CAPE CANAVERAL, Fla. - NASA's space shuttle Discovery is once again ready for launch even if a still unexplained fuel sensor glitch that scrubbed an attempted July 13 space shot occurs again, mission managers said Sunday.
"I'm quite comfortable with where we are," said NASA chief Michael Griffin during a pre-launch briefing here at Kennedy Space Center (KSC) late Sunday. "What you want of NASA is that make the right technical decisions, that we do the right thing to the extent that we can figure that out, which is hard."
Engineers spent nearly two weeks poring over Discovery's engine cut-off (ECO) sensor system after one of four liquid hydrogen fuel sensors inside its external tank failed a standard countdown test on July 13. Their efforts yielded two potential suspects, a wiring grounding issue - since resolved - and potential electromagnetic interference, but the exact cause remains elusive. Under current flight rules, all four hydrogen sensors - which perform as fuel gauges to ensure the orbiter shuts its three main engines down before the external tank runs dry.
"We have literally run every check that we could think of," said Wayne Hale, NASA's deputy shuttle program manager, during the briefing. "We don't think the problem is in the sensor itself...we have to go to cryogenic temperature to find out what's going on next."
Shuttle officials said Discovery and its STS-114 astronaut crew are primed for their scheduled July 26 launch at 10:39 a.m. EDT (1439 GMT), with no technical issues crimping NASA's next launch attempt. But mission managers added that they would okay Discovery's launch even if a sensor failure similar to that which scrubbed initial July 13 attempt, as long as it lines up with a very specific set of circumstances.
"We have not changed the launch commit criteria," Hale said. "It is a deviation."
If launch controllers detect a failure in the No. 2 or the No. 4 hydrogen sensors, whose wiring has been switched to better isolate the fuel gauge problem, shuttle officials are confident that they'll understand the phenomena enough to launch. But if an unforeseen glitch occurs in one of Discovery's four liquid oxygen fuel sensors, or the No. 1 or No. 3 hydrogen sensors, it could scrub the launch and even prevent the orbiter from lifting off before its flight window closes on July 31.
"It begins to look very remote that we'd be able to launch within this window in that situation, but it depends on what we see," Hale said.
Veteran astronaut Eileen Collins, commander of the STS-114 mission, told reporters Friday that she and her crew have paid close attention to the technical and management meetings that convened to solve the ECO sensor glitch.
"It's been very important for us to do that and to really understand the problem," Collins said. "We have...a fantastic team of people working on this."
Earlier today, engineers loaded the cryogenic reactants Discovery's three fuel cells will use to generate power during the shuttle's 12-day mission.
NASA has planned to have at least four launch attempts for Discovery - July 26, July 27, July 29 and July 31 - before its flight window closes at the end of the month. The potential to extend the launch window by one or two days into August will not give launch controllers a fifth space shot opportunity, but could allow more leeway in how they approach another attempted liftoff, NASA test director Jeff Spaulding said.
Discovery's is set to be NASA's first shuttle flight since the loss of the Columbia orbiter, which broke apart during reentry on Feb. 1, 2003 killing its astronaut crew. The STS-114 mission is designed to test out new orbiter inspection tools and methods to increase shuttle flight safety.
Aside from the fuel sensor glitch, NASA is also keeping an eye to the weather, which shows a 40 percent chance of violating launch criteria for the upcoming space shot, shuttle weather officers said.
"If things go our way, you'll see us launch our first shuttle in two and a half years," Hale said.
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