Techniques for Determining the Micrwave Properties of Thermally Degraded Spacecraft Heat Shield Materials

Techniques for determining the microwave properties of spacecraft thermal protection materials, such as subliming ablators, charring ablators, etc., under various thermal conditions are presented. Room temperature dielectric constants and loss tangents of the materials are obtained at several frequencies using standard cavity methods. High temperature dielectric properties can be inferred from real time transmission loss measurements in an arc tunnel. The dielectric properties of cold thermally degraded materials are inferred from transmission measurements in an arc tunnel and on an antenna pattern range. An independent determination of the dielectric properties of thermally degraded materials was obtained by comparing aperture admittance measurements of a ground-plane mounted rectangular waveguide, under the ablation material, with aperture admittance calculations assuming various inhomogeneous dielectric properties. Also included, for comparison, are calculations assuming the conducting char layer is replaced by a perfectly conducting metal sheet.