NASA will not be able to make another attempt to launch Discovery until Sunday at the earliest, giving the astronauts just three days or so to get off the ground before the end of the March launch window.
"It's better to be flying than sitting on the ground," said Launch Director Mike Leinbach. "But our business requires perfection, and our vehicle was not perfect today."
Engineers will not get access to the suspect vent valve until Friday and completing repairs in time for a Sunday launch try may prove challenging Harwood reports.
If the shuttle is not off the ground by March 17, NASA will be forced to delay the 125th shuttle mission to the end of the first week in April, after a Russian mission to rotate crews on the international space station.
Even if Commander Lee Archambault and his crewmates get off Sunday, they will have to give up one of their four planned spacewalks and shorten the flight by two days to make sure the docked phase of their mission is completed ahead of the Russian Soyuz TMA-14 spacecraft's planned March 26 launch from the Baikonur Cosmodrome in Kazakhstan.
If the shuttle launch slips to Monday, the crew could be forced to give up three spacewalks, in part because a Monday launch would require a docking on the fourth day of the mission instead of flight day three as currently planned. LIkewise, a launch on Tuesday would be a one-spacewalk mission.
The primary goals of Discovery's flight are to deliver a fourth and final set of solar arrays to the international space station; to deliver a replacement urine processing assembly for the lab's water recycling system; and to ferry a fresh station crew member - Japanese astronaut Koichi Wakata - to replace outgoing flight engineer Sandra Magnus, who plans to return to Earth aboard Discovery.
With Discovery already running behind schedule because of problems with gaseous hydrogen flow control valves - an issue unrelated to the vent line trouble - today's delay was a frustrating disappointment, Harwood reports. There were no other technical problems of any significance and forecasters were predicting near-perfect weather.
But during the automated procedure to pump a half-million gallons of liquid oxygen and hydrogen rocket fuel into Discovery's huge external tank, sensors detected a significant gaseous hydrogen leak at an umbilical where a vent line hooks up to the side of the tank.
The vent system is used to regulate pressure inside the hydrogen section of the tank as it is being filled for launch. As some of the supercold liquid boils off inside the tank, pressure builds up, the vent valve is opened and gas is routed to a "flare stack" near the pad where it burns harmlessly away.
Similar problems have occurred in the past and Leinbach said it was usually possible to clear the system by cycling the valve a few times. Today, the valve was repeatedly cycled, but the leak did not go away. Because of the threat of fire from free hydrogen near the shuttle, Leinbach ordered engineers to drain the tank, easing the internal pressure that required venting.
"It is the system that maintains pressure inside the hydrogen tank as we're filling up," Leinbach said. "As we put hydrogen in the tank, it does start to boil off and therefore it increases the pressure inside the external tank. We open this valve briefly to let some of that pressure out, it's a very standard thing that happens several times during fueling.
"We were never in any danger of over pressurizing, we were never in any danger of under pressurizing," Leinbach said. "It's just that every time we opened that valve to keep that pressure steady, we saw the leak."
Despite the disappointment, Mission Management Team Chairman Mike Moses said the launch team would take the latest delay in stride and not make another attempt to launch Discovery until it is safe to do so.
"That's life in the space business," Moses said. "Sometimes things happen."
By Bill Harwood