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Development of a Trowelable TPS Cork Material for Reusable Launch Vehicles

Paixão, Sofia; Carvalho, João; Hantz, Christian

Thermal Protection Systems are key elements of space vehicles to keep the structural integrity of interior parts during the high heat loads reached during atmospheric reentry. Over the years, countless types of TPS were developed, with many different types of materials (since ablative to ceramics), all of them with the same goal: to insulate the vehicle surfaces without compromising aerodynamics or payloads of the final system. Thereby, TPS always should be lightweight, with low thermal conductivity and enough structure to withstand shear forces felt during spatial trajectories.

Amorim Cork Composites (ACC), as one of the leaders of cork industry, has been present in different aerospace projects, where cork performs an important role as thermal protection system. Due to all its attributes of being lightweight, with cellular structure that gives it very low thermal conductivity (around 0,05 W/m·K) and also by the whole possibilities of combining it with other materials, it allows the creation of composites with outstanding properties.

The work performed in the scope of the Retro Propulsion Assisted Landing Technologies (RETALT) project comprised the development of a new TPS, capable of being applied in situ, that fulfills the thermal requirements of the reentry trajectory studied for reusable launchers. The development was focused on a trowelable solution (that could be applied in the form of a paste, with a spatula) and to cure directly in the final structure, at room temperature. The final formulation gathers an epoxy matrix with cork granules and a set of additives that have been resulted from a selection process between different types of resins: phenolic, polymeric polyurethane and epoxy, together with different types of additives: fire retardants, rheological and thermal adjuvants.

During the development, the formulations screening criteria was based on ease of application, followed by the characterization of thermal conductivity and thermal capacity measurements, as well as cone calorimeter tests to mainly observe the quality of the surface in terms of cracks and stiffness.

The selected material formulation was then tested in the L2K facilities at DLR, to evaluate its performance under high thermal loads (up to 600 kW/m2). During all the characterizations, the new material was compared with P50, a cork TPS material commercialized by ACC in the form of sheets.

The present document discloses the development process and results achieved along the project as well as the comparison with a TPS material already commercialized and currently used in spatial missions.

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