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."
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.
Preparations on track
Preparations for launch continued on track for launch at 10:39 a.m. ET Tuesday, with no significant issues standing in the way, test director Jeff Spaulding said earlier in the day. Weather officer Kathy Winters put the chances for acceptable weather at 60 percent.
Meanwhile, Discovery's crew of astronauts — headed by the only woman ever to command a shuttle mission, Eileen Collins — continued their own preparations. Because Discovery's landing is now due to take place before sunrise, Collins and shuttle pilot Jim Kelly are practicing night landings, using planes outfitted to duplicate the shuttle's aerodynamics.
Spaulding said the mood among members of the launch team has brightened, after days of focusing so intensely on the glitch that they couldn't look ahead to the launch. "I think now it's finally come to that point where people can realize that we are close, and we really are going to get a shot this time," he said.
However, the key test will come in the wee hours of Tuesday morning, when the launch team starts filling up Discovery's fuel tank with superchilled liquid hydrogen and oxygen. When the low-level sensors are covered with fuel — about a half-hour after fueling begins — managers will begin testing the sensors to make sure they can switch between a "wet" and a "dry" reading.
That "wet/dry" testing would be interrupted for two series of normal automated checkouts — one about an hour before Discovery's crew boards the shuttle, and the other during a T-minus-9-minute hold. The glitch that stopped the first countdown on July 13 could arise at any time during the continuous testing, and that would trigger another series of checks to determine whether the glitch had to do with the tank's sensor or the shuttle's circuitry.
"Some of us are kind of hoping that it recurs," Hale said, so that engineers could definitively track down the source of the problem.
He said one other concern was raised in advance of Sunday's management team meeting: the buildup of ice on the exterior of Discovery's supercold fuel tank during the previous countdown. Mike Wetmore, director of shuttle processing, said the launch team would keep a close eye on the potential icing problem during the current countdown.
If the launch has to be postponed again, mission managers can schedule as many as three more attempts later this month or perhaps even in early August. Then they'd have to wait until September.
12-day mission
Discovery's 12-day mission is aimed at resupplying the space station as well as testing all the safety enhancements that have been made since the shuttle Columbia broke up, killing all seven astronauts aboard. More than 100 cameras will be recording the launch, and so NASA wants to make sure liftoff occurs during daylight. Mission managers also want to get a good look at the fuel tank as it comes off the orbiter, and that's why they're debating exactly when the current launch window will close.
The fuel tank is a key concern, because foam insulation that flew off Columbia's tank just after launch is thought to have knocked a hole in that shuttle's left wing — setting the stage for Columbia's destruction 16 days later when hot atmospheric gases entered through the hole.
During Discovery's mission, astronauts will use cameras and a newly designed extension boom to make a close inspection of the orbiter's protective skin after launch. If serious damage is found, Discovery's crew would take refuge aboard the space station and wait for another shuttle crew to come to the rescue next month.