Sacramento - Apr 16, 2003
The pieces of the puzzle of Columbia's fatal accident are suddenly starting to come together with dramatic speed -- although some major and important details remain puzzling.
The key piece so far has turned out to be its OEX ("Orbiter Experiments") data recorder, found virtually intact in a Hemphill, Texas field on March 19. Many observers (including this reporter) had thought that the hunt for debris in Texas was actually something of a sideshow in the investigation, and that the discovery of any of those isolated pieces of debris photographed coming off it during its earlier passage over the western United States would be the crucial clue -- but so far not one piece of that debris has been located, whereas the data on the recorder has been invaluable.
The OEX recorder was unique to Columbia, a result of its status as the initial "test" vehicle for the Shuttle program. During both launch and reentry, it recorded constant data from 670 different aerodynamics-related sensors for temperature, pressure, vibration and stress -- many of them not linked to the Shuttle's telemetry system.
It turns out that most and perhaps almost all that data is still readable on its tape, and that the recorder continued to function until the exact moment when the Shuttle broke up. But the key data already uncovered on it -- after just a very preliminary examination -- comes from the first few minutes of reentry.
It turns out that the anomalous heating inside the Shuttle's left wing began much earlier than previously thought -- five minutes after it began undergoing any significant air resistance at all, and fully two minutes before the Shuttle's "peak heating" period began.
At that time, a sensor behind one of the large, U-shaped "RCC" thermal-protection panels curving over the wing's leading edge -- which is particularly vulnerable to heating -- began reporting a dramatic rise in temperature. (This was RCC Panel No. 9, located just beyond the point where the wing's leading edge bends out more sharply away from the Shuttle's side.) Three minutes later, at a temperature of fully 232 deg C, the sensor failed.
Forty seconds later, other sensors at various points inside the wing began recording the steady erosive inward progress of a jet of superheated air through the interior of the wing, with some of it also streaming into the left wheel well (probably through a forward vent in the well).
Since this progress began soon after that first sensor -- mounted on the front surface of the wing's internal aluminum support spar -- failed, the conclusion is that the spar itself melted through soon afterwards (aluminum melts at 660 deg C), allowing the superhot air to stream into the wing's interior.
About two minutes after that, observers photographed large pieces of debris (probably RCC panels and tiles) starting to peel away from the Shuttle, and its aerodynamic handling properties began to deteriorate until its loss of control and breakup six minutes later.
The major conclusion drawn by the Columbia Accident Investigation Board ("CAIB") from the tape is that the first breach inside the wing's outer layer of thermal protection began so soon that it could not have been a subtle flaw in a tile or RCC which was enlarged and torn open by aerodynamic stress during reentry -- there had to be some kind of significant hole BEFORE Columbia even began its reentry.
The heating detected by the initial sensor began when the Shuttle was still fully 87 km up -- at a point where the air is so extremely thin that, to quote panel chairman Adm. Harold Gehman, "you've got to have a ton of heat", or rather a big volume of hot air, entering the wing and striking the sensor to cause such a dramatic temperature rise.
This meshes with the second major new discovery of the past few days: tests of the radar reflectivity properties of various kinds of objects that might have separated from the Shuttle now make it near-certain that the object recorded drifting away from it by Air Force radar after it carried out a minor attitude maneuver during its second day in orbit was some kind of important piece off the wing -- not something irrelevant like a thermal blanket from some piece of equipment in the cargo bay.
At its April 1 press briefing the Board said that, of the 30 types of Shuttle-related objects tested so far, the only one that acceptably matched the object's radar profile was one of the "carrier panels" -- tile-covered metal strips about 56 by 10 cm that cover the thin gap between the edges of the RCCs and the outer edges of the regular thermal tiles covering the flat upper and lower surface of the wing.
Any impact damage big enough to detach this panel would almost certainly have produced a big enough hole in the wing's leading-edge thermal protection to explain the quick heat rise recorded on the OEX tape.
And this, in turn, meshes well with the recent conclusion -- from detailed analysis of the fuzzy video camera shots of the big piece of external tank foam hitting the Shuttle during launch -- that it did hit the wing's leading edge. Indeed, the main force of the impact is indicated by those photos to be centered on the front edge of the carrier panel beneath RCC panel No. 6.
As panel member Scott Hubbard says, "If it hit right on the edge [of the carrier panel], you could imagine snapping it off."
Thereafter a stream of superhot air entering near panel 6 could easily have squirted through the narrow tunnel-like gap between the curved RCC panels and the front of the main wing spar to melt the spar. Some recovered pieces of wing debris also suggest this sequence, and the first temperature sensor such a stream would have come to was the one behind panel 9.
Since then, however, further debris discoveries have called this precise determination of the nature of the detached orbiting object into question. Panel 6 itself was found relatively intact, and part of all the other carrier panels from the left wing have now also been found.
The Board now speculates that the object may actually have been either part of a carrier panel -- with its radar reflectivity increased by a still-attached metal bracket -- or part of one of the "RCC" panels itself (although a complete one would be too radar-bright to match the object). Tests are now continuing with more candidate objects to study these possibilities.
But this is a debate strictly over details -- it is now regarded as virtually certain that this foam impact, in some way, caused Columbia's ultimately fatal damage. Assuming that a large piece of orbiting "space garbage" just happened to hit the Shuttle in the same area where the foam hit it during launch is stretching coincidence to the breaking point.
At this point, the only remaining major mystery is how the supposedly soft, lightweight foam could have done such damage. Even given that the estimates of its size have increased slightly -- it's now thought to have been about 62 by 37 by 12 cm -- it should still have weighed only about a kilogram and been relatively soft material.
Some observers, almost from the day of the accident, have suspected that the foam fragment may not have been dry, but may have saturated with rainwater that then froze into solid ice when the external tank was filled with supercold liquid oxygen and hydrogen just before launch. This could have greatly increased the fragment's weight -- since its dry density is only 4% that of water, even mild water saturation could have multiplied its weight severalfold -- as well as making it much harder.
The March 22 "Florida Today" reported the results of a six-week investigation making it look increasingly that this could have happened, despite NASA's initial strong disavowals. Before its launch, Columbia sat on its launch pad for 39 days during very bad weather, during which it was exposed to fully 25 cm of rain and humidity usually above 90 percent.
On its launch day, air temperature was 10 deg C -- but when the external tank was filled with crogenically cold liquid hydrogen and oxygen, the foam insulation itself got a lot colder. Although the entire purpose of the insulation is to keep chunks of ice from forming on the tank's surface and pelting the Shuttle during launch, its outside surface still gets 5 to 17 degrees C. colder than the air outside -- and the inside layers of the foam, of course, get much colder, all the way down to the cryogenically cold walls of the metal fuel tank itself.
This would make little difference if the foam was genuinely waterproof as NASA insists, since NASA's pad teams carefully inspect the tank for any signs of significantly large lumps of external ice forming on the foam's outside surface.
The foam covering most of the tank may have been genuinely waterproof. But there are several reasons to question this.
First, while most of the external tank's sprayed-on foam is coated with a hardened outer "rind", this rind is deliberately trimmed off the foam on the two "bipod ramp" areas -- and the foam fragment apparently fell from the leftmost of these.
Experiments at Florida Tech indicate that some kinds of polyurethane foam without such a rind can triple in weight over a month just by absoring atmospheric moisture in a high-humidity environment.
NASA insists that the External Tank foam is a different, much more water-resistant kind. Scott Sparks testified before the Board on April 7 that samples of this foam, during tests in which it was exposed to 95% humidity for a week, gained only a fraction of 1% of its weight in absorbed water (although it is not clear from the description of the test whether these foam samples had had their rind removed).
And Keith Chong testified on March 6 that, even with the rind trimmed off, only the outer 10% of the foam layer had its cells opened in a way that would allow them to take in water.
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