Speaker
Description
Introduction
The 2023 Decadal Survey [1], published by the National Academies of Sciences, Engineering, and Medicine, concluded that "The highest priority new Flagship mission for the decade 2023–2032 is the Uranus Orbiter and Probe mission". Part of the Uranus Orbiter and Probe (UOP) mission [2] that was used to inform the Decadal study was an entry probe to make scientific measurements of the Uranus atmosphere. A detailed investigation of the entry environments and uncertainties is needed for designing a reliable thermal protection system (TPS) in support of the development of such a probe.
The primarily H$_2$/He upper atmosphere of Uranus poses unique challenges not present for atmospheric entry into Earth or Mars. Most notably, the lack of oxygen removes oxidation as a mechanism for thermochemical ablation of the heatsheild. Entry into the N$_2$/CH$_4$ atmosphere of Titan is similar, but the entry conditions and heating environments are significantly less extreme than those expected when entering any of the Outer Planets. Unfortunately, there has only been one past entry into a primarily H$_2$/He environment, which was the Galileo probe that entered into Jupiter in 1995. However, the entry environments experienced by the Galileo probe were so extreme that lessons learned do not translate well to much lower energy entry conditions expected for a Uranus probe.
There have been limited studies of aeroheating environments or uncertainties of Uranus probes. Work by Tauber et al.[3] investigated the TPS requirements for a probe, similar to the Galileo probe configuration, entering Uranus at 22 and 26 km/s. Palmer et al.[4] investigated the aeroheating environment uncertainties on the same Galileo probe geometry entering Uranus at 22.3 km/s. Neither study considered the probe backshell; both only investigated forebody heating. Also, neither of these two past studies investigated the impact of atmospheric CH$_4$ on radiative heating. %Important lessons from these previous works were leveraged to inform the present study. Most notably, the work by Palmer et al. recommended a coupled flowfield/material response analysis for future work due to the strong contribution of catalytic heating.
The objective of this study is to investigate the radiative heating environments and uncertainties on a Uranus probe. Like the previous studies, the configuration of interest is the Galileo probe [5]. The most recent decadal survey entry conditions are investigated with varying amounts of CH$_4$ in the atmosphere.[2] The entry interface speed of this particular reference mission is about 19 km/s, which is far below previous investigations. Higher energy entry conditions are also explored here to quantify the radiative heating uncertainties should a higher speed entry be realized for a future mission. Radiative heating on both the forebody and backshell of the probe are investigated.
Approach
A coupled flowfield-radiation-material response approach is used to accurately model the flow physics around the probe. The flow field was modeled using the Langley Aerothermodynamic Upwind Relaxation Algorithm (LAURA) software, which is a finite-volume, Navier-Stokes flow solver. LAURA uses a second-order, upwind, discretization scheme with Roe's flux-difference splitting and Yee's Symmetric Total Variation Diminishing (STVD) formulation of the inviscid flux. The gas properties were modeled with a thermochemical nonequilibrium assumption using the two temperature model. Freestream composition was assumed to nominally be 85% H$_2$ and 15% He by volume. The volume fraction of CH$_4$ was increased to as much as 1% and replaced a proportional amount of H$_2$. The coupled radiation and ablation flow field was modeled using a 29 species model that included pyrolysis and char species from the thermal protection system.
Shock-layer radiation was modeled using the High-Temperature Aerothermodynamic Radiation (HARA) code. HARA uses atomic levels and lines obtained from the National Institute of Standards and Technology (NIST) database, Opacity Project databases, and atomic bound-free (photoionization) cross-sections from TOPbase. Radiation mechanisms considered include atomic line emission from H and C, and bands from the H$_2$, CH, C$_2$H, C$_2$H$_2$, C$_2$, C$_3$, CN, and CO molecules. Coupled radiation was modeled in the flowfield energy equations. A ray-tracing approach was applied for computing the radiative heating at the wall, while the tangent-slab approach was used to compute the divergence of the radiative flux. Previous work has shown this coupling strategy is sufficiently accurate for flowfield-radiation coupling.
A one-dimensional material response solver built into LAURA was used to model ablation and pyrolysis of the wall. This material response model was used to determine the energy transfer into a phenolic impregnated carbon ablator (PICA) TPS. The flow solver and the material response model are connected through a surface energy balance and a surface elemental mass balance. A finite rate surface chemistry modeling approach was used. The surface chemistry rate model included surface oxidation, carbonous sublimation, nitridation, and catalytic recombination of N$_2$ and H$_2$.
To investigate the impact of parametric uncertainties in the flowfield and radiation models, an efficient variant of the point-collocation non-intrusive polynomial chaos surrogate method was employed. This approach involved an iterative process to compute the non-zero terms of the underlying polynomial chaos model using only enough evaluations of the flowfield model to converge uncertainty interval predictions and Sobol index values based global nonlinear sensitivity estimates. This strategy enables evaluating hundreds of model parameters with minimal computational expense.
Results
Results from this study show that, during entry along the decadal survey conditions, radiative heating was found to be insignificant on the probe forebody relative to convective heating. Even assuming a 1% by volume fraction of CH$_4$ in the freestream, radiative heating did not significantly increase the total heating on the forebody. However, on the backshell, radiative heating is a significant portion of the total heating across the trajectory. Radiative heating was found to be most sensitive to the production rate of H in the flow field. At the higher entry speeds considered in this study, radiation remains only a small portion of the total heating but becomes increasingly impactful as emission from H$_2$ and H lines increases in the higher temperature, post-shock flow. When CH$_4$ is present, contributions from the CH radical and C$_2$ Swan band further contribute to the total radiative heating around the entire probe. Key sensitivities to model parameters were found to be the production of these participating species, including H, CH, and C$_2$.
Conclusion
The results from this study highlight the relative impact of radiative heating for a probe entering the Uranus atmosphere. Sensitivities to entry conditions and the amount of CH$_4$ in the atmosphere indicated the potential impact of radiative heating across a wide range of entry conditions. While the amount of CH$_4$ in the upper Uranus atmosphere is uncertain, even a small amount can significantly increase the radiative heating an entry probe may experience. Additionally, key modeling uncertainties identified in this work can be used to inform future ground test campaigns to improve modeling capabilities and radiative heating predictions for a future mission.
Bibliography
[1] National Academies of Sciences, Engineering, and Medicine, “Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032”, The National Academies Press, 2023, doi. 10.17226/26522.
[2] Simon, A., Nimmo, F. and Anderson, R., “Uranus Orbiter and Probe: Journey to an Ice Giants System”, National Aeronautics and Space Administration, 2021.
[3] Tauber, M., Wercinski, P., Henline, W., Paterson, J. and Yang, L., “Uranus and Neptune atmospheric-entry probe study”, Journal of Spacecraft and Rockets, Vol. 31, No. 5, pp.799-805, 1994, doi. 10.2514/3.26515.
[4] Palmer, G., Prabhu, D. and Cruden, B. A., “Aeroheating Uncertainties in Uranus and Saturn Entries by the Monte Carlo Method”, Journal of Spacecraft and Rockets, Vol. 51, No. 3, pp. 801-814, 2014, doi. 10.2514/1.A32768.
[5] Milos, F. S., “Galileo Probe Heat Shield Ablation Experiment”, Journal of Spacecraft and Rockets, Vol. 34, No. 6, pp. 705-713, 1997, doi. 10.2514/2.3293.
Summary
The objective of this study is to investigate the radiative heating environments and uncertainties on a Uranus probe. Key results include sensitivity to trace methane in the atmosphere and identification of model parameters that most significantly contribute to radiative heating uncertainty.