The small cracks, uncovered Thursday as technicians used X-ray devices to inspect the external tank, represent the latest bump along the road to launch. The cracks were found following a chain of inspections, repairs, and more inspections dating back to Discovery's scrubbed Nov. 5 liftoff.
That liftoff was canceled after unacceptably high levels of explosive hydrogen were detected outside a crucial fixture on the side of the tank. The fixture transfers excess gas that accumulates as the tank fills to a line that carries the hydrogen to a flare stack, where the excess gas is burned.
IN PICTURES: NASA's Space Shuttle
When technicians returned to the launching pad after the liquid hydrogen and liquid oxygen tanks had been emptied, they noticed cracks in the external tank's foam insulation at one of 108 "stringers" – long U-shaped aluminum ribs that reinforce a section of the tank's outer shell. This section, toward the top of the tank, spans a gap between the hydrogen and oxygen tanks inside.
Once technicians removed the cracked segment foam in anticipation of repairing it, they found that the stringer beneath the foam had cracked, too – shifting enough to fracture the foam. Subsequent inspections showed that a second, adjacent stringer hosted a small crack as well.
Although these cracks have appeared in past tanks and were successfully repaired, this incident was unusual because in the past, the cracks were caught far earlier in the shuttle program's inspection process, mission managers say.
As a result, they wanted to take the time needed to review the inspections on this tank to make sure the process hadn't broken down. And if it hadn't, they wanted to get a clear grasp of the causes that would trigger cracks at the last minute.
The problem appeared on the side of the tank facing the underside of the orbiter, where the shuttle's heat-shedding tiles and carbon-composite edged to the wings could be vulnerable to damage if a piece of foam broke loose and struck them.
In 2003, the shuttle Columbia broke up on reentry, killing its seven-member crew – an accident traced to chunks of foam striking the orbiter's thermal-protection system during ascent.
In this case, technicians were able to repair the stringers on the pad. On Dec. 15, controllers refilled the external tank to see if the repairs held up to the expansion and contraction the tank experiences as it undergoes the extreme temperature changes presented by ultra-cold liquid hydrogen and liquid oxygen.
But managers were uncertain if the problem extended to additional stringers on sides of the tank workers can't reach while the shuttle is on the pad. So on Dec. 21, Discovery headed back to the towering Vehicle Assembly Building for additional inspections.
The cracks technicians uncovered Thursday appeared to vindicate the decision to send Discovery back to the barn for an additional look. Officials say they could decide on Jan. 3 whether to return the orbiter to the pad after weekend repairs or keep Discovery in the Vehicle Assembly Building for additional modifications to the tank.
