How does a heat shield work?

The heat does not come from friction, which is the usual explanation and the wrong one. A capsule arrives far faster than the air can move out of its way, so the air is violently compressed in front of it, and compressing a gas heats it. Squash it that hard and it glows.

That is also why a heat shield is blunt rather than pointed. A blunt shape pushes a shock wave out ahead of itself and holds the hottest gas at arm’s length from the structure. A sharp nose would cut straight through and cook.

Dragon uses PICA-3, an ablative shield. Rather than resisting the heat, it is designed to char and erode, carrying energy away with the material that leaves. It protects the crew by sacrificing itself, which is why ablative shields are inspected — and historically replaced — between flights.

Not everything solves it this way. A Falcon 9 booster has no heat shield at all; it fires its engines into the oncoming air instead, so the worst of the heating happens out in its own exhaust rather than against the airframe.