Live at KSC: NASA hits snag; launch still Sunday

3:45 p.m. Blogger Update: NASA aims to pick up the countdown to launch of Discovery early Sunday and attempt to send up seven astronauts on an Internationl Space Station assembly mission. Countdown preps are running three to four hours behind at this point but NASA still thinks it can make up lost time and launch Discovery as scheduled at 7:43 p.m. Sunday. The Rotating Service Structure is scheduled to back away from the shuttle at 8:30 p.m. tonight. The countdown is slated to pick up at T-Minus 11 hours at 3:18 a.m. and fuel-loading operations are scheduled to begin just before 10:30 a.m. Sunday.

NASA ran into trouble reconnecting a gaseous hydrogen line to shuttle Discovery's external tank, but agency officials still aim to launch shuttle Discovery and seven astronauts Sunday on an International Space Station assembly mission.

Work at Kennedy Space Center's launch pad 39A is about six hours behind schedule, but NASA officials expect to make up lost time and launch Discovery as scheduled at 7:43 p.m. EDT Sunday. There's an 80 percent chance the weather will be acceptable for launch.

Mission managers are meeting at this hour and are expected to give a go-ahead to resume Discovery's stalled countdown at 3:18 a.m. Sunday. A news briefing that had been slated for 3 p.m. now is being moved up to 2:30 p.m. You can watch it live here in The Flame Trench. Simply click the NASA TV box on the righthand side of the page to launch our NASA TV viewer and live coverage.

NASA contractor technicians ran in to trouble after replacing a suspect quick disconnect valve that links a gaseous hydrogen vent line with the shuttle's 15-story external tank. The vent line routes excess hydrogen from the tank to a flare stack within the perimeter of the complex 39A area.

A replacement had been installed along with two associated seals and then technicians noticed that the gaseous hydrogen line and a carrier plate it links to were not aligned properly. The line now must be removed and then reconnected before small explosives designed to separate the line and the tank at liftoff can be reinstalled and tested.

The recovery work is ongoing.

Click here to see a NASA Design Overview powerpoint presentation on the gaseous hydrogen umbilical carrier plate. It details leakage history and a recent seal redesign that was done to lessen the chance of leaks in cases where the quick disconnect valve is misaligned during installation.

NASA scrubbed an initial launch attempt Wednesday when when a higher-than-allowable leak of gaseous hydrogen was detected in the area where the gaseous hydrogen line connects to a gaseous hydrogen umbilical carrier plate. The gaseous hydrogen is highly flammable so NASA has strict criteria for the amount of leakage that is allowable.

The leak was traced to a seven-inch disconnect valve that links the gaseous hydrogen line and the external tank.

NASA still aims to start filling Discovery's giant orange tank Sunday morning with more than a half-million gallons of liquid hydrogen and liquid oxygen.

The liquid hydrogen and liquid oxygen both are cryogenic propellants -- supercold. The liquid hydrogen is Minus 423 degrees Fahrenheit and a certain amount of it boils off -- turning from liquid to gas -- during the course of fuel-loading operations. The quick-connect valve is designed to vent excess gaseous hydrogen as it builds up with the liquid hydrogen tank within the external tank.

The leak last Wednesday came near the end of a three-hour fuel-loading operation. A repeat of a higher-than-allowable leak during launch countdown on Sunday would force a scrub and likely a delay until April 7.

NASA is facing a launch deadline Tuesday. After that, the Discovery mission would be postponed until after an already scheduled Russian launch of a new crew to the International Space Station and a subsequent week-long change-of-command period at the outpost.

A previously scheduled Atlas rocket launch has been indefinitely delayed to give NASA a chance to launch Discovery before the deadline. The weather forecast for Sunday and Monday is generally good, but then a cold front moving into the area could present problems on Tuesday.

Check out the details here in this Official Forecast from the Air Force 45th Space Wing Weather Squadron.

ABOUT THE IMAGES: Click to enlarge the NASA images of the seven-inch quick disconnect valve that links a gaseous hydrogen vent line to the shuttle's external tank. You can also click the enlarged images to get even bigger views.