Preliminary Design Study for Thin-layered Thermal Protection Systems for Future, Reusable Spacecraft Structures Exposed to Moderate Heat Loads Vorkonzeptstudie zu dünnschichtigen Hitzeschutzsystemen für Strukturen zukünftiger, wiederverwendbarer Raumfahrzeuge unter moderater Hitzebelastung

Kurzfassung

To reduce launch costs for space projects, research and development efforts are increasingly focusing on reusable rocket stages. Ablators and reusable Ceramic Matrix Composite solutions are suitable options for providing thermal protection for structures subjected to high thermal loads. For applications subjected to moderate thermal loads (<1000 ◦C), ceramic solutions are often complex, expensive, and oversized; Ablators, on the other hand, are not reusable. The goal of this work is to bridge this technological gap by developing concepts for a passive, reusable, thin-layered thermal protection system designed for use on reusable first-stage rockets and intended to offer economic benefits through its reusability. To achieve this, the relevant requirements regarding material characteristics are identified. Then a database is created that contains relevant material properties and enables the comparison of material candidates that are potentially suitable for the application. Here, particular focus is placed on commercially available high temperature resistant insulation materials. Based on this data collection, design concepts for the thermal protection system are developed and subsequently compared with one another in a systematic evaluation process. The concept development is supported by thermal transient finite element simulations, in which various material combinations and layer arrangements are investigated based on the previously identified material properties. To achieve this, a parameterized, extendable simulation model is set up. This model uses adaptive optimization algorithms to adjust the layer thicknesses of the individual materials within specified limits, based on generic load cases. This allows to determine the optimal solution in terms of the thermal protection systems weight per unit area for various materials and their combinations. Simultaneously, the employed optimization method ensures that the thermal load on the substructure component remains within specified limits. On the basis of the created database, simulation results, and derived concepts, a series of tests is planned and conducted. The goal is twofold: Firstly, material and concepts performance under application-oriented thermal conditions is examined. Secondly, design concept feasibility is assessed. The experimental tests are conducted on the burner test platform at the DLR Institute of Structures and Design. The prototype samples are subjected to controlled thermal loads generated by an oxygen-acetylene flame. The calculations and experiments show that microporous insulation materials and aerogel composites can offer significant advantages over common ceramic fiber products in terms of thermal insulation performance. However, they are only of limited suitability as surface materials due to their weak mechanical properties. Two different approaches to address this problem are tested in accordance to the developed concepts: Modifying the surfaces with high-temperature ceramic paint coating proved to be unpromising, covering it with high-temperature-resistant austenitic steel leads to very good results. To summarize, the current work discusses use-case specific approaches and relevant concepts for the future development of a reusable thermal protection system for moderate thermal loads, including (i) the selection of innovative materials, and (ii) an experimental assessment of performance and feasibility.