ablative material is designed to slowly burn away in a controlled manner, so that heat can be carried away from the spacecraft by the gases generated by the ablative process while the remaining solid material insulates the craft from super-heated gases. SIRCA, Silicone Impregnated Refractory Ceramic Ablator, was used on Space Shuttle and Viking programmes SIRCA has a typical density of 0.2 to 0.4g per cc, and can withstand heat flux of up to 300 W per cm2. Easily machined to custom shape. BLA: silicone based honeycomb ablator. base silicone resin, silica microballoons as filler density: 0.3g per cc withstand temperature up to 1760C ( 3200F) US Patent US6627697B2 whereas SpaceX use PICA-X, Phenolic Impregnated Carbon Ablator based density: 0.27 g per cc withstand over 3000F re-entry speed of Mach 24