Feb. 6, 2003, 10:57PM
By BILL MURPHY
Copyright 2003 Houston Chronicle
Shuttle program manager Ron Dittemore says investigators have largely discounted a theory that a chunk of foam broke off during launch and damaged the shuttle.
A hole in Columbia's left wing most likely caused the drag that
apparently sent the shuttle out of control before it disintegrated, several aerospace engineers who are not involved in NASA's probe said Thursday.
NASA investigators continued to work to determine what caused Columbia to experience a greater degree of drag during its re-entry Saturday than what had been seen on more than 100 previous shuttle flights.
Engineers outside the space agency speculated that a hole that somehow appeared in the wing earlier in the flight could have expanded during re-entry, causing the drag, peeling shuttle parts away. Eyewitnesses have said that Columbia was shedding parts as far away as California.
"The only thing that I can think of that would cause drag would be a hole of 6 inches or a foot or 2 feet in diameter. The hot air is getting in, and it keeps damaging, damaging, damaging. And finally the drag on the left wing is much higher," said Steven Schneider, a Purdue University aeronautics professor who specializes in re-entry heating.
Other possibilities might include loss of the tip of the left wing or a wheel well door, several engineers said.
The key to Columbia's demise may be unlocked by the obscure field of hypersonics -- the study of air flow over objects traveling at speeds greater than Mach 5 and the thermal effects those objects undergo.
Mach 1 is the speed of sound -- 1,100 mph at sea level.
Space shuttle program manager Ron Dittemore on Wednesday said Columbia did not experience drag -- the resistance an object encounters as air flows over it -- until just before the tragedy.
Even in the moments that followed, there were no obvious warning signs that the shuttle was doomed.
Through careful analysis of the final stream of data received from the shuttle, however, NASA investigators have determined that all was not right.
A problem with the left wing began dragging the shuttle's nose to the left. The flight control system compensated for the drag by raising right-wing elevons, the flaps that cause a wing to dip or lift.
Later in the descent, two of the four yaw jets, which help control the shuttle's left and right movements as part of its reaction control subsystem, fired for 1.5 seconds in an attempt to compensate for the drag.
"More and more flight-control muscle is being added to keep the vehicle pointed straight ahead," Dittemore said Wednesday. "And eventually that flight-control muscle is going to run out. You only have so much. And when it does, you eventually will lose control."
Alexander Smits, a Princeton University aerospace engineering professor, said fairly severe damage to the left wing and most likely a hole in its underside may have resulted in the drag.
A hole in the left wing would have caused the air to flow over it less smoothly, a number of experts said, causing the shuttle's nose to pull to the left.
Shuttles have often sustained damage to heat-resistant tiles during takeoff but nonetheless returned safely to Earth. Aerospace experts speculated that Columbia must have suffered something beyond minor tile damage.
The left wing "would have to suffer pretty severe damage to change the lift or drag much," said Noel Clemens, a University of Texas aerospace engineering professor who specializes in supersonic air-flow mechanics.
Schneider, the Purdue professor, said, "Remember there are 22,000 tiles. If you are talking about damage so great that the (computers) are moving the elevons, there has to be some big thing that is wrong. It has to be bigger than a single tile."
NASA is investigating whether missing or damaged tiles caused the drag. Drag "can be indicative of rough tile," Dittemore has said. "It can be indicative for, perhaps, missing tile. We're not sure yet."
Unlike an airplane approaching a runway, a shuttle has little room for error on re-entry. It comes in like a heavy glider at about a 40-degree angle, relying on its tiled underbelly to act as a brake.
After orbiting at about 18,000 mph, it must undergo the tricky maneuver of slowing down to speeds that allow it to land.
The heat on its underside reaches 3,000 degrees, hence the need for the protective tiles.
"Remember this thing is flying in this incredibly hot air," Schneider said. "Imagine a huge blow torch -- that is what the shuttle is flying into. You see the shuttle glowing when it comes in. This is hotter than your fireplace hot."
Such heat could turn a small hole into a much larger one and begin to melt shuttle parts, Schneider said.