I noticed that Parts Heat Resistance are somewhat strange. I had a lander that consisted of the 2-Person Stock Capsule, the Stock Heatshield of Same Diameter with 560 Units of Ablator material, two AIRBRAKES and a few minor parts that shouldnt effect weight or drag significantly.
My reentry orbit was AP about 150.000 and PE about 45.000. Heat shield did not have enough Ablator and the Vessel burnt up.
In a second attempt, i tried to use the last stage (one tank and one engine) as an initial heat shield and found that, while the engines blew up, the empty tank was swallowing all the heat. In the end i could savely land without even using my heatshield (still had all 560 units left). Looks like tank might be very effective heatshields.
I do not have any other mods that should affect aerodynamics and was landing on stock Kerbin. In both cases AIRBRAKES were deployed (and burnt upp quickly) and i did not do any more burns. Reentry Heating was at 100%.