Failed Sensor Delays Shuttle Launching

Today’s scheduled launching of the space shuttle Atlantis was scrubbed with less than an hour left in the countdown because of a faulty sensor in the huge external fuel tank.
NASA will drain the liquid hydrogen and oxygen from the tank and try to fix the problem in time for a launching at 11:15 a.m. Saturday, agency officials said.
The fuel sensor problem is the latest glitch to plague an Atlantis flight that had already been delayed by a lightning strike, Tropical Storm Ernesto and a problem with a power-generating fuel cell.

Though the six astronauts had already boarded the shuttle, the problem with the sensor in the hydrogen sector of the tank meant the spacecraft did not meet NASA’s safety criteria for a launch. The cancellation was “the right thing to do,” a NASA official said.
The weather on Saturday is expected to be as good as it is today, agency officials told the astronauts, which means the launching will be possible if the technical problem can be fixed.
If it cannot go on Saturday, Atlantis will have to be wait at least two weeks, agency officials said, to avoid interference with a Russian Soyuz mission to the Space Station scheduled to lift off on Sept 18.

The sensor in the hydrogen fuel tank is designed to ensure that the spacecraft’s main engines run for the correct amount of time after liftoff. There are three other sensors in the tank, and NASA could have decided to fly with one malfunctioning, but instead chose to adhere strictly to its safety standards.

Today’s cancellation came after space agency officials said Thursday that a problem in an electrical generator presented a “minimum risk.”
The National Aeronautics and Space Administration postponed a launching attempt on Wednesday because of an erratic reading from a pump that chills one of the shuttle’s three fuel cells, which generate electricity. After a detailed study of the readings, engineers determined that the fuel cell was “probably acceptable to fly with” and should not curtail the mission, said N. Wayne Hale Jr., shuttle program director.

A majority of the managers polled at a Thursday afternoon meeting voted to try for this morning, Mr. Hale said, with the only objections based on issues other than safety. The Office of Safety and Mission Assurance at the agency voted for replacing the pump and flying later to ensure that the goals of the mission were achieved, not because of safety concerns, he said.
Mr. Hale said the agency was not bowing to schedule pressure and a closing launching window in pressing ahead. Safety was the primary concern, he said.
“We have stood down two days to make sure we are safe to fly,” he said at an evening news conference at the Kennedy Space Center here. “We have gone the extra mile.”
Atlantis was scheduled to take off on an 11-day mission to resume construction on the International Space Station.
Michael Suffredini, NASA’s space station program manager, said he had spoken with the Russians about avoiding a conflict in the flights. If the Atlantis lifted off on Saturday, it would need to undock from the station on the day of the Soyuz launching instead of the day before, as had been planned.
Mr. Hale said launching at the end of the window would not require the Atlantis to cut its mission short or curtail any of its planned tasks.
The space agency is eager to resume station construction, which has been suspended for almost four years because of the 2003 Columbia accident, which grounded the shuttle fleet. The Atlantis is carrying a $372 million, 30,000-pound, bus-size segment of the station’s backbone that includes a new set of solar power arrays.
On the busy mission, the shuttle’s crew is supposed to conduct three complicated spacewalks to attach the segment and deploy the solar arrays. The next half-dozen shuttle missions to the station are intended to complete a structure that will hold the station’s four main power arrays and a complex wiring and plumbing system.

The agency plans to undertake 16 shuttle missions to complete the assembly of the half-finished space station by 2010, when the three-shuttle fleet is set to retire.
While only one fuel cell is required for a shuttle to operate, NASA rules call for all three units to be working for the shuttle to take off. If one of the units fails in flight, the rules call for ending the mission and returning home early.
Replacing the fuel cell could have delayed any launching attempt by several weeks.
Steve Poulos, manager of shuttle orbiter projects, said that the unit was working and that replacing it would increase the chance of creating new problems. The unit is safe to use, he said.
“Our risk really is going to be minimal,” Mr. Poulos said.

Fuel cell problems have affected several shuttle missions. In 1995, the launching of the Endeavour was delayed eight days so workers could remove and replace a bad fuel cell. A 1997 flight of the Columbia returned to Earth four days after launching when a cell failed. That cell showed erratic readings before taking off, but the Columbia was cleared to fly.