It's important to note that the purpose of a heat shield on a warhead is significantly different from that of a manned vehicle or scientific probe. For the lat

It's important to note that the purpose of a heat shield on a warhead is significantly different from that of a manned vehicle or scientific probe. For the latter, the goal is to safely and efficiently transfer as much kinetic energy as possible (subject to acceleration limits) to the atmosphere so that parachutes will survive being deployed. For a warhead, (disregarding burrowing nukes), they don't care how much kinetic energy is expended since they don't want the warhead touching down. In fact, they would prefer to minimize the amount of time that reentry takes, and so want to minimize the amount of kinetic energy lost to heating the atmosphere. For burrowing nukes, they want the nukes traveling as fast as possible, as long as the impact won't break the nuke, which is somewhere in between the airburst and manned vehicle design cases.