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Comment  (also refer to this link)

2 First piercing of wheel-well area is likely to have been a pin-hole - leading to a focusing of the hot plasma flow.
3 As well as air flowing into the wheel-well, super-heated debris would have been flowing back over the wing. But this may have been due to a wiring bundle failure.
4 Wire-bundle failure
5 pin-hole widens as material is eaten away by heating - affects greater area of wheel-well
6 conductive heating via the wing's aluminum skin; at this point (also) the aileron trim started to wind in and left gear strut temp started to increase.
7 pin-hole plasma entry has widened to permit an unfocused flow of super-heated air.
8 See 6 above. 4mins58secs into re-entry. Slower Conductive heating of a remote fuselage area (connected directly to the point immediately behind the L.E. failure area by a substantial metal sub-spar)
9 wire damage (see link)
10 Hot plasma pooling in wheel-well as breach widens and LE RCC section is almost wholly gone. 13:56:16 and left main gear Unlock Actuator indicates unlocked.
11 One second later - One Tyre (at the bottom in its stowed position) ruptures due to heat-induced pressure build-up.
12 Rupturing tyre blows wheel-well door open slightly (but it is held closed by hydraulic pressure). Less than half a second later the aileron moves to quickly counter yaw and roll caused by door being blown open and then re-closing (due hyd pressure).
13 Half second later "pressure gauge on outer tyre fails" (reflects sensor sampling interval and fact that sensor was blown away by tyre rupture)
14 Sensor shows L Gear deployed (gear blown out of uplocks by tyre burst - but initially retained in wheel-well by hydraulic "UP" pressure)
15 Drag on left wing caused by wheel-well door initially slightly ajar
16 Air pressure on slightly ajar door tears it open/bends it - and Columbia yaws out of control

the NASA Ground Track

GENESIS of a FAILURE

It is becoming apparent that the genesis of this Shuttle accident was not unlike that of the Challenger....too many people (who should have known better) disregarding the environmentals pre-launch.

It has been admitted that the Columbia sat on its launch pad much longer than any other Shuttle before it (39 days) and that the weather was the coldest Florida weather for over 50 years. I recall seeing deep snow on the TV at the time. That's a lot of freeze/melt/re-freeze cycles for the foam insulation - which undoubtedly as a result was rain-soaked, freeze-cracked and was ready to separate in large icy spear-like stalactites under the stresses of max Q (81 secs into launch). Of course, once the liquid hydrogen was loaded, the foam was then frozen in place. It took the post-launch thermodynamic heating to thaw it and it then separated along its cyclic fault-lines. If you look here (link) you will see that it has happened before and damaged the RCC leading edge in a similar fashion to what is here suggested may have happened to STS-107.Once you read into the NASA specifications for the RCC leading edge, it becomes apparent that it was "toughened" - but only against high temperatures (as part of the TPS) - and never against IMPACT. They never took the required precautions against launch debris impact - probably because the spec for its "raison d'etre" was always to revolve around its resistance to re-entry heating.

A likely fix would be to affix a vulcanized (and wedge-shaped) sacrificial strip along the RCC Leading Edge (even a non-aerodynamic and deflective one). That reasonably practical fix would provide launch debris impact protection yet quickly burn away on re-entry. A pointy impact of an icy stalactite on that RCC L.E. probably did what you would expect any pointy impact to do to what's essentially nothing more than a toughened graphite, i.e. shatter it. Because it is only "bolted on" (via inconel attachments), loss of its structural integrity (by shattering) would have led to it being rapidly eroded away. But how?

If you read the NASA blurb here, they describe it as being critically coating-protected against oxidization. Once an RCC section was shattered on launch, that section’s oxidization destruction on re-entry was assured. Unfortunately that section was right ahead of the port wheel-well’s outboard forward corner. Superheated white-hot RCC pieces detaching was what the Owens Valley Astronomer would have seen in the pre-dawn darkness. The underlying aluminium wing surface is only rated to 175 odd degrees. No wonder the wheel-well was pierced and the events in there set in train (see here and here.

Further Reading on the Thermal Protection System is here.

This is an excerpt from the most recent set of modifications made to the Shuttle Fleet. These documents were obtained just prior to NASA pulling all information about the shuttle from the net on that fateful Saturday. Read carefully.

" The area aft of the reinforced carbon-carbon nose cap to the nose landing gear doors has sustained damage (tile slumping) during flight operations from impact during ascent and overheating during re-entry. This area, which previously was covered with high-temperature reusable surface insulation tiles, will now be covered with reinforced carbon-carbon.
The low-temperature thermal protection system tiles on Columbia's midbody, payload bay doors and vertical tail were replaced with advanced flexible reusable surface insulation blankets.
Because of evidence of plasma flow on the lower wing trailing edge and elevon leading edge tiles (wing/elevon cove) at the outboard elevon tip and inboard elevon, the low-temperature tiles are being replaced with fibrous refractory composite insulation (FRCI-12) and high-temperature (HRSI-22) tiles along with gap fillers on Discovery and Atlantis. On Columbia only gap fillers are installed in this area."

External Tank Development

In 1971 ( that's the age of the Shuttle system's design! ), when the decision was made to go with the parallel burn, external tank (ET) configuration, several ET technical issues were not fully anticipated. Specifically, ice formation on the ET was not anticipated to be a problem, although ice formation on cryogenic tanks had been known to result. The ET RFP [ Request For Purchase] did not require insulation for the prevention of ice, but one of the bidders did address and highlight the potential problem of ice being dislodged during ignition of the propulsion systems and during liftoff causing potential damage to the orbiter's TPS [Thermal Protection System].

In late 1973, a realization that ice forming on the external surface of the LO2 tank could be a serious problem resulted in considerable resources being expended to address the problem. In 1974, ice and debris prevention requirements were levied, which specified a minimum of 1 in. of spray-on foam insulation (SOFI) on the LO2 and LH2 external surfaces. The objective was to prevent the formation and shedding of ice from tank surfaces and ground systems, as well as providing thermal protection during ascent.

The Et design that evolved serves several critical functions: the tank carries about 1.6 million lb. of super cold propellants within a skin not more than one quarter inch thick. ...