Monday, February 17, 2003, 5:37 PM EST
Exclusive: Did rough wing break Columbia?
By DEE ANN DIVIS, Science and Technology Editor
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.
"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.
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.
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.
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.
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.
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.
The first known aerodynamic shift occurred on mission
STS-28 [Columbia] 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.
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.
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.
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.
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.
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.
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.
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.
"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."
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).
"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."
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.
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.
"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."
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."
Gibson said Columbia experienced another premature
transition on STS-73.
"There again, the left wing transitioned ahead of the
right wing," he said.
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.
"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."
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.
"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.
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.
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.
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.
"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."
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.
"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."
He said he had raised the question of the roughness to
NASA after STS-28 but the matter was not examined closely.
"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."
Copyright 2003 United Press International. All rights
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