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

Uncertainty Analysis of Carbon Ablation in the VKI Plasmatron

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Investigation of the Gas-surface Interaction of Innovative Carbon Composite Ablators in the VKI Plasmatron

For a new class of low density carbon/resin composite ablators, which has been introduced and successfully applied to flight by the Stardust mission, the process of ablation is not only restricted to the surface but can also occur in-depth of the material if oxygen is able to diffuse into the porous material. The occurrence of such new porous carbon/resin composites requires an important effort in theoretical and experimental investigation for an adequate understanding of the ablation process to enable development and validation of material response models. At the von Karman Institute for Fluid Dynamics (VKI) research activities were developed to establish a methodology for experimental characterization of innovative low-density ablators using the inductively coupled 1.2MW Plasmatron facility. A comprehensive setup of measurement techniques was applied to the facility in order to determine and characterize the in-situ material response of ablative samples in different test conditions. Optical emission spectroscopy was utilized to address the thermo-chemistry of the plasma free-stream and its interaction with the ablating sample. In addition microscopic analysis tools for sample examination, at the carbon fibre length scale (~10μm), are used to investigate the material physics. The degradation behaviour of the material is then being analyzed by scanning electron microscopy to be able to evaluate the depth of degradation and the thinning of the carbon-fibres. In particular, to provide information about the diffusion/reaction competition of oxygen, which controls the oxidation of carbonized resin and exposed fibres in-depth. Material surface properties, as emissivity, are also determined in-situ using an IR-radiometer combined with two-colour pyrometer measurements. Preliminary results showed that nitridation, leading to CN (CN violet & CN red), is highly apparent in pure nitrogen plasma flows but significantly drops when oxygen is involved, speaking for dominant oxidation reactions (CO, CO2, NO). Additionally, different chemical mechanisms were found to occur rather in nitrogen than in air plasma. In such a way, diatomic carbon (C2 Swan) transitions were highly radiating after injection of the sample into N2 plasma but truncated after a few seconds. This was not observed with air as test gas. As expected, oxygen is the driving force to provoke reactions as the system undergoes the ablation process, but its uncertain state of diffusion into the porous material and on the contrary, reactions undergone in the absence of oxygen, necessitate the usage of appropriate micro- and spectroscopic tools

Aerothermal response of ceramic matrix composites to nitrogen plasma at temperatures above 2000 K

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Numerical Investigation of Charring Material Demisability in Atmospheric Entry Conditions

peer reviewedHigh-fidelity models have been developed in the recent years to predict the response of light-weight ablative thermal protection materials used to build heat shields of re-entry capsules. Because the main components of these ablators are typically also present in some spacecraft/satellite components, there is a growing interest in exploring the possible extension of the high-fidelity models to predict demisability of these components. A unified approach simulating the degrading composite material and the high enthalpy flow is extended to treat this type of charring dense material interesting for demise applications. The comparison in between highly porous and dense material shows the numerical challenges to simulate the thermal response of composite carbon fibers materials using this methodology.GSTP #400012271

Material response characterization of a low-density carbon composite ablator in high-enthalpy plasma flows

Future space exploration missions beyond Earth's orbit, such as sample returns from Mars, will use ablative materials for the thermal protection system in order to shield the spacecraft from the severe heating during reentry. In this paper, we present the results of an elaborate test campaign on a lightweight carbon composite ablator with the aim of defining a procedure for material response characterization in a 1.2-MW inductively heated Plasmatron facility, suitable to reproduce the hypersonic flight boundary layer environment. Three different test gases were used, air, nitrogen, and argon, at surface temperatures exceeding 3300 K. A comprehensive experimental setup was developed including a nonintrusive technique to measure surface recession by means of a high-speed camera. Surface degradation was strongly test gas dependent, while mass loss was mainly driven by in-depth decomposition of phenolic resin. Emission spectroscopy helped us identify C2 as a product of dissociating hydrocarbons, as well as cyanogen, suggesting surface nitridation. Melt flow at the surface and silicon emission indicated degradation of the glass microspheres used as additional filler. In air plasma, oxidation was inferred to be the main mechanism for ablation. © 2014 Springer Science+Business Media New York.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Meteoroid atmospheric entry investigated with plasma flow experiments : petrography and geochemistry of the recovered material

Melting experiments attempting to reproduce some of the processes affecting asteroidal and cometary material during atmospheric entry have been performed in a high enthalpy facility. For the first time with the specific experimental setup, the resulting material has been recovered, studied, and compared with natural analogues, focusing on the thermal and redox reactions triggered by interaction between the melt and the atmospheric gases under high temperature and low pressure conditions. Experimental conditions were tested across a range of parameters, such as heat flux, experiment duration, and pressure, using two types of sample holders materials, namely cork and graphite. A basalt served as asteroidal analog and to calibrate the experiments, before melting a H5 ordinary chondrite meteorite. The quenched melt recovered after the experiments has been analyzed by μ-XRF, EDS-SEM, EMPA, LA-ICP-MS, and XANES spectroscopy. The glass formed from the basalt is fairly homogeneous, depleted in highly volatile elements (e.g., Na, K), relatively enriched in moderately siderophile elements (e.g., Co, Ni), and has reached an equilibrium redox state with a lower Fe3+/Fetot ratio than that in the starting material. Spherical objects, enriched in SiO2, Na2O and K2O, were observed, inferring condensation from the vaporized material. Despite instantaneous quenching, the melt formed from the ordinary chondrite shows extensive crystallization of mostly olivine and magnetite, the latter indicative of oxygen fugacity compatible with presence of both Fe2+ and Fe3+. Similar features have been observed in natural meteorite fusion crusts and in micrometeorites, implying that, at least in terms of maximum temperature reached and chemical reactions, the experiments have successfully reproduced the conditions likely encountered by extraterrestrial material following atmospheric entry