By Dee Ann Divis
Science and Technology Editor
From the Science & Technology Desk
Published 2/17/2003 5:37 PM
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WASHINGTON, Feb. 17 (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 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."
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Un modérateur n'est pas là que pour emmerder le monde. Il a aussi un rôle important d'organisation de la section, un peu comme un CRS :o
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