Ceramics as hull

[ClaysGhost]

In this case your's.

Precisely. Actually, aluminium foil might be considered a thermal superconductor, seeing as how it can be heated to a few hundred degrees and then conduct the heat away ridiculously fast.

A good conductor is not a superconductor!

[kojikun]

A good conductor is not a superconductor!

No, this is true. But thermal superconductivity is a very different concept then electrical conductivity. A superconductor, for electricity, conducts electricity without loss to heat. But with a thermal superconductor, it has to conduct heat without loss to.. what? A thermal superconductor must be able to get rid of heat very rapidly (because its conducting it really well, right?). The real question is where do we draw the line between normal thermal conductivity and super thermal conductivity. But because of the nature of thermal conductivity, a thermal superconductor depends both on the conductivity of the material and the shape of the material (a block of aluminium doesnt conduct head away from it at the same rate as a piece of foil of the same mass, due to surface area).

[ClaysGhost]

A good conductor is not a superconductor!

No, this is true. But thermal superconductivity is a very different concept then electrical conductivity.

Then why did you mention room temperature superconductors? That's a goal relevant to electrical superconductors only.

A superconductor, for electricity, conducts electricity without loss to heat. But with a thermal superconductor, it has to conduct heat without loss to.. what?

To the environment. Heat radiated from the superconductor system, surely. It doesn't have to change form to be a loss.

A thermal superconductor must be able to get rid of heat very rapidly (because its conducting it really well, right?).

A good (or, if you prefer, excellent) conductor, then.

The real question is where do we draw the line between normal thermal conductivity and super thermal conductivity. But because of the nature of thermal conductivity, a thermal superconductor depends both on the conductivity of the material and the shape of the material (a block of aluminium doesnt conduct head away from it at the same rate as a piece of foil of the same mass, due to surface area).

My problem with the name "thermal superconductor" is that it's misleading. It suggests many similarities to electrical superconductors, when they don't actually exist. The Meissner effect is irrelevant to thermal conductivity, and hence to a thermal superconductor. The typical electrical superconductor today collapses at middling cold temperatures, leading to a holy grail of room temperature superconductors. It's irrelevant to thermal superconductors. Also, there are no radiation losses from a superconductor carrying a constant current. I doubt the same is true for a thermal superconductor at non-zero temperature.

[kojikun]

Then why did you mention room temperature superconductors? That's a goal relevant to electrical superconductors only.

It was meant to be analogous.

To the environment. Heat radiated from the superconductor system, surely. It doesn't have to change form to be a loss.

Yes but an electrical superconductor tries to allow electricity to flow without impedence. If heat flows without impedences then its the inverse of an electrical superconductor because electrical ones try to prevent loss to heat, but a thermal one tries to OPTIMIZE loss of heat.

A good (or, if you prefer, excellent) conductor, then.

Yes. Thus it would be super.

My problem with the name "thermal superconductor" is that it's misleading. It suggests many similarities to electrical superconductors, when they don't actually exist.

Actually it is similar, atleast in the principle of permitting flow of the thing in question (be it electricity or heat).

The Meissner effect is irrelevant to thermal conductivity, and hence to a thermal superconductor. The typical electrical superconductor today collapses at middling cold temperatures, leading to a holy grail of room temperature superconductors. It's irrelevant to thermal superconductors.

Yep, but we're no longer talking about conductvity of the same thing, so the rules change.

Also, there are no radiation losses from a superconductor carrying a constant current. I doubt the same is true for a thermal superconductor at non-zero temperature.

Well the whole point of a THERMAL superconductor is to permit heat to flow very easilly. Aluminium does this, its a very good conductor of heat because its heat absorption rate is very high (235 W m-1 k-1). Copper is better, tho, with 400 W m-1 k-1. Silver is great at 430 W -1 k-1. But Aluminium is cheap.

I suppose a really good hull would be one in which the surface is very corrugated, with shitloads of fins like its covered with CPU heat sinks, so that it absorbs incoming thermal energy over a large area and can reemit that energy quickly (because its an entire fucking radiator surface ). Under that would be some ceramic material to prevent internal heating. So the metal radiators would distribute the heat over a large surface before it can burn through the ceramic, then reradiate it away quickly.

[Darth Wong]

Yes but an electrical superconductor tries to allow electricity to flow without impedence. If heat flows without impedences then its the inverse of an electrical superconductor because electrical ones try to prevent loss to heat, but a thermal one tries to OPTIMIZE loss of heat.

Incorrect. A thermal superconductor tries to optimize CONDUCTION of heat. There is no reason it must radiate heat to its environment any more quickly than any other substance.

[Sea Skimmer]

Ceramics are utter crap for defeating kinetic energy, it will only make decent armor in combination with an extremely dense material such as DU.

[aerius]

I suppose a really good hull would be one in which the surface is very corrugated, with shitloads of fins like its covered with CPU heat sinks, so that it absorbs incoming thermal energy over a large area and can reemit that energy quickly (because its an entire fucking radiator surface ). Under that would be some ceramic material to prevent internal heating. So the metal radiators would distribute the heat over a large surface before it can burn through the ceramic, then reradiate it away quickly.

Sounds nice in theory, but you're gonna have problems. With a finned surface you are creating weak spots and stress risers galore, every single "valley" or low spot is gonna be one, as is every single joint and edge that makes up the fins. You do not put stress risers and weak spots in armour, you avoid them like the plague. In other words, your making it easier for the enemy to blast your hull apart.

[kojikun]

Incorrect. A thermal superconductor tries to optimize CONDUCTION of heat. There is no reason it must radiate heat to its environment any more quickly than any other substance.

But wouldnt that mean then that if its optimized for conducting heat that it would conduct it away from itself better then non-optimized materials?

Sounds nice in theory, but you're gonna have problems. With a finned surface you are creating weak spots and stress risers galore, every single "valley" or low spot is gonna be one, as is every single joint and edge that makes up the fins. You do not put stress risers and weak spots in armour, you avoid them like the plague. In other words, your making it easier for the enemy to blast your hull apart.

Easy solution: have retractable fins, so that when theyre down they dont add to weak spots. you can then raise them in spots not being targeted.

[Darth Wong]

Incorrect. A thermal superconductor tries to optimize CONDUCTION of heat. There is no reason it must radiate heat to its environment any more quickly than any other substance.

But wouldnt that mean then that if its optimized for conducting heat that it would conduct it away from itself better then non-optimized materials?

It conducts it through itself better than other materials. A superconducting electrical wire does not shed or waste electricity; it moves it around within itself very quickly. Same thing for thermal superconductors. By the way, the best thermal conductors also tend to be the best electrical conductors. It is hardly a mutually exclusive situation.

Easy solution: have retractable fins, so that when theyre down they dont add to weak spots. you can then raise them in spots not being targeted.

Egads, that's even worse. Now you have armour which is criss-crossed with built-in discontinuities, so its strength is shit. Not only that, but the whole point of cooling fins is made moot if they are not continuous with the armour they're trying to cool, since they won't conduct heat across the interface worth shit.

[kojikun]

It conducts it through itself better than other materials. A superconducting electrical wire does not shed or waste electricity; it moves it around within itself very quickly. Same thing for thermal superconductors. By the way, the best thermal conductors also tend to be the best electrical conductors. It is hardly a mutually exclusive situation.

I suppose. But thats also the point, a therma superconducting hull would very quickly distribute heat throughout its entirety, instead of concentrating it. with the heat spread out, it can radiate more quickly thanks to larger surface area.

Egads, that's even worse. Now you have armour which is criss-crossed with built-in discontinuities, so its strength is shit. Not only that, but the whole point of cooling fins is made moot if they are not continuous with the armour they're trying to cool, since they won't conduct heat across the interface worth shit.

Hrmph. Surely theres a way to incorporate a hull that has radiating fins but that isnt weakened. Perhaps the hull would have one layer for strength and a thin layer on top of it with the fins. if the fins are sheared off, they can be replaced, and they wont pull on and weaken the hull beneath it.

[Darth Wong]

It conducts it through itself better than other materials. A superconducting electrical wire does not shed or waste electricity; it moves it around within itself very quickly. Same thing for thermal superconductors. By the way, the best thermal conductors also tend to be the best electrical conductors. It is hardly a mutually exclusive situation.

I suppose. But thats also the point, a therma superconducting hull would very quickly distribute heat throughout its entirety, instead of concentrating it. with the heat spread out, it can radiate more quickly thanks to larger surface area.

Agreed. My beef was with your earlier statement:

... thermal superconductivity is a very different concept then electrical conductivity. A superconductor, for electricity, conducts electricity without loss to heat. But with a thermal superconductor, it has to conduct heat without loss to.. what? A thermal superconductor must be able to get rid of heat very rapidly (because its conducting it really well, right?). The real question is where do we draw the line between normal thermal conductivity and super thermal conductivity. But because of the nature of thermal conductivity, a thermal superconductor depends both on the conductivity of the material and the shape of the material (a block of aluminium doesnt conduct head away from it at the same rate as a piece of foil of the same mass, due to surface area).

An electrical superconductor is a zero-resistance conductor. A thermal superconductor is an isotherm. In both cases, they conduct without delays in time apart from the limits of relativity, although a time delay in the case of an electrical conductor means resistance, which in turn means waste heat. The characteristics which allow one also tend to allow the other.

Also, thermal conductivity has nothing whatsoever to do with the shape of the material; you are confusing heat-exchange efficiency with thermal superconductivity. A theoretical thermally superconductive material will generally be an isotherm; its entire volume will be at the same temperature, limited only by lightspeed conduction of internal energy.

Hrmph. Surely theres a way to incorporate a hull that has radiating fins but that isnt weakened.

Sure. Radius the fillets at the base of the fins so that the fin body breaks before the base develops any stress fractures. Then you can simply accept that the fins will snap off, but the underlying hull will be OK.

[kojikun]

Sure. Radius the fillets at the base of the fins so that the fin body breaks before the base develops any stress fractures. Then you can simply accept that the fins will snap off, but the underlying hull will be OK.

Perfect. So under that we can stick a ceramic layer (a powder for give) to provide insulation. Hows that sound?

[ClaysGhost]

It was meant to be analogous.

I don't think a good electrical conductor is analogous to an electrical superconductor, but that's probably a closer relation than the one you're proposing.

Yes but an electrical superconductor tries to allow electricity to flow without impedence. If heat flows without impedences then its the inverse of an electrical superconductor because electrical ones try to prevent loss to heat, but a thermal one tries to OPTIMIZE loss of heat.

Optimise conduction of heat.

Yes. Thus it would be super.

No. I'm not sure whether your argument is "very good conductor == superconductor" as a point of linguistics (which isn't true for electrical conduction, so why are you saying it's true for thermal conduction?) or a weak analogy.

Actually it is similar, atleast in the principle of permitting flow of the thing in question (be it electricity or heat).

Most any material permits flow of thermal energy. A lot of materials permit the flow of electrical current with some resistance. I wouldn't call either very similar to superconductors. I don't think this similarity principle is the correct one.

Yep, but we're no longer talking about conductvity of the same thing, so the rules change.

So not that similar, then?

Well the whole point of a THERMAL superconductor is to permit heat to flow very easilly. Aluminium does this, its a very good conductor of heat because its heat absorption rate is very high (235 W m-1 k-1). Copper is better, tho, with 400 W m-1 k-1. Silver is great at 430 W -1 k-1. But Aluminium is cheap.

Those are *good* thermal conductors (I've seen suggestions that carbon nanotubes have thermal conductivities of almost 3000W/m/K, but I routinely distrust what I read about carbon nanotubes - they seem to be everyone's favourite super-material of the future). A superconductor is something else. Although you can expect a good electrical conductor to be a good thermal conductor, you can't expect an electrical superconductor to be a thermal superconductor, or even a good thermal conductor. Electrical superconductivity actually seems to reduce thermal conductivity - electrons that have turned into Cooper pairs in a superconductor aren't available for conduction of heat, only current. An oversimplification might be that to transfer heat rapidly, you want to maximise the number of collisions between electrons. To transfer current rapidly, you want to minimise electron collisions.

I suppose a really good hull would be one in which the surface is very corrugated, with shitloads of fins like its covered with CPU heat sinks, so that it absorbs incoming thermal energy over a large area and can reemit that energy quickly (because its an entire fucking radiator surface ). Under that would be some ceramic material to prevent internal heating. So the metal radiators would distribute the heat over a large surface before it can burn through the ceramic, then reradiate it away quickly.

I think that with most plausible weapons, thermal energy could be delivered far more quickly than it could be transferred out and radiated away, ignoring non-thermal effects from weapons like particle beams and high energy lasers. Perhaps large heat capacities will be more valuable.

[kojikun]

clay, mike pointed out my errors. he also pointed out that an isotherm would evenly distribute heat throughout itself. So while you would worry about how quickly it can be transmitte away, its not entirely necessary, because the entire mass of the isotherm hull would have to heat up before it starts to fail.

there are also possible active cooling methods that could turn the incoming heat into light, etc. but thats beyond this thread.