Design studies on the reuseable surface insulation (RSI) systems for the Space Shuttle require accurate data on the thermal conductivity of ceramic-fiber insulations. Well-established laboratory techniques, such as the guarded hot plate experiment, are available for such measurements; however, caution must be exercised in applying laboratory data to widely different re-entry thermal environments. The heat-transfer associated with infrared radiation transmitted through the fibrous materials must be considered. Various analytical models have been developed to account for this radiant heat transfer, but their verification has often been incomplete. A new approach has now been applied in which infrared transmission measurements were made with an optical system that collected all the scattered radiation transmitted through the back surface of fibrous samples. Experimental data were analyzed using a “two-flux” model for the radiation scattered and absorbed in the materials and excellent correlation was obtained between theory and experiment. To account for the wavelength dependence of the scattering cross-sections of many materials, a “four-flux” model was developed. This model was used to compute the contribution of the radiation transmission to the total thermal conductivity of samples measured on the guarded hot plate equipment. The contribution was shown to be as high as 75 percent of the total conductivity at temperatures above 1000 K.

The significance of this result is that data obtained from such instruments should not be used in heat-transfer design computations without regard to the radiation transmission. Such regard will be particularly important in the thermostructural analyses of RSI for the Space Shuttle and other re-entry vehicles.