Mere slit may have been 'Columbia' wing's undoing
© St. Petersburg Times
HOUSTON -- Investigators are looking at new scenarios for how hot gases entered shuttle Columbia's left wing, causing it to disintegrate over Texas on Feb. 1.
Contrary to previous analyses, investigators said Tuesday they now suspect that an object seen floating near Columbia on its second day in space was either a fragment of a carbon-carbon panel that wraps around the wing's edge or a seal from that region.
Investigators think the piece somehow came loose after being damaged by a chunk of foam insulation that flew off the external tank and hit the wing during launch. The impact may have opened a vertical gap between heat-shield panels rather than knocking a hole in them.
The object discovered floating away from the shuttle on its second day in orbit now appears likely to have been all or part of a "T-seal," used to close gaps between the pieces of reinforced carbon that protect the wing's leading edge, officials said.
If it was a seal, the searing hot gases that surround the ship during re-entry would have had a clear path into the shuttle's aluminum skin, said Harold Gehman Jr., chairman of the Columbia Accident Investigation Board.
It has been clear for some time that an opening in the wing allowed the intrusion of deadly superheated air as the orbiter descended toward landing, triggering its destruction. But investigators continue to refine the accident scenario based on accumulating evidence from three sources: the wreckage, onboard sensors and enhancements of imagery obtained during the mission.
Pinpointing where the heat entered the wing as precisely as possible over the next month is important to determining what caused the breach, said Scott Hubbard, a member of the Columbia Accident Investigation Board. "We're closing in on it."
The three different lines of evidence, however, point in slightly different directions on the wing. "The challenge that we've discovered is to try to integrate and reconcile differences between these different data streams," Hubbard said.
A leading candidate as the cause of the breach is foam insulation from the shuttle's external propellant tank that came loose and struck the orbiter about 80 seconds after its Jan. 16 liftoff.
The investigators have scheduled a battery of tests at an independent laboratory, beginning next month, in which a special gun will fire chunks of foam debris at a facsimile of the wing fitted with spare shuttle parts from the suspect leading-edge area, including heat-shield panels made of a composite called reinforced carbon, and the associated bolts, fillers and other materials.
"In order to understand what happened when the foam hit the left wing, we need to come up with the best possible impact tests," Hubbard said. "So we're looking at this data (on the breach) to help inform and structure that test."
Based on painstaking enhancements of the film and video record of the foam impact during launch, with participation by multiple government and private organizations, the latest analysis has moved the possible strike zone outboard to heat-shield panels seven and eight out of 22 such panels on the wing. A previous analysis had focused on panel number six.
A second line of evidence, from onboard sensor measurements of temperature and other variables taken during Columbia's flight, indicates that a substantial heating event occurred at heat-shield panel nine and at the interface between numbers nine and ten, or slightly farther out than the visual record seems to show, Hubbard said.
He noted that the sensor record should grow considerably in coming weeks, as engineers analyze data from a flight data recorder recovered by ground searchers, which has so far yielded readouts from 622 channels.
The final body of evidence is the recovered wreckage, which has grown to represent some 36 percent of the orbiter's dry weight and more than 70,000 pieces, said Gehman, chair of the investigating board. Searchers have been finding crucial left wing pieces in targeted areas outside the primary search grid, he said.
Recovered debris from wing panels eight and nine shows massive deposits of metal, as if aluminum was being melted and sprayed onto it. Also, a nearby carbon-carbon panel had been whittled down from a quarter inch in thickness roughly to the thickness of a dime.
"This kind of heating event indicates long duration, very extreme heating," Hubbard said. "We don't know quite what to make of this yet, other than what I said, a very severe heating event in the intersection between panels eight and nine."
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