Bernd Helber

Ablation of carbon preform in the VKI Plasmatron

Ablation tests of a carbon fiber preform have been carried out in the VKI Plasmatron facility in air plasmas to study the e↵ect of test chamber static pressure on the oxidation behavior of the material and erosion of the carbon fibers. Surface temperatures at a constant cold wall heat flux of 1 MW/m 2 suggested a di↵usion-limited ablation regime for all tests. A higher mass loss and recession rate were measured for ablation tests in a low pressure environment (1.5 kPa). Fiber bundles were found in the preform, embedded between single fibers as illustrated by micrographs. High-speed images showed a strong release of particles during the ablation test. It is assumed that these fiber bundles detach during ablation as the surrounding fibers are oxidized. A tentative explanation is that a low pressures environment may lead to an enhanced release of particles due to intensified di↵usion and higher plasma flow velocities.

Development and Testing of an Ablation Model Based on Plasma Wind Tunnel Experiments

Ablative materials are extensively used in several aerospace applications. Their employ as heat shield for re-entry capsules enables to survive re-entry conditions that would be otherwise unfeasible. The coupled experimental-numerical work is fundamental to grow the understanding of their behavior in operative conditions. This work deals with the development and testing of an ablation model able to reproduce the stagnation-point gas-surface interaction over non-charring carbon-based ablative materials. Numerical tools, specifically developed at the von Karman Institute for Fluid Dynamics for re-entry application studies, are used together in the analysis to obtain relevant quantities as the stagnationpoint surface mass blowing rate and temperature. Data from the experiments performed in the von Karman Institute Plasmatron in both air and nitrogen environment are used to compare with the numerical results and to tailor the ablation model. Test results in nitrogen environment prove that active surface nitridation takes place, and a proper nitridation reaction probability is extracted from the tests using the developed model with a reverse approach. Comparisons with the measured surface temperatures suggest that additional surface phenomena can occur in the low cold-wall heat flux tests. Surface nitrogen recombination, identified as one of these possible mechanisms, is analyzed.

Physico-Chemistry of CN in the Boundary Layer of Graphite in Nitrogen Plasmas

Ablative heat-shield materials are a crucial component of atmospheric entries. Comprehensive experiments in their operative conditions, coupled with numerical work, are fundamental to advance the knowledge of their response for thermal protection system design. Most numerical models describe oxidation and sublimation, but do not treat nitridation of the carbon surface, which may impact surface recession and heat flux. We present experiments of graphite ablation in nitrogen plasmas, produced by the VKI Plasmatron facility, aiming at the estimation of nitridation reaction probabilities of carbon in a relevant temperature range relevant for heat-shield operation, and determination of CN species densities in the boundary layer for comparison and validation of the numerical model. The gas-phase radiative signature was observed using a high resolution spectrometer, combined with a two-dimensional ICCD array. From the rebuilding of local emission intensities, obtained through Abel inversion, local mole fractions of the CN molecule were estimated by comparison to a spectroscopic model. Nitridation reaction probabilities were extracted from the tests using a model including an ablative boundary condition. The computed stagnation-line CN species densities agree very well with those obtained by the experimental approach. An estimation of the translational-rotational and vibrational-electronic temperatures Trot and Tvib of the CN violet system yielded strong evidence that Trot and Tvib are far from thermal equilibrium at low surface temperatures (low Plasmatron power), but seemed to equilibrate at higher surface temperatures. Our investigation is yet inconclusive on the origin of the high deviation of estimated rotational and vibrational temperatures, but the gathered data suggest that non-dissociated nitrogen in the boundary layer might play a significant role in the vibrational excitation of the CN molecule.

Gas/Surface Interaction Study of Low-Density Ablators in Sub- and Supersonic Plasmas

Ablation experiments have been carried out with the carbon-phenolic material AQ61 and a non-pyrolyzing carbon fiber preform in suband supersonic air and nitrogen plasmas in the VKI Plasmatron facility. We performed an in-situ recession analysis, including volumetric ablation, and observed the temperature and radiance of the surface for emissivity estimations, as well as spatial molecular radiation profiles in the boundary layer, which were compared to a numerical approach. Surface temperatures as low as 1600 K allowed observation of the temperature distribution over the surface using an infrared camera. The carbon preform test samples were varied in shape as well as regarding their fiber direction. Sublimation of the surface was reached in supersonic plasma flow at a cold wall heat flux of 9.5 MW/m. Carbon preform emissivities were found to be in the order of 0.86 0.97. A stagnation line description with an ablation boundary condition helped to reproduce experimentally measured boundary layer emission profiles of the CN violet molecule. In spite of the very simplified model, emission intensities were in the same order of magnitude compared to the experimental data. The model was able to reconstruct the location of the maximum emission for two cases. The char layer was examined by Scanning Electron Microscopy, illustrating degradation after nitrogen ablation.

Gas/Surface Interaction Study on Ceramic Matrix Composite Thermal Protection System in the VKI Plasmatron Facility

An experimental campaign dedicated to the characterization of a silicon carbide based thermal protection system is performed in the Plasmatron wind tunnel at the von Karman Institute for Fluid Dynamics. Emissivity, catalycity and oxidation behavior for representative specimens are investigated under a wide set of operating conditions in order to reproduce typical reentry flight situations. Intrusive measurements for flow characterization are used together with infrared techniques and optical emission spectroscopy that provide diagnostic of the test articles surface and the surrounding environment. Experimental data are postprocessed by means of numerical simulations that allow flow enthalpy re building and characterization of the boundary layer chemistry for the different conditions investigated .

Emission Spectroscopic Boundary Layer Investigation during Ablative Material Testing in Plasmatron

Ablative Thermal Protection Systems (TPS) allowed the first humans to safely return to Earth from the moon and are still considered as the only solution for future high-speed reentry missions. But despite the advancements made since Apollo, heat flux prediction remains an imperfect science and engineers resort to safety factors to determine the TPS thickness. This goes at the expense of embarked payload, hampering, for example, sample return missions. Ground testing in plasma wind-tunnels is currently the only affordable possibility for both material qualification and validation of material response codes. The subsonic 1.2MW Inductively Coupled Plasmatron facility at the von Karman Institute for Fluid Dynamics is able to reproduce a wide range of reentry environments. This protocol describes a procedure for the study of the gas/surface interaction on ablative materials in high enthalpy flows and presents sample results of a non-pyrolyzing, ablating carbon fiber precursor. With this publication, the authors envisage the definition of a standard procedure, facilitating comparison with other laboratories and contributing to ongoing efforts to improve heat shield reliability and reduce design uncertainties. The described core techniques are non-intrusive methods to track the material recession with a high-speed camera along with the chemistry in the reactive boundary layer, probed by emission spectroscopy. Although optical emission spectroscopy is limited to line-of-sight measurements and is further constrained to electronically excited atoms and molecules, its simplicity and broad applicability still make it the technique of choice for analysis of the reactive boundary layer. Recession of the ablating sample further requires that the distance of the measurement location with respect to the surface is known at all times during the experiment. Calibration of the optical system of the applied three spectrometers allowed quantitative comparison. At the fiber scale, results from a post-test microscopy analysis are presented.