Speaker
Description
Background: Recent investigations of ice and gas giant entry flows have been motivated by the high priority listing of probe missions to Saturn and Uranus in the 2023-2032 planetary science decadal survey [1]. The atmospheres of these planets are primarily composed of H2 and He, with trace amounts of CH4. The convective and radiative heat loads encountered by these vehicles is controlled by the composition of the gases behind the shock that forms in front of the probe. Therefore, in this work, the chemical-kinetic processes for H2/He/CH4 entry flows are reviewed in three parts: the ro-vibrational relaxation and dissociation of H2, the electronic excitation and ionization of H, and finally the impact of trace CH4.
Ro-vibrational relaxation and dissociation of H2: Behind the shock, the first chemical process to occur is H2 dissociation, which has a direct impact on radiative heating. The reverse process, H2 recombination, has a large impact on convective heating. For several decades, H2 dissociation/recombination has been investigated in lower temperature regimes (300K-5000K) through flow reactors and shock tube experiments, and more recently in higher temperature regimes (>3000K) through computational studies. Unfortunately, limited work has been done to compile the existing data together and extract rates that are accurate across the entire post-shock temperature range of interest. Comparing rates from different sources is complicated by the fact that the dissociation process is coupled to the non-equilibrium relaxation of the ro-vibrational modes of H2. Therefore, in this work, we first quantify the non-thermal and non-Boltzmann effects using a quasi-steady-state (QSS) approximation. Where appropriate, corrections for each effect are applied to the rates reported in the literature. From these updated rates, best fits of the reviewed data are proposed.
Electronic excitation and ionization of H: Behind the shock, the dissociation of H2 is followed by the electronic excitation/ionization of H. A previous experimental study by Cruden and Bogdanoff conducted at the Electric Arc Shock Tube (EAST) facility at NASA Ames revealed that the post-shock ionization/equilibration process occurs slowly, with evidence of non-Boltzmann H state distributions [2]. To reproduce accurately the experimental data, a State-to-State (StS) kinetic model for the excitation/ionization of H is developed [3]. Shock calculations are then performed using a space-marching Euler code, and corresponding radiance values are computed using the radiation code NEQAIR [4]. These calculations are found to reproduce the total integrated radiance observed in the EAST experiments relatively accurately while treating explicitly only 23 species. These results also highlight the importance of accurate heavy particle impact excitation/ionization rates, as the heavy particle impact processes are rate-limiting for the subsequent electron impact processes.
Impact of trace CH4: While ice and gas giant atmospheres are primarily composed of H2/He, recent investigations have suggested that trace CH4 may have a non-negligible impact on radiative heating [2,5-7]. Therefore, CH4 and the subsequent dissociated/ionized species are incorporated into the StS kinetic model. The computed radiance values are then compared to the measured values from the recent EAST test campaign conducted by Cruden and Tibère-Inglesse for H2/He/CH4 mixtures [8].
References:
[1] National Academies of Sciences, E., and Medicine, Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032, The National Academies Press, Washington, DC, 2022. https://doi.org/10.17226/26522.
[2] Cruden, B. A., and Bogdanoff, D. W., “Shock Radiation Tests for Saturn and Uranus Entry Probes,” Journal of Spacecraft and Rockets, Vol. 54, No. 6, 2017, pp. 1246–1257. https://doi.org/10.2514/1.a33891.
[3] Carroll, A. T., Blanquart, G., Brandis, A. M., and Cruden, B. A., “State-Specific Kinetic Modeling for Predictions of Radiative Heating in H2/He Entry Flows,” AIAA Scitech 2024 Forum, American Institute of Aeronautics and Astronautics, 2024. https://doi.org/10.2514/6.2024-1482.
[4] Cruden, B. A., and Brandis, A. M., “Updates to the NEQAIR Radiation Solver,” 6th International Workshop on Radiation of High Temperature Gases in Atmospheric Entry, 2014.
[5] Park, C., “Nonequilibrium Chemistry and Radiation for Neptune Entry,” Journal of Spacecraft and Rockets, Vol. 48, No. 6, 2011, pp. 897–903. https://doi.org/10.2514/1.51810.
[6] Coelho, J., and da Silva, M. L.,“Aerothermodynamic analysis of Neptune ballistic entry and aerocapture flows,” Advances in Space Research, Vol. 71, No. 8, 2023, pp. 3408–3432. https://doi.org/10.1016/j.asr.2022.12.024.
[7] Steer, J., Collen, P. L., Glenn, A. B., Hambidge, C., Doherty, L. J., McGilvray, M., Sopek, T., Loehle, S., and Walpot, L., “Commissioning of Upgrades to T6 to Study Giant Planet Entry,” Journal of Spacecraft and Rockets, 2024, p. 1-18. https://doi.org/10.2514/1.A35893.
[8] Cruden, B. A., and Tibère-Inglesse, A. C., “Impact of Trace CH4 on Shock Layer Radiation in Outer Planet Entry,” AIAA Scitech 2024 Forum, American Institute of Aeronautics and Astronautics, 2024. https://doi.org/10.2514/6.2024-2084.
Summary
A review of the relevant kinetic rates for ice and gas giant entry flows is performed to construct a chemical-kinetic model that can accurately reproduce experimental results.
