Posted by Brian on March 15, 2003 at 17:46:13:
The specific cause has not yet been determined, but it appears most probable that one or more of the Reinforced Carbon Carbon (RCC) panels on the leading edge of the left wing of Columbia sustained some sort of damage before reentry. It may be that a damaged RCC panel came off of the orbiter early in the mission and was observed by an Air Force radar.
As the Columbia re-entered the atmosphere, the broken leading edge generated a vortex, like those normally seen on wingtips, that streamed over the top of the wing. The core of the vortex was heated to incandescence. The radiation of light from the vortex heated the side of the orbiter's fuselage above the wing. The turbulent, superheated airflow downstream of the damaged leading edge began to erode the surfaces of the Thermal Protection System (TPS) tiles.
Hypersonic plasma streamed into the opening in the RCC panels. The plasma impinged directly on the aluminum leading edge of the wing and the stainless steel attachment points for the RCC panels. The aluminum and steel liquified and vaporized. The expanding metal vapor spewed from the gap in the leading edge and was deposited across the underside of the wings and fuselage. Plasma carried some of the metal vapor between the RCC panels and the aluminum structure of the orbiter. Aluminum soot was deposited on the insides of the RCC panels.
The first pieces to fall of the orbiter were probably parts of the TPS tiles and RCC panels.
Fifteen smaller pieces of debris that fell away from the Columbia over California, Nevada, Arizona, and New Mexico were assigned numbers. The sixth piece observed to fall away from Columbia, Debris #6, was particularly bright. It was tracked on radar. They are looking for it near Rachel, Nevada, not far from the Alie-Inn. That area is littered with airplane crashes. Pre-crash contamination is a real problem there. Searchers in that area initially came across metallic parts that showed little evidence of heating. Debris #6 is more likely a badly burned piece of tile or RCC panel.
RCC panel #1 shows heat damage adjacent to RCC panel #2, which has not been found. The greatest damage to the steel RCC panel attachment points is located at panel #2.
Eventually the plasma eroded a hole through the aluminum leading edge under the RCC panels. It streamed into the wing, cutting through the aluminum structure like an acetylene torch. With each change of attitude the plasma stream flowed in a new direction, cutting through other structures in the wing.
Superheated gases filled the voids inside the left wing. It entered the landing gear bay, raising the temperature of several sensors there. The plasma sheared the wires that carried signals from other sensors in the wing and landing gear bay. Debris from the left wheel well indicates that plasma carrying metal vapor was escaping from the landing gear bay through the door seals.
The last voice transmission from the Columbia was received about 32 seconds before the moment that the plasma trail seen in the famous video split into multiple smaller trails. The loss of voice communication occurred when the vertical stabilizer blocked the transmission to the TDRSS relay satellite. It had nothing to do with the imminent break-up.
The loss of voice communication occurred just as Commander Rick Husband was getting his first indications that something was going wrong in the left landing gear well.
Flight data continued to be received for five seconds after the last spoken transmission. During those five seconds, the control system of the orbiter was increasing the force applied to counteract excessive drag from the left wing. Cockpit displays showed Commander Husband that reaction controls were igniting in sequence to counteract the increasing drag from the left wing.
Over the next twenty five seconds, more yellow and red indicators illuminated on the cockpit displays as wires in the left wing were severed by the plasma and the flight control system increased its efforts to maintain the correct attitude. Some of the warning indicators were accompanied by audible alarms.
Small jolts may have been felt by the astronauts as components started breaking away from the left wing.
It may be that the bright light of the plasma on the left side of the orbiter was visible through the cockpit windows. The orange light from the plasma was visible through a window in the mid-deck access hatch on the left side of the fuselage. The rookie astronauts probably did not realize that it was brighter than normal.
The RCC panels and the weakened aluminum structure of the leading edge of the left wing may have begun to collapse inward, rapidly increasing the drag on the left wing.
In a normal reentry, the g-vector reaches a maximum value of 3 gs and points down and forward. As drag from the damage to the left wing increased, the orbiter yawed to the left. The astronauts felt the g-vector moving to the right, forcing them sideways in their seats. The air striking the vertical stablizer rolled the orbiter to the left as the yaw angle increased.
The structure of the left wing failed catastrophically about thirty seconds after the loss of voice communication. A large, brightly glowing piece of debris separated from the orbiter. This piece is referred to as Debris A.
Debris #1 came off of the orbiter about seven minutes before Debris A. Three minutes passed between the observed loss of Debris #15 and Debris A. That's largely due to the lack of ground observers in New Mexico and Western Texas.
Data reception resumed for a couple of seconds at about the time that the left wing failed. Hydraulics in the left wing had been severed and the thruster fuel systems in the rear fuselage were leaking. The fuel tanks in the Orbital Maneuvering System pods had been breached.
The Columbia was yawing to the left at a rate that exceeded 20 degrees per second (the maximum that can be measured by the yaw sensor).
Despite the severe damage to the left wing, the computers were still working and the data path from the rear of the orbiter was still intact. The crew module was still pressurized.
In short order, the right wing and vertical stabilizer were torn from the fuselage. Debris B fell away from the orbiter 15 seconds after Debris A, and Debris C came off one second after that. The magnitude and rate of change of direction of the g-vector in the crew compartment changed violently with the departure of each surface.
The main fuselage remained relatively intact for about 20 seconds after the loss of the left wing. At a rate of yaw of 20 degrees per second, that's enough time to spin around over 360 degrees, more than a complete circle.
The main fuselage broke up three seconds after the departure of Debris C. The plasma trail seen in the videos split into several trails about that time.
One of the brightest glowing trails was streaming from the crew compartment. The crew compartment is a separate structure housed within the airframe of the shuttle. The wings and tail of the shuttle were peeled away from the airframe. The payload bay came apart, but the crew compartment was the strongest part of the structure.
The dynamic pressure of going mach 18 at 200,000 feet altitude is comparable to going 170 miles per hour at sea level. That's not enough to destroy the crew module. Once the rest of the airframe was gone, the aluminum crew compartment survived the initial moments after the break-up. It probably lost pressure, but the crew was protected from the loss of pressure by their space suits.
As the payload bay ripped apart, the connections to the power supply in the mid-fuselage were severed and the lights went out in the crew compartment. The interior of the compartment was illuminated by the light of the plasma outside the windows.
The crew compartment may have rotated chaotically about all three axes as it decelerated. The g-vector inside the compartment spun around chaotically, pulling the astronauts in constantly changing directions. Injuries to the astronauts at this point would have been mainly due to arm flailing and neck snapping, perhaps fatal, maybe not. (Like Chuck Yeager was subjected to when the X-1A went out of control going mach 2.44)
On the other hand, the crew compartment might have stabilized with the heaviest end forward. (The old spherical Vostok capsule depended on its distribution of mass rather than aerodynamics to keep the right end up front.)
Aeodynamic heating weakened the aluminum structure of the crew compartment for several seconds, perhaps nearly a minute, before the airstream breached some point in its skin. In a small fraction of a second, aerodynamic forces peeled the crew compartment apart.
The astronauts in their seats were thrown into the hypersonic airflow, protected only by their pressure suits. The condition of items from the crew compartment indicates that aerodynamic heating was no longer significant at that point in the break-up. No human has ever been subjected to such huge aerodynamic forces.