Shuttle prepared to ‘go’ with a glitch

Alan Boyle Science editor • Profile • E-mail

CAPE CANAVERAL, Fla. - NASA has cleared a plan to launch the space shuttle Discovery even if a mysterious fuel-gauge glitch reappears during the countdown toward a Tuesday launch attempt, mission managers said.

Managers finished hammering out the details of the plan on Sunday afternoon, 11 days after the glitch forced a halt to the countdown for NASA's first shuttle mission in more than two years.

The fuel-gauge glitch involved one of the four low-level sensors in the liquid-hydrogen compartment of the shuttle's fuel tank — which function like the "E" indicator on a car's gas gauge. NASA's current rules require all four sensors to be working at launch, even though the system could get by with just two.

During the first countdown, after the tank was filled, the launch team ran a simulation that should have had the sensors signaling "E," or "dry." Three of the sensors showed that reading, but one troublesome sensor refused to give the expected signal.

At Sunday's news briefing, mission managers voiced confidence that they had addressed the most probable causes of the glitch: grounding problems with the circuitry, or electromagnetic interference.

But if the glitch reappeared "under very closely defined circumstances," the launch team could agree to a one-time deviation from the rules and allow the countdown to go ahead, said deputy shuttle program manager Wayne Hale.

He explained that engineers have switched connections between the sensors and the circuitry in the shuttle, so that the wire that once went to the troublesome sensor (known as No. 2) is now connected to a good sensor (No. 4). If either of those two sensors went out during a pre-launch test, managers could run another series of checks, then give the go-ahead depending on the results, Hale said.

"If we're comfortable that we have a good understanding of the cause, then we can go fly for those specific two cases," Hale told reporters at NASA's Kennedy Space Center. "If anything else happens ... we're going to stop, because that says we really need to do more testing."

Perceptions about ‘safety culture’
Discovery's 12-day mission to the international space station represents the first shuttle flight since the catastrophic breakup of Columbia back in February 2003. Because of that tragedy, NASA has made dozens of upgrades in the shuttle and also has tried to reform its "safety culture."

Time for tribute? Discovery's liftoff on Tuesday is set for 10:39 a.m. ET — the exact launch time used by Columbia on Jan. 16, 2003. The timing is not an intentional tribute, but determined by the trajectory required for a space station rendezvous. MSNBC coverage of Columbia tragedy Spaceflight Now: Launch windows

At Sunday's briefing, NASA's top managers were repeatedly asked whether the decision to accept a glitch might affect how the space agency's commitment to safety would be perceived. NASA Administrator Mike Griffin said he was "quite comfortable with where we are" on the matter. He pointed out that the sensors only come into play if the liquid hydrogen is close to running out before the main engines are shut down — which itself is a rare occurrence — and that engineers have wrung out the most likely causes of the glitch.

"Even if it does recur ... we're still two-failure-tolerant, so it's not a safety-of-flight issue," he said. "What you want of NASA is that we make the right technical decisions, that we do the right thing to the extent that we can figure that out, which is hard."

Griffin acknowledged that balancing the risks of spaceflight can involve "rather arcane matters."

"But in the long run, I think if it's the right thing, we can explain it to you, and you want us doing what's right, not what necessarily is obvious or popular," he told reporters.

Hale said "we're all still struggling a little bit with the ghost of Columbia, and therefore we want to make sure we do it right."

"If it's caused anything, it's caused us to reaffirm the culture change that we've had in the last two years," he said.

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