Abstract: Two types of highly filled, elastomeric silicone-base abiative compositions whose densities range from about 0.20 g/cc to about 0.30 g/cc. One type is a carbon char forming, high thermal efficiency ablator containing at least 92 percent by volume of low density filler with a total filler-to-resin volumetric ratio of at least 16 to 1. The additives comprise ground cork, silica or glass microspheres, phenolic microballons and silica and/or carbon fibers. The second type is a silica char forming, RF-transparent ablator containing at least 90 percent by volume of low density filler with a total filler-to-resin volumetric ratio of at least 11 to 1. The additives comprise silica or glass microspheres and silica fibers.