how does reentry heat really work?

[Laie]

Trying to get to Eve, I encounter some unexpected and counterintuitive heat issues the moment I dip into the atmosphere.

While I presume that the game is getting it all wrong again, I'm open to the notion that my expectations and intuition may be off. Basically, I believe that reentry heat is the result of friction; that is, kinetic energy becomes thermal energy. Is that even true or are there other reentry effects that create a lot of heat, seemingly out of nowhere?

[kurja]

Air is compressed and heated in front of the craft, which then heats the craft. Friction heating isn't nearly as significant.

[fat_cloudz]

Common misconception. Your craft compresses the atmosphere immediately in front of it which increases temperature. The ideal gas law (PV = nRT) is the mathematical explanation; P = pressure, V = volume, n = moles, R = gas constant, T = temp.

More info:

https://en.wikipedia.org/wiki/Ideal_gas_law

https://en.wikipedia.org/wiki/Atmospheric_entry (shock layer gas physics section)

[pincushionman]

Yes, not friction - shocks. The "free-stream" air approaches the object (seriously - it makes more sense if you imagine the object as being fixed and the air as moving) at hypersonic velocity, crosses a shock wave, and is suddenly moving sub-sonically relative to the object. Kenetic energy is proportional to v^2, so all that energy has to become something else, and very quickly. So temperature goes up.

[Leszek]

I would also point out that it is still kinetic energy becomes thermal energy, or rather kinetic energy + gravitational potential energy = thermal energy. I am just pointing this out just in case you where doing calculations somewhere.

[Laie]

I would also point out that it is still kinetic energy becomes thermal energy, or rather kinetic energy + gravitational potential energy = thermal energy. I am just pointing this out just in case you where doing calculations somewhere.

Not exactly calculations. Or only off the top of my head, an educated guess whether it makes any sense at all. What I was really asking was "does it make sense to experience a lot of heat without slowing down"? Whether the exact mechanism is properly called friction or not may be good to know for later occasions, but right now this is not my main concern.

[Nothalogh]

I believe that reentry heat is the result of friction

Heating occurs in the detached blunt body shock ahead of the vehicle, heat is transferred to the vehicle via radiation.

Some heating will take place behind the vehicle as well, due to the convergence of the shock aft of the vehicle, this is why putting a 2.2m heatshield on the end of a Jumbo-64 is ridiculous.

[PakledHostage]

Heating occurs in the detached blunt body shock ahead of the vehicle, heat is transferred to the vehicle via radiation.

How the heat is transfered depends on the gas that the atmosphere is composed of. Radiative heat transfer has a net cooling effect on objects entering the Earth's atmosphere (ever wonder why the hottest parts of the shuttle's tiles were black?), while there's a net heating due to radiative heat transfer during Mars atmospheric entry.

[K^2]

Not exactly calculations. Or only off the top of my head, an educated guess whether it makes any sense at all. What I was really asking was "does it make sense to experience a lot of heat without slowing down"?

Keep in mind that kinetic energy is quadratic in velocity. The faster you're moving the less of a change in velocity you need to experience for heatting to occur. And as someone pointed out, gravity is in that equation too. It's entirely possible to be generating increadible amounts of heat and still be speeding up if you're going down.

Radiative heat transfer has a net cooling effect on objects entering the Earth's atmosphere, while there's a net heating due to radiative heat transfer during Mars atmospheric entry.

We're catching up, though.