Shuttle-Centaur canceled.

Shuttle-Centaur canceled

Even as NASA grapples with the task of getting the space shuttle flying again, safety concerns have prompted the agency to cancel plans for launching shuttle-deployed spacecraft with a powerful upper-stage rocket that had been developed specifically for the purpose -- and on which $674 million had already been spent. Called the Centaur, the rocket uses the same volatile liquid-hydrogen and liquid-oxygen propellants that fuel the shuttle's own main engines, and which destroyed the shuttlecraft Challenger when they exploded on Jan. 28.

The shuttle version of the Centaur is a variation on one that has been in use for a decade atop unmanned, expendable rockets for such tasks as sending the Viking spacecraft to Mars. Its first uses from the shuttle were to have been in May, when Centaurs were to have launced the Galileo orbiter-and-probe mission to Jupiter and the European Space Agency's Ulysses mission on a course that would swing around Jupiter on the way to a flight over the poles of the sun. Other shuttle-lofted Centaurs were planned for a U.S. Venus-radar-mapping spacecraft called Galileo, now under construction, as well as for several payloads for the Department of Defense.

Banning the Centaur from the shuttle has not caused the cancellation of any of these missions, but it requires adopting other ways to launch them. Even before the shuttle explosion, the Air Force was layin plans for a backup system, ordering a variation of its own unmanned Titan 34D7 rocket that could carry the Centaur as an upper-stage. Ten of them are already on order, and the Air Force hopes to get more. As for NASA, the agency is now studying a variety of options, including the use of Titan rockets with the Centaur, as well as of the shuttle itself but with a different upper-stage that is fueled by solid propellants instead of the touchy hydrogen/oxygen mixture. Possibilities under study include modifications of the Air force's existing Inertial Upper Stage booster, as well as a commercial booster called the Transfer Orbit Stage, being developed by Orbital Sciences Corp. in Vienna, Va.

A number of people had voiced safety concerns about the shuttle-Centaur even before the rocket got the development go-ahead in 1981, but NASA concluded at the time that the safety margin was adequate. The issue rose again after the shuttle explosion, however, prompting investigation in an independent study by the surveys and investigation staff of the House subcomittee that deals with NASA appropriations. Among the safety issues raised was the possibility that if a space shuttle carrying a Centaur had to abort its mission and make an emergency landing, it would be necessary first to dump the Centaur propellants overboard -- through valves that had shown reliability problems in recent testing.

"Although the shuttle-Centaur decision was ver difficult to make," says NASA Administrator James C. Fletcher, "it is the proper thing to do and this is the time to do it."