After a lengthy fueling delay because of stormy weather, launch of the shuttle Endeavour on a space station mission was scrubbed Wednesday when a presumably repaired hydrogen vent line umbilical began leaking potentially dangerous vapor for the second launch try in a row.
Given the apparent severity of the problem and the planned launch of NASA's Lunar Reconnaissance Orbiter on Thursday or Friday atop an Atlas 5 rocket, Endeavour will be grounded until at least July 11 when the next shuttle/space station launch window opens.
"We've got to step back and try to understand this problem, and we will do that," said LeRoy Cain, chairman of NASA's Mission Management Team. "It's going to take us a little time...The team will be relentless in terms of trying to go understand what's going on with this system. We'll fix it and we'll move forward once we have determined we can get in a safe configuration to go fly."
The vent line in question, 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 on 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 in 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, well above the 40,000 parts per million limit. Additional vent valve cycles also were out of limits.
Finally, at 1:55 a.m. EDT, launch director Pete Nickolenko, overseeing his first countdown, 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."
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--took the delay in stride and planned to fly back to Houston later in the day.
"It's a reminder that spaceflight is NOT routine," Polansky said in a Twitter feed.
The goals of Endeavour's mission are to attach an experiment platform to a Japanese research module, to replace aging solar array batteries, to deliver critical spare parts and to ferry Kopra to the station to replace Japanese astronaut Koichi Wakata.
Wakata, who traveled to the space station in March aboard Discovery, was informed of Endeavour's launch delay--and his extended mission--earlier Wednesday.
"OK, it is great that this international crew will continue," he said. "Another month for me!"
With Endeavour now delayed to at least July 11, launch of the next space station assembly flight, currently targeted for August 7, likely will be delayed as well. And this assumes that engineers can resolve the umbilical plate problem in time for a July 11 launch. With Wednesday'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 misalignment issues or isolated damage to a seal.
"It's too early for me to give you any idea of level of confidence," Cain said. "The direction we gave to the team today after our scrub was we need to step back from this problem and try to understand what is different in our process, if anything. Somehow, we've introduced some other variable or some change, albeit very small. But our sense is something has changed and something is different and we need to go re-evaluate."