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by aperiodic 5314 days ago
Contrary to popular belief, the reason that spacecraft heat up upon re-entry is gas compression, not friction (remember the ideal gas law, PV = nRT). The gas would still be compressed by the electrostatic barrier, so the heating would still occur.
1 comments
CamperBob 5314 days ago
Is there really a difference between heat generated by friction and heat generated by gas compression, at the end of the day?
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ars 5313 days ago
Of course. They work very differently.

If you have heat from friction you expect the heat along the length of the ship.

If you have heat from compression you expect it at any point where the ship is not parallel to the air - but not along the length of it.

Heat from friction can be aided by a low friction surface material.

Heat from compression can be aided by not having any surface very perpendicular to the air (spread out the change in angle over a distance).

With heat from friction you want a short ship - so just have the angled surface and be done with it.

Heat from compression would benefit from a longer ship so you have space to gradually change the angle of a surface.

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CamperBob 5313 days ago
If you have heat from friction you expect the heat along the length of the ship.

How in the world does that follow? Heat from friction will be generated in proportion to cos(angle) between the airflow and the surface at any given point, just like heat from compression would. It will flow along the ship's surface and soak into its interior volume just as if it were generated by compression.

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ars 5313 days ago
Um, air can't flow at an angle to a surface. It can only flow parallel to it.
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CamperBob 5313 days ago
But what's important is the behavior of the airflow at the point where it hits the surface and is forced to both compress and diverge. That's where the majority of the heat is presumably generated, right? Maybe that's where I'm misunderstanding the issue.
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ars 5313 days ago
That heat is created by compression, not by friction.

And I would not automatically say that's where the majority is. Some is created there from compression, some from friction by flowing along the length of the ship.

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