Aerothermal Characterization of Silicon Carbide-Based TPS in High Enthalpy Airflow

Inductively-coupled plasma generators provide an ideal environment to reproduce the aerothermal heating experienced by a spacecraft re-entering a planetary atmosphere. The flight boundary layer chemistry is duplicated around a TPS model, ensuring a similarity between the flight and ground stagnation-point heat flux. Experiments conducted in an induction plasmatron on silicon carbide-based thermal protection materials will be described. Several specimens are tested under a wide range of pressure and temperature conditions and investigated by means of infrared radiometry and optical emission spectroscopy. The plasma to which the materials are exposed is characterized in details by calorimetric and Pitot pressure measurements, and numerically rebuilt by means of a nonequilibrium boundary layer model. The presentation will focus on the thermophysical properties of the material and their dependency on the testing environment. In particular, we will discuss the oxidation features of silicon carbide which are detected both via emission spectroscopy and post-test reflectivity measurements

Hypersonic Aerothermochemistry Duplication in Ground Plasma Facilities: A Flight-to-Ground Approach

peer reviewedHigh conservative safety margins, applied to the design of spacecraft thermal protection systems for planetary entry, need to be reduced for higher efficiency of future space missions. Ground testing of such protection systems is of great importance during the design phase. This study covers a methodology for simulating the complex hypersonic entry aerothermochemistry in a plasma wind tunnel for a given spacecraft geometry without any assumption on axisymmetry or bluntness. A demonstration of this proposed methodology is made on the Qubesat for Aerothermodynamic Research and Measurements on AblatioN, QARMAN mission, which is a rectangular reentry CubeSat with a cork-based ablative thermal protection system in the front unit. The reacting boundary-layer profiles of the hypersonic entry probe compare well with the ones developing at the stagnation region of the plasma test model, defined with the proposed flight-to-ground duplication method

Affected depth and effective reactivity in porous thermal protection materials for atmospheric re-entry

International audienceAffected depth and effective reactivity in porous thermal protection materials for atmospheric re-entr

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

Experimental measurements of radio signal attenuation and Faraday rotation in an inductively coupled plasma facility

Spacecraft entering a planetary atmosphere are surrounded by a plasma layer containing high levels of ionization due to the high temperatures on the shock layer. The high electron number densities cause attenuation and rotation of the emitted signal, leading to communication blackout. This work presents experimental measurements of radio signal attenuation and Faraday rotation due to an ionized plasma flow. These measurements are conducted at the VKI Plasmatron using circularly polarized directive horn lens antennas with a waveguide orthomode transducer. Clear attenuations are observed when the signal is propagating through the plasma, and Faraday rotation measurements show a good agreement with the theoretical estimation.Diana Luıs research is funded by a doctoral fellowship (2021.04930.BD) granted by Fundaçao para a Ciencia e Tecnologia (FCT Portugal). The MEESST project is funded by the European Union’s Horizon 2020 research and innovation programme under grant agreement No 899298.Postprint (author's final draft

Experimental methodology for the accurate stochastic calibration of catalytic recombination affecting reusable spacecraft thermal protection systems

This work focuses on the development of a dedicated experimental methodology that allows for a better stochastic characterization of catalytic recombination parameters for reusable ceramic matrix composite materials when dealing with uncertain measurements and model parameters. As one of the critical factors affecting the performance of such materials, the contribution to the heat flux of the exothermic recombination reactions at the vehicle surface must be carefully assessed. In this work, we first use synthetic data to test whether or not the proposed experimental methodology brings any advantages in terms of uncertainty reduction on the sought out parameters compared to more traditional experimental approaches in the literature. The evaluation is done through the use of a Bayesian framework developed in a previous work with the advantage of being able to fully and objectively characterize the uncertainty on the calibrated parameters. The synthetic dataset is adapted for testing ceramic matrix composites by carefully choosing adequate auxiliary materials whose heat flux measurements have the capability of reducing the resulting uncertainty on the catalytic parameter of the thermal protection material itself when tested under the same flow conditions. We then propose a comprehensive set of real wind tunnel testing cases for which stochastic analyses are carried out. The physical model used for the estimations consists of a 1D boundary layer solver along the stagnation line in which the chemical production term included in the surface mass balance depends on the catalytic recombination efficiency. All catalytic parameters of the auxiliary and thermal protection materials are calibrated jointly with the boundary conditions of the experiments. The testing methodology confirms to be a reliable experimental approach for characterizing these materials while reducing the uncertainty on the calibrated catalytic efficiencies by more than 50 %. An account of the posteriors summary statistics is provided to enrich the current state-of-the-art experimental databases

Effect of electron number densities on the radio signal propagation in an inductively coupled plasma facility

Spacecraft entering a planetary atmosphere are surrounded by a plasma layer containing high levels of ionization, due to the extreme temperatures in the shock layer. The high electron number densities cause attenuation of the electromagnetic waves emitted by the on-board antennas, leading to communication blackout for several minutes. This work presents experimental measurements of signal propagation through an ionized plasma flow. The measurements are conducted at the VKI plasma wind tunnel (Plasmatron) using conical horn antennas transmitting in the Ka-band, between 33 and 40 GHz. Testing conditions at 15, 50 and 100 mbar, and powers between 100 and 600 kW cover a broad range of the testing envelope of the Plasmatron as well as a broad range of atmospheric entry conditions. The transmitting antenna is characterized at the UPC anechoic chamber, obtaining the radiation patterns, beamwidth, and gain at the boresight direction; and an optical ray tracing technique is used to describe the electromagnetic waves propagation in the plasma flowfield inside of the Plasmatron chamber. The signal propagation measurements show clear attenuation when the signal is propagating through the plasma, varying between 2 and 15 dB depending on the testing conditions. This attenuation increases with electron number densities, which are driven by the Plasmatron power and pressure settings. Preliminary evidence of Faraday rotation effects caused by the plasma is also observed.Diana Luís research is funded by a doctoral fellowship (2021.04930.BD) granted by Fundação para a Ciência e Tecnologia (FCT Portugal). The research of Vincent Fitzgerald Giangaspero is supported by SB PhD fellowship 1SA8219N of the Research Foundation - Flanders (FWO). The resources and services used for the BORAT simulations were provided by the VSC (Flemish Supercomputer Center), funded by the Research Foundation - Flanders (FWO) and the Flemish Government. The MEESST project is funded by the European Union’s Horizon 2020 research and innovation programme under grant agreement No 899298.Peer ReviewedPostprint (published version

A Magnetohydrodynamic enhanced entry system for space transportation: MEESST

This paper outlines the initial development of a novel magnetohydrodynamic (MHD) plasma control system which aims at mitigating shock-induced heating and the radio-frequency communication blackout typically encountered during (re-)entry into planetary atmospheres. An international consortium comprising universities, SMEs, research institutions, and industry has been formed in order to develop this technology within the MEESST project. The latter is funded by the Future and Emerging Technologies (FET) program of the European Commission’s Horizon 2020 scheme (grant no. 899298). Atmospheric entry imposes one of the harshest environments which a spacecraft can experience. The combination of hypersonic velocities and the rapid compression of atmospheric particles by the spacecraft leads to high-enthalpy, partially ionised gases forming around the vehicle. This inhibits radio communications and induces high thermal loads on the spacecraft surface. For the former problem, spacecraft can sometimes rely on satellite constellations for communicating through the plasma wake and therefore preventing the blackout. On the other hand, expensive, heavy, and non-reusable thermal protection systems (TPS) are needed to dissipate the severe thermal loads. Such TPS can represent up to 30% of an entry vehicles weight, and especially for manned missions they can reduce the cost- efficiency by sacrificing payload mass. Such systems are also prone to failure, putting the lives of astronauts at risk. The use of electromagnetic fields to exploit MHD principles has long been considered as an attractive solution for tackling the problems described above. By pushing the boundary layer of the ionized gas layer away from the spacecraft, the thermal loads can be reduced, while also opening a magnetic window for radio communications and mitigating the blackout phenomenon. The application of this MHD-enabled system has previously not been demonstrated in realistic conditions due to the required large magnetic fields (on the order of Tesla or more), which for conventional technologies would demand exceptionally heavy and power-hungry electromagnets. High-temperature superconductors (HTS) have reached a level of industrial maturity sufficient for them to act as a key enabling technology for this application. Thanks to superior current densities, HTS coils can offer the necessary low weight and compactness required for space applications, with the ability to generate the strong magnetic fields needed for entry purposes. This paper provides an overview of the MEESST project, including its goals, methodology and some preliminary design considerations