Malfunction Delays Shuttle Another Week

KENNEDY SPACE CENTER, Fla. —  Space shuttle Endeavour’s science mission, which already had been delayed by O-ring repairs, was put off for another week Thursday when an electricity-generating unit overheated hours before liftoff.

NASA said the problem was probably caused by a stuck valve in one of the shuttle’s three fuel cells, which burn liquid hydrogen and oxygen to generate electricity used by the spacecraft throughout a flight.

The 255-pound cells are so critical that failure of even one in orbit would force a shuttle to return to Earth immediately.

Endeavour had not yet been fueled and the five astronauts had not yet boarded when the launch was scrubbed with 7 1/2 hours to go. Thunderstorms probably would have delayed liftoff even if everything had worked properly.

NASA will try again Sept. 7 to launch Endeavour after replacing the failed unit with a used, $3-million spare.

The mission to release a pair of science satellites already was nearly a month late because of work to replace the thermal insulation surrounding O-ring seals in the solid rocket boosters.

Hot rocket gas singed O-rings in the boosters of two other shuttles earlier this summer, forcing the repairs on Endeavour.

The Challenger disaster in 1986 was blamed on hot gas escaping through a set of O-rings.