NASA Aims For Friday Launch Try

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

In a bit of an unusual twist, the 18-story shuttle and its crew would have two opportunities to launch that day -- the first at 12:22 a.m. and the next at 11:59 p.m. -- times when Discovery could be sent up into the same orbit as the International Space Station.

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 -- 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 under way.

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 cryogenic 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.

Engineers think the problem might actually be coming from a sensor that is designed to indicate whether or not the valve is open or closed. So they will run several tests today and Thursday, opening and closing the valve, to see whether the valve or the sensor is the root of the problem.

If the sensor is the culprit, NASA could proceed with launch attempts on Friday. If not, NASA would have to replace the valve, which would force a lengthier delay, likely until October.

NASA must launch Discovery by Sunday for the astronauts to complete their station outfitting mission and depart in advance of the debut launch Sept. 10 of a robotic Japanese cargo carrier.

The supplies and equipment being hauled to the station by Discovery and the Japanese freighter are critical to maintaining a full staff of six on the outpost.

NASA also would wait until after a planned Soyuz crew rotation mission that will stretch from late September through the early October as well as the subsequent departure of the Japanese cargo carrier and the arrival of a robotic Russian space freighter.

The next launch opportunity for Discovery would be Oct. 17.

A shuttle mission to the station now set for launch Nov. 12 would slip to Dec. 11 or Dec. 12. The delay also would make it more difficult for NASA to complete seven remaining shuttle missions before scheduled fleet retirement in late 2010.