Engineers readying the shuttle Discovery for launch Wednesday on a space station resupply mission are troubleshooting an apparent electrical problem with circuitry associated with a backup main engine computer.

Credit: Justin Ray/Spaceflight Now

Engineers said earlier Tuesday the problem had been resolved, but another glitch in the system raised concerns and additional troubleshooting was ordered.

Assuming the problem is resolved, Discovery will be fueled early Wednesday for a launch attempt at 3:52:13 p.m. EDT, roughly the moment Earth's rotation carries pad 39A into the plane of the space station's orbit.

Forecasters are predicting a 70 percent chance of acceptable weather with a slight chance of low clouds and showers. The forecast drops to 70 percent "no-go" Thursday as a front moves through the area and improves to 70 percent acceptable again on Friday.

Early Tuesday, engineers ran into a problem with the backup computer controller mounted on main engine No. 3. Troubleshooting indicated the problem was related to "transient contamination" in a circuit breaker and NASA Test Director Steve Payne told reporters in a 10:30 a.m. status briefing that after troubleshooting and power cycles, the controller powered up normally and that engineers were closing out paperwork.

But at almost the same time, another anomaly occurred. Sources said a brief, unexpected voltage drop was observed while nothing else of any significance was going on.

Engineers are troubleshooting the issue to determine if the behavior is consistent with contamination in a circuit breaker or whether it indicates a real problem with the backup engine controller or trouble elsewhere in the shuttle's complex circuitry.

"It still could be associated with the same circuit breaker," an official said. "It's just too soon to say."

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Each of the shuttle's three hydrogen-fueled main engines is equipped with a two-channel controller that monitors engine operation 50 times a second. That telemetry stream is critical during ascent and NASA flight rules require full redundancy for a launch to proceed.

NASA's mission management team planned to meet at 3 p.m. to discuss the results of ongoing troubleshooting. Engine controllers can be replaced at the launch pad, but it's not clear whether a swap-out and retest could be completed before the current launch window closes Nov. 7. The next available launch period is Dec. 1-5.

But officials said talk of a controller swap out was premature. As of this writing, engineers were not sure where the problem was located or what, if anything, might be needed to clear Discovery for launch Wednesday.

Assuming Discovery is cleared to fly as is, engineers will begin loading a half-million gallons of rocket fuel into Discovery's external tank starting around 6:27 a.m. Wednesday.

Discovery's six-member all-veteran crew -- commander Steven Lindsey, pilot Eric Boe, Michael Barratt, Nicole Stott and spacewalkers Timothy Kopra Alvin Drew -- plans to begin strapping in around 12:32 p.m. to await launch on the 133rd shuttle mission, Discovery's 39th and final flight.

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