HOUSTON Space shuttle Columbia was crippled by super-heated air, that rushed inside the space planes left wing, blow-torching the aluminum super-structure then jetting into and out of the vehicles left wheel well area.
That scenario is a leading candidate for why Columbia fell apart as it sped to Earth on February 1, killing its seven-person crew and peppering multiple states with tons of falling wreckage. But as to what caused the breach of the space planes exterior, and exactly where that breach occurred remains an enigma.
Unexpected gyrations
The Columbia Accident Investigation Board (CAIB) held the second part of a two-day public hearing here March 18, asking experts to explore what may have taken place as the space plane made its final and fatal reentry.
Stephen Labbe, Chief of NASA Johnson Space Centers (JSC) Applied Aeroscience and Computational Fluid Dynamics Branch, reviewed "piece-wise integration" of heating and aerodynamic modeling with sensor data relayed from Columbia as it plowed through the upper atmosphere.
Labbe detailed the valiant effort of Columbias reaction control jets to counter a series of unexpected gyrations of the space plane at high altitude. Some computer simulations suggest a huge hole may have formed in the planes left wing this while bits and pieces of Columbia were being tossed off.
"Everything weve seen about the flight says that [Columbias] flight control system was doing the proper response to these changes," Labbe said. "Despite the damage, the flight control system was still commanding the vehicle to do exactly what guidance was telling it to do."
Leading edge candidate
Labbe said one future task concentrates on the possibility that several missing reinforced carbon-carbon (RCC) panels -- material used on the leading edge of the space planes wings allowed hot gases to seep into Columbias left wing.
To date, Labbe said, trying to link computer models, as well as wind tunnel tests with actual heat loads seen and relayed sensor data has proven elusive.
The prospect that tiny dings in RCC -- essentially weakened spots -- might permit reentry heat to eventually drill through protective material and strike the shuttles aluminum skeleton, was reviewed by Christopher Madden, Deputy Chief of JSCs Thermal Design Branch.
"What were talking about here is what does eventual mean? How many seconds is eventual?," said Sheila Widnall, a CAIB member and an aerodynamics expert from the Massachusetts Institute of Technology in Cambridge.
Columbias internal reentry
Widnall said that aluminum, like that used throughout a space shuttle, when exposed to super-heating, will oxidize meaning it burns. Instead of being a shield, it can become a fuel. Attempting to model what might have taken place as hot gases blasted inside Columbias left wing will be extremely difficult, she said.
"Nobody would ever build a reentry vehicle out of aluminum. So clearly you're trying to do the kind of calculations never thought about," Widnall cautioned Madden, as well as Jose Caram, NASA JSC Aerospace Engineer of the Aeroscience and Flight Mechanics Division.
Widnall said Columbia, once its outer skin was breached, faced an "internal reentry problem."
The complex set of interactions between Columbia, shock waves, boundary layers, and other high-speed phenomenon must be explored, said John Bertin, a professor of aeronautics at the U.S. Air Force Academy in Colorado Springs.
Those assessments are made all the more difficult to understand given maneuvers of Columbia itself and its worsening structural health as it sped through thin to ever-thicker atmosphere, Bertin said. "Its a very unique and very harsh environment," he added.
Return to flight?
Steven Wallace, CAIB member and Director of Accident Investigation for the Federal Aviation Administration (FAA), said that NASAs recent statements about return to flight are "notional" speculative based on getting fixes underway starting today.
Wallace said that NASA faces two different levels of return to flight considerations. One is short-term predicated on assuring the continued viability of the International Space Station.
Beyond that, Wallace said, the CAIB may reframe the debate as to why send humans into space. "Manned spaceflight is an order of magnitude or two more dangerous than operating aircraft in a hostile war zone," he explained in a post-hearing meeting with reporters.
Squeezing out data
As for whether NASA study groups may come up with different recommendations than the independent board, Chairman of the CAIB, retired Admiral Hal Gehman said: "Theres only one investigation and thats oursall the NASA people work for us."
"Our report does not go to the NASA Administrator, it goes to the White House and to Congress, as well as to the NASA Administrator," Gehman said. "We arent going to be the adjudicators of return to flight but we will make recommendations as to parameters of return to flightand those parameters might be too expensive."
Gehman said he remains confident that the CAIB will ascertain the root cause of Columbias downfall.
"Aircraft accidents with less data than this have been solved before. The debris is showing us things. Were still squeezing data out of the telemetry," Gehman said.
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