Credit to u/estanminar
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What really helped me visualize the problem of reentry was when it was explained in terms of kinetic energy vs enthalpy of evaporation. At orbital velocity, one kg of matter has more than enough kinetic energy to vaporize that kg of matter. Which means an object reentering will always have more than enough kinetic energy built up to vaporize it completely. All that kinetic energy has to be slowly bled off in a way that not only doesn’t vaporize it, but doesn’t even melt it. Or for human-rated craft, in a way that it doesn’t even get hot enough to hurt the delicate humans onboard.
Well, if your heatshield is ablative, you just resign yourself to the fact that part of your spacecraft will vapourize, but you can use its enthalpy of vapourization to absorb the kinetic energy and protect the rest of the craft. IIRC, all currently-operational human-rated spacecraft (Soyuz, Dragon, Shenzhou, Orion, Starliner) use ablative heatshields.
Ablative heatshields are inherently single-use though, and the goal of Starship is to be rapidly and fully reusable. The Space Shuttle’s heatshield was refurbishable, but not rapidly reusable. As for Buran, I’m not sure that it flew enough flights to get data on heatshield durability.