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Debris theory disputed { February 8 2003 } Original Source Link: (May no longer be active)
http://www.washingtonpost.com/wp-dyn/articles/A42593-2003Feb7.htmlhttp://www.washingtonpost.com/wp-dyn/articles/A42593-2003Feb7.html
Debris-Hit Finding Is Disputed
Reports Challenge NASA Assumptions on Launch
By Rick Weiss
Washington Post Staff Writer
Saturday, February 8, 2003; Page A01
The conclusion by NASA engineers that Columbia probably did not suffer serious damage from debris during its Jan. 16 launch was based on two key assumptions that no one could confirm as true, according to details in a pair of reports produced for NASA by Boeing Co. engineers during the flight.
The two assumptions -- that the shuttle was hit by a piece of insulating foam and not by ice, and that the leading edge of the wing was not damaged by the impact -- remain unproved. Indeed, NASA has explicitly acknowledged that ice damage has not been ruled out and, according to one of the Boeing reports, there was "large uncertainty" as to where the debris hit the spacecraft.
The Boeing analyses, obtained by The Washington Post, indicate that if either assumption was wrong, the flight could well be in serious jeopardy. Yet the second of the two reports -- the only one to make an overall safety prediction -- concludes with the reassuring words: "safe return indicated."
Paul Fischbeck, a professor at Carnegie Mellon University and coauthor of a NASA-funded report on launch debris and critical mishaps, said the reports left him shaken when he viewed them yesterday. "This says that if only a single tile is lost, you're already on the edge," under the conditions scientists think existed during the debris hit, he said. "And it also shows that, in fact, multiple tile damage is likely. Looking at this did not exactly make my day."
"This looks like a case of people trying to fool themselves into not being worried," said a former shuttle engineer who spoke on condition of anonymity yesterday after looking at the report.
Had the debris that hit the wing 80 seconds into flight been not foam but ice, one of the Boeing reports indicates, it could easily have breached the structural integrity of the wing -- even the super hard reinforced carbon-carbon material used in the shuttle's most heat-exposed areas. That would leave the wing unprotected from the nearly 3,000-degree Fahrenheit temperatures reached during reentry.
And even if the debris were a piece of foam, it might have inflicted serious damage and undermined flight safety if the impact were just a few inches further forward than the engineers assumed, said two independent engineers and a former NASA flight controller who looked at the reports for The Post.
All told, the reports, portions of which have been reported by Florida newspapers, suggest NASA engineers may not have considered a wide enough array of possibilities as they decided whether the crew members were at risk, the experts said. But recognizing that it is always easier to find fault after the ending is known, they added, the reports show at a minimum how difficult it was for engineers to calculate the degree of risk for Columbia and her crew.
One report, for example, acknowledges that as engineers tried to predict the effects of a 1,200-cubic-inch to 1,920-cubic-inch piece of foam hitting the shuttle, they had to rely on data relating to an impact with a 3-cubic-inch piece of foam -- a degree of extrapolation so extreme as to make prediction very difficult.
"Oh my gosh," one aeronautical engineer said yesterday when shown that element of the report. "You have to be very cautious when you're extrapolating that much."
NASA officials late yesterday said they were too busy with other queries to answer detailed questions about the reports.
The first of the two reports was produced Jan. 21, the sixth day of the mission. It laid the outlines of a "predicted impact area," about 11 feet by 17 feet, within which the actual impact was predicted to have occurred. The forward-most boundary of that predicted impact area is about 20 inches to 24 inches behind the left wing's leading edge.
The report also noted that the speed and angle of impact would vary depending on exactly where within the predicted area of impact the impact occurred. It concluded that debris from the shuttle's external fuel tank hit the shuttle at up to 730 feet per second and at angles of impact mostly less than 20 degrees -- both a little lower than originally predicted in a Jan. 17 report.
That was reassuring because slower speeds and lower angles generally mean less damage. But the Jan. 21 report noted that certain areas within the predicted impact area could have sustained an impact as steep as 22 degrees -- especially near the wing's leading edge, because the leading edge of the wing is itself angled.
The report also noted that "large uncertainty in trajectory computation does not allow a good prediction of the impact area." In other words, the team could not be certain that impact really occurred within the bounds they were modeling.
That is significant, outside engineers said, because the speed and angle of impact would be considerably higher in some wing regions just a little outside the area that was studied.
Looking outward toward the wing's tip, for example, air velocities get higher. And looking forward the 20 inches or so toward the wing's leading edge, the angle of impact gets steeper. Yet no analysis appears to have been done in the less likely, more distant periphery of that area.
A second Boeing report, dated Jan. 23, further emphasizes why these uncertainties were significant. At one point it analyzes the effects of an impact by a piece of ice, about the same size as the piece of debris that was seen breaking off the external tank. It shows how deeply such a piece of ice would penetrate the shuttle's tough carbon-carbon coating at 720 feet per second and at various angles of impact.
The analysis shows that at an angle of only 15 degrees, the ice would almost penetrate the entire thickness of the carbon-carbon coating, leaving only three one-thousandths of an inch left. Anything steeper than 15 degrees would fully penetrate the crucial coating.
The report concludes that a 15-degree hit from ice would be safe -- albeit barely -- and since foam is less dense than ice, an impact at 15 degrees with foam would also be safe. It extrapolates through calculations not shown that even a foam impact of 21 degrees would be safe.
But the report does not address the possibility that the impact involved ice, or might have occurred a few inches further forward, toward the wing's leading edge, where the angle of impact would be even steeper. There, engineers said, even a piece of foam might have inflicted fatal damage.
The report also suggests that the shuttle's protective ceramic tiles could be seriously compromised by a foam hit like the one Boeing modeled. But it concluded that "safe return [is] indicated even with significant tile damage."
Fischbeck said that overall, the margin of safety the team accepted was thinner than he would have been comfortable with. And Robert K. Weatherwax, president of Sierra Energy and Risk Assessment, a company in Roseville, Calif., that has done safety analyses on the shuttle, said he believed the team erred by discounting the possibility of damage on or near the leading edge.
One detail about the Jan. 23 report struck several engineers as especially odd. It is a section that describes six different scenarios in which tiles are lost from various parts of the wing within the predicted area of impact. Under the "Results" column, the conclusion is "No issue," for the first four, meaning not of concern.
For the last two scenarios, one involving damage to tiles in the lower wing area and another involving a loss of tiles near the landing gear door, the "Results" column is left blank.
Staff reporter R. Jeffrey Smith, special correspondent William Harwood and staff researcher Lucy Shackelford contributed to this report.
© 2003 The Washington Post Company
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