Mission managers set the new launch target after reviewing the status of Discovery's new fuel control valves, which were
replaced last week. The shuttle is now slated to launch toward the International Space Station on March 11 at 9:20 p.m. EDT (0120 March 12 GMT) pending a final discussion by top NASA officials on Friday.Kyle Herring, a NASA spokesperson at the agency's Johnson Space Center in Houston, told SPACE.com that Discovery's new launch target - up one day from March 12 - is feasible because mission managers opted to forgo an extra modification to the shuttle's engine plumbing that would take more time to develop.
"All of the other analysis, the impact testing and the fact that we've got poppets and fuel control valves that are essentially crack-free, gave everyone comfort that we could fly without a modification," he said.
Discovery's two-week construction flight to the International Space Station has been delayed since Feb. 12 due to concerns with the shuttle's three fuel control valves. The valves, one for each main engine, are designed to work in concert to maintain pressure in the shuttle's liquid hydrogen reservoir inside its attached external tank.
To maintain a stable pressure during launch, metal poppets in the valves pop up as needed - much like lawn sprinkler heads - to route gaseous hydrogen from the shuttle's aft-mounted engines through a set of plumbing lines and into the external tank.
When NASA's shuttle Endeavour launched last November, one of its fuel valves cracked and chipped. The spacecraft's two other valves compensated for the damaged one and the shuttle reached orbit without incident.
But NASA wanted to be sure that a similar problem, if it occurred during Discovery's launch, would not cause catastrophic damage by puncturing the shuttle's vital plumbing lines or overpressurizing its hydrogen tank. A plumbing line break near the shuttle's aft could cause an emergency engine shutdown, while an overpressurized tank could end up venting the flammable gas overboard during launch, according to a NASA document.
Last week, NASA set a tentative March 12 launch target for Discovery as engineers replaced the shuttle's fuel valves with newer ones that had flown fewer times.
NASA must launch Discovery by about March 13 in order to complete its 14-day mission before the arrival of a previously scheduled Russian Soyuz spacecraft carrying the space station's next crew later this month. If Discovery does not launch by then, NASA would stand down until April 7 to launch after the space station crew change.
Commanded by veteran astronaut Lee Archambault, Discovery's STS-119 crew will deliver the final segment of the space station's backbone-like main truss and the last pair of U.S. solar wings for the outpost. Four spacewalks are scheduled for the two-week mission.
Discovery will also ferry Japanese astronaut Koichi Wakata to the space station to join the outpost's three-person crew. Wakata will replace NASA astronaut Sandra Magnus as a flight engineer for the Expedition 18 mission, then joint the incoming Expedition 19 crew when it arrives in late March.
Magnus, who has lived aboard the station since last November, is due to return to Earth aboard Discovery after four months living and working aboard the orbital laboratory.
Discovery's STS-119 mission is the first of up to six NASA shuttle missions scheduled to fly in 2009. The others include the final shuttle mission to overhaul the Hubble Space Telescope – slated to launch no earlier than May 12 – and a series of space station construction flights.
SPACE.com -- NASA Moves Shuttle Launch Target Up to March 11
