HOUSTON - (KRT) - The chunk that hit the Columbia's left wing after takeoff appeared to come from the area of a foam patch applied by hand at Kennedy Space Center - a problem zone during at least four previous missions, NASA documents show.

A spokesman said Wednesday that the hand patching is a subject of "particular concern" to the panel investigating the disaster, along with the space agency's ongoing efforts to perfect the shuttle's foam insulation, even as Columbia took flight.

NASA experts have warned that dislodged patches from the massive external tank could damage the shuttle's belly tiles and subject the craft to an abnormal rise in temperatures similar to that seen in the Columbia's sensors just before it disintegrated over Texas on Feb. 1.

Compared with most of the tank foam, such "closeout" areas are denser and more likely to accumulate ice, and they had shown a tendency to come off and rip adjacent foam "acreage," the documents show.

Over the years, NASA has devised measures to detect and reduce loose closeout foam, but it was unclear this week whether those techniques were employed on the last Columbia mission or a flight last October, when sister shuttle Atlantis lost a chunk of foam from a similar area.

"Things having to do with the external tank are getting early focus," said Steve Nesbitt, a spokesman for the Columbia Accident Investigation Board. "How the hand-applied installation is done on the external tank is a matter of particular concern."

NASA officials did not return calls Wednesday.

The board has not specified a cause for the disaster but has repeatedly mentioned the breakaway foam as a possible cause of a breach in the shuttle's aluminum skin, which allowed superheated gases to invade Columbia's structure and ultimately doom the craft.

Most of the insulation is precision machine-sprayed onto the external fuel tank during manufacture at the Michoud Assembly Facility in New Orleans, where crash investigators have been examining every facet of tank construction.

However, handcrafted patches are specified for 27 areas on the tank where various tubes and hardware are attached, including five closeout patches that are applied at Kennedy, according to a manufacturer's manual for the tank.

The manual states that the closeouts are "most critical" on areas like the external tank. There, supercold temperatures can lead to loss of foam on ascent from "cryopumping," when air is sucked into voids in the foam and converted to liquid, which pops the insulation loose.

NASA officials have said their analysis, performed while Columbia was in flight on its 16-day mission, showed that the external tank's foam coating was too light to have done fatal damage. They have estimated the weight of that breakaway piece at 2.67 pounds.

But a NASA ice inspection handbook warns that defects and voids - and ice - will be more common in closeout areas, where foam is denser and "cannot be processed by the tightly controlled automated mixing and application processes."

"Defects are more likely to occur around closeout areas, areas which are hand-worked {ellipsis} ," the manual says.

NASA officials have said that the foam dislodged from the area of the bipod, the V-shaped strut that attaches the front end of the shuttle to the tank.

The bipod closeout was specifically cited in the ice manual as a likely point of ice and frost formation.

A report by NASA experts of a 1995 test of laser, or shearographic, detection of loose foam listed the bipod closeout area as the subject of "greatest concern" among potential foam loss areas. It's not known whether the shearographic methods discussed in that study were used to analyze the foam on Columbia.

That same report noted difficulties inherent in hand-patching areas that have been exposed to dirt or the elements.

"The foam expands unevenly, causing voids, and surface contamination on the substrate may cause debonds," the report says.

The closeouts at Kennedy are made with two types of material, in a "clean room" enclosure after the tank is mated to the shuttle in the Vehicle Assembly Building. That typically happens about three weeks before rollout to the launch pad, according to a NASA engineer who worked on the closeouts until he left the agency in the late 1990s.

Just inside the two points where the bipod bolts to the tank, two bare metal mounting brackets stick out, where ground service equipment fastens to the tank to pull it snug to the shuttle. The two mounting points are just below the ribbed band known as the intertank.

After the tank and shuttle are mated, engineers install a small retaining device over the two bare metal spots, then pour liquid foam behind that dam to fill the gaps to the level of the surrounding, factory-installed insulation.

The first NASA-reported loss of bipod closeout foam was on the June 1983 launch of Challenger. That was followed by a similar foam loss on the January 1990 flight of Columbia. No records are available from those flights about the size of the foam chunk or damage to the shuttles.

A little more than two years later, the Columbia again suffered bipod foam loss, that time from both closeouts, during a June 1992 launch. A 6-inch divot was missing from the right closeout, and the left closeout popped loose, taking with it a chunk of intertank foam. That piece measured 20 inches by 10 inches by several inches deep, according to a debris and ice assessment prepared after the mission.

The chunk was blamed for a 9-by-4 1/2-inch damage area on three underbelly tiles near the back of the left wing.

That 1992 foam chunk was slightly smaller than the estimated size of the piece seen hitting the Columbia on its last mission: 26 inches by 16 inches. NASA officials have never explained their estimate that the piece was 6 inches thick, at least twice the prescribed thickness for foam in the bipod area.

In an apparently successful attempt to prevent a recurrence, the engineers reported, they "vented" the intertank foam around the bipod patch on the next three missions that year, drilling small holes around the patch to let gas escape.

It could not be determined whether that venting technique was continued or whether it was used on Columbia's last mission.

Venting of the intertank was used in the late 1990s to address foam "popcorning," splintering of tiny foam shards from the factory-applied insulation. Popcorning escalated in 1997, after NASA changed its foam formulae for environmental reasons.

However, a NASA technical document detailing the venting solution shows that it did not call for venting in the bipod area of the intertank.

Analysis of the foam loss 80 seconds into Columbia's Jan. 16 launch is complicated by the absence of photos of the affected area.

Normally, as the empty tank is released to burn up in the atmosphere, the surface facing the shuttle is photographed by a high-speed movie camera in the belly of the ship. That film is stored on the shuttle and brought home.

Also, crew members typically take still photographs as the tank slips away into space. Those photos on some missions have been made with a digital camera and transmitted to NASA before re-entry, but all of the tank images on Columbia were recorded on film now presumed lost.

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Dallas Morning News staff writers Allen Pusey and Bruce Nichols contributed to this report. from this link

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