NASA and contractor engineers are working "around the clock" to find out why one of two circuits used to detonate the massive bolts holding the shuttle Atlantis to the launch pad failed to fire earlier this month. While the healthy circuit operated normally and all eight of the "hold down" bolts anchoring the shuttle vehicle to the pad detonated as required, a failure in such a "crit-1" system is of some concern. As such, the issue must be resolved before the shuttle Endeavour can be cleared for launch Nov. 10 or 11 on the next space station assembly flight.

A close up view of the top nut on a solid rocket booster hold-down post. Photo: NASA TV/Spaceflight Now

NASA managers plan to meet Thursday for a traditional flight readiness review to assess Endeavour's ground processing and to set an official launch date. Liftoff from pad 39A at the Kennedy Space Center currently is targeted for between midnight and 4 a.m. EST Nov. 10. But NASA managers may opt to delay the flight a day to give the station crew a bit of a breather and more time to shift their sleep cycles between the departure of a Soyuz deliver crew on Nov. 9 and the arrival of Endeavour. The new Soyuz lifeboat is scheduled for launch to the station at 10:11 p.m. EST Tuesday. The three-man crew aboard the new Soyuz will return to Earth Nov. 9 aboard the Soyuz currently docked to the outpost.

While the station crew can certainly use a break between the Soyuz visit and the arrival of Endeavour, NASA is unlikely to delay the shuttle's launch by more than one day. Lead flight director Paul Dye said today a variety of factors, including two unmanned rocket launches and the changing angle between the sun and the plane of the station's orbit, leave NASA little choice when all is said and done.

"We always like to get up as soon as we can, as soon as we're ready," Dye said. "The (station) crew is doing a good job of getting ahead for the (shuttle) mission and there is a sleep shift, but it's in the right direction. We have four days, something like that, starting on the 10th."

The maiden flight of an unmanned Boeing Delta 4 rocket is targeted for Nov. 16 and because all rockets launched from the East Coast use the same Air Force tracking equipment, Endeavour must wait its turn. For new rockets, the Eastern Range provides three days of launch opportunities. Complicating matters, NASA plans to launch a new TDRS communications satellite Nov. 20 atop an Atlas 2A rocket.

One wild card in NASA's launch planning is the ongoing investigation into what went wrong with the booster hold-down post circuitry during Atlantis' launching Oct. 7. Two fully redundant circuits are in place to fire the explosive bolts holding the shuttle "stack" to the pad - four massive 25-inch long, seven-and-a-half-inch-wide bolts at the base of each booster. Either circuit can deliver the power necessary to detonate the small explosives that fracture the nuts and free the shuttle for flight. During Atlantis' launch, only one circuit fired, leaving the crew one failure away from a potentially catastrophic event.

File footage shows technicans inserting the hold-down post into the solid rocket booster aft skirt. Photo: NASA TV/Spaceflight Now

Engineers initially believed the problem involved a problem with circuitry in the shuttle's mobile launch platform. But those circuits checked out fine, as did the two master events controllers in Atlantis.

"We've only had the vehicle back for a week or so to be looking at this," Dye said. "So far, and they're still doing a lot of testing, so far they really have not found the smoking gun, they haven't gone in and found a disconnected cable or two pins that didn't mate. So they're still looking at it.

"As a precaution, they have already replaced a lot of wire harnesses and connectors and I think there's a lot of confidence that while we don't have the complete answer right now, they'll have an answer or build some confidence in the system via testing between now and flight. So right now, it's still an ongoing investigation, they're going to talk about it at the FRR (Thursday). The folks are working around the clock on it trying to come to a conclusion. So we'll know more when we know more."

Whether NASA managers will set an official launch date Thursday or wait until more data is available on the hold-down post issue is not yet known. But it may not be possible to clearly identify what caused the Oct. 7 failure.

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