Shuttle engineers are studying what, if anything, to do about an unexpected reading from one of four liquid hydrogen main engine cutoff - ECO - sensors in Discovery's external fuel tank, officials said today. The sensors play a critical role during the climb to space by ensuring a shuttle's main engines shut down normally before draining the ship's external tank. A malfunction could trigger an early engine shutdown or let the powerplants run too long.

Discovery's external fuel tank is lifted to a checkout bay inside the Vehicle Assembly Building last week for pre-flight preparations. Credit: NASA-KSC

Discovery's launch last year was delayed because of ECO sensor problems that cropped up during pre-flight fueling tests. While engineers were never able to conclusively resolve the issue, data showed the problem was not generic and NASA managers approved a rule change that would have permitted Discovery to fly with three of four operational ECO sensors if the same problem showed up again. As it turned out, the rule change was not needed. On launch day, the sensors behaved normally.

This time around, a possible problem was noticed before external tank No. 119 - the one Discovery will use on its next flight - was shipped to Florida from Lockheed Martin's Michoud Assembly Facility near New Orleans. The tank arrived at the Kennedy Space Center last week.

During an all-systems electrical check at Michoud, a NASA spokesman said, liquid hydrogen ECO sensor No. 3 showed a 2-ohm shift from what engineers expected. The test was carried out again and ECO-3 showed the same reading.

"They haven't done anything since," said a NASA spokesman at the Johnson Space Center in Houston. "They're down there (at the Kennedy Space Center) processing the tank (for launch) and they're still talking about it."

The unexpected reading is within or very close to design specifications and it's not yet clear if anything will need to be done. If the sensors have to be replaced, engineers will need between one and three weeks to get inside the tank and make the swap.

"But you don't know if you've corrected the problem because you don't know if it's in the sensors or the connector and wiring behind it," the NASA spokesman said.

In the meantime, engineers are reviewing possible flight rationale to fly with three of four sensors much like the agency was prepared to do with Discovery's last launch.

An independent web site that covers NASA operations reported today that launch would be delayed for an ECO sensor swap out. But no such decisions have been made, according to shuttle program manager Wayne Hale. The issue has not yet even been elevated to Hale's level and it is far from clear what, if any, impact it might have on plans to launch Discovery during a 12-day window starting May 10. The next window opens July 1.

But Discovery's processing schedule has no built-in contingency time, and an ECO sensor swap-out would clearly pose an additional threat to the May window.

Hale told the shuttle team Monday, sources said, that the May launch target is threatened on a variety of fronts and that figuring out whether it makes sense to reshape foam insulation making up so-called ice/frost ramps on the exterior of the tank remains a long pole in the tent (see earlier status reports for details). But Hale said he plans to stick with May 10 while engineers collect additional data.

A discussion on whether to modify the shape of the ice/frost ramps is planned for March 23.

Background on the ECO sensor problems that cropped up during Discovery's STS-114 launch campaign last year can be found in our pre-flight archive.

A description of the sensors and how they work is available from CBS News and Spaceflight Now here.