Shock Layer Radiation Research Articles

Commissioning of Upgrades to T6 to Study Giant Planet Entry

The scientific potential of a mission to the ice giants is well recognized and has been identified by NASA and ESA as a high priority on several occasions, most recently in the 2023–2032 Decadal Survey. The payload capacity of such a spacecraft is limited by the heat shield thickness, which must be sized conservatively due to a lack of reliable data for convective and radiative heat flux along the proposed entry trajectories. Major upgrades to the Oxford T6 Stalker Tunnel have been commissioned that allow study of giant planet entry trajectories, including a flammable gas handling system, a Mach 10 expansion nozzle, and a steel shock tube with optical access. Initial testing has been completed in shock tube and expansion tunnel modes, with peak shock speeds of 18.9 km/s achieved. Convective heat flux and surface pressure were measured at several locations on a 45° sphere cone model in expansion tunnel mode. Measurements of the radiating shock layer were made in shock tube mode to assess the effect of CH4 concentration. This work establishes the first high-enthalpy giant planet entry test bed in Europe.

Radiative Heat Transfer Measurements of Titan Atmospheric Entry in a Shock Tube

Measurements were performed in the T6 Stalker facility operating in Aluminum Shock Tube mode for conditions relevant to Titan entry. Spatially and spectrally resolved radiation emitted from a high-temperature test gas behind a normal shock was recorded by means of emission spectroscopy. For Titan atmospheric entry, the main radiator of interest is cyano radical, formed in the nonequilibrium region behind the shock. The tests reported in this work measured radiation at velocities from 3.1 to 8.5 km/s and freestream pressures of 13, 20, and 133 Pa at a nominal composition of 98% N2 and 2% CH4. These shock layer radiation experiments employed an optical emission spectroscopy system on each side of the facility to allow two spectral regions to be measured simultaneously for each test, covering the spectral range of 200–900 nm. The present work provides a new, comprehensive benchmark set of data relevant to Titan entry studies.

Analysis of Mars 2020 Entry Vehicle Aerothermal Flight Data

The Mars 2020 entry vehicle’s thermal protection system included measurements of in-depth temperature on the heatshield and backshell, and surface heating on the backshell. This paper describes the flight data and resulting analysis: subsurface temperatures at 11 heatshield and 6 backshell locations, measured total heat flux at two backshell locations, and a radiometer on the backshell to measure shock layer radiation. All temperatures were within allowable material limits. Total surface heat flux was derived from the in-depth temperature measurements at the thermocouple locations. Turbulent boundary-layer conditions occurred at nine heatshield locations and the radiometer provided evidence that heating from shock layer radiation significantly exceeded convective heating on the backshell. Computational results on the reconstructed entry trajectory are compared to the flight data. After boundary-layer transition, total heat flux from algebraic turbulence model calculations qualitatively matches the reconstructed heating, but with lower peak magnitudes. On the backshell, laminar heat flux predictions generally exceed the as-flown values by less than , with shock layer radiation predicted to contribute the majority of heating. At the radiometer location, the maximum predicted radiative heat flux exceeds the measurement, with the measured value likely suppressed by ablation products on the sapphire window.

Absolute Measurements of Air Shock-Layer Radiation in the T6 Aluminium Shock Tube

This paper presents the first experimental measurements of shock-layer radiation from a new high-enthalpy ground-test facility: the T6 Aluminium Shock Tube mode. A dual-channel imaging emission spectroscopy system was used to record spatially and spectrally resolved, absolute radiation data from air shock layers at velocities ranging from 7 to . The presented conditions are designed to provide overlap with other experimental datasets in the literature, from both the NASA Electric Arc Shock Tube and an atmospheric plasma torch. Comparisons with these data (as well as computational tools) was favorable, thereby benchmarking the data from the new shock tube against established sources. The measurements made in this paper also confirm that T6 is now the first European facility capable of performing such superorbital shock-layer radiation studies, thereby providing a new capability to support current and future missions in the solar system.

Experimental and Computational Fluid Dynamics Study of Hayabusa Reentry Peak Heating

In this work, near-infrared and ultraviolet spectroscopic data collected during the Hayabusa spacecraft reentry is used as the basis for validating numerical and experimental studies using the compressible flow computational fluid dynamics code Eilmer3 and the University of Queensland’s X2 expansion tube. Spectra generated numerically from simulations of the full-scale vehicle at the selected trajectory point at 1 s after peak total heating were compared with experimentally captured spectra over a binary-scaled and total-enthalpy-matched radiating shock layer around a 1/5-scale Hayabusa model, and both sets of data were compared with spectra obtained from flight. For the infrared results, experimental and simulated spectra, as well as simulated and flight spectra, compared well within an uncertainty margin of between 20 and 90% depending on the particular case being compared. Experimental and simulated ultraviolet results did not compare well; however, comparisons between simulated and flight ultraviolet spectra were more favorable, comparing well within an uncertainty of around 50%.

Experimental validation of a test gas substitution for simulating non-equilibrium giant planet entry conditions in impulse facilities

Scaled giant planet entry testing is beyond the simulation capability of most impulse facilities, which limits the ground testing data available for computational model validation. Substituting the He in their predominately H/He atmospheres with Ne has been used in the previous experiments. This enables higher-temperature shock layers to be recreated using the same facility performance. For binary hydrogen dissociation and ionization reactions, the substitution gives similarity with non-equilibrium processes, but has never been experimentally validated by direct comparison with H/He relaxation data in a region where the facility simulation capabilities in using H/He and H/Ne overlap. This work demonstrates and validates a scaling method based on the substitution in terms of flow relaxation. A H/Ne condition was designed to recreate the post-shock relaxation of a H/He condition in our X2 expansion tube. The two conditions were experimentally tested with cylindrical test models, and shock layer radiation from the Balmer series was measured. The spectroscopic data using the H/Ne condition show a successful recreation of the non-equilibrium processes of the target H/He condition, experimentally validating the scaling method. High-speed images demonstrate that the associated radiation field is also correctly reproduced. With the use of this substitution, the facility can now be used to simulate high-speed giant planet entries with confidence, and the available ground testing envelope is extended to much higher speeds.

Using Aerothermodynamic Similarity to Experimentally Study Nonequilibrium Giant Planet Entry

Peak heating points for the entry into Uranus, Neptune, and Saturn exhibit highly nonequilibrium shock layers, and are not well understood. This paper describes an experimental study in the X2 expansion tube at the University of Queensland, using a condition with a higher concentration of helium than exists on Saturn, with a shock speed of , to simulate radiation during blunt body Saturn entry at . High speed imaging and emission spectrometry were employed to measure shock layer radiation. Axial and radial spectroscopic measurements were performed through planes along and perpendicular to stagnation streamline, and an Abel inversion was used to quantify the radial radiation profile. Regions at different nonequilibrium stages were identified by radiation measurements. Nonequilibrium shock layers with a slow ionization rate, which were observed in other Saturn entry radiation tests, were successfully recreated. This paper shows increasing the amount of helium enables expansion tubes or other facilities to simulate nonequilibrium giant planet entry, thus providing more ground testing data for future study.

Radiative transfer in porous carbon-fiber materials for thermal protection systems

Highly porous carbon-fiber materials are used in ablative heat shields for atmospheric re-entry. These carbon fiber based materials perform well against heat convection and conduction, and have high radiation absorption. During re-entry, spacecrafts are exposed to shock layer radiation, which provides a considerable share of the heat load faced by their shields. In this work, models for reflectance and heat transfer through FiberForm, a porous carbon-fiber material, are described. The models are based on the multiphase approach and were validated by comparison with experimental data on reflectance and on effective high-temperature thermal conductivity. The morphology of FiberForm was reconstructed from X-ray tomography data and was used as an input for the models. Transmission electron microscopy and diffraction analyses of FiberForm revealed the presence of an amorphous phase and graphite nanocrystalline grains smaller than 10 nm, with no preferential crystallographic orientation. A strong influence of anisotropy in fiber orientation was observed in measured reflectance and has been included in modeling. The porosity and the extinction coefficient were obtained from X-ray tomography images. The radiative transfer modeling indicates that 90% of the incident radiation at wavelengths around 1 μm is absorbed in the 200–300 μm thick outermost surface layer.

In situ ultraviolet shock radiance measurements using GaN-on-sapphire photodetectors.

Gallium nitride (GaN)-on-sapphire photodetectors are used to measure the ultraviolet (UV) radiance behind a shock wave in support of atmospheric entry sensing technologies. DC spectral response characterization of the GaN-based photodetectors shows a peak response around 365 nm with an UV/visible rejection of an order of magnitude. To conduct in situ measurements of UV shock-layer radiation, the GaN-based photodetectors were installed, without protective packaging, in the test section of a shock tube. The measured UV radiation, in terms of incident optical power on the photodetectors, is in excellent agreement with average UV radiation measured by the shock tube facility spectrometers. Furthermore, the device response after being subjected to the shock wave is unaltered, suggesting that the GaN-based material platform is suitable for implementation in aerospace and other harsh environment sensing applications.

Earth Reentry Flow over a Phenolic Aeroshell in the X2 Expansion Tube

The effects of phenolic decomposition on shock layer radiation in air were investigated in the X2 expansion tube. By subjecting a carbon phenolic aeroshell to a flow with a velocity of , decomposing resin on the model surface was allowed to react with the boundary-layer gases. Emission spectroscopy was used to measure radiation emitted by the gas in parts of the ultraviolet, visible, and near-infrared spectrum and then compared with control measurements taken with a steel model. With the composite model in place, measured spectral radiance was seen to increase for CN violet band radiation but remained unchanged for atomic emissions in the visible and near-infrared spectrum. Recorded data were then compared with spectra produced by two-dimensional axisymmetric computational fluid dynamics simulations parsed to the NEQAIR radiation solver. Both the applied chemistry models overpredicted CN violet band radiation, but excellent comparison was found for the band. Good comparisons were also achieved for the O triplet (777 nm) and atomic N line intensities, between 740 and 750 nm. However, peak intensities were underestimated by the numerical simulations in the visible spectrum. Background radiation and broadening of the spectral wings were not accurately accounted for by simulations.

Temperature characterization of a radiating gas layer using digital-single-lens-reflex-camera-based two-color ratio pyrometry.

The two-color ratio pyrometry technique using a digital single-lens reflex camera has been used to measure the time-averaged and path-integrated temperature distribution in the radiating shock layer in a high-enthalpy flow. A 70mm diameter cylindrical body with a 70mm long spike was placed in a hypersonic shock tunnel, and the region behind the shock layer was investigated. The systematic error due to contributions from line emissions was corrected by monitoring the emission spectrum from this region using a spectrometer. The relative contributions due to line emissions on R, G, and B channels of the camera were 7.4%, 2.2%, and 0.4%, respectively. The temperature contours obtained clearly distinguished regions of highest temperature. The maximum absolute temperature obtained in the experiment was ∼2920 K±55 K, which was 20% lower than the stagnation temperature. This lower value is expected due to line-of-sight integration, time averaging, and losses in the flow. Strategies to overcome these limitations are also suggested in the paper.

Venus Entry Flow over a Decomposing Aeroshell in X2 Expansion Tube

The effects of phenolic decomposition on shock-layer radiation were investigated experimentally in X2 expansion tube for a Venus entry flow. A carbon–phenolic composite aeroshell was subjected to a flow with a flight equivalent velocity of . Emission spectroscopy was used to measure boundary-layer radiation in parts of the ultraviolet (380–480 nm) and visible (620–700 nm) spectrum, and it was compared to control measurements taken with a cold steel model. With the composite model in place, the calibrated spectral radiance measured was seen to increase for the O (645.598 nm) atomic line and the (391.22 nm) band head, but it decreased for the Swan band (420–480 nm). The recorded data were then compared to numerical spectra produced by two-dimensional axisymmetric computational fluid dynamic simulations coupled to a radiation solver. Both of the applied chemistry models overpredicted CN violet band radiation, which demonstrated strong self-absorption at these conditions. Better comparisons were achieved for the Swan band radiation. At visible wavelengths, peak intensities were underestimated by the numerical simulations. Several possible reasons were hypothesized for these discrepancies.

Shock Radiation Tests for Saturn and Uranus Entry Probes

This paper describes a test series in the Electric Arc Shock Tube at NASA Ames Research Center with the objective of quantifying shock-layer radiative heating magnitudes for future probe entries into Saturn and Uranus atmospheres. Normal shock waves are measured in hydrogen–helium mixtures ( by volume) at freestream pressures between 13 and 66 Pa (0.1 and 0.5 torr) and velocities from . No shock-layer radiation is detected within measurement limits below , a finding consistent with predictions for Uranus entries. Between 25 and , radiance is quantified from the vacuum ultraviolet through near infrared, with focus on the Lyman- and Balmer series lines of hydrogen. Shock profiles are analyzed for electron number density and electronic state distribution. The shocks do not equilibrate over several centimeters and in many cases the state distributions are non-Boltzmann. Radiation data are compared to simulations of Decadal Survey entries for Saturn and shown to be as much as eight times lower than predicted with the Boltzmann radiation model. Radiance is observed in front of the shock layer, the characteristics of which match the expected diffusion length.

Blunt-Body Aerothermodynamic Database from High-Enthalpy Carbon-Dioxide Testing in an Expansion Tunnel

An extensive database of heating, pressure, and flowfield measurements on a 70 deg sphere-cone blunt-body geometry in high-enthalpy carbon-dioxide flow has been generated through testing in an expansion tunnel. This database is intended to support development and validation of computational tools and methods to be employed in the design of future Mars missions. The test was conducted in an expansion tunnel in order to avoid uncertainties in the definition of freestream conditions noted in previous studies performed in reflected shock tunnels. Data were obtained across a wide range of test velocity/density conditions that produced various physical phenomena of interest, including laminar and transitional/turbulent boundary layers, nonreacting to completely dissociated postshock gas composition and shock-layer radiation. Flowfield computations were performed at the test conditions, and comparisons were made with the experimental data. Based on these comparisons, it was recommended that computational uncertainties on surface heating and pressure for laminar reacting-gas environments could be reduced to and , respectively. However, for flows with turbulence and shock-layer radiation, sufficient validation-quality data were not obtained in this study to make any conclusions with respect to uncertainties for those phenomena, which highlighted the need for further research in these areas.

Carbon Ablative Shock-Layer Radiation with High Surface Temperatures

Despite the prominence of carbon-based materials for use in thermal protection systems, much uncertainty remains in predicting thermochemical ablation rates at high surface temperatures. To address this issue, experiments using preheated graphite models with surface temperatures up to 3300 K were conducted in the X2 expansion tunnel at the University of Queensland. Calibrated shock-layer emission measurements in the wavelength region from 353 to 391 nm were taken to observe the effect of surface temperature on radiation from the CN violet and bands. Numerical simulations were conducted using US3D with modified Park as well as Zhluktov and Abe surface thermochemistry models. Lines of sight extracted from the flowfield data were simulated in NEQAIR to reproduce the experimental radiance profiles. It was found that the experimental CN radiance showed no significant dependence on surface temperature, whereas the numerical simulations predicted a monotonic increase with temperature for all surface models. Several potential mechanisms for these discrepancies have been identified and discussed.

Radiative Heating During Mars Science Laboratory Entry: Simulation, Ground Test, and Flight

The heat shield of the Mars Science Laboratory was equipped with thermocouple stacks to measure in-depth heating of the thermal protection system during atmospheric entry. The heat load derived from the thermocouples in the stagnation region was found to be 33% lower than corresponding postflight predictions of convective heating alone. It was hypothesized that this difference could be attributed to radiation from the shock-heated gas, a mechanism not considered in preflight analyses of flowfields. To test the hypothesis and quantify the contribution of shock-layer radiation to total surface heating, ground tests and simulations (both flow and radiation) were performed at several points along the best-estimated entry trajectory of the Mars Science Laboratory. The present paper provides an assessment of the quality of the radiation model and its impact to stagnation point heating. The impact of radiative heating is shown to account for 43% of the heat load discrepancy. Additional possible factors behind the remaining discrepancy are discussed.

Numerical Modeling of Earth Reentry Flow with Surface Ablation

Vehicles entering planetary atmospheres at high speed require an ablative heat shield to withstand the high thermal energy flux to the body. The interaction between the ablative products and the flowfield is not well characterized. In this study, numerical simulations were conducted to investigate the influence of carbon ablation on shock-layer radiation. Data collected from experiments performed in the X-2 expansion tunnel at the University of Queensland were used to compare to the simulations. The model was a short half-cylinder made of isomolded graphite and was tested in Earth entry flow. The graphite model was heated within a temperature range of 1770–2410 K over the course of the experimental campaign. The radiation emitted from the CN violet bands was measured by ultraviolet spectrometry in a spectral range from 353 to 391 nm. The simulations used the Park as well as Zhluktov and Abe finite-rate surface kinetic models for determining the chemical state of the ablating boundary layer. The Park model consistently overpredicted the radiative heat fluxes, but better comparison was achieved when a modified nitridation rate was used. The Zhluktov and Abe predictions displayed good agreement with the measured radiative heat fluxes at 1770 K but remained well below the measured values at the higher wall temperatures. The lack of a nitridation mechanism in the Zhluktov and Abe model is suggested as the reason for these underpredictions.

Development of a Nonequilibrium Finite-Rate Ablation Model for Radiating Earth Reentry Flows

Thermal protection system design for atmospheric reentry vehicles remains a challenging and complex problem. Recent advances in computational modeling of air–carbon interactions consider competing finite-rate reactions on a limited number of available surface sites. One of the most advanced kinetic models is due to Zhluktov and Abe. However, the Zhluktov and Abe model only describes the oxidation and sublimation of carbon and has no nitridation mechanism. The following study develops several modifications to the Zhluktov and Abe air–carbon model that account for all three reaction mechanisms with the goal of improving cyanogen shock-layer radiation predictions to recent experimental results. First, the study examines two possible paths for carbon nitridation and then assesses the augmented surface reaction model in a representative blunt-body reentry flow. Second, a sensitivity analysis is performed to determine which surface reactions have the most impact on altering cyanogen radiance predictions. It was found that modifications to the oxidation mechanisms significantly improved the comparisons and the nitridation mechanism had a minor effect. Specifically, it was determined that carbon monoxide must be the principal oxidation product at high surface temperatures instead of carbon dioxide, as the Zhluktov and Abe model had originally predicted. More detailed measurements of both the carbon oxidation and nitridation reactions at higher surface temperatures are required to further validate and improve the rate parameters derived in this study.

Vacuum Ultraviolet Spectral Measurements for Superorbital Earth Entry in X2 Expansion Tube

Experiments were conducted using the X2 expansion tube at two conditions representative of flight velocities of 10.0 and for a vehicle entry into Earth’s atmosphere. Calibrated spectral measurements were made of radiating shock layers across and through the surface of a model. These measurements provided a unique dataset for the validation of computational codes used in the calculation of incident vacuum ultraviolet radiative heat flux. Computational simulations at equilibrium gas chemistry were conducted using the Specair program and showed good agreement with the measurements.

Mars Science Laboratory Heat Shield Aerothermodynamics: Design and Reconstruction

The Mars Science Laboratory heat shield was designed to withstand a fully turbulent heat pulse using information from ground testing and computational analysis on a preflight design trajectory. Instrumentation on the flight heat shield measured in-depth temperatures to permit reconstruction of the surface heating. The data indicate that boundary-layer transition occurred at five of seven measurement locations before peak heating. Data oscillations at three pressure measurement locations may also indicate transition. This paper presents the heat shield temperature and pressure data, possible explanations for the timing of boundary-layer transition, and a comparison of reconstructed and computational heating on the actual trajectory. A smooth-wall boundary-layer Reynolds number that was used to predict transition is compared with observed transition at various heat shield locations. A single transition Reynolds number criterion does not uniformly explain the timing of boundary-layer transition observed during flight. A roughness-based Reynolds number suggests that transition due to discrete or distributed roughness elements occurred. However, the distributed roughness height from acreage heat shield material would have needed to be larger than expected. The instrumentation confirmed the predicted location of maximum turbulent heat flux near the lee-side shoulder. The reconstructed heat flux at that location is bounded by smooth-wall turbulent convective heating, indicating that a significant augmentation due to surface roughness did not occur. Turbulent heating on the downstream side of the heat shield nose exceeded smooth-wall convective levels, assuming a supercatalytic surface, indicating that roughness may have augmented heating. The stagnation region heating also exceeded calculated convective heating; the cause of elevated heating may be attributed to a combination of shock-layer radiation and a heating augmentation of unknown origin that was also evident in ground test data.