Heat Shield Conductivity

[MatixCubix]

Hi, I was wondering about the Heat Shields that the spacecraft use when it decend from space, Well, I have a question about how the heat dissipates and gets transmitted through the heat shield, I know that it should have alot of specific heat so it its tempeture dosent increase to much, but what really intrigues me is how is the conductivity of the heat shield I really dont know. Are there different forms of heat shields? how different they are from each other?

[K^2]

Heat shields are made from materials, often ceramics, that are very poor heat conductors. Almost all of the heat is carried away from the shield's surface. The heat may be carried away by radiation, like with the Space Shuttle tiles, or by sublimation of the shield, as with many types of ablative shielding.

[MartGonzo]

An interesting point on heat shields is that ICBM's used to use uranium as both the heat shield and part of the explosive component of the weapon because uranium is extremely resistant to heat, theses were known as fission, fusion, fission devices

They worked by firing two uranium slugs at each other to create a fission reaction, that in turn would create the energy needed to set of a fusion reaction in a surrounding layer of hydrogen and finally a fission reaction in the uranium of the heat shield.

[K^2]

A gun type lighting a secondary? I call shenanigans. Do you have a reference on that?

[rhoark]

Heat shields aren't very conductive, but that heat will still enter the spacecraft if not mitigated in some way. That could be ejecting the heat shield (Soyuz), landing it in the ocean (Apollo), or boiling off on-board coolant (Shuttle).

[Mazon Del]

A gun type lighting a secondary? I call shenanigans. Do you have a reference on that?

Managed to find one.

http://en.wikipedia.org/wiki/Joe_4

It is the Truba (RDS-6T) design. "The RDS-6t was a two-stage gun-type bomb with a deuterium-tritium secondary and was similar to the U.S. “classical Super†design."

[xenomorph555]

Heat shields aren't very conductive, but that heat will still enter the spacecraft if not mitigated in some way. That could be ejecting the heat shield (Soyuz), landing it in the ocean (Apollo), or boiling off on-board coolant (Shuttle).

Soyuz don't eject heat shields to remove heat, they eject them to reveal the soft impact rockets. Apollo didn't land in water to remove heat, it did it because it's easier to land in water then on land.

[MatixCubix]

Oh, ok thanks, another thing, is the heat shield directly connected (if connected) to the spacecraft, or there is another layer of some other material between them?

[EzinX]

If the heat shield is an extremely poor conductor, then the outer surface of the shield will remain red hot and will conduct it's heat directly to the cooler air flowing over it once the spacecraft gets to a lower altitude.

Interestingly, there's a paradox here. Imagine the heat shield has a conductivity of 0. No heat at all flows through it. Then, the outer surface atoms of the heat shield will soak up all of the heat from the air pushed aside by your craft on reentry. Since only a layer of atoms just 1 thick has absorbed any heat, once the spacecraft slows to reasonable speeds, you could EVA out and possibly put your bare hand on the heat shield because a 1 layer thick atomic layer, no matter how high the temperature, has very low actual heat stored in it. This is the same reason you can grab wax paper used in baking in the oven with your bare hands, even if the oven temp is 500 Fahrenheit. It would also cool itself off very rapidly.

Now, there's another interesting paradox. If the heat shield has zero conductivity, when it heats up enough, the outer layer will be so incredibly hot that atoms start ablating off. So your heat shield needs a very high melting point - ideally, a higher melting point than the temperature of the plasma that flows against the heat shield.

So Deadly reentry is dead wrong in it's heatshield model, huh. What would really happen if you reenter too hot is that your heat shield would begin sublimating to gas (not to mention the structural stress on the spacecraft and crew) but it would not do this unless you are entering much hotter than a "typical" reentry on Earth.

[MatixCubix]

Oh, ok but lets talk using energy, so we can agree, when the spacecraft goes into a colission trajectory towards the atmosphere, it has some kinetic energy and some gravitational energy, as it goes down, part of the potential energy is transfomed into kinetic, when the spacecraft possitions itself to reentry, and starts, it starts to gain heat energy, part lost from the kinetic energy, now due to the material of the heat shield that I assume that would have a great specific heat, its tempeture wouldn't raise as much as so if I put another material with lower specific heat, now when you said that "no matter how high the temperature, has very low actual heat stored in it" you mean that in fact due that because a low fraction of the mass of the heat shield was exposed then its energy would still be low or you mean that the specific heat of the heat shield was low, another thing, returing what I said about energy, well part of the kinetic energy is transformed into heat energy, where is heat energy? is acumulating in the first layer of the heat shield or is dissipating in form of radiation, of what you said I would assume that part of the heat was dissipated because, you said that it would't be so "hot" but brings the question if conductivity 0 exists, haha

[EzinX]

Oh, ok but lets talk using energy, so we can agree, when the spacecraft goes into a colission trajectory towards the atmosphere, it has some kinetic energy and some gravitational energy, as it goes down, part of the potential energy is transfomed into kinetic, when the spacecraft possitions itself to reentry, and starts, it starts to gain heat energy, part lost from the kinetic energy, now due to the material of the heat shield that I assume that would have a great specific heat, its tempeture wouldn't raise as much as so if I put another material with lower specific heat, now when you said that "no matter how high the temperature, has very low actual heat stored in it" you mean that in fact due that because a low fraction of the mass of the heat shield was exposed then its energy would still be low or you mean that the specific heat of the heat shield was low, another thing, returing what I said about energy, well part of the kinetic energy is transformed into heat energy, where is heat energy? is acumulating in the first layer of the heat shield or is dissipating in form of radiation, of what you said I would assume that part of the heat was dissipated because, you said that it would't be so "hot" but brings the question if conductivity 0 exists, haha

1. The specific heat doesn't matter if you're talking about a thin layer. This is also the reason why real life heat shields apparently don't contain that much heat energy after reentry, because the material did not conduct heat and it did not ablate away. Instead, the surface layer got incredibly hot and conducted that heat to the cooler air once the craft is on the parachute phase.

With that said, most of the energy that was in the craft (KE + PE) would have been carried away by the air striking the heat shield and flowing around it during the hot part of reentry.

[MatixCubix]

Oh ok, that makes sense, so the specifc heat dosen't matter at all if we are talking of that type of heat shield?

[EzinX]

Oh ok, that makes sense, so the specifc heat dosen't matter at all if we are talking of that type of heat shield?

It matters, but I don't think it's the important factors for designing a heat shield. I think the important factors are :

1. Thermal Conductivity

2. Melting point

3. General all around durability to shock, air pressure, temperature extremes, etc.

Specific heat doesn't affect anything other than it affects how much heat is stored in the shield at touchdown..

[K^2]

Managed to find one.

http://en.wikipedia.org/wiki/Joe_4

It is the Truba (RDS-6T) design. "The RDS-6t was a two-stage gun-type bomb with a deuterium-tritium secondary and was similar to the U.S. “classical Super†design."

According to the article, which agrees with what I recall of the Soviet nuclear weapon history, Truba was never actually completed or tested. Sloika was selected as the more likely to actually work, which it did.

I suppose, it's interesting that people even tried that, but I still call foul on MartGonzo's post. I can imagine a TU device using one wall of its uranium tamper as part of the shield, but even that seems questionable.

[Mazon Del]

Fair enough, I was more looking for something that met the idea of "someone tried" for that concept.

[MartGonzo]

According to the article, which agrees with what I recall of the Soviet nuclear weapon history, Truba was never actually completed or tested. Sloika was selected as the more likely to actually work, which it did.

I suppose, it's interesting that people even tried that, but I still call foul on MartGonzo's post. I can imagine a TU device using one wall of its uranium tamper as part of the shield, but even that seems questionable.

sorry I didn't realise that the offhand post I made would be commented on so I haven't been following the thread.

'fraid I can't offer any proof exactly and I have no first hand knowledge

just repeating something that was mentioned in a documentary about nuclear weapons I watched a while ago.

I can't remember what it was called but I do remember it was narrated by William Shatner if that's any help.

EDIT - this was it

http://en.wikipedia.org/wiki/Trinity_and_Beyond

Edited December 10, 2014 by MartGonzo

[rhoark]

Soyuz don't eject heat shields to remove heat, they eject them to reveal the soft impact rockets. Apollo didn't land in water to remove heat, it did it because it's easier to land in water then on land.

It's all part of an interconnected system. Other concerns dictated which heat removal strategy was used, but one of them had to be used.