High purity (hyperpure silica <10 ppm and Astroquartz <500 ppm) three dimensionally woven silica-silica composite materials were evaluated for potential use as a tough reflective heat shield for planetary entry probes. A special weave design was selected to minimize light piping effects through the heat shield thickness. Various Y-yarn weave spacings (0.15, 0.23 and 0.32 cm) were evaluated. Densification efficiency was determined with an 0.7 pm particle size high purity silica binder (1 ppm total metallic impurities) as an initial screening operation. The 0.23 cm Y-yarn spacing was selected as the optimum spacing. Spectral hemispherical reflectance was measured from 0.2 to 2.5 microns at room temperature. Reflectance of three dimensional hyperpure silica-silica composite (0.23 cm Y-7arn spacing) was higher than reflectance of three dimensional Astroquartz silica-silica composite at all three Y-yarn spacings and at all wavelengths measured. Mechanical properties were measured in beam flexure. Because of a lower specific gravity (1.3 to 1.4), the strength results were lower than the slip cast hyperpure fused silica (1.6 gm/cc) evaluated under NASA CR152118. Thermal conductivity of the hyperpure silica-silica composite was measured from room temperature to 908°C. With improved densification techniques a viable Jupiter atmospheric entry probe heat shield could be fabricated from 3D woven silica-silica materials. The lower density material evaluated had very promising reflectance characteristics and improved toughness but the strength did not meet the design requirements.
KeywordsHeat Shield Fiber Spacing Silica Composite Auxiliary Heater Light Piping
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