Medium-density ablative composites: processing, characterisation and thermal response under moderate atmospheric re-entry heating conditions

Medium-density foam composites based on silica fibre-filled phenolic syntactic foams were processed and characterised for mechanical, dynamic mechanical and thermophysical properties, and they were evaluated as Thermal Protection Systems (TPSs) materials by way of experiment and simulated thermal response studies under atmospheric re-entry conditions. Ablative composites with different specific gravities were processed by varying the volume fraction of the constituents. Tensile strength increased with fibre concentration and showed a maximum corresponding to 15% by volume of silica fibre and decreased on further addition, whereas flexural and compressive strength increased with increase in volume percentage of silica fibre. The mechanical properties of the fibre reinforced system were superior compared to those of bare phenolic syntactic foams. Storage modulus was considerably improved by the addition of fibre whilst the glass transition temperature was unaffected. The compositional dependency of the ablative composites on their thermophysical properties and thermal degradation behaviour was also examined. The thermal response of the ablatives was studied by simulating a moderate atmospheric re-entry heat flux history on the specimen with a maximum heat flux of about 15–18% of the stagnation heating. The thermal response was measured, and the material surface behaviour, mass loss and flammability were studied. For fibre fractions corresponding to various specific gravities, the thermal simulation experiments were studied, and it was observed that the char strength and its structural integrity were satisfactory for a specific gravity of 0.5. The maximum backwall temperature measured was 110 °C for a test duration of 500 s, and this meets the structural temperature constraint of 150 °C at the interface. The thermal response was numerically modelled, and fairly good comparison was obtained with the experimental results. This validated the accuracy of the measurement of the thermophysical properties and delivered the medium-density ablative TPS system qualified for application in atmospheric re-entry.