In this study, carbon aerogel based lightweight composite ablators (CALCAs) with different densities and microstructures were prepared using the vacuum impregnation method. The mechanical strength and morphology of CALCAs were investigated. Ablation-insulation performance of the samples was also evaluated by an oxyacetylene flame test with a constant heating rate of 2.5 MW/m 2 . Moreover, the structural evolution during the ablation was studied and the ablation mechanism of CALCAs was proposed. The porous insulators showed good compressive strength (3.2-9 MPa) and low thermal conductivity (lower than 0.35 W/mK). Besides the tunable density, the low recession rate (as low as 0.117 mm/s), and good ablation-insulation performance (175 C at 25 mm and 125 at 35 mm in-depth), these lightweight porous heat shielding composites are promising candidates to use as the thermal protection systems for aerospace applications.

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Enhancement of thermal stability‐insulation performance of hyper porous materials is the premier issue to design of novel porous thermal protection systems. Boron‐containing monolithic novolac xerogels (BCNXs) were synthesized using sol–gel networking of novolac resin with hexamethylenetetramine (HMTA) and boric acid at the solvent saturated vapor atmosphere (SSVA). The aim was to elucidate the effect of higher crosslinking density and thermal stable boron containing chemical bonds on the microstructure, thermal conductivity, and thermal oxidation stability of novolac xerogels. The results of FESEM and BET analysis showed that the microstructural characteristics of xerogels are significantly depend on the HMTA and boric acid concentration. The thermogravimetric results were analyzed using characteristic kinetic temperature (CKT)‐characteristic kinetic temperature range (CKTR) approximations. The effect of micromorphology of xerogels on the thermal conductivity was investigated. The effective thermal conductivity of samples were in the range of 0.031–0.048 W/m K.