Shuttle Endeavour lifts off from the Kennedy Space Center on November 30. Photo: NASA-KSC

During launch of shuttle Endeavour November 30, one of two explosive cartridges used to separate a strut holding the base of the orbiter's left-side solid-fuel booster to the ship's external fuel tank failed to fire.

A redundant "NASA standard initiator," or detonator, inside the strut in question did fire, however, and the booster separated cleanly.

But troubleshooting could impact preparations for launching the next space station assembly mission, a long-awaited flight by shuttle Atlantis in late January to deliver the U.S. laboratory module, Destiny, to the orbital outpost.

Atlantis is scheduled to be hauled from its hangar to pad 39A at the Kennedy Space Center early Monday. If that schedule holds up, the Destiny module will be moved into the pad's payload changeout room Jan. 2 for installation in Atlantis' cargo bay.

But the orbiter's rollout Monday could be held up pending analysis of what went wrong during Endeavour's launch.

Diagram of the solid rocket booster aft attach struts. Image: NASA

The shuttle's two solid-fuel boosters fire for two minutes and four seconds, providing most of the orbiter's initial liftoff power. When internal chamber pressure drops to 50 pounds per square inch, the boosters are jettisoned to parachute into the Atlantic Ocean.

After recovery, disassembly and inspection, the four segments making up each booster are shipped back to rocket-builder Thiokol in Utah to be reloaded with propellent for another flight.

During inspections of Endeavour's left-side booster late this week, engineers discovered one of two detonators in one of the rocket's three aft struts -- the lower strut -- had failed to fire.

"I think they found it last night at the Cape when they were going over the SRBs," said Endeavour's lead flight director, William Reeves. "The bolt does have redundant initiators on it and it's a bolt that can separate from either end to separate the strut.

"The way I understand it - and I don't have all the details yet - one initiator didn't fire and the other one did the job. So that's enough said at this point. There will be a thorough examination of it and an analysis of it to determine why that one didn't fire. But the redundant system did perform its job."

Here's a description of the separation system from NASA's on-line Shuttle Reference Manual:

The SRBs separate from the external tank within 30 milliseconds of the ordnance firing command.

The forward attachment point consists of a ball (SRB) and socket (ET) held together by one bolt. The bolt contains one NSD pressure cartridge at each end. The forward attachment point also carries the range safety system cross-strap wiring connecting each SRB RSS and the ET RSS with each other.

The aft attachment points consist of three separate struts: upper, diagonal and lower. Each strut contains one bolt with an NSD pressure cartridge at each end. The upper strut also carries the umbilical interface between its SRB and the external tank and on to the orbiter.

There are four booster separation motors on each end of each SRB. The BSMs separate the SRBs from the external tank. The solid rocket motors in each cluster of four are ignited by firing redundant NSD pressure cartridges into redundant confined detonating fuse manifolds.

But given the critical nature of the system, NASA and contractor engineers want to find out what went wrong with the one that failed to make sure similar problems -- or worse -- don't affect downstream flights.