Space Shuttle Launch Postponed

A fuel sensor that has given NASA headaches in the past acted up Friday morning forcing a delay of space shuttle Atlantis' liftoff.

NASA managers continued to discuss problems with one of four engine cutoff (ECO) sensors in the shuttle's external tank up until 45 minutes before the launch.

"We had a lot of discussion. ... We follow the rules," said launch director Mike Leinbach. "Ought to feel good that we did that."

The next launch opportunity is 11:15 a.m. EDT Saturday morning, but that window is also short, just five minutes.

If the launch is not completed by Saturday, NASA could open up launching opportunities in late September and early October by relaxing a requirement that launches take place in daylight.

The Atlantis astronauts were already aboard the shuttle, reports CBS News correspondent Peter King.

In order to fly Friday, NASA would have needed to waive a rule requiring all four sensors work properly, said NASA spokesman Bruce Buckingham.

The fuel gauges are designed to prevent the main engines from running too long or not long enough during the climb to space. If two sensors fail, a main engine would shut down. An engine shutdown at the wrong time could prove catastrophic, forcing the astronauts to attempt a risky emergency landing overseas, or leading to a ruptured engine.

ECO sensor No. 3 in the base of the hydrogen section of the huge external tank apparently "failed wet" early Friday during a test, reports CBS News Space Consultant Bill Harwood. Computer commands were sent to simulate a dry condition as part of a now-standard health check and the sensor continued to indicate wet.

Twenty-four propellant sensors are used in the shuttle's external tank, 12 each in the oxygen and hydrogen sections. Eight are used in each tank to measure the amount of propellant present before launch. Four ECO sensors in each tank are part of a backup system intended to make sure the ship's engines don't shut down too early, resulting in an abort, or run too long, draining the tank dry with potentially catastrophic results.

NASA's original launch commit criteria required three operational ECO sensors for a countdown to proceed. But in the wake of the 1986 Challenger disaster, the LCC was amended to four-of-four because of concerns two sensors could be knocked out by a single failure in an upstream electronic black box known as a multiplexer-demultiplexer. The single-point failure later was corrected, but the four-of-four launch rule remained on the books.

Because of ECO sensor problems going into me first post-Columbia mission, NASA managers ultimately developed an "exception" to the four-of-four rule that would permit a launch if A) a hydrogen sensor failed wet; and B) engineers could show the problem didn't originate in the multiplexer-demultiplexer avionics system.

As originally written, the flight rule exception called for standing down a day and if, during a second launch try, the same electronic "signature" was seen, the team could proceed with launch. Because of the way the system works, two more ECO sensors would have to fail wet to pose the threat of running the tank dry.

The problem is particularly vexing because Atlantis' tank is equipped with ECO sensors that underwent extensive inspections before installation.

Earlier this year, the launch of Discovery was delayed by almost two months so four hydrogen fuel tank sensors could be replaced after one was found to be faulty. A similar problem briefly delayed last summer's launch of Discovery on the first shuttle flight since the Columbia disaster in 2003.

NASA managers Thursday evening cleared another problem that had delayed the takeoff: a short in a motor in one of the shuttle's three electricity-generating fuel cells.

Atlantis was supposed to launch Aug. 27, but was delayed first by a lightning strike at the launch pad, then by the approach of Tropical Storm Ernesto. It was held up again Wednesday morning when the fuel cell problem arose right before the shuttle's tank was filled.

But the wait for this 11-day construction mission goes back far longer for Atlantis and its six-astronaut crew.

One of the two girders that Atlantis is hauling up to the international space station has been waiting at Kennedy Space Center for nearly seven years. Atlantis' mission is to bring up two girders and solar panels — weighing 17½ tons — to the orbital outpost.

Originally, the mission was slated for May 2003, but the February 2003 Columbia accident put the mission on hold for more than three years. Atlantis, which has flown 26 times, has not launched since October 2002.

Atlantis' astronauts will restart construction on the half-built international space station for the first time since the Columbia disaster.

The Russians are launching a three-person Soyuz capsule to the space station on Sept. 18, and Atlantis has to leave the station before the Soyuz arrives to prevent a cosmic traffic jam.

An ECO failure would not be a safety problem and would only require a shortened mission, said shuttle program manager Wayne Hale.