xanfus

Shunter, i may assume, Kerbin emulates Earth atmosphere by placing high temperature layer high enough. This is area, where highly energetic ionospheric electrons occur. Moving through the area at high speed leads to heat buildup due to fast absorption of electrons' kinetic energy. Stay at that height long enough, and you get warm anyway. Supersonic flight well below leads to significant relative heat buildup due to adiabatic compression of rushing air.