Arc-jet Facility Research Articles

Pyrolysis front detection in carbon phenolic composites using x-ray computed tomography

Carbon phenolic composites are used as thermal protection systems (TPS) materials on space capsules to protect them from the hot aerothermal environment. The phenolic resin in the composite material decomposes (pyrolyzes) at low temperatures resulting in a pyrolysis front within the material. The detection of the pyrolysis front after exposure to heat has historically been achieved by physically sectioning cross-sections of the material. We combine the phase contrast retrieval method to reconstruct x-ray computed tomography scans along with image convolution to identify the pyrolysis front in carbon phenolic composites. Unlike the standard filtered back projection method that captures only the carbon phase, the phase contrast retrieval method uses both the attenuation coefficients and refractive indices to illuminate all three phases (carbon, resin, and voids) of carbon phenolic composites. Image convolution is applied on scans reconstructed using the phase contrast retrieval method to develop a density map of the composite to locate the pyrolysis front. The analysis is performed on a sample of phenolic impregnated carbon ablator that was tested in an arc-jet facility. For the sample analyzed, the depth of the pyrolysis front from the surface of the sample is calculated to be 2.150 ± 0.148 mm. Although the proposed approach is applied to detect the pyrolysis front, the tools can be used to illuminate the structure of any carbon phenolic composite, and we propose the use of the phase contrast retrieval method as a methodological standard to analyze carbon phenolic composites used on space capsules.

Transient Plasma Wind Tunnel Testing Under Thermomechanical Loads for Reentry Break-Up Analysis

This paper introduces a novel approach for transient plasma wind tunnel testing in an arcjet facility, enabling the experimental simulation of the aerothermal loads along reentry trajectory segments. Such experiments newly allow for the investigation of the causes of break-up events. The trajectory segment is simulated by duplicating the time-resolved profiles of the characteristic flow parameters, namely, heat flux and stagnation pressure as well as the mechanical load, which represents the aerodynamic forces. In arcjet facilities, the parameters that govern the plasma condition are split into variable and constant parameters, which define the available altitude range that can be replicated. The variable conditions arc current and ambient pressure were determined with steady-state trajectory points and traces determined between these conditions resulting in a full trajectory segment. The test time then relates directly to an altitude on the trajectory segment, which allows for an experimental determination of the main break-up altitude. The methodology is applied to a trajectory segment of the transport vehicle CYGNUS OA6, covering an altitude range from 85 to 70 km. A transient break-up experiment on an aluminum bar yielded a failure at an altitude corresponding to 77.6 km, showing a thermomechanical characteristic. These results align well with the observed break-up altitude of CYGNUS OA6, showing the potential of the proposed method.

Uncertainty Analysis of Slug Calorimeters in the NASA HyMETS Arc-Jet Facility

The objective of this work is to perform an uncertainty analysis of the deduced stagnation heat flux environment on a slug calorimeter in the Hypersonic Materials Environmental Test System arc-jet facility located at NASA Langley Research Center. Analytical solutions are developed for boundary-value problems on the slug sensor accounting for nonideal effects, including spatial variation in the slug heat flux, multidimensional thermal conduction, and backface losses, which depart from the state-of-the-art method derived from the American Society of Testing and Materials. The analytical solutions are compared and optimized to experimental backface slug thermocouple data for a high- and low-enthalpy test condition. The results indicate that the epistemic uncertainty of the deduced stagnation heat flux on the slug calorimeter is at most ±2.5% for both conditions. Using a numerical approach to evaluate the aleatory uncertainty component in the slug stagnation heat flux, there is a compromise between the sample size and filter frequency of slug backface thermal data points when evaluating the deduced stagnation heat flux standard deviation. The total (mixed) uncertainty in the deduced stagnation heat flux is determined to be up to ±4%, which is at least a 60% reduction from the standard uncertainty used in the state-of-the-art method.

Finite-rate and equilibrium study of graphite ablation under arc-jet conditions

Arc-jet facilities play a primary role in recreating aerothermal conditions experienced by atmospheric entry vehicles and are widely used to test the performance of thermal protection materials. In this work, we utilize a developed coupled framework between an overset flow solver CHAMPS NBS-Cart, and a material solver KATS-MR to study the ablation of graphite under arc-jet conditions. We implement a 12-species gas phase model to accurately represent the air-carbon mixture, including argon species present in the flow. The gas phase is modeled with a two-temperature thermo-chemical non-equilibrium model without considering electronic and ionization effects. The gas-surface interactions are modeled with a newly developed air-carbon ablation model accounting for oxidation, nitridation, and recombination reactions. In addition, the model is augmented with carbon sublimation reactions experienced at high heating conditions. The chemical state at the surface is tightly coupled with the flow solver, resulting in the improved accuracy and effectiveness of the simulation. The coupled approach is applied to study two experimental test cases conducted at the IHF arc-jet facility at NASA Ames. The predicted results are validated against measured recession, surface, and in-depth temperatures and compared to the prediction of the uncoupled, equilibrium-based approach. Finally, the accuracy of the prediction is explored with respect to the environmental properties, such as the diffusion coefficient, and material thermal conductivity.

Perspectives and Recent Progresses on the Simulation of the Entry into the Atmospheres of the Outer Ice Giants

The relatively recent decision of NASA and ESA to plan new missions to the so-called Ice Giants, namely Uranus and Neptune, has prompted a resurgence of interest in the experimental analysis of the aero-heating environment that probes entering such atmospheres would experience. In the present study, arc-jet facilities, previously used to simulate space flight in the atmospheres of Earth, Mars, and Titan, are considered as a relevant basis for the implementation of a more complex framework adequately accounting for the atmospheric features of the Ice Giants. It is shown that the key to the successful realization of such an endeavor is a new operating mode for the plasma torch (relying on a nitrogen–hydrogen mixture) together with the inclusion of a new gas control unit, a new mixing chamber to generate relevant gas mixtures (mimicking to a sufficient extent the Ice Giants atmosphere) and a new thermo-chemical model of the overall flow process. The outcomes of some initial tests are presented to demonstrate the adequacy and performances of the implemented approach with respect to typical entry conditions related to these two planets.

Aftbody Heat Flux Measurements During Mars 2020 Entry

The Mars Entry, Descent, and Landing Instrumentation 2 (MEDLI2) sensor suite on the Mars 2020 mission included two total heat flux sensors and one radiometer on the backshell to directly measure the aftbody aerothermal environments during entry into the Martian atmosphere. All three sensors successfully returned aftbody entry heating measurements. Comparisons between the total heat flux sensor measurements and predictions by NASA simulation tools (DPLR/NEQAIR) show excellent agreement and provide confidence in the models. The radiometer measured significant radiative heating, but compared to the model predictions the signal was attenuated by 48% at the end of the entry heat pulse. The loss of signal is attributed to blockage by thermal protection system (TPS) ablation product deposits on the radiometer window. Ground-based testing in the NASA Ames arcjet facilities was conducted to understand the impact of ablation product deposits on the measured radiometer signal. A discussion of the test results, how flight-like the test conditions were, and future work to further characterize the effect of TPS ablation product deposits on the radiometer performance are presented. In addition to measuring the entry heat pulse, all three sensors were sensitive enough to measure solar radiation during cruise, the radiometer measured solar flux during the entry heat pulse, and the leeside total heat flux sensor picked up the descent reaction control system firings.

An open carbon–phenolic ablator for scientific exploration

Space exploration missions rely on ablative heat shields for the thermal protection of spacecraft during atmospheric entry flights. While dedicated research is needed for future missions, the scientific community has limited access to ablative materials typically used in aerospace. In this paper, we report the development of the HEFDiG Ablation-Research Laboratory Experiment Material (HARLEM), a carbon–phenolic ablator designed to supply the need for ablative materials in laboratory experiments. HARLEM is manufactured using polyacrylonitrile-based carbon fiber preforms and a simplified processing route for phenolic impregnation. We characterized the thermal protection performance of HARLEM in arcjet experiments conducted in the plasma wind tunnel PWK1 of the Institute of Space Systems at the University of Stuttgart. We assessed the performance of the new material by measuring surface recession rate and temperature using photogrammetry and thermography setups during the experiments, respectively. Our results show that HARLEM’s thermal protection performance is comparable to legacy carbon–phenolic ablators that have been validated in different arcjet facilities or in-flight, as demonstrated by calculations of the effective heat of ablation and scanning electron microscopy of as-produced samples. In-house manufacturing of carbon–phenolic ablators enables the addition of embedded diagnostics to ablators, allowing for the acquisition of data on internal pressure and more sophisticated pyrolysis analysis techniques.

Measurement of Skin Friction in the University of Texas at Arlington Arcjet

Arcjet flows are known to have significant uncertainties associated with the fundamental flow quantities. A detailed characterization is critical to augment the current understanding of these complex flows as well as developing, screening, and qualifying thermal protection systems. Classically, estimations of skin friction have been made indirectly using heat flux analogies. These techniques, however, are marred by high levels of measurement uncertainty and require a priori assumptions about the flowfield. Directly measuring the wall shear stress removes the demand for these assumptions. A direct measurement of shear stress in supersonic high-enthalpy flow at the Office of Naval Research/University of Texas at Arlington arcjet facility has been conducted. The measured wall shear stress was (average combined calibration uncertainty), which matches well with estimations for laminar flow. Two shear stress sensors separated by a distance of 1.905 cm (0.75 in.) were integrated into a 20 deg half-angle shear-wedge platform. The two sensors tracked together within 10% of each other, implying a uniform flowfield over the region studied. Approximately 1.5 s into the test window, a steady increase in skin friction was observed. The cause of this nonphysically accurate increase in the shear stress measurement is discussed.

Spallation of Carbon Ablators in Arcjet Facility Experiments

Spallation is the mostly undesirable phenomenon where solid particles are ejected from ablative materials leaving less mass to be decomposed at the ablator surface. The exact mechanisms and conditions that promote spallation as well as the extent of it are unclear to this date. This paper presents the results of an extensive test campaign targeted at measuring spallation of carbonaceous ablator materials in an arc heated air flow of . The employed diagnostic methods are photogrammetric surface measurements, high-speed imaging, thermography, and two-color-ratio pyrometry using the raw digital single-lens reflex camera images from the photogrammetry setup. Data from the high-speed camera show a much higher spallation rate for the carbon preform Calcarb than for carbon-phenolic ablators. Temperature measurements show a difference of up to 300 K between carbon fibers and surrounding matrix. The pyrolysis gas from the phenolic resin is found to decrease the spallation rate of the carbon-phenolic ablator ZURAM compared to Calcarb, and the pyrolysis gas pressure slightly increases the distance that particles travel upstream after ejection from the front surface.

EFESTO - advancing European hypersonic inflatable heatshield technology for Earth recovery and Mars high-mass delivery missions

The European Union H2020 EFESTO project is coordinated by DEIMOS Space with the end goals of improving the European TRL of Inflatable Heat Shields for re-entry vehicles (from 3 to 4/5) and paving the way towards further improvements (TRL 6 with a future In-Orbit Demonstrator). This paper presents the project objectives and provides with a general overview of the activities ongoing and planned for the next two years, promoting its position in the frame of a European re-entry technology roadmap. EFESTO aims at (1) the definition of critical space mission scenarios (Earth and Mars applications) enabled by the use of advanced inflatable Thermal Protection Systems (TPS), (2) characterization of the operative environment and (3) validation by tests of both the flexible materials needed for the thermal protection (flexible thermal blanket will be tested in arcjet facility in both Earth and Martian environments) and the inflatable structure at 1:2 scale (exploring the morphing dynamics and materials response from packed to fully inflated configuration). These results will be injected into the consolidated design of a future In-Orbit Demonstrator (IOD) mission.

Evaluation of Sublimation Kinetics for Phenolic Impregnated Carbon Ablator

Recession analyses of stagnation tests conducted in NASA Ames Research Center arcjet facilities with surface heat fluxes high enough for carbon char to sublimate are performed and compared with data. The axisymmetric recession computations are based on both the finite-rate gas–surface interaction model and the standard chemical equilibrium surface ablation model. The finite-rate gas–surface interaction model is studied using three different sets of sublimation coefficients, which are obtained through direct measurements. The standard chemical equilibrium ablation model is established through the tables. The ablating material used in these arcjet tests is phenolic impregnated carbon ablator. The prediction of axisymmetric surface recession is achieved by implicitly coupling a material thermal response code and a flow code. The material thermal response code used in this study is the Two-Dimensional Implicit Thermal-Response and Ablation program, which predicts charring material thermal response and shape change. The flow code solves the reacting Navier–Stokes equations using the data parallel line relaxation method. The recession predictions for three selected arcjet conditions with various base radii are presented. The effect of sticking coefficient for carbon sublimation on recession prediction is investigated, and the prediction discrepancy between finite-rate and chemical equilibrium models is examined and discussed in detail.

Plasma Torch Testing of Thermal Protection Materials (TPMs)

This article presents an introduction to ground testing of thermal protection materials (TPMs) used to protect spacecraft and hypersonic vehicles. These test facilities must simulate the aerothermodynamic environments expected during flight. Often these tests have been done in arc jet facilities. However, inductively coupled plasma (ICP) torches can generate the relevant test flow field and have advantages. Guidance for using these facilities for thermal protection material performance verification and flight qualification is provided.

Material Response Modeling of Melt Flow-Vapor Ablation for Iron

The ability to accurately predict melt-dominated ablation of meteoritic materials is important for risk assessment of near-Earth asteroids and survivability of reentering space debris. Despite previous computational studies that involve melt ablation for glassy materials, the application of incompressible gas boundary-layer theory to melt/vapor formation has largely been applied to limited surface heating rates and vapor-dominated regimes. A melt ablation formulation near stagnation regions, for crystalline and glassy materials, is implemented here and validated with ground experiments performed for iron under hypersonic flow conditions. The mathematical model does not neglect temperature gradients in the molten layer or assume domination of either melt or vapor formation. The implementation of this comprehensive melt/vapor ablation numerical model helps provide domain-independent engineering recession estimates. By using computational fluid dynamic-driven surface properties and radial basis function interpolation-driven dynamic meshes to predict stagnation-point melt ablation, it is shown that melt initiation is predicted within 10% of pyrometer data and that simulated recession is dominated by melt flow contributions at the extreme operating limits of a hypersonic arc-jet test facility. The melt thickness shows a strong parabolic dependence on heat transfer coefficient variation. Shape change validation results of test articles accounting for this variation are discussed.

Technique Development for Investigating Axisymmetric Ablation Models in Hypersonic Impulse Facilities

Arcjet facilities have been used extensively for ablation and materials characterization but arcjets do not correctly simulate the hypervelocity flow during reentry. Impulse facilities can simulate aspects of hypervelocity reentry flows, but cannot duplicate the extended-duration thermal environments. To address such deficiencies in short-duration testing environments, a novel plasma technique for preheating axisymmetric samples of heat-shield materials has been developed. The heating technique has been demonstrated using a short-duration blow-down arrangement at the University of Southern Queensland. A 50-mm-diameter graphite disk sample with a uniform thickness of 2 mm was heated from the downstream side with plasma to approximately 2500 K, and then it was exposed to a cold Mach 4.5 flow. The disk was mounted on an experimental probe that was very similar to the European standard probe, normal to the flow. Future applications for the technique are expected to be found in expansion tube facilities that can simulate the true flow energy under reentry conditions.

Compact fiber-coupled UV-NIR hyperspectral imaging sensor for characterizing ultra-high temperature ceramic materials.

A compact fiber-coupled hyperspectral imaging sensor (HSIS) operating within the range of ultraviolet to near-infrared (UV-NIR) wavelengths is designed and developed for the remote recording of two-dimensional (2D) spectrally resolved thermal radiation and chemiluminescent emission from ultra-high-temperature ceramics (UHTCs). Using simulations, the entire system is optimized to improve the collection efficiency and minimize aberrations. The design, construction, and characterization of the HSIS sensor are discussed in detail. We present the 2D spectrally resolved measurements of the simultaneous thermal radiation and BO2∗ chemiluminescent emission from a commonly used UHTC (HfB2-SiC) material under high-heat-flux conditions. Our results show that BO2∗ chemiluminescence corresponds directly to material ablation and can be used to track the formation of the protective heat-resistant glass/oxide layer. Furthermore, the temperature measurements demonstrate the heat distribution properties of the sample and indicate the locations at which BO2∗ chemiluminescence is possible. These results highlight the application prospects of the compact fiber-coupled HSIS for high-temperature material characterization in practical arc-jet facilities with limited optical access.

Magnetohydrodynamic Drag Measurements in an Expansion Tunnel with Argon Test Gas

Magnetohydrodynamic (MHD) aerobraking experiments were conducted in the large-scale X3 expansion tunnel at the University of Queensland. The test gas was argon flowing at approximately for a quasi-steady period of . A neodymium spherical permanent magnet, suspended by string and enclosed within a 65-mm-diameter spherical forebody, was used to generate the magnetic field. An accelerometer was attached to the rear of the magnet to measure streamwise motion, allowing the Lorentz force on the magnet to be deduced according to Newton’s second law. The stagnation point magnetic flux density was varied by changing the internal streamwise location of the magnet within the forebody. Experimental results demonstrate a linear increase in MHD drag with magnetic field strength, which is in agreement with previous results from arcjet facilities. However, the MHD drag force was much lower in these experiments, even after accounting for shock density ratio and viscosity differences. This discrepancy could be due to the different freestream state generated by the two types of facilities.

A model for meteoroid ablation including melting and vaporization

Meteor influx and its physical properties can be estimated based on observations using heuristic models for ablation. These models suffer from a lack of validation and physical description of the flow and material fields, as well as their coupling. In this paper, we develop a model for meteoroid ablation including melting and vaporization, where both flow and material are coupled. We apply the model to ground experiments carried out at the arc jet facilities at NASA Ames Research Center. These experiments reveal a substantial effect of the shear ablation on the ablation process, which the heuristic models do not describe. Due to scarce data on physicochemical material properties at high temperatures, we perform a sensitivity analysis, showing that the material thermal conductivity and viscosity are essential parameters for the shear ablation process. We also observe that the direct evaporation phenomenon is negligible compared to the mass loss by shear ablation. Our models are used to compute an effective heat of ablation six times smaller than the one reported in the literature. These models can be applied to study meteor trajectories and to improve coefficients used in heuristic models.

Oxidation response of a SiCf/SiC CMC with a HfB2-based coating in an arc jet test

ABSTRACTThe behaviour of a SiC-fibre-reinforced composite with a graded matrix was investigated in an arc jet facility. The graded matrix was comprised of several layers of silicon carbide that transition to a mixed SiC-HfB2 top layer and was sealed with an HfB2-SiC coating. Samples were exposed to maximum specific total enthalpy hypersonic flow between 14 and 17 MJ kg−1 under stagnation pressures between 2700 and 3050 Pa for hold times of between 137 and 395 s. The resulting sample surface temperatures ranged between 1600 and 1950°C. Spectral emissivity at about 1 μm was calculated using a pyrometer capable of switching between one and two color modes and was shown to decrease with oxidation and removal of SiO2 from the surface. The impacts of the surface chemistry changes during oxidation and of active oxidation were investigated.

Testing ultra-high-temperature ceramics for thermal protection and rocket applications

ABSTRACTThis work deals with experimental characterisation of ultra-high-temperature ceramic materials for aerospace applications. Current research activities are focused on development and testing of new-class ultra-high-temperature ceramic matrix composites for near-zero ablation Thermal Protection Systems and near-zero erosion rocket nozzles. To investigate materials behaviour in atmospheric re-entry conditions, relevant tests are carried out on samples with different shapes with a supersonic arc-jet facility, with specific total enthalpies higher than 20 MJ kg−1 and temperatures over 2000 K in a gas atmosphere with high concentration of atomic oxygen. The Aerospace Propulsion Laboratory allows investigating rocket components or subsystems manufactured in innovative materials, such as nozzles and nozzle inserts, but also small specimens exposed to the rocket exhaust plume, in highly relevant operating conditions. Computational models for numerical simulations of high-enthalpy flows, in both arc-jet and propulsion environments, and thermal analyses of the material samples are developed to support the experimental activities.

Computational Predictions of the Hypersonic Material Environmental Test System Arcjet Facility

The Hypersonic Materials Environmental Test System arcjet facility located at the NASA Langley Research Center in Hampton, Virginia, is primarily used for the research, development, and evaluation of high-temperature thermal protection systems for hypersonic vehicles and reentry systems. To improve testing capabilities and knowledge of the test article environment, a detailed three-dimensional model of the arcjet nozzle and the freejet portion of the flowfield is developed. The computational fluid dynamics model takes into account nonuniform inflow state profiles at the nozzle inlet as well as catalytic recombination efficiency effects at the probe surface. The results of the numerical simulations are compared to the calibrated pitot pressure and the stagnation-point heat flux for three test conditions at low, medium, and high enthalpies. Comparing the results and the test data indicates a partially catalytic copper surface on the heat flux probe of about 10% recombination efficiency and a 2–3 kPa pressure drop from the total pressure measured at the plenum section in front of the nozzle. With these assumptions, the predictions are within the uncertainty of the stagnation pressure and heat flux measurements. The predicted velocity conditions at the nozzle exit are also compared and show good agreement with the radial and axial velocimetry data.