Spaceflight Now

Plume may have entered wheel well from within wing
Posted: March 7, 2003

NASA engineers struggling to match up telemetry from the shuttle Columbia's left wing and hot gas flow patterns found in wing debris increasingly suspect a plume of hot gas may have entered the wing from a breach at or near the leading edge area, close to the ship's fuselage, and worked its way into the left main landing gear wheel well. NASA sources say wreckage from the underbelly of the shuttle just adjacent to the left landing gear door indicates hot gas from inside the wheel well may have spewed out around the inboard edge of the door as the catastrophe unfolded.

A source close to the independent Columbia Accident Investigation Board outlined a similar scenario today, but he said other explanations are possible and that board members have not yet drawn any conclusions about where the breach occurred or what might have caused it in the first place.

But the source said several lines of evidence point to the possibility of a breach toward the front of the wing that burned its way deeper inside toward the wheel well as Columbia crossed above California and the southwest. Investigators are trying to determine the location of the breach and the behavior of the presumed internal plume of plasma by making various assumptions and then determining which scenarios best fit the pattern of sensor failures and elevated temperatures that were recorded during the shuttle's final minutes.

Such scenarios also predict certain types of heat-related damage that can then be checked against actual wreckage as it is recovered. That work is far from complete. But the source familiar with the accident board's investigation outlined one such scenario to provide a sense of how engineers are working through the problem.

Assume the shuttle began shedding debris, possibly one or more of the reinforced carbon carbon panels making up the leading of the left wing, early on as Columbia re-entered the atmosphere. As the shuttle plunged into the atmosphere and began heating up, the initially small problem area would worsen. Perhaps additional RCC panels, or nearby heat-shield tiles loosened and fell away as the disaster unfolded. Multiple "debris shedding" events were photographed and videotaped by ground observers as Columbia raced across the west-southwestern United States.

Under this scenario, at some point the forward edge of the wing just behind the RCC panels was breached, allowing hot plasma to burn its way into the unpressurized wing volume in front of the left main landing gear wheel well box. As the shuttle soared through a steep right bank to bleed off energy, the internal plume initially impinged on the lower part of the forward face of the wheel well box near where it merges with the aluminum skin of the lower wing. Then, as the shuttle banked to the left, the plume direction may have changed, possibly allowing the hot gas to burn through electrical cables routed around the upper front face of the wheel well.

Eventually, the plume entered the wheel well itself, possibly impinging on one or both tires and bouncing off to another part of the well. The board source said a plasma jet in the thin air where the shuttle was flying at the time can do that. Then at some point after that, hot air inside the wheel well may have worked its way out, through seals around the main landing gear door.

"My reading when I look at the tile around the door, that one piece we've been talking about, it looks to me that the plume is coming out," the source said. "What's happening is it's burned through the seals and it's coming out, not going in."

NASA sources said a similar analysis was presented at a Mishap Response Team meeting today at the Johnson Space Center in Houston.

But another board source stressed other explanations are possible and said some investigators have not yet ruled out a breach in the door seal area.

"I don't think anything right now is THE leading scenario because we're looking at a group of scenarios that, based on the evidence, are probably more likely than other scenarios," the source said, adding that initial scenarios in aircraft disasters frequently get discarded as an investigation proceeds.

Complicating the picture in Columbia's case, engineers have not yet conclusively determined whether the damage seen in many pieces of debris occurred before the breakup or later, as the result of atmospheric heating.

Assuming, however, a left wing leading edge burn through, the smooth flow of air over the wing would have been disrupted, possibly generating a hot vortex that then spun back along the left side of the shuttle's fuselage, above the left wing. Upper surface tiles and blankets may have been ripped away, showing up as debris shedding events to observers below. And such a vortex would explain elevated temperatures detected by sensors well above the wing on the side of the fuselage.

"There's a lot of stuff coming off that thing and nobody's saying anything," the board source said of Columbia's crew and flight controllers in Houston. "Why is that happening?"

A blurry picture of Columbia shot through a small Air Force telescope in New Mexico roughly a minute before the shuttle's breakup over Texas shows what appears to be major damage along the front of the left wing. The picture is very blurry, but whatever is going on in that region of the wing appears at or near the leading edge close to the fuselage.

"It looks like a large number of RCC panels are missing," the board source said.

The forward edge of the left wing is protected by 22 U-shaped RCC panels that protect the wing spar from extreme heating during entry while at the same time directing a smooth flow of air back over the upper and lower surface of the wing. RCC panel No. 1, located where the leading edge merges with the chine area of the fuselage, has been recovered. Sources say it is heavily damaged with a large area missing where it would have butted up against panel No. 2. That panel - No. 2 - has not been found. A panel that could be No. 3 or No. 4 has been found, with deposits of aluminum and steel slag on its inner surface. Panel No. 5 and others located farther outboard along the wing also have been recovered, as has a large portion of the wing tip area. Surprisingly, that piece of wreckage shows few signs of extreme heating or stress.

But the board source said if the breach occurred at or near RCC panel No. 2, the plasma would have impinged the forward face of the wing spar, possibly blowing molten metal back toward the front as it burned its way inside. As the breach worsened, the aerodynamic characteristics of the wing would have been affected, producing increasing drag. Columbia's flight computers initially counteracted this drag by adjusting roll trim with the ship's elevons and later, by firing four right-firing maneuvering jets.

Commander Rick Husband made a final call to mission control at 8:59:32 a.m., radioing "Roger, uh, buh..." He may have been about to say the word "both" or "before" in reference to a landing gear tire pressure alarm. The transmission was cut off when the antenna being used to reach a NASA communications satellite over the Indian Ocean was blocked by Columbia's vertical stabilizer. Another five seconds of telemetry made it through, showing the shuttle was still on course and under control up to that moment.

Then, even the telemetry stream was blocked. Twenty-five seconds later, a final two seconds of telemetry was received. The data showed the shuttle's fuselage was still intact and that it's major systems were operating normally. But there was no data from the left wing and the shuttle's hydraulic system was out of fluid, indicating the triply redundant hydraulic lines in the wing were ruptured.

In addition, the telemetry showed multiple computer alarms indicating severe problems with Columbia's left-side orbital maneuvering system rocket pod. The craft was in an "uncommanded" attitude, or orientation, and yawing quickly to one side. Eventually, the craft yawed out of control and broke up.

But the initiating cause of the disaster is still unknown.

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