Speaker
Description
I. Introduction
On February 18, 2021, the Mars 2020 Perseverance rover successfully entered the Martian atmosphere and landed safely on the surface. During entry, the Mars 2020 aeroshell was outfitted with a sensor suite, known as Mars Entry, Descent, and Landing Instrumentation 2 (MEDLI2), which measured pressures, in-depth temperatures, and surface heat fluxes at various points on the forebody heatshield and backshell [1]. The integrated sensor plug subsystem included two total heat flux sensors and one radiometer which was used to directly measure the radiative and convective heating on the backshell of the vehicle. The radiometer and one total heat flux sensor were located on the leeside shoulder of the backshell near the predicted peak radiative heating location while the other total heat flux sensor was located at a similar radial location on the windside shoulder of the backshell. A detailed analysis of the measurements recorded by these three sensors are reported by Miller et al. [2].
Prior to flight, an experimental campaign in the NASA Ames Research Center’s miniature arc jet (mARC) facility was performed in order to assess the possibility that ablation products from the forebody thermal protection system (TPS), convected downstream to the radiometer location, could deposit on the sapphire window of the radiometer and lead to a loss of the radiation signal [3]. These tests demonstrated signal losses of around 20 %, though questions remained about the applicability of the results to flight. In particular, while the expected heat loads on the forebody and aftbody TPS samples were nearly matched during the tests, the heating profile and length scales were very different from the flight conditions. Moreover, due to limitations on the mARC facility, the composition of the test gas contained significantly more nitrogen than the Martian atmosphere. Subsequent test campaigns performed in the NASA ARC’s Panel Test Facility arc jet demonstrated signal losses as high as 76 % [2]. Post-flight analysis of the measured total and radiative heat flux measurements suggest an actual signal loss of around 50 %, assuming the predicted convective and radiative heating rates are correct (total predicted heat flux varied by about 12 % from flight data) [2]. Based on these results, the largest uncertainty in the radiometer measurement is the change in the radiometer window transmissivity due to ablation product deposition, and more work is required to fully understand the flight radiometer measurements.
II. Methodology
In this work, we present a methodology for numerically predicting the MEDLI2 radiometer signal loss due to ablation product deposition, taking into account the actual time and length scales seen during the flight. The overall strategy is based on coupled ablation, radiation, and flow field solutions for the Mars 2020 vehicle over the Best Estimated Trajectory (BET). As a post-processing step, the film growth on the radiometer window is estimated using a detailed surface chemistry model, accounting for the key processes believed to be important for the change in transmissivity of the window. The predicted signal loss is then obtained from predicted irradiance on the vehicle surface at the radiometer location and the computed optical properties of the deposited film.
The coupled ablation, radiation, and flowfield solutions are obtained using the LAURA/HARA code [4], assuming two-temperature thermochemical nonequilibrium with the following 16 species: CO2, CO, N2, C, O, N, CN, O2, C2, C3, C5, H, H2, CH, C2H, and C2H2. The ablation rates are computed using locally 1D in-depth material response solutions based on the methodology of [5] for a PICA TPS, where the elemental composition of PICA is taken to be [C,H,O,N] = [0.403, 0.144, 0.435, 0.018] for the pyrolysis gas and 100 % carbon for the char. Only the forebody surface is modeled with an ablating boundary condition. The backshell is modelled using a fully catalytic wall, which is appropriate for SLA-561V insulation on the backshell.
The simulation framework for ablation product deposition compiled in this work is based on existing vapor deposition models developed for carbon nanotube growth [6, 7], silicon carbide coating [8, 9], petrochemistry [10], and the production of thin solid films for electronic devices [11, 12]. These diverse applications consider essentially the same fundamental process, although each with different chemical species. The model utilizes a detailed surface chemistry mechanism which describes the transition from ablation products in the gas phase to adsorbed surface species, and then the deposition of these surface species to the solid film. The total number of available surface sites is modeled as a conserved, material-dependent value, taken to be 1 × 10−9 mol/cm2 for the sapphire window as suggested by Zhluktov and Abe [13]. The surface site compositions are determined using a steady-state approach which allows for the determination of the deposition rate for the bulk phases.
Using the detailed surface chemistry model, the film deposition rates are integrated over the BET to yield an average film composition and thickness. The transmissivity of the film is then determined at each time point through exponentiation of the negative product of the film thickness, density, and absorption cross-section. Where possible, the spectral absorption cross-sections for each bulk phase are taken from relevant literature sources. For the dominant contribution of OH(b), we have utilized the cross-sections from Goullet et al. [14]. Once determined, the film transmissivity is then coupled with the measured transmissivity of the pristine window and the predicted incoming irradiance and integrated over the spectral range of the radiometer to determine the signal loss. The presentation will provide comparisons between this predicted signal loss with the rebuilt flight data and discuss how further improvements to the modeling framework could reduce uncertainties of future radiometer signal loss predictions.
References
[1] White,T.R., Mahzari,M., Miller,R.A., Tang,C.Y., Monk,J., Santos,J.A.B., Karlgaard,C.D., Alpert,H.S., Wright,H.S., and Kuhl, C., “Mars Entry Instrumentation Flight Data and Mars 2020 Entry Environments,” AIAA SciTech 2022 Forum, AIAA, 2022. https://doi.org/10.2514/6.2022-0011.
[2] Miller, R. A., Tang, C. Y., White, T. R., and Cruden, B. A., “MEDLI2: MISP Measured Aftbody Aerothermal Environments,” AIAA SciTech 2022 Forum, AIAA, 2022. https://doi.org/10.2514/6.2022-0551.
[3] Miller,R.A., Tang,C., McGlaughlin,M.S., White,T.R., Ho,T.S., MacDonald,M., and Cruden,B.A., “Characterizationofa Radiometer Window for Mars Aftbody Heating Including Ablation Product Deposition Using a Miniature Arc Jet,” 2018 Joint Thermophysics and Heat Transfer Conference, AIAA, 2018. https://doi.org/10.2514/6.2018-3590.
[4] Thompson, K., Hollis, B. R., Johnston, C. O., Kleb, B., Lessard, V., and Mazaheri, A., “LAURA Users Manual v5.6,” TM 2020-220566, NASA, 2020.
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[11] Coltrin,M.E.,Kee,R.J.,andMiller,J.A.,“A Mathematical Model of the Coupled Fluid Mechanics and Chemical Kinetics in a Chemical Vapor Deposition Reactor,” Solid State Science and Technology, Vol. 131, 1984, pp. 425–434.
[12] Wang,Y.,andPollard,R.,“An Approach for Modeling Surface Reaction Kinetics in Chemical Vapor Deposition Processes,” Journal of the Electrochemical Society, Vol. 142, 1995, pp. 1712–1725.
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Summary
We present a numerical methodology for predicting the MEDLI2 radiometer signal loss due to ablation product deposition on the radiometer window. The model uses a finite-rate gas surface interaction mechanism to predict the deposition rate of ablation products, coupled with an optical model of the resulting film to determine the spectral transmissivity of the radiometer window, contaminated with the ablation products, over the best estimated trajectory of the Mars 2020 atmospheric entry.
