Discovery's Launch Date Shifts as Engineers Investigate Sensor Glitch

A diagram of the engine cutoff (ECO) sensors inside the external tanks used by NASA's space shuttle. CREDIT: NASA.

CAPE CANAVERAL, Fla. - NASA's plans to launch the space shuttle Discovery are stretching further into July as engineers struggle to understand a fuel sensor anomaly that scrubbed an attempted space shot Wednesday.

If the problem is something as simple as a case of loose wiring, NASA could launch Discovery - its first orbiter to fly since the 2003 Columbia tragedy - at about 2:14:21 p.m. EDT (1814:21 GMT) on Sunday, July 17, but the chance of that is remote at best, NASA officials said here Thursday.

"If we were to get extremely lucky, it is theoretically possible we could still launch on Sunday," said Wayne Hale, deputy manager of NASA's shuttle program, during a press briefing here at Kennedy Space Center (KSC). "This represents a really optimistic, good luck scenario which I think is not very credible."

Shuttle engineers are more likely to spend several days troubleshooting Discovery's external tank fuel sensor system in hopes of isolating the problem, Hale added.

In that case the launch could push into early next week, shuttle officials said. While Discovery's STS-114 astronaut crew - which was strapping into the orbiter when their launch was scrubbed - will stay on at KSC for the time being, they will likely return to NASA's Johnson Space Center if the liftoff date stretches further out, they added. Discovery must launch by July 31 to make its current launch window. The next opportunity to launch after that runs between Sept. 9-24.

Discovery's mission management team expects to meet at 3:30 p.m. EDT (1030 GMT) Friday for an update on roubleshooting plans for Discovery. A press briefing is expected pending that meeting, NASA officials said.

During July 13 countdown for Discovery's STS-114 mission, launch controllers detected a failure in one of four engine cut-off (ECO) sensors in the liquid hydrogen section of the orbiter's external tank. In a countdown test designed to force the sensors to report an empty fuel tank, one sensor reported it full - or 'wet'. Since all four must perform perfectly in order to launch, flight controllers called off the launch attempt.

The sensors track the amount of liquid hydrogen propellant so the orbiter can shut down its three main engines before propellant runs out. A similar sensor suite performs the same function for the external tank's liquid oxygen supply.

As pad engineers emptied Discovery's fuel tank late Wednesday, the faulty fuel sensor continued to report a full tank despite the absence of liquid hydrogen. In later tests, the sensor reported the proper, dry tank status.

In even more subsequent tests, engineers were able to get the sensor to again report a full external tank before it settled into normal operations, Hale said.

"We now have an intermittent, transient kind of failure which is the worst kind of thing to troubleshoot," Hale said.

Engineers are still unclear whether the fuel sensor glitch resides with the actual sensor itself, the wiring between it and Discovery or the sensor box inside the orbiter that processes the fuel readings. The box does contain transistors that engineers have found faulty in the past, but the malfunction seen Wednesday is not indicative of a transistor failure, shuttle managers said.

"The indications are not consistent with what a transistor failure would indicate," Hale said. "That's part of the trouble here...if we really thought that was the smoking gun, I think we'd have something to go after."

For now, all troubleshooting plans assume that fixes to Discovery's ECO sensor system can be made at the pad, with mission managers hoping they will not have to roll the shuttle launch stack back to the 52-story Vehicle Assembly Building for work or send engineers inside the fume-filled liquid hydrogen tank.

"We would like to not go into the liquid hydrogen tank," Hale said, adding that special precautions and care must be taken to ensure engineers safe access. "It becomes an operation you'd just as soon not do."

Shuttle officials and engineers hope to determine a specific troubleshooting plan for Discovery tonight and decide whether to begin unloading the orbiter's cryogenic fuel cell reactants to allow pad workers safe access to its aft section where the sensor box resides.

"At this point we are following the normal 72-hour scrub turnaround process," Michael Wetmore, NASA's director of space shuttle processing at KSC, told reporters in the briefing.

But NASA cannot keep its launch team on alert indefinitely, and if the mission is not expected to launch by early next week, the team will shift away from a countdown hold status and into full troubleshooting mode.

"Early next week is the break point at which we need to get out of the countdown procedure and get everybody some crew rest," Wetmore said.

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