KENNEDY SPACE CENTER, Fla.--The shuttle Discovery's external tank was loaded with more than a half million gallons of supercold liquid oxygen and hydrogen rocket fuel today in a critical test to learn more about what caused cracks in structural ribs, or stringers, that were discovered after a November 5 launch attempt.

The cracks were repaired, but engineers want to pin down the root cause to make sure the huge tank is structurally sound and able to withstand the rigors of another launch attempt.

Wiring from dozens of sensors can be seen draped around the shuttle Discovery&39's external tank during a fueling test today.

The fueling test began at 7 a.m. EST and ended at 2:25 p.m. with the countdown holding at the T-minus 31-second mark. The liquid hydrogen and oxygen tanks were pressurized as they would be in a real launch countdown to collect additional data on the tank's performance.

A detailed assessment of the data collected during today's test is not expected until later. But a senior shuttle manager said readings from temperature sensors attached to the skin of the tank closely matched predictions from computer models and no unusual stresses were noted from strain gauges on or near the stringers in question.

An inspection by engineers at the launch pad found no obvious problems. One of two stereo cameras focused on thousands of small dot-like targets to detect any movement in the structure due to shrinkage or other factors apparently moved during the test and it's not yet clear if those data will be useful. The other camera apparently worked normally.

After draining the propellants, engineers planned to move a rotating gantry back around the space shuttle about 10:30 p.m. today in preparation for work to ready the ship for roll back to the Vehicle Assembly Building early Tuesday. Once inside the VAB, X-ray equipment will be used to look for any signs of cracks or other damage that might have escaped detection to this point.

NASA managers plan to meet Tuesday to discuss whether to order installation of structural stiffeners on 36 stringers, nine to either side of the two booster-attachment thrust panels, that experience the most stress during launch.

If all goes well and no other major problems are found, NASA hopes to haul Discovery back to the launch pad in mid-January for work to ready the ship for launch February 3 on a space station resupply mission.