CAPE CANAVERAL, Fla. - NASA is back on the trail of an erratic fuel sensor glitch that forced the agency to delay launch plans for the space shuttle Atlantis until early January, mission managers said Sunday.
The failure of a fuel gauge-like sensor in Atlantis' 15-story external tank during a countdown test today forced NASA to call off a planned afternoon launch for the second time. Engineers will now scour the sensor system in hopes of pinning down the malfunction in time for a potential Jan. 2 launch attempt.
"In this case, with the vehicle and the system at the pad, we feel like we need to find root cause," said LeRoy Cain, head of Atlantis' mission management team, after the launch attempt here at the agency's Kennedy Space Center spaceport. "And we're going to make every effort to do that."
Cain said an engineering team is expected to draw up possible troubleshooting efforts while Atlantis is on the launch pad and present a preliminary plan to mission managers on Tuesday.
Known as engine cut-off sensors, the fuel gauges monitor the levels of super-chilled liquid hydrogen and liquid oxygen propellant in Atlantis' external tank. They serve as a backup system to shut down the shuttle's three main engines during flight before the fuel tank runs dry.
During a Thursday launch attempt, two of the four engine cut-off sensors monitoring Atlantis' liquid hydrogen supply failed a standard countdown test, with a third failing later. NASA flight rules require at least three working sensors to launch.
Because the sensors have behaved erratically in the past, working sometimes then failing at others, NASA tightened its launch rules to require all four suspect units to function properly before Atlantis could fly. Mission managers also cut the shuttle's five-minute launch window to just a single minute to conserve fuel as an extra safety measure in case the sensors failed during liftoff.
"We gave it a good try," said Doug Lyons, Atlantis' STS-122 launch director. "I think the team is disappointed, but highly motivated to go help this [engineering] team to track this problem."
NASA has been plagued by intermittent glitches with shuttle engine cut-off sensors sine 2005, when the space agency resumed orbiter flights following the Columbia accident. If the engines continue to fire with a dry tank, they could rip apart and cause catastrophic damage, though several other failures in addition to the sensor glitch must occur for that possibility, NASA has said.
Engineers, last year, switched to new engine cut-off sensors and attached additional instruments to monitor their performance, only to see the issues resurface aboard Atlantis.
"I think this has been sort a cloud that has always been over us," said William Gerstenmaier, NASA's associate administrator for space operations. "We thought we had it fixed when we changed out the engine cut-off sensors before."
Gerstenmaier said there is currently enough padding in NASA's flight schedule to hunt for the glitch and launch Atlantis' mission before the planned Feb. 14 liftoff of the shuttle Endeavour to carry part of Japan's Kibo laboratory to the ISS next year. But with up to 12 more shuttle missions on tap to complete the ISS, plus one more to service the Hubble Space Telescope, NASA hopes to settle the fuel sensor glitch once and for all in order to avoid more delays.
The agency plans to retire its aging three-shuttle fleet by September 2010 once space station construction is complete.
"So this can be a huge advantage to us to get this understood and move forward," Gerstenmaier said.
Commanded by veteran shuttle flyer Stephen Frick, Atlantis' seven-astronaut crew planned to haul the European Space Agency's Columbus laboratory to the International Space Station (ISS) to mark NASA's fourth construction flight of the year. The shuttle's STS-122 mission was slated to run about 11 days and include at least three spacewalks to attach the 1.4 billion Euro ($2 billion) Columbus lab to the ISS.
NASA had a slim, week-long window that closes Thursday in which to launch Atlantis due to the angles between the sun and the ISS while the shuttle is docked at the space station. While the window reopens around Dec. 30, mission managers have said they would wait until Jan. 2 to avoid software concerns with flying during the year-end change.
"We want to thank everyone who worked so hard to get us into space this launch window," Frick and his STS-122 crewmates said in a group statement. "We were ready to fly, but understand that these types of technical challenges are part of the space program."
The astronauts will depart for NASA's Johnson Space Center in Houston at 5:00 p.m. EST (2200 GMT) and resume training for their space station construction flight.
"We hope everyone gets some well-deserved rest, and we will be back to try again when the vehicle is ready," the astronauts said.