Discovery Engineers to Complete Checks of Empty External Tank

CAPE CANAVERAL, Fla. - NASA will restart the countdown for the space shuttle Discovery Saturday, with plans to launch the orbiter spaceward on July 26 after more than week of work to pin down a fuel sensor glitch, mission managers said late Wednesday.

"Right now we think we have eliminated all the common causes," shuttle program manager Bill Parsons said of the glitch during a press briefing here at NASA's Kennedy Space Center (KSC). "We believe we've done everything we possible could on the vehicle."

Discovery's STS-114 mission, NASA's first shuttle flight since the 2003 Columbia disaster, is now set to launch at 10:39 a.m. EDT (1439 GMT) on July 26.

Wednesday's announcement comes more than a week after launch controllers scrubbed Discovery's attempted July 13 space shot. They has discovered that a fuel sensor, one of four that monitor liquid hydrogen levels inside the shuttle's external tank, failed during a standard countdown test. Known as engine cut-off (ECO) sensors, the sensor system is designed to track fuel levels during launch and ensure Discovery's three main engines shut down properly before the external tank runs dry. If the engines keep firing without fuel, it would prove disastrous for the orbiter and its crew.

"This has been a very, very thorough effort that we've been through," said John Muratore, manager of shuttle systems engineering and integration, during the briefing. "We've used every kind of analysis technique and test technique that we can find."

Shuttle engineers investigating the anomaly believe that electromagnetic interference from existing or new hardware, such as additional cameras or heaters attached to the bipod fitting that connects Discovery to its external tank, and a small grounding issue with sensor wiring may be the culprit. Additional tests over the next 48 hours should address those areas, with the launch countdown set to begin at about 12:00 p.m. EDT (1600 GMT) Saturday, Parson said.

But engineers have only a limited time to complete their troubleshooting efforts inside Discovery's aft compartment, where the electronics box that processes ECO sensor readings resides.

"About 20 hours into the countdown is when you do the cryogenic loading for the [shuttle's] fuel cells," said Michael Wetmore, director of space shuttle processing at KSC. "We'd absolutely have to be out of there by then."

While all four hydrogen ECO sensors are required to perform properly under current flight rules, shuttle officials said they could make an "exception" for Discovery on launch day if they see a glitch that they fully understand.

"We expect to have four of four sensors," Parsons said. "If we can understand that failure and it was a known failure that we expected...then we might very well be willing to go fly with three of four sensors, there's good flight rationale behind it."

If shuttle officials see a sensor failure that they do not expect then they'd have to reassess the situation, which could take more than 24 hours or push the launch outside of July entirely, Parsons added.

Discovery's launch window runs through July 31, with the next flight opportunity opening on Sept. 9.

New launch protocols instituted after the Columbia accident call for optimum lighting levels for the myriad of still and video cameras watching the shuttle's ascent, as well as good lighting for the STS-114 crew when they photograph the external tank just after it separates from the orbiter. Still more daylight is needed during Discovery's International Space Station (ISS) rendezvous, where station crewmembers will photograph its tile-lined belly to record and transmit its condition to ground-based engineers.

Columbia disintegrated during atmospheric reentry, killing its seven-astronaut crew, on Feb. 1, 2003.

Launch officials said that if nothing bars a July 26 space shot, flight controllers should have at least four opportunities to launch Discovery. Additional dates for the flight - should it fail to fly Tuesday - include the 27^th, 29^th and 31^st of July.

"We've got a great amount of work in front of us," Parson said. "But we've all agreed this work is doable, and that it all takes us to a launch."

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