaur as the best option. Magellan was tentatively scheduled for launch in April 1988. 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. Telephone conversations with General Dynamics regarding the project had to be conducted over secure lines. Having the USAF on board had saved the project from cancellation, but the USAF 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, which required further testing to ascertain the effect of the resulting shock.