NASA's Shuttle Strategy

Bill Harwood On Preparations For Discovery's Launch

(CBS) 
The launch window is defined by the space station's orbit, which is tilted 51.6 degrees to the equator. The shuttle has enough power to launch five minutes to either side of the moment Earth's rotation carries the launch pad into the plane of the station's orbit.

The issue is more complicated than usual in Discovery's case because of a NASA-imposed requirement to launch in daylight - and to make sure the external tank separates in daylight half a world away - to ensure good photography of the shuttle.

Given the inherent pressure of a tight launch window, the fuel sensor problem has been particularly vexing.

One of four liquid hydrogen main engine cutoff - ECO - sensors failed to operate properly during a routine pre-launch test July 13, forcing NASA managers to delay the first post-Columbia shuttle flight. The ECO sensors are part of a backup system that ensures the shuttle's main engines shut down normally before the tank runs out of fuel. NASA launch rules require all four ECO sensors to be operating before a countdown can proceed.

Despite exhaustive, around-the-clock tests, engineers were never able to find the problem that caused ECO sensor No. 2 to misbehave. Even so, they believe the testing completed to date proves the problem, whatever the cause, is not a generic defect and as such, they are confident it will not affect the other sensors.

"We have literally run every check that we can think of that people could suggest to us to try to find this problem and so far, no repeat," said Hale. "So we have developed a plan that says we have to go to cryogenic temperature to find out what's going on next.

"We also need to turn on all the equipment in the orbiter and the launch pad area to see if there's any electromagnetic interference that we could not check piece wise earlier. So both of those things together say we are ready to go the launch countdown configuration to see our next level of checking."

NASA's original launch rule 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 was corrected during Discovery's last overhaul, but the four-of-four launch rule remained on the books.

And that rule will be in place Tuesday. But the mission management team is prepared to sign an exception to the rule - permitting Discovery to launch with three of four ECO sensors - depending on what happens after the tank is loaded with fuel.