NASA Shoots for Saturday Launch

HOUSTON, Dec. 67 2007 — -- After experiencing trouble with two fuel sensors on Thursday, NASA said it will try to launch the space shuttle Atlantis again on Saturday. Engineers are using Friday on tests to learn more about the sensors' failure.

NASA has had plenty of headaches with the sensors. On Thursday morning, it scrubbed Atlantis's first launch attempt because the sensors were misbehaving.

They are known as ECO sensors, short for Engine Cut-Off. Mounted inside the base of the shuttle's giant orange external tank, they are designed to monitor fuel levels and shut down an orbiter's three main engines before its fuel tank runs dry.

Without them, high-power pumps in the shuttle's engines might continue to run, even with no fuel to pump. Engineers say if that were to happen, damage to the shuttle could be severe or even fatal.

The last four shuttle launch attempts have been plagued by wiring defects in the manufacturing of some ECO sensors, which could lead them to falsely report the fuel tank was empty. That could force an early engine shutdown before a shuttle reaches its proper orbit.

Faulty sensors forced NASA to delay the launch of the Shuttle Discovery in 2005 on the first flight after the Columbia accident in 2003. The shuttle's external tank was redesigned after the Columbia astronauts were killed and there was concern that some engineering error was causing the sensors to malfunction.

Atlantis' hydrogen tank was 80 percent full Thursday morning when the first signs of trouble appeared.

With time running out to solve the problem and launch today, launch director Doug Lyons said he was forced to scrub. But he said he is optimistic Atlantis will launch before its current launch window closes Dec. 14.

The window for this mission, labeled STS-122, is very short, so engineers are racing to figure out what is wrong with the sensors. Shuttles can only fly to the International Space Station during specific periods, limited by the station's orbit relative to Earth and sun.

So what's the problem with the ECO sensors? Is it a computer glitch or a hardware failure?

NASA is hoping to launch four shuttle flights this year. Few observers thought NASA could pull it off. Four shuttle flights in 2007 would be the most missions flown in one year since the Columbia accident.