NASA delays shuttle flight a fourth time

By Irene Klotz

CAPE CANAVERAL, Florida (Reuters) - NASA delayed its first space shuttle launch of the year for a fourth time on Friday after failing to resolve concerns about a potential problem with fuel pressurization valves.

Shuttle Discovery had been scheduled to lift off on February 27 for a 14-day flight to outfit the International Space Station with a final set of solar wing panels. The flight, already delayed three times for safety reviews, was not immediately rescheduled.

The issue that has delayed the mission involves three small valves in the shuttle's rear engine compartment that are needed to keep the external fuel tank properly pressurized during the 8-1/2-minute climb into orbit.

During the last shuttle mission in November a tiny piece of one valve cracked.

There was no danger to the shuttle or its crew, but the U.S. space agency wants to be sure a reoccurrence would not have a different outcome.

"We need to understand a little bit more the consequences if a piece comes off," said Bill Gerstenmaier, NASA's associate administrator for space operations. "When we're in this mode we need to be careful about jumping to conclusions and picking solutions."

Program mangers will spend a few days next week assessing options before reconvening to set a new target launch date. To avoid a schedule conflict with a Russian-led station mission, NASA would not launch the shuttle between about March 13 and April 7.

"This is just one of those things in a complicated vehicle like this," said program manager John Shannon.

The long-term solution is to redesign the valves, he added.

NASA has nine shuttle flights remaining to complete construction of the station and make a final servicing call to the Hubble Space Telescope. The fleet is scheduled to be retired in 2010.

(Editing by Sandra Maler)