Shuttle fueling test planned to assess leak fix

A slight misalignment in the way a vent port on the shuttle Endeavour's external tank was built into the structure is the leading candidate for what caused gaseous hydrogen leaks that derailed two launch attempts June 13 and 17, the shuttle program manager said Friday.

To find out, engineers at the Kennedy Space Center are making plans for a fueling test at launch pad 39A. Endeavour's tank will be loaded with super-cold hydrogen rocket fuel and sensors will measure leakage where the vent line connects to the side of the tank. The leaks typically show up when the mechanism is subjected to cryogenic temperatures.

NASA initially attempted to launch Endeavour June 13. But the night before, as the shuttle's tank was nearing its full load, sensors detected a significant gaseous hydrogen leak at the ground umbilical carrier plate where the vent line attaches to the tank with a quick-disconnect fitting.

While some leakage at the quick-disconnect fitting is acceptable, hydrogen concentrations higher than 40,000 parts per million are grounds for calling off a countdown.

That limit was exceeded during both of Endeavour's fuelings and after the second scrub, the flight was put on hold to make way for Thursday's launch of NASA's Lunar Reconnaissance Orbiter from the Cape Canaveral Air Force Station.

The Progress can "loiter" in orbit for five days, but it must dock by July 29. And that means Endeavour must take off by July 14 to complete its 16-day space station assembly mission in time to undock before the Progress arrives.

Shannon said three engineering teams have been set up to resolve the vent line seal issue. One team will begin precise measurements of the vent line interface this weekend before taking the mechanism apart next week. Those measurements are needed to confirm the root cause hypothesis.

"The way the umbilical line that carries the hydrogen away from the tank (is attached) to the tank is there's a plate that's bolted on with a pyrotechnic bolt. And there's a receiving plate that's on the external tank and those two are lined up, there are two little pieces of metal that go down to these hinge pins that keep it from moving side to side. And there's a Teflon seal ... on the inside of that flange coming out of the tank. The line going into it pressure fits in."

"What has happened, we are pretty sure, is that when you put that external plate with the line on it onto the ET flange, there's a pyrotechnic bolt that holds those two together. It's above that round pipe. That whole system can rotate about that pyro bolt. If the two plates are in perfect alignment, it's not going to rotate, it'll just move slightly up and down. But since it is cocked a little bit ... it pulls the entire structure to the right (when the hardware contracts at cryogenic temperatures) and that allows a leak on the left side."

Shannon said engineers noted 18 to 20 gaseous hydrogen leaks at the ground umbilical carrier plate interface during previous fuelings but in all but two of those cases, cycling the vent valve caused enough vibration to help the seal seat itself.

If higher-than-allowable leakage is seen during the fueling test using the alternative seal design, Shannon said the team may have to consider more extensive work to remove, realign and re-attach the vent line flange on the external tank. In that case, Endeavour likely would be moved to a different tank and launch would face a more significant delay.