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?.