Surface temperature measurements of a low-emissivity woven flexible TPS under high-enthalpy plasma conditions

Inflatable aerodynamic decelerators for atmospheric entry missions rely on Flexible Thermal Protection System (FTPS). These systems require the use of woven materials, which must be characterised under representative reentry conditions. Accurate assessment of the material thermal behaviour requires knowledge of radiative properties, particularly emissivity. However, limited information is available regarding the emissivity of Refrex 1420 HC, the woven material employed in this study. To address this, the present work investigates the radiative response of the FTPS stack exposed to high-enthalpy air in a plasma wind tunnel facility. The samples were evaluated in two configurations: a stagnation-point and a flat-plate setup. Several radiometric instruments operating at different wavelengths were employed to measure surface temperature and emissivity as a function of wavelength and temperature. Nevertheless, measuring surface temperatures proved to be challenging due to the material’s low and strongly wavelength-dependent emissivity. This increases susceptibility to parasitic radiation, primarily due to reflections from the plasma torch at the material surface. Testing in an off-stagnation flat-plate configuration proved effective in reducing plasma reflection. In this configuration, the plasma flow is directed tangentially to the material surface, and the radiometric instruments are aligned approximately normal to the surface. As a result, the contribution of reflected plasma radiation to the measured signal is mitigated. Surface temperatures were obtained from different instruments pointed at the same measurement location. The agreement observed, particularly in the flat-plate configuration, between the instruments demonstrates the consistency of the adopted wavelength-dependent emissivity values. This is further supported by one-dimensional numerical simulations of the stagnation-point tests.