Cape Canaveral, Fla. – In a disappointing setback for NASA’s effort to return the space shuttle to flight, mission planners called off the launch of Discovery two hours before scheduled liftoff Wednesday, citing a faulty fuel gauge in the vehicle’s external tank.
Engineers could not pinpoint the source of the glitch, which affected a sensor that shuts down the spacecraft’s engines if the fuel level gets too low. Mission planners said the earliest Discovery could fly would be Saturday afternoon, though the problem could set the flight back weeks or months.
Shuttle engineers have struggled since April with the kind of sensor that caused trouble Wednesday. Two of the sensors malfunctioned during testing at that time – a failure technicians never fully understood, though the glitch seemed to resolve itself after engineers replaced circuitry on the orbiter.
The latest problem raised a new mystery. When flight controllers sent a signal that should have switched the four fuel sensors to a reading of “empty,” one failed to respond.
“All I can say is shucks,” said shuttle-program deputy manager Wayne Hale.
Launch director Mike Leinbach informed a disappointed crew that the launch would be scrubbed.
“I appreciate all we have been through together, but this one is not going to result in a launch attempt today,” Leinbach told shuttle commander Eileen Collins, who was already strapped into the cockpit.
This is the second time NASA has had to postpone a planned Discovery launch date. In May, managers scrubbed a launch because of a separate issue with the external tank that required installation of a modified tank.
Asked if the failures are an embarrassment for NASA, Rep. Sherwood Boehlert, R-N.Y., said that, if anything, the delays are a good sign.
“Today constitutes a success,” said Boehlert, chairman of the U.S. House Committee on Science, which oversees NASA. “The success was the NASA team identifying a problem. Think about the alternative.”
Shuttle engineers said that if the spacecraft were to fly without working fuel gauges, the engines might not shut off in the event that the tank ran out of hydrogen propellant sooner than expected. That would result in the engines running dry – something they were not designed to do.
The sensors “are there to protect us in case we run out of gas,” Hale said. He said it took the shuttle managers about five minutes to decide that the faulty sensors meant the mission could not go forward.
Mission planners have not given up on the possibility that the problem could be resolved soon. They said the astronauts will remain in Florida, and engineers will conserve energy in the shuttle’s fuel cells so the spacecraft could launch in the next few days.
The launch window runs through July 31. If the shuttle does not launch by then, the next opportunity would not arise until September.
At several safety meetings, including one Tuesday, NASA technicians had questioned whether the shuttle should fly at all until engineers understood what had caused the earlier fuel-sensor problems.
“We had a long discussion during the prelaunch time period about how confident we were about the safety of flying with what we call an unexplained anomaly,” Hale said.
