High Temperature Insulations Precise Thermal Conductivity Measurements - Executive Summary

Abstract

Reliable dimensioning of highly efficient thermal protection for entry or re-entry applications depends on an accurate knowledge of the thermal conductivity of the considered materials. The fibre and foam insulations, as well as ablator char, used in such thermal protection systems are porous materials, where the thermal conductivity varies with both, applied temperature and ambient pres-sure. Precise thermal conductivity measurements in the lower tempera-ture range and for isotropic insulations are state-of-the art. The transfer of well-established lower temperature measurement principles to very high temperatures and to non-isotropic insu-lations is a challenge. DLR owns a facil-ity of adequate size which enables thermal conductivity measurements at high temperatures up to 1600°C and for controllable ambient pressures between 1 mbar and ambient pressure. This facility has been used in previous projects, however, the quality of steady state thermal conductivity measurements was not satis-factorily in particular for reduced pressure levels. This is caused by uncontrolled lateral heat ex-changes at the test specimen surface. Control of these lateral heat exchanges is therefore the key for improvement of measurement accuracy. With this in mind, a test setup for measurements under varying temperatures up to 1600°C and for pressures between 1 mbar and 1 bar has been developed as part of ESA’s Innovation Trian-gle Initiative (ITI). This test setup is not only applicable to today’s most advanced insulations, i.e. to nanoporous insulations and to anisotropic insulation systems like IMI, but also to the direct measurement of ablator char thermal conductivity.