Physics Question
 

I often have physics questions but fail to explore them before I forget.

Driving home I started thinking about heat generation on spacecraft reentry vehicles. The Gemini and Apollo reentry vehicles used ablative heat shields to deflect and dissipate heat. The shuttle used tiles to insulate against the heat. I don't know what strategies are used in current reentry vehicles.

My question is, what is the primary cause of heating on the surface of the reentry vehicle? We know that compressing air increases the temperature of it, but I've heard that air friction on the surface is what causes the extreme temperatures. Which action is most responsible for heating the surface of the object? How much does air compression contribute to heating, if at all? Are there other significant sources of heating that I'm not thinking of?

It's mostly the friction: The spacecraft is going VERY fast (appr 30k km/h) when reentering the atmosphere, so that is the relative speed with which air molecules are hitting the surface. I won't go into equations on how much heat energy each collision produces, but as the craft falls deeper into the atmosphere it becomes denser, so there are more and more collisions.

Friction is the main way for the multi-ton spacecraft to slow down (from 30k to ~200 km/h), and that braking energy is focused on a relatively small surface area. A parachute is used only towards the end of the fall.

I found this on the temperatures that spaceships experience during reentry:
What are the top temperatures occurring during reentry?

Actually, adiabatic compression heats up the skin of the re-entering vehicle. Consider that the re-entering vehicle initially enters the atmosphere at some 25 times the speed of sound. The shock wave generated is actually a wave of compressed air that is adiabatically compressed.

Requesting an in depth explanation of heat created during atmospheric reentry

Logically, the nose and leading edge heat more than the rest, while it's all subject to skin friction.

Compression wins.

I think it is both compression and friction - compressed air has greater friction with the space ship. And the greater friction between the molecules that make up the air, is what heats it up.

Inter-molecular spacing — win-win!