What is this "ablator"?

[RandomRyan]

heres the question.

ablators are heat shields, yeah i know that.

but what is it made of?some kind of reinforced carbon-carbon heat tiles?

(reinforced carbon-carbon is actually the heat shields for the Space Shuttle(thats all i know about heat shields))

Thanks in advance

[Kryten]

The point of the carbon-carbon composite is it doesn't ablate. Ablative materials are usually phenolic plastics or cork (yes, the wood).

[Camacha]

Isn't the beauty of a traditional ablative shield that it works with pretty much any material that does not burn off instantly? As long as you are burning through your shield, it works.

[SargeRho]

The point of an ablative heat shield is that material slowly boils way, and in doing so, carries away heat. C-C Tiles don't ablate, they're just crazy heat resistant. And as Kryten said, ablative heat shields are usually made of phenolic plastics or cork.

Edited June 16, 2015 by SargeRho

[peadar1987]

You want something that conducts heat poorly, boils at the right temperature, and has an extremely high latent heat of vaporisation.

[Streetwind]

The Kerbals seem to prefer cork, from the color of their heat shields. Serves them quite well

[xenomorph555]

What do the Russians use for the soyuz? I know that the Shenzhou uses cork but could never find info on the Soyuz heatshield.

[shynung]

ablators are heat shields, yeah i know that.

but what is it made of?

According to this, mostly resin(plastic)-impregnated carbon-fiber.

The Space Shuttle, meanwhile, uses a material called LI-900, which is actually made out of silica(glass) fibers, and have 94% of is volume being air. Reinforced carbon-carbon is used on the wing leading edges and the nosecone.

[Red Iron Crown]

^^ The shuttle's heat tiles are not ablative as I understand it.

[shynung]

^^ The shuttle's heat tiles are not ablative as I understand it.

They're not, they're mainly heat sinks. Also doubles as insulation.

Edited June 16, 2015 by shynung

[pincushionman]

The shuttle used heat sink because ablation would alter the airflow surface during flight.* Which injects uncertainty into an already-complex problem.

They were also supposed to be re-useable, which ablator always is not.

* for a sufficiently broad definition of "fly"

[PakledHostage]

There's a bit of a discission about ablative heat shield materials on the Atmospheric Entry wikipedia article. I was familiar with the Phenolic impregnated carbon ablator (PICA) that was used on Mars Science Lab and Stardust's heat shields, but I wasn't familiar with the SLA-561V ablative compound used on earlier Mars landers. Both MSL and Stardust's atmospheric entries were particularly demanding. Stardust because it was the highest speed entry ever and MSL because it was a direct entry from a hyperbolic trajectory by a massive lander. And even though an entry like MSLs into Mars' atmosphere was at lower speed than typical Earth atmospheric entry from LEO, I have read that Mars' carbon dioxide atmosphere reflects heat back to the heat shield during entry, rather than allowing it to cool by radiative heat transfer.

[Armchair Rocket Scientist]

Stardust because it was the highest speed entry ever

Only the fastest into Earth's atmosphere: the Galileo Atmospheric Probe entered Jupiter's atmosphere nearly four times faster.

[PakledHostage]

I didn't realize it was that much higher, but it makes sense when you think about it. Thanks for the correction.

[magnemoe]

Only the fastest into Earth's atmosphere: the Galileo Atmospheric Probe entered Jupiter's atmosphere nearly four times faster.

Makes sense it entered fast, not only had it far to travel but Jupiter gravity would accelerate it a lot before entering the atmosphere.

[wdsmith3]

the ablative material is essentially a sacrificial layer of materials that boil off of the surface of the heat shield. The material boils off and caries waste heat away from the spacecraft essentially cooling the heat shield.

Imagine a hot summer day and you spray water on your neck. The water absorbs heat from your skin and the air around you. When the water absorbs enough heat, it evaporates and caries that heat energy away from your body. In this example the water is the "ablative material"

[Mad Rocket Scientist]

Here's the wiki page on the Apollo and Orion ablator: https://en.wikipedia.org/wiki/AVCOAT

It is epoxy novolac resin with special additives in a fiberglass honeycomb matrix.

It looks like KSP's shield:

Also: https://en.wikipedia.org/wiki/Atmospheric_entry#Ablative

[magnemoe]

The shuttle used heat sink because ablation would alter the airflow surface during flight.* Which injects uncertainty into an already-complex problem.

They were also supposed to be re-useable, which ablator always is not.

* for a sufficiently broad definition of "fly"

As I understand dragon and dragon2 heatshields are ablators and reuseable. might be that they are an combination, the ablate a little.

[Kryten]

Dragon heatshield material would have to be reapplied for reuse.

[SargeRho]

I think Dragon's heat shield can be reused 10 times for LEO reentry.

[Armchair Rocket Scientist]

the ablative material is essentially a sacrificial layer of materials that boil off of the surface of the heat shield. The material boils off and caries waste heat away from the spacecraft essentially cooling the heat shield.

Imagine a hot summer day and you spray water on your neck. The water absorbs heat from your skin and the air around you. When the water absorbs enough heat, it evaporates and caries that heat energy away from your body. In this example the water is the "ablative material"

Technically, IIRC ablative cooling only involves the vaporization or melting of a solid. What you're describing is evaporative cooling, which involves the use of a liquid.

Incidentally, the human body automatically "sprays" water on its skin to cool down. That's what sweating is.

A better water-based analogy for an ablative heat shield would be if you wet your finger before touching it to a dangerously hot surface (such as a red hot metal bar). The film of water, evaporates, absorbing almost all of the heat transferred from the metal. This buys you a little time before your skin gets hot enough to cause injury.

This is pretty much exactly how ablative heat shields work: for a brief exposure to high heat fluxes, the vast majority of the heat will be sunk into vaporizing the ablative material. On the other hand, an evaporative cooling system like sweat can actually reduce an object's temperature by absorbing heat from that object and evaporating. If you tried using an ablator to deal with internal waste heat, you'd end up cooking yourself since most ablative materials are also excellent thermal insulators, and would heat up from the inside out. On the other hand, ablative cooling is used to deal with waste heat in some rocket engines, like the RS-68. However, ablative cooling is only useful for rocket engines because the nature of the design means the combustion chamber and nozzle are exposed to a fast-moving stream of very hot gas.

[B787_300]

They're not, they're mainly heat sinks. Also doubles as insulation.

they are NOT heat sinks... they are just really really good insulators.

An ablative is any substance that burns away and releases gas. The benefit of the gas is that it helps carry heat away from the structure being protected and the char is a decent insulator until it gets blasted off by the air. The reason the Shuttle did not use ablative materials on the underside (the top side white fabric looking thing is an ablator, but normally doesn't get hot enough for it to start ablating) is that an ablator will not hold a very aerodynamic shape when it is ablating. IE the ablator will not burn off equally and leave divots and pits in the material which are really really bad for aerodynamics. On simple capsule designs maintaining such a perfect aerodynamic "mold line" is not as important.

There were old ideas on using large metal (copper normally) heat sinks that would just absorb the heat, but were much to heavy.

If you want to read more there is a free EBook by NASA on the subject that can be found at https://www.nasa.gov/connect/ebooks/coming_home_detail.html

[shynung]

they are NOT heat sinks... they are just really really good insulators.

It actually is. A landed Space Shuttle has to be left alone on the runway for several hours for said shields to cool down.

[B787_300]

Shynung that is because they are really good insulators. a heat sink is different than an insulator, the whole point of the shuttle tiles are to keep the interior structure cool, a heat sink as discussed in the ebook i linked would have been way too heavy and still transmitted too much heat to the interior structure necessitating exotic alloys and a more expensive construction. Yes you are right that the shuttle had to be left to cool down, that is because the interior of the blocks heated up and stayed heated. There are plenty of you tube videos of people holding the red hot blocks of shuttle tiles shortly after they came out of an furnace. a true heat sink would cool down much much faster and would not be able to be held.