Question about thermal conductivity

[Junghalli]

I was thinking about the problem of disposing of waste heat on spacecraft, specifically spacecraft designed to enter an atmosphere, and it occured to me that a great solution to this problem would be to find a material that was thermally conductive in only one direction. If your ship is designed to enter atmosphere and has a powerful engine it produces a design contradiction: you want your outer hull to keep the heat of reentry out of the ship on one hand, but on the other hand you don't want it to trap waste heat in. If you could make a material that would somehow let heat out but not in it would solve this problem.

Is this possible? Or am I talking the physics equivalent "sky taste like vanilla" here?

[Kuroneko]

Sure, it in principle on could construct various anisotropic materials that have this property. An easy example is quartz, although extending that sort of thing to a sufficiently large macroscopic level would be difficult. An even more extreme but theoretically possible example would be nanotubes.

[Wyrm]

I'd be careful how I phrase this. Kuroneko said such materials exist, but you nonetheless have to make it clear that the material does not violate thermodynamic laws; in short, the material must still conduct heat from hot to cold, even if it gives preferential treatment in one direction. You can't make this material pump heat from the cool interior to the hot reentry face without input of work, but you can still preferentially have the material pump heat faster to the cold side of the spacecraft more than the hot side of the spacecraft lets heat soak in.

Provided that you have a cold side, that is. Does someone who knows something about reentering spacecraft know if there is such thing upon reentry?

[Kuroneko]

It should also be noted that I assumed that the OP meant to ask about different thermal conductivities along different axes, rather than truly zero conductivity in one direction (as that would make the "sky taste like vanilla"), and that there is no implication that either quartz or nanotubes are realisitic materials for spacecraft hulls--merely that they are easy examples of anisotropic thermal conductivity. It's possible at least in principle to have a hull made of a material that conducts heat much better along surface of the hull itself rather than inward; the top would obviously tend to be much cooler during re-entry.

On another note, much of the same effect should be achievable simply by layering a good thermal conductor on top of a good insulator. It would be even better to have an extra insulator plate on top of that on the re-entry side of the craft.

[Junghalli]

in short, the material must still conduct heat from hot to cold, even if it gives preferential treatment in one direction.

Of course. The idea was that it allows waste heat to move easily from the inside of the hull to the outside, but still can act as a decent heat shield on reentry. I.e. it would let heat out but not in. Of course this means if your ship isn't generating huge amounts of waste heat it would make rather lousy insulation and force life support to work extra hard... I guess you could have a second layer of insulation on the inside of the hull and then run coolant pipes directly underneath it.

Provided that you have a cold side, that is. Does someone who knows something about reentering spacecraft know if there is such thing upon reentry?

The cold side is the inside of the ship. Otherwise the heat of friction from reentry would kill the crew.

On another note, much of the same effect should be achievable simply by layering a good thermal conductor on top of a good insulator. It would be even better to have an extra insulator plate on top of that on the re-entry side of the craft.

I'm kicking myself for not thinking of that as we speak...

[Surlethe]

On another note, much of the same effect should be achievable simply by layering a good thermal conductor on top of a good insulator. It would be even better to have an extra insulator plate on top of that on the re-entry side of the craft.

Isn't that how the space shuttle's designed?

[Kuroneko]

Isn't that how the space shuttle's designed?

Definitely on the extra insulation layer on the re-entry side, but I don't know the interior. It would, however, make a lot of sense to construct it that way, so I wouldn't be at all surprised.