A low-to-medium mass density ablative thermal protection system (TPS) comprises a phenolic-based core member comprising a honeycombed plurality of cells separated by cell walls; and an ablator material comprising a carbonaceous material and a phenolic resin material filling the cells. The ablator material may further comprise silica and phenolic resin microspheres. Embodiments include fabricating the ablative TPS by filling the cells of the core member with a liquid slurry or dry mixture of the ablator material, followed by degassing, drying, and curing of the ablator material.