NASA managers are converging on a repair plan to fix small cracks in space shuttle fuel lines that could lead to a resumption of shuttle flights by late September or early October. During an all-day meeting Wednesday, shuttle managers reviewed repair plans and a variety of possible launch options. While no final decisions were made, the most favored option calls for delaying the shuttle Columbia's flight on a 16-day science mission until at least early December, after two space station assembly flights.

Under that scenario, the shuttle Atlantis would take off between Sept. 26 and Oct. 10 to deliver an outboard solar array truss segment to the space station. The shuttle Endeavour, carrying another truss section, would follow suit between Nov. 2 and Nov. 6. Columbia, originally scheduled for liftoff July 19, would slip to no earlier than Dec. 3.

But Sept. 26 is a "best case" launch date, sources say, and it assumes engineers begin re-installing Atlantis' main engines by Aug. 5. That work could easily be delayed depending on the work required to fix the hydrogen flow liner cracks that grounded the shuttle fleet in the first place. While the dates might change, shuttle and station program managers have agreed the upcoming station flights have priority and it now appears unlikely Columbia will launch before either station mission.

The launch schedule is complicated by Russian plans to launch a new Soyuz lifeboat to the station Oct. 22. Because of complex mission control issues, a Soyuz cannot be launched until a shuttle flight is back on the ground. That means Atlantis, assuming the current manifest option is finalized, must get off the ground by Oct. 10.

The next shuttle launch window opens on Nov. 2 and closes Nov. 6. That's because NASA does not want a shuttle in orbit during the Leonids meteor shower Nov. 19. Even though Endeavour's mission currently is scheduled to last 10 days, NASA always protects against the possibility of bad weather that could keep a crew in orbit an additional two days. For a launch on Nov. 6, that policy would provide landing opportunities through Nov. 18.

NASA managers briefly considered the possibility of launching Endeavour before Atlantis, but Endeavour's ground processing cannot be moved up. It now appears all but certain Atlantis will fly before Endeavour, although the exact launch dates remain uncertain.

Wild cards in all of this include:

• A possible Soyuz taxi flight delay, either to give N'Sync's Lance Bass more time to train (assuming he finalizes a contract for a seat on the flight) or to process the launch vehicle. An on-time launch Oct. 22 would permit a daylight landing at the end of the 10-day flight. The Russians could, however, waive their daytime landing requirement depending on training or processing issues. In that case, the Soyuz taxi crew could be launched as late as Nov. 1 and still get home in the station's current Soyuz before that spacecraft reaches its 200-day orbital lifetime limit on Nov. 11.
  
  
• Beta angle cutouts: Station assembly flights cannot be launched during certain periods when the angle between the sun and the plane of the lab's orbit is such that solar power generation is minimized. Just for the record, one such beta angle cutout falls between Oct. 15-24; the next cutout occurs between Dec. 9 and Dec. 25.
  
  
• Engineers still do not know for sure what caused the flow liner cracks found in all four of NASA's orbiters. If additional inspections are required, or if current repair plans fall through, the shuttle fleet likely would remain grounded until at least early November, if not longer.

NASA originally planned to close out the year with three flights: Columbia on July 19; Atlantis on Aug. 22; and Endeavour on Oct. 6.

The station's current crew - Expedition 5 commander Valery Korzun, Sergei Treschev and Peggy Whitson - plans to return to Earth aboard Endeavour after four-and-a-half months in space.

But during routine inspections of the shuttle Atlantis on June 17, engineers discovered a small crack in a liner inside the 12-inch-wide liquid hydrogen feed line leading to main engine No. 1. Two more cracks in the same flow liner were found the next day.

Similar cracks then were found aboard Discovery and then Columbia, at which point shuttle program manager Ronald Dittemore stopped processing for the July flight. Last week, cracks were found in flow liners aboard Endeavour.

The concern is that if a crack worsened, a piece of debris could break off and get sucked into a main engine with possibly catastrophic results. Dittemore stopped Columbia's processing and, in effect, grounded the fleet pending the outcome of an exhaustive engineering analysis.

That work continues and engineers have yet to determine the cause of the cracks. But they are converging on a plan to fix them. While some engineers believe the shuttle can be safely launched as is, it now appears likely Dittemore will order repairs. A meeting to make a decision one way or the other is expected before the end of the month.