Development and Verification of Improved Composite Material Demise Models for Ground Risk Prediction Software Tools

The Carbon Fiber Reinforced Polymer (CFRP) represents an example of complex, non-monolithic compounds, which are becoming progressively more prevalent in many aerospace applications. This work of thesis develops a numerical model describing the CFRP ablation behavior during an atmospheric uncontrolled reentry on Earth from Low Earth Orbits (LEO) regions. The modelling results are compared to the results that have been obtained experimentally at the Institute of Space Systems (IRS) of the University of Stuttgart. This work is part of the Clean Space initiative promoted by the European Space Agency (ESA): this initiative aims to maintain the Earth’s orbital environment as a safe area and debris free (https://www.esa.int/Our_Activities/Space_Safety/Clean_Space). In order to achieve this objective, a new approach, known as Design for Demise (D4D), has been introduced: it refers to the intentional design of space system hardware that at the end of their life cycle have the characteristic to completely burn up or “ablate” during an atmospheric uncontrolled reentry decreasing, therefore, the amount of surviving parts reaching the ground and the correlated casualty risk. In the demisable behavior’s analysis of the CFRP, a cylindrical disk of 0,00407 m length and 0,02655 m diameter has been examined in three different simulation cases: a) low heat fluxes conditions (the reference value used for the aerothermal heat flux is 260 kW/m2), where the sample is subjected mainly to pyrolysis outgassing; b) medium heat fluxes conditions (the reference value used for the aerothermal heat flux is 520 kW/m2), where the sample presents an initial pyrolysis outgassing followed by a gradual oxidation; and c) high heat fluxes (the reference value used for the aerothermal heat flux is 1400 kW/m2) where both the pyrolysis of the epoxy matrix and the oxidation process of the char proceed at higher rate. The aerothermal heat flux reference values are in agreement with the values that have been adopted during the experiments at IRS. Firstly, the developed numerical ablation model investigates on the temperature profiles of the CFRP sample with respect to the time of exposition to reentry conditions and along all its thickness. Particular attention has been addressed to analyze the trends of the temperature profiles related to the temperature’s growth on the sample’s boundary surfaces which includes: a) the surface that appears to be directly exposed to the aerothermal heat flux (surface referred in the following chapters as “front surface”), and b) the rear surface, that is not directly exposed to the flux (surface known in the following study as “back surface”). The modelled temperature profiles have been found to be in good agreement with that obtained experimentally. In particular, the modelled vs. laboratory tests comparison showed a good agreement in terms of temperature profile’s growth rates and highest temperature values achieved at the end of exposition time to heat flux conditions that are similar to those that would be observed during an uncontrolled reentry to the Earth from LEO regions. The second part of the analysis is focused on modelling the reduction of the CFRP sample’s thickness during different reentry conditions: the sample thickness’s reduction is primarily due to the oxidation process of the carbon char that takes place when the CFRP is under high temperatures conditions (temperatures higher than the activation temperature of the oxidation process, which is 1160 °K). This additional analysis concerns the cases when the sample is exposed to medium and high heat fluxes, that are conditions during which the oxidation process takes place: a) in the medium heat flux case, the thickness’s decrease results to be equal to 46,81% at the end of the test, whereas b) in the high heat flux condition, about 60% of the thickness is lost at the end of the simulation