NASA Delays Launch Another Day as Officials Probe Sensor Failure

NASA officials pushed back the earliest possible date to launch the shuttle Discovery by another day, to Sunday, saying they still had no idea what caused a sensor failure in the shuttle’s external fuel tank.

Even a Sunday launch would require an “extremely lucky” break, N. Wayne Hale Jr., deputy shuttle program manager, said at a news conference Thursday.

“If we go in and wiggle some wires and it works, then we can go Sunday,” Hale said.

More likely, the launch would be delayed at least into the middle of next week, the space agency said.

NASA officials met for about two hours to draw up a troubleshooting plan to resolve the problem that resulted in the scrubbing of Discovery’s launch Wednesday.

If engineers have to go inside the external tank or the aft section of the orbiter, the delays could grow longer, Hale said.

The current launch window, which would allow Discovery to reach the International Space Station, closes July 31. Another opportunity would not open until September.

Such a delay could affect future shuttle flights and the space station assembly schedule.

Another big unknown is the unpredictable Florida weather in summer. Sunday forecasts called for a 40% chance of rain and lightning, increasing to a 60% chance of rain by Tuesday, Air Force weather officials said.

The malfunction involved one of four sensors on the hydrogen side of the external fuel tank. Four similar sensors monitor liquid oxygen levels in the tank.

During a routine test 2 1/2 hours before launch, the sensor failed to respond correctly to commands from mission control.

A dozen engineering teams across the country have been mobilized to unravel the cause of the failure.

The sensor, known as an engine cutoff sensor, or ECO, is a critical component designed to shut off the shuttle’s main engines just before the fuel is depleted.

A malfunctioning sensor could allow the engines to continue firing after the fuel is gone or prematurely shut off the engines, preventing the orbiter from reaching space.

No similar failures had occurred over the shuttle program’s history until April, when another sensor failed in a test.

As part of a routine run-up to the originally scheduled May launch of Discovery, engineers loaded liquid hydrogen into the external tank. They discovered one of the ECO sensors in the hydrogen tank was emitting false signals.

A second tank test succeeded after engineers investigated the problem.

But they were never convinced they had isolated the original failure.

“It is an unfortunate part of the business that we have unexplained anomalies,” Hale said.

Rather than spend more time finding the cause of the failure in April, they decided to swap the entire external tank to fix the sensor problem and install a de-icing heater on a fuel line.

Those repairs led NASA officials to postpone Discovery’s launch from May to July.

The latest failure has raised concerns among some outside experts that NASA should have determined the cause of the sensor problem before proceeding with the July launch.

Hale said he wanted to find the cause of the problem, but when questioned repeatedly about it at the news conference Thursday, he would not rule out the possibility that it would remain an unexplained anomaly.