- WASHINGTON (UPI) -- Increased
drag -- possibly caused by roughness on Columbia's left wing identified
years ago and worsened by time and debris -- may have been enough to pull
the shuttle into a fatal sideways flight angle on re-entry, experts have
told United Press International.
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- "It could very well be" that the combined drag
from these different sources was enough to cause the demise of Columbia,
said John Anderson, a leading aerodynamics expert.
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- The damage from the insulation that hit the left wing
during liftoff "just might have been enough to throw things over the
edge," he said.
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- At least twice before, as Columbia returned to Earth
from missions, its left wing experienced a critical aerodynamic shift too
early -- prematurely increasing heating and drag on that wing, former shuttle
commander Navy Capt. Robert (Hoot) Gibson, now retired, told UPI.
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- NASA knew about the early aerodynamic shifts at the time
and was told by Gibson about a particular roughness he had discovered on
the surface of Columbia's left wing. Experts confirmed to UPI that the
roughness might have caused the premature aerodynamic shift.
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- The increase in drag on the left wing, particularly if
made worse by tile damage, may have been enough to cause the vehicle to
fly sideways, something the vehicle might not survive, said Anderson, the
curator for aerodynamics at the National Air and Space Museum.
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- Columbia disintegrated on Feb. 1 as it returned from
STS-107, NASA's designation of the 16-day mission devoted to scientific
research. Remains of Columbia's seven-member crew have been recovered,
along with thousands of pieces of shuttle debris scattered across the southwestern
United States.
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- The first known aerodynamic shift occurred on mission
STS-28 in 1989 and was studied carefully by Gibson, an aeronautical engineer
who helped investigate the Challenger disaster and redesign shuttle's solid
rocket boosters. The Challenger exploded on liftoff in 1986, killing its
seven-member crew. Gibson was also commander during four shuttle missions
and piloted a fifth mission.
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- Gibson told UPI he found the surface of Columbia's wings
was two-to-four times rougher than the wings of the three other shuttles
-- Atlantis, Discovery and Endeavour -- and that Columbia's left wing was
50 percent rougher than its right. He suspected the roughness caused the
1989 shift and another in 1995.
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- NASA engineers did not pay much attention to Gibson's
concern in 1989, he said, finding another cause for the shift. Other experts
told UPI, however, that such roughness could trigger a premature aerodynamic
shift, leading to additional heating and drag.
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- Columbia experienced both additional drag and heating
on its left wing before it broke up. By itself such an increase in drag
is probably not enough to destroy the vehicle -- explaining why the vehicle
returned safely in 1989 and 1995.
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- During its final, fatal mission, however, the drag could
have been worsened by tile damage caused when insulation from the external
tank broke off early in the launch, striking its left wing. It is also
possible the roughness on the tiles got worse over time, again increasing
drag, confirmed Anderson, who has 40 years of experience in high-speed
aerodynamics, hypersonic aerodynamics and aerodynamic heating.
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- Even with replacements over the years, 70 percent of
Columbia's tiles were the originals made by Lockheed Martin, according
to a document approved by NASA and released by the prime shuttle contractor,
United Space Alliance, on Feb. 3, two days after the accident.
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- Anderson told UPI the combined sources of drag could
pull the shuttle enough to the left that it was essentially flying sideways
-- at which point the uneven forces on it could break it apart.
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- A NASA press release issued Feb. 15 said that two more
yaw jets than originally thought -- for a total of four -- were firing
as the shuttle sped towards its landing site in Florida.
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- "The flight control system was detecting drag...
on the left side of the orbiter," said a NASA spokesman. "To
compensate for the drag the automatic computers onboard commanded these
jets on the left side of the orbiter to fire. ... Just like if you were
driving a car on ice and you started skidding -- you would turn the wheel
in the other direction to compensate for the skid."
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- The three axes of flight are roll (tilting of one wingtip
up and the other down), pitch (movement of the nose up or down), and yaw
(turning of the nose to the right or left).
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- "Anything to cause increased drag on that left wing
would certainly have caused it to yaw," Anderson said. "The shuttle
is designed to fly straight. It is not designed to fly sideways. That would
have been absolute disaster if something had yawed it so much that it was
basically trying to fly sideways."
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- As a space shuttle re-enters Earth's upper atmosphere,
the initial movement of air over the wings is smooth and orderly -- called
laminar flow. At a key point in the flight, called the boundary layer transition,
the increasing speed causes the smooth flow to breakup into eddies, becoming
"turbulent flow." The shift increases heating and drag on the
wings.
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- The temperature jumps, Gibson said, because "the
turbulent flow mixes the air much more effectively at the surface, which
brings hotter air in contact with the wing -- so you see higher temperature
in the course of the entry.
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- "That's the way they determined (for STS-28) that
they had an early boundary layer transition," explained Gibson, "the
temperature profiles (were) hotter than they (were) accustomed to."
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- Normally the shuttle's wings transition from laminar
to turbulent flow at 1,200 seconds into re-entry, Gibson said. "On
STS-28, on Columbia, that transition happened at 900 seconds -- 300 seconds
early. As you might expect, the left wing saw a significantly higher heating
environment than the rest of the orbiter."
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- Gibson said Columbia experienced another premature transition
on STS-73.
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- "There again, the left wing transitioned ahead of
the right wing," he said.
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- Gibson, who had experienced a close call during a mission
less than a year before, took a special interest in an early boundary layer
transition on the left wing of Columbia during flight STS-28.
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- "I pulled together the data from all the orbiters,"
Gibson told UPI. "I saw that Columbia was two-to-four times rougher
overall (than the other orbiters) and the left wing was rougher than the
right wing by 50 percent."
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- Surface roughness is a factor in aerodynamics and, in
this case, has to do with the gaps between the shuttle tiles and the "step,"
or difference in height, between one tile and its neighbor. NASA measured
such roughness early in the program, Anderson said.
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- "A rougher surface on the wings will cause premature
transition," said Brian Landrum, a professor of mechanical and aerospace
engineering at the University of Alabama in Huntsville.
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- The roughness of the wing is indicated by a measurement
called the K equivalent, derived by combining data on the gaps and steps
with information on the airflow, Landrum said. Small differences in the
K factor can be significant, he said, comparing the roughness to grains
of sand.
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- NASA spokesmen, citing the crush of news media requests
after the loss of Columbia, could not provide information on the roughness
of Columbia's wings or an early boundary layer transition during Columbia's
last mission.
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- Gibson said NASA had determined in 1989 that protruding
gap-fillers caused the early transition. The fillers are pieces that fit
between the tiles to seal all the gaps between them.
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- "There were a couple of protruding gap fillers,"
said Gibson, (but) I always wondered if (the gap fillers were) part of
the problem and the surface roughness was the rest of the problem."
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- Gibson said he was not as familiar with STS-73 because
he was in the process of leaving NASA at the time. He pointed out, however,
that the characteristics of the tiles might have changed and that the early
transition did not happen on every flight. He had, however kept his files
from STS-28, he said, and had reviewed them before discussing the matter
with UPI.
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- "The one time that I looked at all the data very
thoroughly it was for one flight. That was for STS-28 and it happened on
the left wing, which was the bumpy wing."
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- He said he had raised the question of the roughness to
NASA after STS-28 but the matter was not examined closely.
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- "Nobody ever thought a whole lot about the wing
roughness except me," said Gibson. "Because I was not a NASA
engineer, I was an astronaut, nobody paid much attention to it."
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- Copyright © 2001-2003 United Press International
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