Broken valve grounds shuttle until at least Friday

CAPE CANAVERAL -- Discovery and seven astronauts will remain grounded until at least Friday so technicians can test a faulty propulsion system valve that failed as the shuttle was being fueled Tuesday for its second launch attempt.

In a bit of an unusual twist, the 18-story shuttle and its crew would have two opportunities to launch Friday — the first at 12:22 a.m. and the next at 11:59 p.m. Eastern.

NASA also would have opportunities at 11:33 p.m. Saturday and then 11:11 p.m. Sunday — the 25th anniversary of Discovery's first flight — to send Discovery up to the International Space Station before a looming deadline that would trigger a delay until mid-October.

NASA in that case would yield to previously planned Japanese and Russian missions to the station. But senior managers say NASA still has a chance to get Discovery's mission underway.

The valve failure cropped up as NASA was just about to finish loading supercold liquid hydrogen and liquid oxygen into Discovery's 15-story external tank in advance of an early Wednesday launch attempt. Thunderstorms forced NASA to scrap an initial attempt early Tuesday.

Engineers pouring over data in NASA's Launch Control Center saw that the liquid hydrogen fill-and-drain valve was not operating as intended.

The eight-inch-diameter valve is one of four that control the flow of liquid hydrogen and liquid oxygen from the shuttle?s external tank through the orbiter's main propulsion system and then into its three main engines.

Engineers could not be certain the valve would open and close on command. They feared the valve might close and remain closed. In that case, it would be very difficult for NASA to drain millions of gallons of croygenic hydrogen from the tank.

NASA safety rules prohibit a launch with the valve stuck closed. The valve is one of two that can dump excess hydrogen after the shuttle reaches orbit.

An accumulation of highly flammable hydrogen in the shuttle's engine compartment would be an explosive hazard.