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

• It does not say it could crack the glass, just that it will erode it. Oct 16 '15 at 14:17
• @JanHudec The implication is that eventually the weakening of the glass would cause a car window to crack. If it cracks enough it would eventually shatter because of the pressure being exerted on the windshield, right? Oct 16 '15 at 14:22
• ... it's stronger glass ... why would planes use the same type of windshield as cars? Oct 16 '15 at 22:36
• Interesting information: SR-71 Blackbird's "windshield" would reach 600 degrees Celsius during hypersonic flight. Oct 17 '15 at 10:34
• Sometimes a question comes along that you never think about because it's "obvious" but which leads to very interesting answers -- good work. Oct 18 '15 at 0:49

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

(source)

In Place

(source)

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°C and the exterior of the nose area would be closer to 260°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.

(source)

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.

(source)

• That hail damage picture is intense. I wonder how the nose can look that bad but the engines and nacelles seem to be relatively unharmed? (not really expecting an answer) Thanks for the picture and link! Oct 16 '15 at 16:13
• Looks like Randall Munroe (whom I admire greatly) might have missed a chance to write that the car windshield in question (and the rest of the car) would heat up and suffer other damage before rain would be a factor - although he did point out that the car as a whole would encounter a lot of trouble even reaching such a speed. Oct 16 '15 at 17:01
• 80,000 ft is well into the stratosphere and there are no clouds there, ever. Even normal flight levels ~32,000–42,000 ft are above most clouds (average altitude of tropopause is 36,000 ft). Oct 16 '15 at 18:19
• Note that it's "Concorde", not "the Concorde". Oct 17 '15 at 7:58
• @JPhi1618 The nose houses the weather radar and therefore needs to be made of a material transparent to radar. That means no metal. The engines and their nacelles are a bit more sturdy than that. Oct 17 '15 at 23:07

The first trick to make windshields stronger is used also in aerodynamics: Sweep reduces the effects of speed. Giving the windshield an inclination both back- and sideways will reduce the impact energy of whatever hits it in flight direction.

X-15 carried by a B-52 (picture source). Note the angled windows: They were oriented at 75° to the air stream, so an object hitting them at 2,000 m/s (Mach 6.65 at altitude) would "feel" like an impact at 500 m/s against a surface at right angle. Not that it would rain at 80 km up, so this was mostly for aerodynamics.

And the second trick is used always if things are at the risk of breaking: Make them thicker! The thickness of a supersonic fighter canopy is approximately 20 mm and that of an airliner even 50 mm, whereas your average car glass is only 4 to 6 mm thick. Also, windshields are multi-layered: In airliners as in cars, they are made from tempered glass and polyvinyl butyrate, and the spherical canopies of fighter jets made from polycarbonate, polyurethane and acrylic.

Finally, the goal of the engineers is only to make the structure withstand the impacts over a certain time, which can be only minutes for supersonic flight. Birdstrikes are one-time events, but rain impact is an ongoing process which erodes any surface over time. The single raindrop or hail impact will only cause local, microscopic damage, but enough of them over time will wear down any structure.

• 50mm?! That sounds like it would cause some non-negligible refraction. Oct 18 '15 at 6:43
• @shortstheory: That is why they are flat. Curved windows are thinner and sometimes even need grinding to get their optical quality right. Oct 18 '15 at 7:19

How do aircraft windshields not crack when flying through precipitation at supersonic speeds?

You are talking about a phenomenon called rain erosion at supersonic speeds.

Just because a small steel ball of mass m, traveling at supersonic speeds, can crack a windshield, it does not mean a water droplet of the same mass and velocity will produce the same effect even if it has the same energy.

A simple explanation can be given based on elastic and inelastic collisions theory, you know from school. The steel ball hitting the glass is in the elastic collision case characterized by high interaction forces, at least up to the moment the glass breaks. The water drop is in the inelastic situation, the impact forces being lower.

We are in the case of a soft ball and a stone of the same mass hitting a window with the same speed. In the first case the glass survives in the second breaks.

• Not sure where you attended secondary school, but elastic and inelastic collision theory certainly was not covered in my high school! Since I didn't study physics or math in college, I didn't learn much about it there, either. :( Oct 16 '15 at 17:48
• About inelastic collisions you can read here: en.wikipedia.org/wiki/Inelastic_collision . The main idea is that the kinetic energy of the droplet relative to the plane converts in heat quickly in a fraction of a millisecond as the droplet smashes against the window.and so the water drop loses its high speed. Oct 16 '15 at 18:40
• Highschool elastic and inelastic collision theory is generally useful, but ignores supersonic shockwaves. Note that supersonic in this context refers to the speed of sound in the colliding materials (glass/water), not the sound of speed in air. The former is several times higher. Oct 19 '15 at 11:08
• In my case (further maths A level in the UK), elastic and inelastic collisions were covered in high school as far as conservation of momentum and energy, but not not to the detail level of shockwaves and interaction forces. Jan 9 '18 at 12:47

Because aircraft don't regularly fly at such speeds at altitudes where this might be a concern.

The quoted problem (hitting a drop of water faster then the speed of sound in water) is never faced by any real aircraft. Even if because of the shock-wave there was no windshield in existence that would survive that impact, it wouldn't pose a problem, because no aircraft flies at Mach 4 at an altitude where it can encounter rain.

Actually, no aircraft was ever capable of cruising at Mach 4. Besides some experimental prototypes, no aircraft could even fly at Mach 4.

Even the handful of aircraft which can reach about Mach 3, can only do it high in the stratosphere, where the air is much thinner and there is no rain.

Therefore, hitting a raindrop at Mach 4 is not an issue, because no aircraft can fly that fast. Actually, no aircraft flies much faster than Mach 1 at altitudes where rain is possible. Even the planned future aircraft which will be capable of sustained flight over Mach 4, will fly at that speed only when high above the clouds, because they would not be able to fly that fast where the air is thicker.

EDIT:

Even if the problems would arise at Mach 1, aircraft regularly fly supersonic only in the stratosphere, where there is no rain. Actually, very few aircraft fly supersonic: airliners (except for the Concorde) are all subsonic, and even the Concorde only flew supersonic when it was flying high enough. Military aircraft, although many of them capable of supersonic flight, rarely ever fly supersonic, and even then only for short bursts.

So, summarized:

• supersonic aircraft fly usually not much above the speed of sound and so they can easily slow down to subsonic speeds when encountering rain. (maybe someone can help out whether there was anything in the Concorde flight manual about the maximum allowed speed in rain)

• aircraft rarely (if ever) fly supersonic at altitudes where rain is a concern.

• The question is saying that the craft is flying at supersonic speeds for air, I should probably go back and clarify that... Oct 16 '15 at 17:41
• Yes, but the problem Randall Monroe mentions only arises when flying at the supersonic speed for water.
– vsz
Oct 16 '15 at 17:43
• I think you ought to read the full article. But, to clarify, he is saying that when you collide with water at mach 1, the pressure wave travels through the water droplet at 4 times the speed of sound (in water). Which is much faster than energy normally travels trough water. Oct 16 '15 at 18:01
• @JayCarr, the article is ambiguous, as is saying Mach 1 and speed of sound, because they can all mean either in air or in water. The article does, however, at one place mention 500 m/s, which is ~M1.47 at sea level and ~M1.67 at 36,000 ft in air in ISA (sound is slower when it is colder). Oct 16 '15 at 18:29
• @JayCarr: As I understood it, the droplet hits the glass at a speed which is supersonic in air but subsonic in water. Therefore, the pressure wave can travel into the droplet, which happens at the speed of sound in water. This pressure change now will squirt water sideways, and this happens at water's supersonic speed. It is this sideways squirted water which causes most erosion. Oct 18 '15 at 7:24

Also bear in mind that when a droplet of water approaches a supersonic airplane windshield, it first encounters a supersonic skin layer of air, which would tend to buffer and deflect some component of the force before it hit the windshield itself. That is not to say that objects are prevented from hitting the glass...they do it all the time...but rather to say that much of the force will be dissipated, diminishing the urgency of the problem.

• Not just that; in all (?) supersonic aircraft the windshield and the whole cockpit is behind the nose cone of the aircraft itself. The nose creates the first shockwave, and the stream behind it is already slower and hotter and has higher pressure. This alone may be enough to evaporate droplets, though I don't know for sure.
– Zeus
Jul 6 '16 at 6:01