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Advanced Ablation Characterization and Modelling

Periodic Report Summary 1 - ABLAMOD (Advanced Ablation Characterization and Modelling)

Project Context and Objectives:
During entry into a planetary atmosphere, a vehicle is subjected to severe heating caused by extremely high gas temperatures in the surrounding shock layer. Historically, an ablative thermal protection system has been used to protect the vehicle (Apollo, Galileo, Huygens, Viking and many others). In the last two decades space related material research activities in Europe has almost completely shifted to reusable thermal protection systems. Since during that period most European missions were heading to low Earth orbit, there has been no major space related research and development on ablative materials and the physico-chemical modelling of ablation processes. More recent as well as future missions have more often considered the exploration of other planets or sample return to Earth, as e.g. the American Stardust mission, Japanese Hayabusa mission or the European ExoMars mission. Therefore, a drift back to ablator materials has been observed.
A material is called ablative due to a certain thermophysical behaviour, not due to a certain microscopic structure. There are many different ablative materials with strongly varying mechanical and thermal properties. From the variety a certain ablative material can be chosen depending on the levels of heat flux and pressure that are expected for a certain space mission.
The discontinuity in R&D activities on ablators becomes obvious in the level of complexity of models for the simulation of ablation processes. In many other physical and technical fields the level of complexity of simulation models has significantly increased during the last decades, e.g. in fluid dynamics or structural mechanics three-dimensional time-resolving simulation models have been developed and validated and are state-of-the-art models in technical application. Existing ablation models, however, are still equivalent to those of the late 1980s which are based on a one-dimensional treatment and simplifying considerations that were developed by Kendall et al. [1] in 1968. Commonly the numerical core is very simple, and the models neglect all aspects of fluid-structure interaction inside the porous charred ablator. Only conductive heat transfer and pyrolysis are modelled in detail inside the solid. No material anisotropy is considered, and the material is considered to exist in three states: virgin, char and reacting with a front between the zones. For the virgin and char material data is required, and is interpolated for the reacting zone.
The validity of the existing models very strongly depends on the availability of mechanical and thermal properties of both virgin and charred material. These properties must be provided for the complete temperature range. Usually, not all required data can be provided from laboratory measurements, and therefore the results very strongly depend on empirically derived test data. For example, the thermal decomposition rates of a material are derived from thermal gravimetric analysis (TGA). This method is restricted to low heating rates and it does not give specific information on particular chemical processes, but provides global data only. Experience has shown that under realistic heating rates physico-chemical processes on the materials’ surface as well as inside the porous char proceed differently from laboratory measurements. The modelling is not sophisticated enough to consider the real processes, but must account for grossly averaged bulk properties which are matched to macroscopic experimental data. The properties used are not necessarily reflective of the true (mesoscale) material behaviour, but are designed to represent the overall material behaviour. Therefore a minor change to the material requires a new full dataset as the material properties cannot be extrapolated within the model due to the lack of representation of the physics.
Main advancements of the ABLAMOD project can be summarized as follows:
- A multi-scale modelling approach will be used in order to achieve a significantly higher fidelity ablation simulation, since the available European tools have significant shortcomings,
- A modular approach to the modelling of the complex ablation process handling different physical phenomena separately will be constructed. The modules will be transport properties, radiation, internal flow, gas surface interaction (internal and external). These modules will be integrated and coupled into a multi-scale ablator modelling code which will provide a significant enhancement on current models. The modular approach allows future improvements to be easily incorporated. This approach to ablator modelling is unique in Europe.
- All three main ablation materials, i.e. carbon phenolic, silica phenolic and cork phenolic will be investigated partially in the same flow environment. The instrumentation of the samples for the tests will be the same. For the first time all relevant ablator types will be studied and compared at the same conditions within one project in Europe.
- The modelling work needs high quality experimental data in flight relevant test conditions for development and validation. Therefore experiments will be carried out in two very powerful European long duration high enthalpy facilities. Careful attention is paid to the characterization of the high enthalpy flow using laser based advanced diagnostic techniques and Emission Spectroscopy including infrared spectroscopy. This will provide a unique and comprehensive dataset on the flow conditions.
- In addition to the aerothermal test data, material properties like density, thermal conductivity, heat capacity and porosity before and after testing will be provided for the validation of modelling. The internal micro-structure of the material samples will be measured with a computer tomography system. This is the key to the understanding of the meso-scale processes which drive the ablator response.
- Material sample development, pre and post test modelling, synthesis of the data and extrapolation to flight will be carried out by two industrial teams, which are involved in key European space transportation programmes.

Project Results:
- Requirements on material samples and characterization have been defined and reported in D21.1.
- All cork based material samples have been manufactured and provided to the test centers (D22.1). The instrumentation of the tests for ablation tests will be carried out by DLR in May’14.
- Silicon based material samples for material characterization at AIT and ÖGI have been manufactured and provided. The instrumentation of the tests for ablation tests will be carried out by Avio in May’14 (D22.2).
- Carbon based material samples for material characterization at AIT and ÖGI have been manufactured and provided. Samples for ablation tests and their instrumentation will be carried out in May’14 (D22.3 and D23.1).
- Characterization of crock based and silicon based material samples have been completed. The characterization work on carbon based materials is in progress and will be completed in the coming 2-3 weeks. The first version of the reports and the data base are available (D24.1 and D24.2).
- Test plan for validation experiments has been created and distributed to the project partners (D41.1).
- CFD simulation of the flow parameters in L3K and SCIROCCO has been performed and reported (D51.1). The electronic data base is also available (D51.2).
- Pre-test modelling of validation experiments using the baseline ablation code FABL iss in progress. A preliminary report and data base has been issued (D52.1 and D52.2).
- A project ftp server and project portal on web-site base have been established (D 61.1).

Potential Impact:
- Ability for the definition of new thermal shielding and lower risk return strategies for European missions or mission components using the new ABLAMOD design tool, which is validated with dedicated experimental data of two key European facilities.
- This new tool will lead to enhancement of the technological spin-off effects between European non-space (defence, automotive, propulsion, etc) and space industries by using it for less complex (from modelling point of view) CMC based thermal protection systems,
- Creation of an experimental and numerical data base of all relevant ablator materials (cork based, silicon based and carbon based ablators) at the same conditions within one project for future European space missions,
- Decreasing the quite high safety margins of ablative thermal protection systems, and. decreasing the costs resulting from improved prediction and characterization tools,
- Improvement of the European non-dependent access to space ability by developing a high fidelity simulation tool for the design of ablative thermal protection systems,
- Improvement of future European space missions allowing them to carry more scientific payloads or instrumentation due to the more reliable and non-conservative mass budget estimation
- Education of young European scientists due to the use of the results for Ph. D, M.Sc and B.Sc thesis,

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