NASA aiming for Tuesday launch after engineers zero in on problem

Cause of sensor failure is believed to be a glitch in electrical grounding

July 21, 2005|By John Johnson Jr. | John Johnson Jr.,LOS ANGELES TIMES

NASA officials say they might have uncovered the problem behind a faulty fuel sensor on the shuttle Discovery and will resume the countdown to launch the spacecraft Tuesday.

After nearly a week of troubleshooting the problem that scrubbed the planned July 13 launch of the shuttle, officials said they had narrowed the cause of the failure to a possible electrical grounding problem in a piece of electronic equipment called the point sensor box.

They also said they were looking at the possibility that changes to the external fuel tank made after the 2003 Columbia accident could be the cause of the electrical problem, a possibility NASA had earlier dismissed.

"This has been a very, very thorough effort," program manager Bill Parsons said at a news conference yesterday at Kennedy Space Center in Florida.

Parsons said troubleshooting would continue and, if further testing proved the current theory, the countdown would resume Saturday for a Tuesday launch at 10:39 a.m.

Discovery and its seven-member crew were within 2 1/2 hours of launch when one of four fuel sensors responded incorrectly to commands.

Since then, a dozen engineering teams have been analyzing the shuttle's cables, wiring and sensors. This week, the teams began zeroing in on the point sensor box, where electronics cards receive signals from the sensors in the fuel tank and pass them on to the shuttle's computer.

The troubleshooters also are suspicious of a heater installed on the external fuel tank after the Columbia accident to prevent the formation of ice on the tank's surface. In one test, the sensor failed only seconds after the heater was turned on.

To test their current theories, workers will swap positions of the problem sensor with one that has consistently worked. They also are reworking some of the grounding areas.

Managers said for the first time that they might be willing to launch the shuttle with only three of the four sensors operating correctly.

The sensors are designed to cut off the main engines before they run out of fuel, something that could cause a catastrophic engine failure. But if they cut off too soon, that could prevent the shuttle from reaching orbit.

In the early days of the shuttle program, three working sensors was considered sufficiently safe to allow a launch. That was changed after the Challenger accident in 1986.

Engineering chief John Muratore said his teams of engineers have analyzed 350 items in seeking the cause of the electrical glitch. During that time, they have simulated filling the fuel tank 10 times to try to re-create the conditions under which the sensor failed.

The tests were conducted with the tank empty. Under launch conditions, it is filled with liquid oxygen and liquid hydrogen fuel, cooled to minus-423 degrees Fahrenheit.

NASA engineers have opted not to perform a full test with the tanks filled. Instead, they will reinitiate the countdown, fill the tank and test the sensors.

Officials said they might proceed with a launch even if they encountered another failure, as long as they were confident they had identified the problem as minor.

The Los Angeles Times is a Tribune Publishing newspaper.

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