NASA plans shuttle fuel tank test

The space shuttle Atlantis stands atop Launch Pad 39A after launch was delayed because of a sensor problem in the external tank in Cape Canaveral, Florida, December 6, 2007.

CAPE CANAVERAL, Florida (Reuters) - NASA will fill shuttle Atlantis' fuel tank for a third time next week in a test intended to resolve sensor problems that scuttled two attempts to launch the ship to the International Space Station, the program manager said on Tuesday.

Depending on the results of the test, which is scheduled for December 18, NASA is targeting Atlantis' launch for January 2. The shuttle will carry Europe's Columbus science laboratory to the space station.

Technicians will splice jumper cables into about 100 feet

of wiring that runs between fuel sensors in the shuttle's tank and the ship's engine compartment. The cables will be connected to instruments to monitor electrical circuits as the sensors are checked during fueling.

The sensors, which operate like dipsticks, are part of a backup system to cut off the shuttle's three hydrogen-fueled main engines in case a leak or other problem causes the fuel tank to run dry during the 8.5-minute climb to orbit. Running the engines without fuel could trigger a catastrophic explosion.

"We think we have a high degree of confidence of pinpointing the location where we're having our problem," program manager Wayne Hale told reporters on a conference call.

Launch attempts on Thursday and Sunday were canceled after one or more sensors failed routine tests as the tank was being filled.

NASA had a spate of problems with the fuel sensors as it struggled to return the shuttle fleet to flight following the 2003 Columbia accident. Managers believed the issue had been successfully resolved until the problem cropped up again on Thursday.

SENSORS

Hale wrote in an internal e-mail on Friday that it is likely the sensor-driven engine cutoff system, which is a backup to the flight computers' main cutoff system, has been unreliable since the shuttles began flying in 1981.

"It seems to me likely that we have been flying the entire history of the program with a false sense of security," Hale wrote in an internal e-mail obtained by Aviation Week. "That is a really sobering thought."

Hale wrote the e-mail in reply to one from Bill McArthur, the astronaut who heads the Space Shuttle Safety and Mission Assurance Office at the Johnson Space Center. Aviation Week posted the e-mails on its Web site on Tuesday.

In his notes, Hale also raised the prospect of flying the shuttles without the sensors working. He backed off the prospect on Tuesday, saying the situation had changed after Atlantis' sensors failed again during the second launch attempt.

Previously, the sensors, possibly conditioned in some way after soaking in the liquid hydrogen, worked fine on second launch attempts.

"There's quite a lot that we can do," Hale said.

(Editing by Cynthia Osterman)