NASA began a critical fueling test on the space shuttle Endeavour early Wednesday to see if new repairs have plugged a potentially dangerous hydrogen gas leak that thwarted two recent launch attempts.

During the test at the Kennedy Space Center in Florida, NASA is loading Endeavour's massive external tank with the 526,000 gallons of super-chilled liquid hydrogen and liquid oxygen while watching a hydrogen gas vent line for excessive leaks. The test is expected to determine whether Endeavour will be ready for a July 11 launch attempt, or have to stand down for lengthy additional repairs.

"We think we've got this licked," said NASA test director Steve Payne during the fueling test. "This should solve it, and the test we're doing today is to verify it before we go launch next week."

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Shuttle engineers traced the flammable gas leak that thwarted consecutive launch attempts on June 13 and June 17 to a misalignment on an umbilical plate attached to a Teflon seal leading to a vent line on Endeavour's 15-story fuel tank. The leak was in a line that siphons the hydrogen gas, which is created as the shuttle's cryogenic liquid hydrogen propellant boils off, away from the launch pad so it can be burned off safely, instead of posing an explosion risk during liftoff.

Leak repairs

To plug the leak, NASA engineers replaced the single-piece Teflon seal with a two-piece version and attempted to fix the misalignment with special washer-like shims.

Payne said shuttle engineers have been working through weekends and around the clock, when weather permitted, to repair Endeavour in time for today's test.

"I think we've got this figured out," he added.

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NASA began pumping Endeavour full of fuel at about 7:00 a.m. EDT (1100 GMT) at its seaside launch pad in Cape Canaveral, Fla. By 9:00 a.m. EDT (1300 GMT), when the tank is about 98 percent full, NASA should know whether the repairs to Endeavour's hydrogen gas vent line are successful.

If the leak reappears, NASA will have to decide whether additional modifications can be made at the launch pad, or whether Endeavour will have to be rolled back to the cavernous Vehicle Assembly Building to switch to a different external tank, a move that could lead to a longer launch delay.

"They're going to have to reassess where they are at that point," NASA spokesperson Allard Beutel told SPACE.com.

Endeavour is currently slated to launch at 7:39 p.m. EDT (2339 GMT) on July 11 to begin a planned 16-day mission to the International Space Station. The shuttle's seven-astronaut crew plans to deliver the last piece of the station's massive Japanese lab Kibo, perform five rigorous spacewalks and replace one member of the outpost's six-man crew during the marathon spaceflight.

Stuck knob freed

While today's fueling test is performed on Endeavour, another team of engineers is inspecting a flight deck window on the shuttle Atlantis, which flew a successful mission to the Hubble Space Telescope in May.

A work light knob had inadvertently wedged itself between the window and shuttle dashboard while Atlantis was in space, forcing engineers on Earth to pressurize the orbiter's cabin to expand it slightly in order to remove the part. A team will inspect the window for damage to determine if needs repairs or replacement.

Atlantis is slated to launch supplies to the International Space Station on Nov. 12.

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