Reentry: What Causes Structure Heating of Spacecraft?

[Clausius2]

What is the principal cause of spacecraft structure heating when reentering in the atmosphere?.

When I took my course in shock waves I thought in it like the most probably cause. The temperature increases behind of a shock wave surface of incidence (am I allright?). So there would be an optimal pitch angle for which the oblique shock wave generated does not damage spacecraft structure, or it balances temperature distribution so that maximum is reached in thermal armoured zones (shuttle's bottom).

Some people talk about atmosphere friction against structure, but for me it is the same concept of shock wave formation. Or would be possible a spacecraft desintegration when it falls subsonic?. In this case I think it would not be possible, except of an excesive thermal flux when passing trough thermosphere.

If my explanation is correct, thus there would not be a critical angle for simmetrical spacecraft , like Spirit host vehicle (it had conical shape when falling to Mars surface?.

If you are not agree, then what is the cause of critical angle for reentry existence?.

 

[kurious]

Too steep an angle gives overheating.
Too low an angle makes the spacecraft bounce off the atmosphere back into space.

 

[aerobilal]

well, you see the problem of heating is threefold. First, the shockwave is almost always detatched due to nose bluntness(space shuttle).Thus it is a curved bow shock rather than a attatched oblique shock(unless the nose is pointed). hence you have heating due to slowing of the air behind the shock. Second, there is definitely frictional heating by the flow past the ship (remember the reynold-colsburn anology?). In fact the nose on the spaceship entering at mach 36 is blunted since if the shock was allowed to remain attatched the heating due to the fast moving flow's friction would be much more. Third at these high temperatures the shock wave radiates (remember the white glow columbia crash?).
Thus there is a critical angle which would ensure that the point of maximum flux(a combination of the three) would move to the underside.
BILAL, Pakistan.

 

[Clausius2]

Now I don't remember the Reynolds-Coburn analogy.

Your answer looks like very logical. So, is it correct that critical angle is a function of spacecraft geometry and speed exclusively?.

Has a cone-shaped spacecraft any critical angle different of 0º?.

What is the reason for bouncing off the atmosphere for low pitch angles?. This seems to be a simple question, but I cannot imagine any bouncing when a body crashes into a fluid (yes, I have seen that yet when I throw a plain stone to the water, but I can't explain that.).

Anybody knows how the hell NASA can calculate the critical angle?. It seems to me a heavy task.

 

[remcook]

Anybody knows how the hell NASA can calculate the critical angle?. It seems to me a heavy task.

they also use windtunnels to test the basic flow. you don't have to calculate much then.

 

[aerobilal]

hello clasusius:
i am glad you appreciated the answer.
1)The Reynold Colsburn anology means that fluid friction due to viscosity is directly propotional to the amount of convective heating(heating by means of flowing fluid)

2)Of course , as u mentioned ,the underside is armored against heating hence we would like the points/area of maximum flux to move underside, hence a positive(nose up) angle of attack is more favorable.

3) You said it. the pebble bounces off becouse it doesnot have enough momentum normal to the pond surface to penetrate the surface tension. Similarly, the space shuttle would bounce off at low pitch, however this skidding off would generate extra heating. In fact at one time scientists were contemplating "skidding reentry" to slow down the shuttle before reentry. However itwas abandoned due to extra heating.

4) NASA i believe uses CFD for visualizing the "red" portions and try to move it to the underside all the time by iterating for angle of attack.

BILAL
Pakistan

 

[LURCH]

It is my understanding that the heating caused when an object enters the atmosphere is not principally caused by friction, but rather by compression. Perhaps this is saying the same thing as the "heating because of a shock wave" that you mentioned. Any gas increases in temperature when compressed, the atmosphere in front of an object traveling at high speed is compressed so much the temperature becomes very high.

 

[Clausius2]

Ok, you have the same point of view. Compression-friction and heating are linked concepts in reentry.

 

[griffin]

I know that it sounds insignificant because of the compression and the shockwave effect, but it is also a lot hotter in the highter regins of our atmosphere.

(I don't know how significant it is though)