The effects of radiation on ablative heat shields during atmospheric entry

Abstract

Spacecraft employ composite materials as Thermal Protection Systems (TPS) to survive entering a planetary atmosphere at hypersonic speeds. Under intense convective and radiative heating from the surrounding shock layer, these composites decompose and erode, transferring heat away from the payload. A risk-averse approach has long saturated the space industry, with the selection of unoptimised, dense, flight-qualified materials taking priority over novel TPS, tailored to the mission at hand. However, demanding flight trajectories and greater payload carrying capacity required on future missions call for contemporary research into lightweight composites. A better understanding of the high temperature response of these materials is needed to improve TPS sizing and optimisation. This thesis uses a combination of experimental testing and numerical simulations to understand the effects of thermal radiation on these composites, both inside the material and upon interacting with the surrounding aerothermal environment. TPS material testing in arc-jets can be complemented by the use of impulse facilities to characterise flight-equivalent radiative heating in multiple atmospheres. Being a recent development, ablation testing in expansion tubes has so far used non-flight geometries and/or conditions. A methodology was therefore established to design and manufacture scaled composite aeroshells in the laboratory. The models were subjected to Earth and Venus hypersonic conditions and decomposed upon contact with the flow, allowing carbonaceous species to mix with the surrounding hot plasma. Radiation from the boundary layer was then measured using emission spectroscopy and compared to data collected from experiments using a cold steel model. Computational fluid dynamics, finite-rate surface kinetics and radiation databases were then validated based on these spectral measurements. Results most suited for comparison with experimental data were mainly obtained using a combination of Park's reaction schemes, Suzuki's reduced nitridation surface kinetic rate and the NEQAIR radiation code for both flow conditions. Visible radiation was heavily underestimated by numerical models, while generated UV and IR spectra compared well with measurements. Two unique datasets for Earth and Venus entry were thus created through experiment and numerical analysis. The recent investigation into the volumetric nature of ablation makes understanding internal radiation of TPS materials a priority. This term is rarely included in contemporary thermal response codes. To accurately characterise effective morphological and radiative properties, pore-level simulations were carried out on real TPS material geometries, recorded using high resolution synchrotron tomography. A library of the calculated properties, such as spectrally resolved extinction and scattering coefficients and scattering phase function, was used to calculate macroscopic optical properties of a semi-infinite slab of each material. A greater increase in absorptance during pyrolysis was seen for the modern medium density carbon phenolic than for the dense graphite. Combined with these properties, volumetric view factors were used to evaluate the radiative flux inside the TPS material, which was then supplied to the PATO thermal response code. Internal radiation was shown to have a demonstrable effect through the comparison of simulation and flight data, promoting its inclusion in future modelling.