Вот, что, кстати, аферюги пишут про различия в подходах к теплозащите в разных программах:
During the late 1950s and into the 1960s, the three major human space programs—Mercury, Dyna-Soar, and Apollo—faced very different reentry concerns.
Mercury would perform a ballistic reentry from a low-Earth orbit (or a ballistic trajectory for the suborbital flights). The heating rates and durations it faced were much the same as the ballistic missiles, and the total size of the capsule was not much larger or heavier. Thus, the solutions used to protect the missile warheads were generally adequate for the Mercury capsule.
Dyna-Soar was a winged vehicle that used a lifting reentry, which presented a lower heat-ing rate than did ballistic reentry but for a much longer duration. The desire to use the glider for multiple missions dictated a metallic heat shield almost by default.
Apollo needed to withstand superorbital entries after its lunar flybys, resulting in a heating load an order of magnitude greater than Mercury, the ballistic missiles, or Dyna-Soar. Because Apollo would fly a semilifting entry, the heating duration was also quite long, although it was not as extreme as that of Dyna-Soar.
Engineers at NASA and prime contractor North American Aviation estimated that the total heat at the stagnation point might be as high as 120,000 BTU per square foot, with stagnation point heating rates of 520 BTU per square foot per second. It was clear that the only material that could withstand the heating at the stagnation point was an advanced ablator. The best material for the rest of the heat shield, and indeed the rest of the capsule, was less clear. For instance, Faget, one of the principle designers of Mercury, wanted to use René 41 and beryllium shingles on the afterbody, as had been done with Mercury, but the heating estimates were higher than could be tolerated by a shingle of any practicable thickness.
Т.е., в капсулах Аполло нагрев был на порядок выше, чем на Меркьюри, да еще впридачу был более продолжительным. Разработчики рассматривали возможность использования металлических плиток для защиты боковых поверхностей капсулы, однако расчеты показали, что для нужных рабочих температур плитки выйдут слишком массивными.
