Launch Still On Despite Glitch

The countdown continued Sunday for a scheduled Tuesday launch of the space shuttle Discovery, as program managers said they would be willing to blast off even if a balky fuel sensor failed again.

But NASA officials admitted the choice to go ahead despite an unexplained glitch in the shuttle electronics could appear questionable since the agency was still trying to shed an image of sloppiness after the loss of Columbia in 2003.

“I wake up every day and ask myself, ‘Are we pushing too hard,’ ” N. Wayne Hale Jr., deputy shuttle program manager, said at a news briefing at Kennedy Space Center in Florida. “We are still struggling with the ghosts of Columbia.”

NASA Administrator Michael D. Griffin denied the agency was suffering from launch fever.

“We’re trying to make reasoned judgments,” he said.

Columbia’s seven astronauts died in 2003 when the orbiter broke up as it reentered Earth’s atmosphere.

An investigatory panel later said the accident was caused by a piece of lightweight foam that fell off the external fuel tank and struck the left wing. The panel also cited a NASA culture that, among other things, was preoccupied with meeting launch schedules.

Discovery, the first launch attempt since the Columbia tragedy, was originally scheduled to lift off July 13. But a faulty sensor in the liquid hydrogen portion of the fuel tank caused the launch to be scrubbed. Since then, a dozen teams of engineers around the country have been working long hours to find the source of the trouble.

After testing hundreds of possibilities in the so-called fault-tree between the sensors and the orbiter’s computer, investigators believe they have narrowed the cause to either a problem with the sensor or wiring in the point sensor box, a complicated piece of electronics that receives signals from the sensors at the bottom of the fuel tank.

There also is concern that changes made to the fuel tank after Columbia, including the addition of heaters on the surface to prevent the formation of ice, could be causing electromagnetic interference that is shorting out the sensor.

The sensors, officially called Engine Cut-Off, or ECO, sensors, are designed to shut off the main engines before they run out of fuel, potentially destroying the engines. If they cut off the engines too soon, they could prevent the orbiter from reaching space, an equally dangerous event.

There are four sensors in the liquid hydrogen part of the tank. At one time, rules allowed a launch with only three reliable sensors. That changed after the Challenger accident in 1986.

Shuttle managers said Sunday they would only go back to the previous rules under certain, well-defined conditions. Specifically, the launch team would have to be convinced the glitch was localized to one sensor, or the wiring to a single sensor, thus assuring that other sensors wouldn’t also fail.

The launch is scheduled for 7:39 a.m. PDT Tuesday. About 10 hours before the launch, technicians will begin filling the fuel tanks with liquid oxygen and liquid hydrogen, chilled to minus 423 degrees Fahrenheit.

Once the hydrogen sensors are immersed in the cryogenic fuel, launch managers will send a series of commands to see if one or more fails again.

They also will turn on all the orbiter’s electronics to see if that causes an electromagnetic problem.