How do aircraft windshields not crack when flying at supersonic speeds through precipitation? Randall Munroe (author of XKCD and What If), once wrote a What If about how fast one would have to drive a car in order to have rain water shatter the windshield.

In the post he mentions that water hitting a windshield at supersonic speeds has the potential to shatter the glass.**

The article, unfortunately, is only talking about automobile glass.  And while I'm sure it's true that automobile glass would shatter if rain hit it at super sonic speeds, it's pretty clear to me that the glass on supersonic aircraft is not shattering when it flies through precipitation.

So, what is done to a super sonic aircraft's windshield to keep it from shattering when flying through precipitation?



** Here's a block quote of all the pertinent text in the post from Munroe.  I figured I'd post it for those who are Too Lazy™ to go read the article.  (Though, honestly, you ought to.  It's informative and entertaining.)


  Here's what happens when a raindrop hits a glass surface at high speed:
  When the droplet makes contact with the surface, a shockwave travels back up through the droplet...
  
  ...Normally, this shockwave would move at the speed of sound within the liquid—about 1300 m/s, four times faster than in air. However, at high impact speeds, this shockwave actually moves substantially faster than the speed of sound in water.
  
  The water is squeezed between the incoming drop and the glass surface, which makes it squirt sideways in all directions. These jets of water can move even faster than the original (already supersonic) droplet, and even faster than the shockwaves we mentioned...
  
  ...The sharp pulse from the shockwave can crack glass...
  
  ...In addition to the direct downward pressure, the water jetting sideways can cause damage, too. If the material has any microscopic holes, cracks, or bumps, those jets can strike them and create new cracks or widen existing ones.
  
  source: What If #93, Randall Munroe
The Concorde had 2 discrete windshields for different stages of flight. During subsonic flight (mainly take off and landing) the nose was drooped and the inner windshield was exposed. For supersonic flight the Concorde would raise its drop nose which also contained an outer windshield that would cover the inner windows. This outer windshield was made of special heat resistant glass that was also tinted. It can be assumed that this outer windshield could also withstand any pressure exerted during supersonic flight. The bigger problem in the Concorde was heat: the nose had to have fuel pumped through it just to keep it cool enough. For what it's worth the Concorde flew at about 50,000ft when it was supersonic; this is well above most weather systems.

Drooped

 
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In Place 


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If you are interested, there is an old paper you can find here that covers windshield failures and lists the Concorde as the #2 aircraft for it (747 comes in at #1) over the years the paper covers. Interestingly enough here is a table of causes:  



Heat fatigue and manufacturing defects (what I assume they mean by saying the windshield itself is the issue) are the primary cause of the issue according to this data. Here is one of the patents filed for what appears to be the Concorde windshield; it's an interesting read!

Going further, if we look at the most extreme case of supersonic planes, the SR-71 Blackbird, the inside of the windshield was in excess of 120&deg;C and the exterior of the nose area would be closer to 260&deg;C during supersonic flight. This would turn any water into vapor if it came into contact with the surface. While I don't know the specifics of that, the Blackbird flew at close to 80,000ft, so it too was above most (if not all) weather. I can't yet find an official quote of what that windshield was made of, but heat was still a larger issue than hitting rain. 


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Here is an interesting paper on what happens to water droplets when they suddenly encounter a high-speed airstream. It might suggest that such an impact with an aircraft would also cause vaporization, aside from the heat of the surface causing it. 

Even subsonic aircraft windshields are extremely strong; they can take continuous hits from solid hail chunks and not shatter. 


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