CAPE CANAVERAL, Fla. - NASA is fueling Discovery's external tank this morning in a bid to troubleshoot problems with critical engine sensors and valves.

NASA managers gave the go-ahead to start fueling Discovery around 5:30 a.m. EDT. Supercold propellants are slowly pumping into the 15-story tank.

It will take the launch team at Kennedy Space Center about three hours to fill the tank with more than a half million gallons of liquid oxygen and liquid hydrogen.

The shuttle team will take advantage of the fueling test to get another look at how and where ice forms on the tank in addition to how the redesigned foam insulation performs in general.

However, the primary reason for doing a second tanking test is to figure out a problem that cropped up during a similar test in April.

During that test, sensors that serve as fuel gauges and a pressure-relief valve failed to operate properly. The sensors produced an intermittent stream of data, and the valve opened and shut more than expected.

NASA engineers want to pinpoint the causes of the problems and take any corrective action before launching the agency's first post-Columbia shuttle mission, which now is scheduled to lift off between July 13 and July 31.

The fuel-depletion sensors constantly measure the amount of propellant left in the 15-story tank as the shuttle makes its nine-minute climb into orbit.

The relief valve is designed to keep pressures within the tank at proper levels during fuel-loading operations and flight.

NASA plans to move Discovery from launch pad 39B to the KSC Vehicle Assembly Building as early as Tuesday so the ship can be outfitted with a new external tank equipped with a fuel pipeline heater.

Managers decided to make that move after dangerous amounts of ice built up around a 70-foot propellant line on the outside of Discovery's tank during last month's fuel-loading test.

NASA feared ice could break off during launch and damage the shuttle's fragile thermal tiles and wing panels.

Serious damage to a wing panel doomed Columbia and its seven astronauts as they tried to return to the Earth in February 2003.

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