CAIB accepts, agrees with NASA failure scenario
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: May 6, 2003
For the first time, the Columbia Accident Investigation Board has endorsed a detailed failure scenario developed by NASA and contractor engineers that traces the shuttle's destruction to a breach in the ship's left wing at or near leading edge panels 8 and 9. That scenario, first reported by CBS News on April 20, assumes Columbia began its ill-fated Feb. 1 descent to Earth with a breach in the leading edge and that hot gas ate its way into the interior of the wing less than eight minutes after the orbiter fell into the discernible atmosphere 76 miles above the Pacific Ocean.
How large the initial breach might have been is not yet known. But the day after Columbia's launching, military radars tracked an object separating from the orbiter. Investigators believe the "flight day 2 mystery object" left an opening of some sort that provided a direct path for hot air to enter a cavity behind the U-shaped reinforced carbon carbon panels making up the leading edge of the left wing. More than two dozen shuttle components have been tested to find the best match with the radar data and today, experts testified before the CAIB that only two candidates are still on the table (a third has yet to be tested).
One is a large section of an RCC panel measuring up to 120 square inches. The other is a T-seal, one of 22 such fasteners used to lock the RCC panels together. Under the scenario developed by NASA and the CAIB, the mystery object could be either the T-seal between RCC panels 8 and 9 or a large section of the lower portions of the RCC panels themselves.
"From all the testing and analysis we've done, we feel RCC T-seals as a class cannot be excluded and RCC, what we call acreage, or pieces of the panel, cannot be excluded," said Steve Rickman, chief of the thermal design branch at the Johnson Space Center. "But there is another point to be made there, that the panel acreage itself would have to be on the order of 0.33 inches thick for it to have the correct ballistics. ... It turns out that on the lower panel acreage in the panel 8-to-9 region you do have RCC panel acreage that is of this thickness."
The loss of a T-seal would have left a slot-like gap between panels 8 and 9 just a few inches across as opposed to the much larger hole represented by the loss of a section of RCC panel measuring between 90 and 120 square inches. Engineers currently are trying to determine how big the actual breach must have been to permit the entrance of enough heat to explain the timing of sensor readings and dozens of subsequent sensor failures.
"My understanding is that NASA is, in fact, doing specific analyses for those different shapes, sort of two-dimensional analysis," said board member Sheila Widnall. "My understanding is the hole sizes they've been using to date are quite a bit smaller than the RCC panel that was suggested in the Wright Patterson radar tests. So I think more analysis is clearly required."
Board chairman Harold Gehman said "this is one of the key areas we're going to continue to focus on."
"The way I like to describe it is that the breach that was there at the time of entry has to be big enough to cause the heat scenario that we saw but it also has to be small enough to permit the orbiter to get all the way to Texas. Keeping in mind that we've got some bounds in there and we've got a very, very rich timeline, I believe we've got a good chance of achieving the analysis it's going to take to be more specific about this. We're talking about weeks of work here. It's just plain hard work."
But it is critical work and it will play a major role in how NASA and CAIB investigators design upcoming tests at the Southwest Research Institute in San Antonia, Texas, to fire external tank foam insulation into leading edge components in a bid to simulate what went wrong in the first place.
As is well known by now, just 82 seconds after liftoff Jan. 16, a large piece of foam broke away from Columbia's external fuel tank. The debris originated at or near the left bipod ramp, an aerodynamically shaped area of foam just in front of a strut that helps hold the nose of the shuttle to the tank. At least three distinct pieces can be seen falling away from the bipod area in enhanced footage from ground cameras. But only one - the largest - actually hit the left wing. Experts testified today the large piece measured 24 inches by 15 inches with an uncertainty of 3 inches. The thickness of the debris has not been determined but it is believed to be relatively thin. The impact velocity was somewhere between 610 feet per second and 840 feet per second, or between 416 mph and 573 mph.
The debris hit the left wing in a "footprint" centered on the lower side of RCC panels 6 through 9. It's possible the impact damaged the T-seal between panels 8 and 9 - or cracked one of the RCC panels - and that the damaged component finally shook free the day after launch, leaving an opening into the cavity behind the leading edge. Investigators have not yet come up with a credible mechanism to explain how the damaged component was able to remain in place for a full day and then separated on flight day two.
But for their part, Gehman said the board has deliberately excluded the foam impact as a direct cause of the disaster in its working hypothesis of what went wrong.
"We were careful not to say the foam knocked a hole in the leading edge of the orbiter because we can't prove it," he said. "Now that's not to say we don't believe that's what happened, but we were careful here to base our working scenario on agreed facts and right now, we aren't willing to make that kind of a statement."
Whatever caused the breach in the leading edge, the pathway for hot air was large enough to trigger a remarkably rapid series of events. The shuttle entered the atmosphere at 8:44:09 a.m. Less than eight minutes later, at 8:52:05 a.m., the shuttle began responding to unusual aerodynamic forces. At some point between that moment and 8:52:16 a.m. - 11 seconds later - the hot air had burned its way through the wing spar and into the wing's interior. One minute later, by 8:53:10 a.m., 120 sensors had dropped off line as the super-heated air burned through wiring inside the wing just behind the spar.
That data was recorded on board and not transmitted to the ground. The first realtime indication of a possible problem in the left wing came at just about that same moment - 8:53:10 a.m. - showing up on a computer display monitored by the mechanical systems officer in mission control. By that point, the wing was in severe distress and complete failure was just seven minutes away.
The upcoming tests at the Southwest Research Institute, scheduled to begin in early June, "will demonstrate to us whether or not we have a plausible scenario," Gehman said. "But it doesn't seem to me that it will prove anything one way or the other."
Gehman said the board hopes to begin writing its report later this month. No more hearings are currently scheduled for Houston but the board plans to relocate in Washington next month and one or more hearings may be held there to flesh out questions about NASA's management and operating philosophy. Gehman said it will not matter in the long run whether investigators ever conclusively link the foam impact with the leading edge breach.
"We can make fairly good case of what we think the return-to-flight criteria should be with or without any positive knowledge or positive proof that the foam caused the accident," he said. "Because we're going to make return-to-flight recommendations that are designed to enhance the safety of the orbiter in every way we find that it needs to be enhanced.
"Just fixing the foam alone won't do it. So I'm not the least bit concerned that our inability to make a positive statement with proof that the foam knocked a hole in the leading edge of the orbiter in any way slows us up or in any way restricts what we need to do in order to come up with a criteria for return to flight. Most of our work on return-to-flight issues has to do with the fact that the safety margins have been changed over the years and we're going to try to restore those safety margins back to at least as far as we're comfortable with."
Lest there be any doubt Gehman is serious about addressing a wide-range of issues in the board's final report, he stressed again today that "we're looking at this program and these shuttles in a very, very broad way. We have to, because we don't have a single point failure like the O-rings (that caused the Challenger disaster). And therefore we're going to come up with a broad range of recommendations, which taken together, we believe, will make the program safer. The fact that we don't have a single causal event doesn't bother me in the least. ... It may not be quite so easy to explain, but practice wise and function wise, it doesn't bother me in the least."
He also said the board plans to address the overall risk of flying the shuttle to stimulate public debate.
"The board is going to attempt to characterize the true risk in our own words," Gehman said. "Whether or not we put a number on that, the board hasn't decided. But we are going to attempt to describe for our constituents - the Congress, the administration, the astronauts and the people of the United States - what the risk is in this enterprise. It's not zero, it's not anywhere near zero.
"I don't know that the board would be interested in putting a number on it. ... Whether or not we pass judgment on any number that NASA uses remains to be seen. But we will attempt to characterize the risk in our own terms and if it differs from NASA's, so be it. But that will be one of our goals, to restate the risks in terms that there can be a good public policy debate on whether or not we should be doing this or not."
Readers are encouraged to review the April 20 CBS News status report below for details about NASA's failure scenario. Here is the text of a CAIB news release today summarizing the board's conclusions to date:
Columbia Accident Investigation Board Releases Working Scenario"By building a working hypothesis, it really enables us to focus the testing, it enables us to much more narrowly direct the analysis, it really produces a very synergistic effect on different disciplines," Gehman said.
"The real value of this is that it tells us what to do now and where to go next," he said. "The foam impact testing is very important to us, we have to continue the very, very hard work on the aerothermal analyses to correlate some of these events that we have really good data on but yet we can't quite fully understand everything that's happening, like how long does it take to burn through Kapton wiring, how long does it take to create a knife edge (burn pattern) in a piece of RCC that's a third of an inch thick, how long does it take to burn through aluminum skin and those kinds of things."
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