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I was reading about crew capsules and the reentering into earths atmosphere.
Since this generates loads of heat that the crew and capsule needs to be protected from heat shielding is used. I read this is mostly made of aluminium with a ceramic coating (please correct me if I am wrong).
But then I thought since the isolating properties and weight must be an important factor why isn't this made from Aerogel?

Aerogel is extremely light, very isolating.
So why is it not used? Has it todo with its brittleness, cost?

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    $\begingroup$ Apollo and Gemini did not use a ceramic coating. $\endgroup$ – Uwe Apr 3 at 17:02
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    $\begingroup$ aerogel is a good insulator but ridiculously hard to make. if you look at nile reds video (youtu.be/Y0HfmYBlF8g?t=40) you see while the flame doesn't burn him the gel flakes away rather fast which means it would need to be thicker and therefore much harder to make. $\endgroup$ – Topcode Apr 3 at 17:45
  • $\begingroup$ @Topcode, so if i understand correctly it has to do with the technical difficulties of making the aerogel? I have seen Nile reds video hence the question :) I thought maybe professionals can make aerogel a bit easier and in larger quantities.. $\endgroup$ – FutureCake Apr 3 at 22:25
  • $\begingroup$ @FutureCake in theory you could make hexagons to tessellate together to form the shield but it would be impractical because it wasn't designed to be a heat shield, its better as a lightweight insulator. most heat shields are ablative which means they burn off while we can see that aerogel does do this it does it rather fast which means more layers and therefore more cost. you could use it as an insulator after a non ablative heat shield for added security but making it be a heat shield on its own in impractical. $\endgroup$ – Topcode Apr 3 at 22:49
  • $\begingroup$ Commercial aerogel is still crazy expensive. $\endgroup$ – ikrase Apr 4 at 4:12
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Aerogel doesn't have much mechanical strength. Orbital velocity is say 7800 m/s and you have to decelerate over say 300 seconds. Even if it's done carefully there are 2 or 3 gees of peak deceleration.

So imagine your spacecraft sitting on the ground resting on its aerogel heat shield with two or three more spacecraft stacked on top of it. It will likely be instantly crushed into aerodust.

I don't know the details of the highest melting point silica gel possible but silica aerogel is said to melt at 1473 K or about 1100 C and that seems to be in the right ballpark though a bit lower than the temperatures required for some areas of the Space Shuttle for example.

The heat shield also needs to be opaque to near infrared light because the plasma sheath is really hot and radiative heating of the heat shield by the sheath is a big problem. Ablative heat shields generate a gas layer that is opaque to infrared and ceramic tiles are opaque to this as well. If the aerogel was transparent to even the shorter IR wavelengths then its thermal insulating properties wouldn't be so helpful.

As velocity increases, both convective and radiative heating increase. At very high speeds, radiative heating will come to quickly dominate the convective heat fluxes, as convective heating is proportional to the velocity cubed, while radiative heating is proportional to the velocity exponentiated to the eighth power. Radiative heating—which is highly wavelength dependent—thus predominates very early in atmospheric entry while convection predominates in the later phases

See also What aspects of reentry heating 'scale as the 8th power'?

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    $\begingroup$ "crushed into aerodust" I do like that! $\endgroup$ – Uwe Apr 4 at 9:04
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Aerogel is potentially a very, very useful insulating material for spacecraft, but it does not have the properties that would make it suitable as a heat shield (at least, not as the only or primary component of the outer layer of one.)

  • A heat shield needs to be somewhat physically strong. It flies through the air at supersonic speed and supports the weight of the capsule when decelerating at 2 G's or more. Most silica aerogels are not merely brittle, but extremely fragile as well as being brittle. Contrast typical ablators, which are made of plastic or composites (commonly, some poor technician has to squirt liquid plastic into every single cell of a sheet of composite honeycomb), or the Space Shuttle tiles, which are pretty fragile, but do not fail in such a brittle manner (they are more like styrofoam).

  • Aerogel is very expensive. While space exploration tends to operate on quasi-unlimited budgets, it's probably more expensive than the liquid plastic that ablators are made of (overtime pay for honeycomb-filling worker bees notwithstanding), Shuttle tiles, or sheets of Hastelloy.

  • A heat shield must be able to be bonded to the hull of the spacecraft, which is typically made of a metal that has a different coefficient of thermal expansion and is not completely rigid. This was a bit of an issue with shuttle tiles (they needed to have gaps between them, stuffed with pads of heat-resistant felt, to allow flex) and would be much, much worse with proportionally more brittle and fragile aerogel.

  • The very fine structure of silica aerogel will potentially be subject to creep and densification when heated near its melting point, at which point it will lose many of its desirable properties.

  • Aerogel is subject to thermal shock, and will tend to break when one side is heated to high temperatures and the other side is not.

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