Methodology for Ablation Investigations in the VKI Plasmatron Facility: Preliminary Results with a Carbon Fiber Preform

Following the current developments of a new class of low-density, carbon/resin composite ablators, new efforts were initiated at the VKI on ablation research to understand the complex material response under reentry conditions and to develop and validate new material response models. Promising experimental results were obtained by testing the low-density monolytic composite ablator (MonA) in the 1.2MW inductively heated VKI Plasmatron facility. The application of a high speed camera with short exposure times (2μs) enabled in-situ analysis of both (3D) surface recession and spallation and further made it possible to demonstrate the outgassing effects of pyrolizing ablators. A change in the surrounding gas phase was observed, which is likely due to outgassing products keeping away the hot surrounding plasma before burn-off in the boundary layer. Time-resolved emission spectroscopy helped to identify carbonic species and to capture thermo-chemical effects.

This knowledge was then translated into the development of a testing methodology for charring, low-density ablators in order to investigate the material response in the reactive boundary layer. The successful application of emission spectroscopy encouraged the extension of the setup by two more emission spectrometers for not only temporal but also spatial observations. The extracted experimental data will be employed for comparison with model estimates enabling validation of a newly developed stagnation line formulation for ablation thermochemistry. It was further understood that a proper examination of tested samples has to be performed, especially of the subsurface char layer, which is subjected to ablation. Degradation of the carbon fibers can vary with pressure and surface temperature due to the changing diffusion mechanisms of oxygen that can weaken the internal structure, leading to spallation and mechanical failure. This necessitates ablation tests in combination with microscopic analysis tools (SEM/EDX) for sample examination at the carbon fiber length scale (~10μm).

Such microscale characterization was recently started at the VKI: A low-density carbon fiber prefom (without phenolic impregnation) was tested in the Plasmatron facility at varying static pressures from 1.5-20kPa at a constant cold wall heat flux of 1MW/m2, resulting in surface temperatures of around 2000K. Surprisingly, it was found that recession and mass loss of the test specimen was highest at low static pressure (1.5kPa). Furthermore, high-speed-imaging as well as conventional photography revealed strong release of particles into the flow field, probably assignable to spallation.

Micrographs showed that packages of glued fibers (fiber bundles) are embedded in between randomly oriented, individual fibers. It is therefore assumed that ablation of the individual fibers leads to detachment of such whole fiber bundles. It was further found that in an ablation environment of 10kPa ablation lead to an icicle shape on a top layer of 250μm of the fibers with constant thinning, whereas at low pressure (1.5kPa), the fibers showed strong oxidation degradation over their whole length (650μm). Computed diffusion coefficients of atomic oxygen in the boundary layer were more than ten times higher in the case of 1.5kPa compared to 20kPa. This, together with a much lower atomic oxygen concentration at 1.5kPa (decreasing the fiber’s reactivity) may allow oxygen to penetrate into the internal material structure. More investigation on both experimental and numerical level is required to confirm those trends. A comprehensive test campaign on a fully developed low-density ablator, ASTERM, is planned for spring 2012 at the VKI.