Life Cycle Performance of a Thermal Protection System C/SiC with a Multi-Layer Surface Protection Coating for Reusable Launch Vehicles
R. Knoche (Sp), D. Koch, Universität Bremen (Germany); U. Trabandt, EADS ST, Bremen (Germany); G. Grathwohl, Universität Bremen (Germany)
The operation of reusable launch vehicles (RLV) under severe environmental conditions especially during the re-entry phase requires sophisticated thermal protecting systems (TPS). Ceramic matrix composites (CMC) with carbon fibres offer good thermal barrier effects and provide high strength and stiffness up to temperatures of 2000°C under inert conditions. These C/SiC composites are also favoured due to their much higher creep resistance compared to oxide CMCs and SiC/SiC composites. The drawback of C/SiC, however, is the poor oxidation resistance. Therefore, the challenge of using these materials as a TPS lies in the prevention of the carbon fibres from oxidative degradation during re-entry. This is realized by an outer oxidation protecting system (OPS) whose functional reliability primarily determines the mechanical behaviour of the composite under complex loading conditions.
The present study focuses on the life cycle performance of a C/SiC TPS with a self-healing multi-layer OPS suitable for highest thermal load areas as e.g. nose caps or wing leading edges. The test procedure is defined to approach re-entry conditions, based on data of the experiment vehicle X-38 and the RLV concept “Hopper”, as close as possible. Thus re-entry conditions are simulated by thermal cycles in a temperature range between 400°C and 1450°C. In addition, tensile stress at approx. 20% of the reference rupture strength is applied to the specimens in order to open cracks within the OPS induced by residual stresses. Four different configurations are investigated: initial state (reference), low speed impact (e.g. tool dropping during assembly), rain erosion, and repaired OPS (after being damaged by rain or impact). The effectiveness and limitations of the TPS is determined by comparing mass loss, mechanical properties (e. g. residual strength), and microstructure after various numbers of thermo-mechanical cycles. Strain measurements throughout cycling additionally allow monitoring significant changes in compliance during simulated re-entries. Oxidative and mechanical degradation can therefore be related to the number of thermo-mechanical cycles. Finally, the performance of the C/SiC TPS will be discussed under consideration of the pre-damage level, critical temperature gradient, and fatigue behaviour.
