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Christopher O Johnston

Publications and source records attributed to Christopher O Johnston.

25 records · Page 2

Assessment of Mars 2020 Forebody Heating Predictions with Coupled Material Response

The objective of this work was to assess the impact of flow field - material response cou-pling on reproducing temperatures within the Mars 2020 forebody thermal protection system.Both equilibrium and nonequilibrium coupled material response approaches were considered.Turbulent transition estimates were included in the flow field modeling based on transitiontimes inferred from flight measurements. A coupled equilibrium ablation model was foundto overpredict the in-depth temperatures near the surface by over 100 K in the stagnationregion and by about 200 K near the leeside shoulder, which was subject to turbulent heatingaugmentation. For nonequilibrium ablation, two finite rate surface chemistry models wereused to model the chemical process on the vehicle surface. The first was a model consistingof N2catalysis and CO2recombination model, and the second was a model consisting of onlyN2and O2catalysis. Near the stagnation point, the N2and CO2model was found to be withinabout 10 K from the peak in-depth temperature near the surface; however, overpredictionof the in-depth temperatures was observed early in the trajectory, similar to the equilibriumablation model. The N2and O2catalysis model was found to significantly underpredict nearsurface in-depth temperatures.

Thomas K West↗

Advances in Entry Modeling for Impact Risk Assessment

A summary of recent advancements in the detailed modeling of asteroid atmospheric entry processes made through NASA’s Asteroid Threat Assessment Project (ATAP) is presented. Understanding, and accurately modeling these processes and their associated uncertainties is critical to predicting all downstream impact effects, such as blast wave and thermal damage footprints. Furthermore, there is (perhaps thankfully) a dearth of empirical data for large impactors of the kind that would pose a threat to human populations, on which to anchor and/or validate models used in risk assessments. Therefore, we must rely heavily on detailed theoretical and numerical modeling to develop robust assessments for decision makers. To that end, ATAP has made some significant progress in advancing the capabilities in this area. Two areas in particular are highlighted in the present work: meteoroid ablation mechanisms, and bolide luminosity. The first research area – meteoroid ablation mechanisms – has focused on performing novel high-enthalpy wind tunnel experiments on meteorites and meteorite analogs, and utilizing the resulting data to develop high-fidelity models for impactor mass loss at scale. These efforts have resulted in several insights. Of note, these data suggest a differential vaporization process where volatiles are liberated preferentially when the asteroidal material is subject to high heat, while refractory components remain in the molten layer on the surface. A numerical model has been developed which considers this phenomena, and its effect on the bulk impactors effective heat of ablation is examined. The second research thrust that is discussed is focused on accurate modeling of impactor radiation phenomena. While another submission to the conference will discuss the application of our approach to thermal ground damage modeling, here, we present an overview of our extensive efforts to utilize available ground- and space-based observations of large bolides (~1m diameter, and above) to inform and validate our detailed modeling approaches. These methods have been shown to accurately reconstruct the detailed spectra for the Benesov bolide, as well as approximate the burn footprint for the Tunguska event. Recent effort has focused on reconciling light curve data from multiple sources (all-sky camera networks, GLM, US government sensors), using our validated model, and providing a calibrated model for luminous efficiency which can then be utilized to infer impactor properties such as shape, mass, and composition. This work will be demonstrated through an exemplar case study focusing on a large bolide event with observational data (e.g. Chelyabinsk, Flensburg, etc.). Finally, our team’s assessment on the current maturity of atmospheric entry models, and priorities for future research will be provided.

ATAP↗

Backshell Radiation Measurements in the EAST Facility for Titan Entry

A new test series was performed in the Electric Arc Shock Tube (EAST) facility, with the aim of reproducing conditions which will be encountered in the backshell of the Dragonfly spacecraft. Measurements of shock-layer radiation from CN Violet, CN Red and C 2 Swan bands are made in mixtures of 2.2% CH 4 in N 2 (by mole) heated by incident shock waves spanning from 3 to 7 km/s. This study focuses on the spectrally and spatially resolved measurement of absolute radiance made with four optical emission spectrometers spanning 220-1400 nm. Comparison with CFD simulations and equilibrium conditions were made. The predicted radiation was found to be between -40% to 100% from the measurements for the nonequilibrium peak radiance, increasing with velocity. At 7 cm/150 μs from the shock front, the discrepancies appear to be larger, ranging from -50 to 200%. Comparison between measured and predicted temperatures and number densities of CN indicate shortcomings in the non-Boltzmann model of CN.

Augustin Tibere-Inglesse↗

Mars Sample Return Earth Entry System Woven Roughness Mach 6 Aeroheating Test

The Mars Sample Return Earth Entry System is the first entry vehicle designed for a NASA flagship mission to utilize a woven thermal protection system as well as a 52.5 degree sphere cone forebody geometry. As a result, no heritage aeroheating experimental data set exists to compare with computational predictions. In order to obtain a set of validation data for computational models, an extensive wind tunnel test campaign was funded by the Mars Sample Return Earth Entry System project. The first entry in the test campaign was completed in April of 2023, which was an investigation of turbulent heating augmentation due to woven thermal protection system roughness on the 52.5 degree Mars Sample Return - Earth Entry System vehicle forebody geometry. This test entry produced the first ever experimental aeroheating data on a 52.5 degree sphere-cone geometry as well as the first NASA experimental aeroheating data on a woven surface roughness pattern. Data obtained from this test campaign is being directly leveraged to support the NASA flight program, as experimental results are being utilized to validate the computational models which characterize the aeroheating environment that the vehicle experiences and predict turbulent surface heating levels and margins across the woven surface.

Jonathan S Cheatwood↗

An Approach for Practical Grid-Resolved Roughness Aerothermodynamic Simulations for Woven Heatshield Surfaces

An approach for simulating the aerothermal environment over a patterned roughness surface within an otherwise axisymmetric flow is developed. The patterned roughness is simplified into a sinusoidal shape that captures the basic features of a charred 3MDCP ablator. Two orientations of this pattern, which are equally flight relevant due to the forming process, enable periodic boundary conditions along the edges of a grid spanning a single roughness element. This simplified three-dimensional grid represents the minimum problem size for a patterned roughness simulation, which enables computationally efficient grid-resolved roughness simulations. This approach is validated by simulating the Langley Mach 6 measurements made on a similar sinusoidal surface, which results in convective heating within 5% of the experimental data. Applying this approach to Mars Sample Return (MSR) Earth Entry System (EES) flight cases, considering simulations with both 11 species air and 30 species air with ablation products, results in heating augmentation that is slightly lower than the widely-used Dahm correlation approach, with the difference being dependent on pattern orientation. This result provides the only insight into the behavior of the roughness augmentation at the high temperature reacting flow conditions present for EES flight cases, which are not captured by available ground test measurements. For shear augmentation, the simulations revealed that the surface-parallel pressure component is dominant. For arc-jet tests targeted to flight values for shear, the higher arc-jet pressures required to match flight values for shear result in up to a 400% increase in the rough wall shear value, which is not captured by the Dahm correlation. This indicates that, to avoid significant over-testing, grid-resolved roughness simulations are required to determine arc-jet conditions that produce flight shear levels in rough-wall scenarios.

Christopher O Johnston↗

Including Radiative Heating for the Design of the Orion Backshell for Artemis-1: A Narrative

The backshell radiative heating for Earth entry was ignored primarily because of the negligible Fire and Apollo 4 backshell radiometer measurements and negligible radiative heating resulting from the assumed upper-limit Boltzmann assumption of atomic electronic levels. These assumptions are shown to be erroneous in the recombining expanding flow characteristic of the wake, where the slow finite-rate recombination of electrons and ions leads to the Saha-Boltzmann distribution being larger than the Boltzmann distribution.

aftbody radiation↗

Aeroheating Testing of the Mars Sample Return Earth Entry System with Surface Roughness

The Mars Sample Return Earth Entry System is a mission concept which would be the first NASA entry vehicle to utilize a woven thermal protection system as well as a 52.5 degree sphere-cone forebody geometry. Due to its novel design, historic experimental data sets were insufficient to validate the models used to characterize the expected aerothermal environment. A wind tunnel test campaign was funded by the Mars Sample Return Earth Entry System project in the NASA Langley 20-Inch Mach 6 Air Tunnel in order to obtain validation data. The December 2023 test entry of this campaign sought to obtain thermographic data to capture the impact of supersonic flow near woven surface roughness elements on convective heating augmentation, which was the largest remaining uncertainty in aerothermal design of the Earth Entry System. Data obtained were in family with prior results obtained on identical woven patterns without supersonic flow present at roughness elements, and it was found that this presence of supersonic flow did not have a noticeable effect on surface convective heating augmentation. Data were directly leveraged to support the NASA Mars Sample Return flight program and this data set is valuable for validating computational solutions on roughness-resolved grids.

Mars Sample Return↗