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Wednesday, 17 June 2009

Hydrogen leak scrubs shuttle Endeavour launch again

Posted: June 17, 2009
After a lengthy fueling delay because of stormy weather, launch of the shuttle Endeavour on a space station assembly mission was scrubbed early Wednesday when a presumably repaired hydrogen vent line umbilical began leaking potentially dangerous vapor.
The vent line, attached to the side of the shuttle's external fuel tank, carries hydrogen gas away from the shuttle so it can be safely dissipated. A leak in the same mechanism scrubbed a launch attempt last Saturday.
Engineers replaced a seal in the gaseous hydrogen vent line umbilical plate Sunday and Monday and NASA managers were hopeful that would resolve the problem. A seal replacement worked last March when the shuttle Discovery was grounded by a similar leak.
While Endeavour's vent line passed leak checks at ambient temperatures, the seal replacement apparently wasn't enough to resolve the problem. During the initial stages of fueling, engineers observed a relatively small leak rate that, while unexpected, was within specification.
But as the tank filled and the temperature of the vent line kept dropping, the leak worsened. Engineers stopped the flow of hydrogen and cycled a valve in the system in hopes of clearing the problem, but they were not successful.
They then resumed hydrogen "fast fill" operations to collect additional data. When the vent valve was opened again, however, higher-than-allowable levels of hydrogen gas were detected, up to 60,000 parts per million. Additional vent valve cycles also were out of limits.
Finally, at 1:55 a.m. EDT, Launch Director Pete Nickolenko, overseeing his first countdown, reluctantly ordered a scrub.
"We are scrubbing the launch attempt for today," said launch commentator Mike Curie. "The troubleshooting efforts have not resulted in a significant decrease in the amount of gaseous hydrogen that's being detected outside of the ground umbilical carrier plate, the same area where we experienced a leak the last launch attempt.
"STS-127 Launch Director Pete Nickolenko has just given the team a 'go' to scrub the launch attempt after two more attempts to open the vent valve once again displayed much higher than expected amounts of gaseous hydrogen."
Because of a conflict with the launch of NASA's Lunar Reconnaissance Orbiter, scheduled for takeoff Friday atop a United Launch Alliance Atlas 5 rocket, NASA is not expected to get another chance to launch Endeavour until July 11 at the earliest.
The shuttle's normal launch window extends through Saturday and a launch on Sunday would be possible if mission managers eliminate one of the crew's five planned spacewalks. The window is defined by temperature constraints related to the space station's orbit.
But the Air Force Eastern Range, which provides required telemetry and tracking support, cannot reset its systems in time to support a shuttle launch Sunday even if the LRO mission took off on time Friday.
At least that's what reporters were told earlier. NASA managers have not yet said when another attempt to launch Endeavour might be possible.
Either way, it was a frustrating disappointment to Endeavour's crew - commander Mark Polansky, pilot Douglas Hurley, Canadian flight engineer Julie Payette, David Wolf, Christopher Cassidy, Thomas Marshburn and space station flight engineer Timothy Kopra - and to the NASA launch teams at Kennedy and the Johnson Space Center in Houston.
If Endeavour is, in fact, delayed to July 11, launch of the next space station assembly flight, currently targeted for Aug. 7, would be delayed as well in a downstream domino effect.
And that assumes engineers can resolve the umbilical plate problem in time for a July 11 launch. With today's scrub, leaks have developed at the vent line interface in three of the last five shuttle fuelings, indicating a potentially more serious problem than a misaligned seal.

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