Shuttle silica ceramics black tiles

Question

Since other members wrote, that my my original question here Shuttle silica ceramics black tiles? was too broad I modified it to asked just one thing, while the other questions I will asked later.

Shuttle silica ceramics black tiles, were mostly air and so fragile that you can break, crush them with the force of your hands (quote from NASA documentary). They were usually damaged by ice in upper atmosphere during ascend and from some old quora post potentially by micrometeoroids, space debris in space and heat during reentry. In 1996 they were introduced new stronger black tiles https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120016878.pdf but because problems with their weight and heat conductivity they were used only on some parts of the Orbiter. These new tiles could survive ascend ice impact without any damage (there is picture in link above page 11, how they look after 3 flights vs old tiles) but someone here at Space Stack-Exchange wrote that non of Shuttle tiles survive more than 10 flights. Also from NASA documentary, during reentry Shuttle must survive not only lot of reentry heating, but also lot of aerodynamics pressure.

My question :

If those early silica ceramics black tiles were so fragile how can they survive AD pressure during reentry? Did shockwave protect them from direct impact?

Answer 1

The tiles were not as fragile as you think. While the back sides had the consistency of styrofoam, they were covered with a reaction-cured glass coating that made the surface smooth and hard.

High-temperature reusable surface insulation tiles used a black borosilicate glass coating that had an emittance value greater than 0.8 and covered areas of the vehicle in which temperatures reached up to 1,260°C (2,300°F).

Impacts could break the glass coating - and that was a problem indeed - but they were designed to take the dynamic pressure up to the TPS design limit of 640 lbf / ft^2.

Highly loaded tiles had the back side of the tile densified as well.

Accommodating these stiff spots for the more highly loaded tiles was met by locally densifying the underside of the tile. NASA applied a solution of colloidal silica particles to the non-coated tile underside and baked in an oven at 1,926°C (3,500°F) for 3 hours. The densified layer produced measured about 0.3 cm (0.1 in.) in thickness and increased the weight of a typical 15-by-15-cm (6-by-6-in.) tile by only 27 grams (0.06 pounds). For load distribution, the densified layer served as a structural plate that distributed the concentrated strain isolation pad loads evenly into the weaker, unmodified reusable surface insulation tiles.

Here's a discussion of tile loading and the failure requirements:

The overriding challenge was to ensure the strength integrity of the tiles had a probability of tile failure of no greater than 1/10^8. To accomplish this magnitude of system reliability and still minimize the weight, it was necessary to define the detailed loads and environments on each tile. To verify the integrity of the Thermal Protection System tile design, each tile experienced stresses induced by the following combined sources:

• Substrate or structure out-of-plane displacement
• Aerodynamic loads on the tile
• Tile accelerations due to vibration and acoustics
• Mismatch between tile and structure at installation
• Thermal gradients in the tile
• Residual stress due to tile manufacture
• Substrate in-plane displacement

All quotes from Wings In Orbit: Thermal Protection Systems

It is incorrect that no tile survived more than 10 missions. See What was the operational lifetime of a shuttle tile?