CAPE CANAVERAL, Fla. -- NASA conducted a second fueling test yesterday on the space shuttle Discovery to try to figure out why sensors and valves did not work properly during a previous run-through.
The test was in preparation for the launch of Discovery in July on the first shuttle flight since the Columbia disaster nearly 2 1/2 years ago.
Discovery's external tank was filled with 500,000 gallons of liquid hydrogen and oxygen, and a countdown was simulated.
Everything went without a hitch this time, but engineers evaluating the preliminary data were still unsure why the liquid hydrogen sensors gave intermittent readings and why a pressurization relief valve opened and closed more times than normal during last month's test.
The sensors act like fuel gauges that notify the shuttle's main engines to shut down when propellants reach a certain level. The valve opens and closes to ensure the liquid hydrogen stays at the correct temperature.
''We didn't find any smoking gun," said Bill Parsons, manager of the space shuttle program. ''At this point, the conclusion that you might come to is that we had some kind of connection that wasn't exactly right . . . and, therefore, we've cleared this up."
NASA Administrator Michael Griffin, who took the top job with the National Aeronautics and Space Administration last month and was visiting Kennedy Space Center yesterday, said he did not believe there were any major hurdles to returning to flight.
''I've seen just the normal bumps in the road that you have to get past, especially when you haven't flown in two years and several months," he said.
The second test occurred just days before Discovery will be rolled back into the hangar to replace its tank with a safer, updated model. Also, a heater will be installed on the new tank to prevent the buildup of ice once the super-cold fuel is pumped in.
Engineering tests found that ice falling off the tank could be as dangerous as the chunk of foam insulation that doomed Columbia.
The danger of ice and the sensor-and-valve problems prompted NASA to postpone Discovery's launch from late May to mid-July.
The board that investigated the Columbia accident criticized NASA, saying it stifled dissent. Griffin said the decision to postpone Discovery's launch shows that the space agency is changing its culture.