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Actually, I think it is most likely that the precipitating structural failure occured long before the ultimate breakup... perhaps over California as based on initial telemetry anomolies occuring 6 minutes before the breakup over Texas.

On another thread I discussed how documenting the location of each piece of debris on the ground will be an important part of "reverse engineering" the breakup.

They will back that into and correlate it with the various video records showing the dispersion of the major structural sections which were shattered and began to scatter imediately after the catastrophic failure.

The sudden appearance of the huge diamond-shaped cloud is most likely the actual catatrophic failure of the orbiter, when it suddenly transferred a huge amount of its energy into the atmosphere (like a frangible projectile), that most likely generated the shock wave heard at the ground as an explosion.

I have been under the shuttle during a night landing several years ago, at about 4:30AM near Navasota, Texas. The normal, true sonic boom (an unmistakeable characteristic double-thud) was barely audible even at that very quiet time of night/early morning, and reached my ears after the shuttle had gone from being near straight overhead to about 40 degrees or less above the horizon (I made notes at the time but haven't located my notebook yet to get the exact timing.)

During this landing I observe it almost completely from horizon to horizon. At night the plasma ball left a glowing orange trail behind the shuttle that persisted for several minutes. It swirled and looked just like a dust cloud behind a car driving fast down a dirt road. Except of course that at the altitude the shuttle was at, the observable glowing trail was several miles wide.

When the shuttle dissappeared in the haze above the eastern horizon, the westernmost part of the trail was still barely visible, and the orange glowing trail stretched across almost the entire sky... it was awesome.

But over Texas, the shuttle's normal sonic boom was barely audible, most likely due to both the distance and the rarified air at that altitude.

The huge explosion heard (my brother who lives in Longview, TX heard it - he thought a chemical plant he lives only 4 miles from had blown up - the blast was that powerful) was the tremendous shock wave generated when the shuttle mechanically disintegrated and transferred a huge pulse of energy into the atmosphere. This is almost certainly the event appearing as the sudden blossoming of the huge diamond-shaped cloud, which continued across the sky with chunks dispersing from it.

Other things that could have failed, such as an APU explosion, etc., may have been the precipitating event.

But if tile damage occured at launch, then it is not difficult to see a series of events unfolding over a period of minutes, which would lead to loss of a section of a control surface, or buckling/failure of a structural member in a wing, and then a subsequent cascade of events until the catastrophic disintegration occurred.

Because of the extreme nose high atitude during the period of re-entry we are discussing (40 degrees), the shuttle tail is completely shielded from any "airstream". It is in the vacuum trailing the shuttle as it punches through the atmosphere, moving so fast that the rarified air at that altitude is blasted away from the path, and doesn't close behind the shuttle until well after it has passed.

The shuttle in this phase of the landing actually "tacks" significantly to the left and right to disperse energy, but all of these motions are controlled by thrusters - not aerodynamic control surfaces.

The elevons and tail don't begin to function as control surfaces until the final manuever phase, well past Alabama / Fla panhandle, when the speed has dropped to about 1700 mph and the altitude is about 90,000 feet (5 - 6 minutes until landing.

Clearly, the flight control surfaces (indeed, ALL surfaces) were destroyed due to either loss of attitude control, or a significant mechanical/structural integrity failue.

I hope that they can recover enough debris to reveal by what they find, how far it is separated, and what they DON'T find (because it was vaporized, or failed over California / Az / NMexico) that they CAN deduce what happened.

My bet is on tile failure due to launch damage.

Just like cold weather launches, O-ring seals and Challenger. And running out of luck.

There has been tile damage before but it has always been minor and caused no real propblem. In other words, we have been lucky.

This time, if the damage that occurred WAS too severe - if the "poof" they saw was tile destruction that penetrated to the skin - then the die was cast... the failure inevitable.

= = = = = = = = = =

Our condolences to the families of these astronauts - our respect and gratitude to the astronauts themselves, who have given their lives on the trail to space, like the pioneers of all ages - those who venture over the horizon to open a new frontier, a new future for those who follow.

MFLTMN

... from a NASA info site...

Roll Schedule - as the orbiter descends through the atmosphere to a level where air pressure has built sufficiently and slows to where heating has subsided somewhat, it begins a series of four steep banks to slow down. The shuttle, in essence, fishtails through the atmosphere as it descends to dissipate its speed. The first couple of banks that the shuttle performs can often be very steep, as much as 80 degrees, that result in the shuttle's side facing toward the ground. The second, third and fourth banks are referred to as "roll reversals," since they basically reverse the shuttle's roll angle, i.e. from 80 degrees left to 70 degrees right. It is important to understand that although the shuttle is performing these steep banks, its angle of attack -- the angle of its nose toward the oncoming air pressure -- is very high, at 40 degrees for much of the entry, to protect the spacecraft from the intense heat that is generated. The angle gradually decreases, i.e. the nose is slowly brought down, as the shuttle descends and slows.

This occurs from 400,000 feet to about 100,000 feet AGL