NASA studies shuttle fuel tank sensor issue

NASA and contractor engineers are studying unexpected readings from a critical fuel-depletion sensor in shuttle Discovery's external tank, agency officials said today.

But it's unclear whether the issue will prompt a delay in NASA's plans to launch its second post-Columbia test flight around May 10.

Kyle Herring, a spokesman for NASA's Johnson Space Center in Houston, said the readings cropped up during an electrical checkout of Discovery's tank before it was shipped to Kennedy Space Center.

The readings showed a slight shift in the sensor's resistance to electrical current. The level of resistance is key to accurate sensor readings. Engineers are trying to determine why the readings -- which still were within specifications -- came out different than expected.

The sensor is one of four liquid hydrogen engine cut-off (ECO) sensors within shuttle external tanks. They provide a back-up means of making certain the shuttle's three liquid-fueled main engines shut down properly after an ascent into orbit.

A sensor failure could prompt a premature engine shut down, which could lead to an unprecedented emergency landing attempt, or allow engines to run dry, potentially triggering an inflight catastrophe.

ECO sensor problems during fueling tests last spring prompted NASA to delay its first post-Columbia shuttle mission to July.

Herring and June Malone, a spokeswoman for NASA's Marshall Flight Center in Huntsville, Ala., said it is unclear now whether the sensors within Discovery's tank would have to be replaced.

Replacement work would be invasive, requiring technicians to enter the tank through a manhole in its aft dome. Prior to shipping the tank from its factory, managers determined the work -- if ultimately deemed necessary -- would be more easily accomplished in a checkout cell in the KSC Vehicle Assembly Building.

Early estimates -- which have not yet been refined -- show the work might take anywhere between five days and three to four weeks.

NASA is aiming to launch Discovery around May 10, or a week into a window that extends from May 3 through May 22. Its next opportunity would come in July.