The flexible external insulation is part of the HERMES thermal protection system. It is designed to cover the central and upper fuselage and upper wing sections. In the central fuselage area, extreme temperatures up to 800°C can be expected. The FEI configuration chosen is described with respect to thermal properties.The thermal development of the FEI is performed in iterative steps of analytical evaluations, experimental investigations and adaptation/improvement of the analytical approach to the test results. During the pre-development work, a two-dimensional thermal mathematical SINDA model of the FEI has been established with separate functions describing radiative, gas and solid heat exchange. The results of transient analyses with respect to external heat loads during re-entry are presented.Calorimetric tests have been performed measuring the heat transfer fractions separately. Test and analysis results are in good agreement.