Posted on 02/09/2003 9:32:53 PM PST by TLBSHOW
Shuttle Testing Suggested Wings Were Vulnerable
ASHINGTON, Feb. 9 Studies conducted by NASA over the last four years concluded that damage to the brittle, heat-shedding material on the leading edge of the space shuttle Columbia's wings posed one of the highest risks of a catastrophic accident.
The studies focused largely on the tremendous damage that could be caused in the unlikely event that a tiny meteoroid or other bit of orbital debris hit the leading edge of a wing, which is made of a lightweight material called reinforced carbon-carbon. That is still one of the theories about what might have happened to the Columbia eight days ago.
But in interviews, engineers for NASA and one of its leading contractors said there was comparatively little testing to determine if slower-moving debris perhaps of the kind that fell off the shuttle's external tank about 80 seconds after liftoff on Jan. 16 posed a similar hazard.
As a result, said one engineer familiar with the discussions that took place at NASA in mid-January, the engineers who saw little risk from the debris that hit the Columbia's left wing had scant information to back up their assertion.
"People came to the conclusion that whatever damage happened was tolerable, but it's not clear that was based on any solid data," said the engineer, who spoke on the condition of anonymity because NASA had not allowed them to speak to reporters without prior authorization. "The testing data just wasn't there."
No one has concluded that damage to the leading edge was responsible for the Columbia's disintegration, or even that the foam insulation that struck the wing and probably hit the leading edge, had hurt the orbiter.
But much attention since the accident has focused on the possibility of some kind of devastating damage to the leading edge of the left wing, or the heat-resistant tiles.
An analysis by the Boeing Company concluded that the underside of the leading edge had probably been hit, along with a large swath of the panels.
The area of the wing identified as the likely site of the debris strike in the company's Jan. 21 presentation to NASA nearly matches an area of the wing that appears to be jagged in an unclear silhouette photograph taken of the orbiter through an Air Force telescope less than a minute before it disintegrated on Feb. 1. Sensors in the wing were knocked out, one by one, fueling suspicions that as heat spread through the structure, it burned through wiring.
Over the weekend, NASA officials declined to say if a section of leading-edge material found west of Fort Worth late last week had come from the same left wing. That mystery should not be difficult for NASA to decipher: the leading-edge material was still attached to tiles that are identified with unique serial numbers that indicate their precise position on the orbiter.
If the wreckage turns out to be part of the left wing, it would indicate that the wing was among the first pieces of the orbiter to break off and fall to earth.
"It is one of many approaches we're looking at," Sean O'Keefe, the administrator of NASA, said today in an interview on CNN. Damage from the foam insulation, he added, is "still a very current theory," though he cautioned that "nothing is off the table."
The engineering teams investigating the shuttle disaster took the day off, their first since disaster struck, and will resume their work on Monday, a NASA spokesman said.
Among the things they will be looking at is an Air Force radar image of the shuttle gathered at Eglin Air Force Base in Florida on Jan. 17, one day into Columbia's flight. The radar data appears to show a small object moving away from the shuttle at about five meters per second, a NASA spokesman said today.
It was not known whether the object was something ejected from the shuttle, like waste water, or a piece of the exterior of the craft. The object could also have been a meteorite or space debris that struck Columbia or passed very close. The radar is able to detect objects as small as a softball, officials said.The spokesman, David Drachlis, said that no water dumps were planned for that day and that there was no record of any other object being discarded by the shuttle. He said the object entered the earth's atmosphere two days later and disintegrated.
"We're not sure whether it came off the spacecraft or was space debris," Mr. Drachlis said. "And it's too early to determine whether Columbia was struck by space debris."
Even while that mystery remains, there is new focus on the properties of the materials that protect the wings from heat.
The reinforced carbon-carbon, R.C.C. in the lingo of NASA engineers, covers the orbiter's nosecap and makes up the 22 panels that protect the edge of each wing. It has never been known to fail to protect a shuttle, during more than 100 flights. Unlike the faulty O-rings that brought down the Challenger in 1986, there appears to be no history of near-disaster with the material. But damage has been recorded, especially on a flight of the Atlantis in March 1992.
But starting in 1999, two NASA research centers began publishing studies examining pinholes and cracks in the material, which they noted could result in "potential burn through," with catastrophic results for the orbiter and its crew. A study published by the Johnson Space Center in March 2000 noted that if hit by orbital debris moving at what scientists call "hypervelocity," in the area of 21,000 feet per second or faster, the leading edges of the wing "pose the highest risk for critical failure."
That risk was, statistics suggested, far higher than if any other component of the orbiter was struck by orbital debris.
A team of four engineers, two from NASA and two from Boeing North America, conducted tests at NASA to simulate high-velocity strikes on the material, similar to firing a bullet at a board to determine what kind of damage it wreaks. The results were striking: while the entry hole was small, on the backside the material blew apart, in a hole that would clearly widen under the intense heat of re-entering the atmosphere.
Under some conditions, the report concluded, those holes could provide a pathway for intense heat to enter the metal structure of the wing.
The report acknowledged that harm could come to the carbon-carbon from "launch debris, runway debris from landing, and handling and service damage," but the study did not simulate that kind of strike.
Reached at his home in Houston on Saturday, Donald M. Curry, an engineer who worked on the study, declined to comment without NASA's approval. But Mr. Curry cautioned against extrapolating from his study the damage that could be done by materials moving at slower speeds.
Another engineer familiar with the study, however, said he was mystified about how engineers from Boeing, one of the lead contractors for the shuttle, could conclude after the Columbia was launched last month that the large piece of foam had done no damage to the leading edge.
The Boeing study showed that the lightweight foam was about the size of a couch cushion and that it hit at a speed of around 700 feet per second, a tiny fraction of the speed of orbital debris.
Boeing noted in one of its presentations that there was no "test data available" of the impact of material like the insulation on the carbon-carbon surface.
Still, Boeing came to a reassuring conclusion that was echoed by NASA in its daily mission reports. The company estimated that the damage to the leading edge probably only scratched the surface by a depth of a quarter of an inch or so, and it concluded: "safe return indicated."
As a result, said one engineer familiar with the discussions that took place at NASA in mid-January, the engineers who saw little risk from the debris that hit the Columbia's left wing had scant information to back up their assertion. "People came to the conclusion that whatever damage happened was tolerable, but it's not clear that was based on any solid data," said the engineer, who spoke on the condition of anonymity because NASA had not allowed them to speak to reporters without prior authorization. "The testing data just wasn't there." |
Troubling.... |
Over the weekend, NASA officials declined to say if a section of leading-edge material found west of Fort Worth late last week had come from the same left wing. That mystery should not be difficult for NASA to decipher: the leading-edge material was still attached to tiles that are identified with unique serial numbers that indicate their precise position on the orbiter. |
That has a very familiar ring to it... perhaps they were told to take off their engineer hats and put on their management hats... :-(
Duuuuh.... somehow this makes sense...
My mantra to everybody I have spoken to over the past week has been "The heat shielding tiles have always been the weakest link of the whole Shuttle project." Not a brilliant statement, I know, but sometimes the most obvious things go unnoticed. The leading edge RCC is part of that system. I, for one, have been amazed that they can work at all, but they have. If this accident is not traced to some failure of the shielding, I'll eat my hat. They are simply too critical a component, and many ways they can fail.
That being said, I am not aware of a better system. Perhaps we simply have to accept that there will be occasional failures.
NASA has worked on a rescue craft that looks like a smaller version of the shuttle that uses something other than tiles I believe. Work on the project was dropped before a real reentry test could be done.
I guess he doesn't want to be interviewed without one of his minders present.
Since that was the first day of flight, I doubt that Columbia had done any water dumps at all. If that's true the object was unlikely to be ice. I also wonder if ice could have survived two days in orbit without shielding.
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