By NBC News space analyst
Special to msnbc.com

Undetected corrosion that had weakened Columbia’s left wing could be the still-sought “missing link” between the otherwise-harmless debris impact during launch and the eventual vehicle loss during descent, a veteran shuttle engineer claims.

In recent days, NASA officials have expressed their frustrated bafflement over the observed debris impact on Columbia’s left wing. They have repeated studies made during the flight and still come up with results that show the worst-case damage is still far short of a mortal wound that could have prompted the catastrophic failure of the wing.

If the falling insulating foam were the triggering event, some additional factor or factors must have been present, they say, to multiply its harmfulness. Accident investigators continue to seek such factors and evaluate them.

'One possible explanation'
In a two-page memo sent to the NASA space engineering office in Houston, obtained exclusively by MSNBC.com, veteran space shuttle engineer Ray Erikson offers “one possible explanation”: “corrosion of the leading edge spars” on the left wing had already so weakened that structure that the small additional damage from the debris was then enough to cause the later failure.

In this suggested scenario, the loss of several tiles just behind the left wing leading edge led to significant heating (“not catastrophic by itself”) that is conducted into the support structure for the leading edge panels. Aluminum is notorious for weakening rapidly at even relatively small temperature increases, and combined with pre-existing corrosion this could have caused a bolt to break free and one leading edge panel to separate. From that point, vehicle loss was inevitable.

Erikson provided details of actual corrosion problems that he personally observed, and expressed concerns — echoed in other documents MSNBC.com has found — that in later years this hazard may not have been adequately appreciated.

Moreover, Erikson explained that the source of the corrosion was unique to Columbia and that the wing designs of the remaining three Orbiters — Discovery, Atlantis, and Endeavour — had been improved to the point that they are immune to this particular hazard and hence need no modifications to fly safely in the near future.

The most severe entry heating for space shuttles is on the nose and along the leading edge of the wings, especially where they join the fuselage. Reenforced carbon-carbon (RCC) panels are bolted on here, with shuttle tiles arrayed downwind of them.

By unhappy coincidence, wrote Erikson, the bolts were composed of a material that reacted badly with the aluminum mounting structure. When exposed to salt water, the metal in the bolts and the metal in the mounting structure acted like a weak chemical battery. This led to a gradual eating away of the aluminum structure.

The bolts were made of Inconel 718 alloy and of A286 corrosion-resistent steel (CRES), and they were not affected. But the structure, fabricated from 2024 aluminum honeycomb, did corrode when salt spray from the nearby ocean was dissolved in rainwater and seeped into the structure while Columbia was on the launch pad.

Launchings had begun in 1981. “In 1983,” Erikson wrote, “corrosion was detected around the attach points” of the panels. The damage was discovered during careful post-flight interior inspections of the wings.

Aluminum splints were glued to the corroded areas, a process that was repeated for several years while Erikson was working at the launch site. He recalled that access to this area was so tight that only one person on his team, “a tiny English fellow,” could even crawl into the wing to perform inspections and repair. Erikson did not know what subsequent repair and replacement was performed after he left in 1985.

Corrosion concerns persist
But the question of detecting and repairing corrosion has continued. In March 2000, the “Space Shuttle Independent Assessment Team” set up by NASA, reported that “hidden corrosion problems require a proactive inspection program with practical and reliable non-destructive evaluation techniques; at this point, this inspection is done on a randomized basis.” It urged that an assessment of the impact of hidden (or inaccessible) corrosion and the repairs of identified corrosion on the integrity of the Orbiter structure should to be made” by the end of 2000.

Further, the report listed as one of its “long term” suggestions that “Inspection techniques for locating corrosion . . . in inaccessible areas should be developed.” This implied that they realized such techniques currently did not exist.

The actual NASA response to these suggestions is not available. However, there is an “Orbiter Corrosion Control Review Board” that is dedicated to this very issue.

A retired NASA engineer who worked the corrosion issue anonymously assured MSNBC.com that corrosion was a known problem that was “examined very, very closely — even minor white deposits were looked at with a lot of interest.” He stated flatly: “That this could get out of control to the point of structural catastrophe would be inconceivable to me.”

Yet as recently as 2001, corrosion was raised during considerations of Columbia’s major modifications performed at the Palmdale, Calif., manufacturing facility and later at the Kennedy Space Center. When asked if deferring vehicle inspections to a new team in Florida would “incur additional risks,” Richard Blomberg, chairman of NASA’s “Aerospace Safety Advisory Panel,” said: “Personally, I don’t think if we slowed down on the corrosion inspections for a while, that there would be any immediate safety concerns.” However, he expressed concerns about longer-term issues.

The actual role — if any — of corrosion in the Columbia catastrophe remains to be verified, and both maintenance records and recovered debris may be adequate to disprove or prove this hypothesis. But space engineers feel that it will be some ‘missing link’ like this — if not this particular suggestion — that will finally make sense out of this disaster.

Neither Erikson nor current NASA engineering officials could be reached for comment.

James Oberg a former shuttle flight controller, is an author and NBC analyst. Among his books is a history of the U.S.-Russian space alliance titled “Star-Crossed Orbits.”

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