Sensors near a repaired hydrogen vent line attached to the shuttle Endeavour's external tank detected only the slightest traces of free hydrogen during a critical fueling test Wednesday, officials said, clearing the way for another launch attempt July 11.
The 7-inch vent line and the ground umbilical carrier plate used to connect it to a port on the side of the external tank will remain in their current configuration and engineers are confident the system will be leak-free when Endeavour is fueled for launch on a space station assembly mission.
"We're in really good shape," said Mike Moses, the shuttle program launch integration manager at the Kennedy Space Center. "We're going to try on the 11th...We got it lined up just right and it doesn't leak."
The vent line is used to carry excess hydrogen gas away from the shuttle when the tank is filled with super-cold propellant. A valve used to route hydrogen to the vent line is closed a few minutes before launch when the tank is pressurized for flight.
Endeavour was grounded June 13 and 17 when sensors near the umbilical attachment plate detected hydrogen concentrations of more than 60,000 parts per million, or 6 percent. The allowable concentration near the shuttle is 4 percent.
After the second launch scrub, engineers collected detailed measurements and concluded the problem was caused by an alignment issue between the hydrogen vent port on the tank and the vent line interface. To ensure a tight fit, engineers replaced a rigid Teflon seal with a more flexible design, modified the umbilical plate mounting pins, and installed washer-like shims to counteract the alignment issue.
During the June launch attempts, the leaks occurred after the hydrogen section of the external tank was nearly full and fueling operations were transitioning from "fast fill" to "topping." In both cases, the leaks exceeded 60,000 parts per million.
During today's test, sensors detected a barely measurable 12 parts per million, a level so low it's not considered a sign of leakage.
"In this case, there were absolutely no leak indications and when we did transition all the way through the replenish operations, there were absolutely no leak indications whatsoever noted," said Launch Director Pete Nickolenko. The 12 parts per million reading was right at the limits of detectability, he said, and "we're calling that system tight, we show that as no leaks."
Moses said the same techniques used to ensure a near-perfect alignment of the vent line hardware will be used on all subsequent flights to prevent any repeat of the leaks that grounded Endeavour last month.
Assuming no other problems develop, NASA now plans to restart Endeavour's countdown at 10 p.m. EDT next Wednesday, setting up a launch attempt at 7:39:33 p.m. EDT Saturday, July 11. On board will be commander Mark Polansky, pilot Douglas Hurley, Canadian flight engineer Julie Payette, David Wolf, Christopher Cassidy, Thomas Marshburn and space station flight engineer Timothy Kopra.
Assuming an on-time liftoff, Polansky plans to guide the shuttle to a docking with the International Space Station at 3:25 p.m. EDT on July 13. Five spacewalks are planned between July 14 and July 23, starting in the early afternoon to late morning U.S. time, to install a Japanese experiment platform, to replace aging solar array batteries, and to store spare parts.
Kopra will remain behind aboard the space station when Endeavour undocks on July 25, becoming part of the Expedition 20 crew. He will replace Japanese astronaut Koichi Wakata, who was launched to the station in March and who will return to Earth aboard Endeavour. Landing at the Kennedy Space Center is targeted for 12:15 p.m. EDT on July 27.
As it now stands, NASA will only have four days to get the shuttle off the ground or the flight will slip to July 27 because of a critical Russian Progress space station resupply mission scheduled for launch July 24.
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 mission in time to undock by July 27, making way for the Progress.
Because of Endeavour's problems getting off the ground, the next flight in the station assembly sequence, mission STS-128, has slipped from August 6 to around August 18. NASA plans to close out the year by launching the shuttle Atlantis on November 12, although that flight may slip into December when all is said and done.
Engineers recently ran into an unusual problem with Atlantis when an astronaut work light attachment knob was lost during the shuttle's recent Hubble Space Telescope servicing mission. The hardware ended up lodged between a cockpit window pane and a flight deck instrument console. When the shuttle returned to Earth, the pressurized crew module contracted slightly and the knob was pinned against the glass.
The knob eventually was extracted by pressurizing the crew module and using dry ice to cool the metal enough to cause a slight amount of shrinkage. But the inner pressure pane in window No. 5 must now be inspected to make sure it did not suffer any structural damage.
Engineers are hopeful the window can be flown as is because replacing the inner pressure pane would take four to six months. Access is extremely tight and cockpit instrumentation would have to be disconnected, reconnected and retested after a window swap out.
With the shuttle program scheduled for retirement at the end of next year, a six-month schedule hit for one orbiter would require major replanning. But Moses said today NASA cannot complete the space station with just two shuttles, Endeavour and Discovery, and if Atlantis needs a window replacement, it will get one.
But engineers are hopeful it won't come to that.