Effect of additives on ablation of phenolic-silica composites.

Abstract

The behavior of three additives, Cr2O3, Fe, and Fe2O3, incorporated into the resin phase of a typical phenolic-silica charring-ablatiye material, was investigated. The performance, as indicated by the over-all char depth, of additive-cont aining ablators relative to that of identically fabricated control material was evaluated during exposure to two different environments, provided by an arc-imaging furnace and by a small hybrid rocket motor employing polymethylmethacrylate/oxygen propellants. Important improvements in ablator performance were observed, particularly in the more extreme hybrid-rocket-motor tests. Char samples were examined by means of scanning-electron microscopy. Composites containing Cr2O3 exhibited the least reduction in char depth. The Cr2O3 particles did not appear to interact appreciably with either carbon or silica during exposure in the rocket motor. The effect of Fe was an over-all char-depth reduction of about 10%. Microscopically observed interactions between the Fe phase and silica were highly supportive of a mechanism of silica reduction by carbon dissolved in iron, as postulated. The largest reduction in the char depth, ranging up to 30%, was observed in composites containing Fe2O3. Results of microscopic examinations were consistent with the hypothesis that the decrease in char depth was due mainly to heat absorption by endothermic reduction of iron oxides. Nomenclature Kp = chemical equilibrium constant based on partial pressures N = mole fraction p = partial pressure a = activity y = activity coefficient AH = heat of reaction