Parametric Sensitivity Analysis and Uncertainty Quantification of Ice Giant Aerocapture Aerothermodynamics

Accurate characterization of the aerothermodynamic environment for Ice Giant aerocapture is necessary for thermal protection system design and overall mission reliability. However, uncertainties in the models used to simulate these flows produce substantial variations in the predicted aerothermodynamic quantities of interest, particularly the radiative heating. There is speculation over the effect of freestream methane on the aerothermal environments, and the amount of methane present in Ice Giant atmospheres is not well known. As such, this work presents a sensitivity analysis and uncertainty quantification of the convective and radiative heating distributions, and aerodynamics of an entry vehicle along an Ice Giant aerocapture trajectory. Sources of uncertainty include chemical reaction rates, transport properties, vibrational-relaxation rates, and the freestream methane concentration. A non-intrusive, pointcollocated polynomial chaos expansion technique is employed to compute Sobol indices and estimate epistemic uncertainty bounds of the aerothermal quantities of interest. The convective heating is most sensitive to transport properties and the rate of the hydrogen electron-impact ionization (EII) reaction. The stagnation region is dominated by dissociation/ionization phenomena, while the shoulder region is dominated by recombination. Overall uncertainty in the convective-heating distributions ranges from 4.40 to 11.7%. Radiative-heating uncertainty is much larger, with upper bounds exceeding 160% of the nominal predictions. A slight sensitivity of the radiative heating to the freestream methane concentration is observed, but the majority of the uncertainty at the stagnation point and along the frustum is attributed to the dissociation of H-2 with partner H. At lower altitudes, the radiative heating at the shoulder of the vehicle is highly sensitive to the reverse direction of the EII reaction rate due to its impact on the electronic state population of H. This behavior decreases as the altitude increases due to decreased levels of ionization. Aerodynamic coefficients are relatively insensitive to the CFD input parameters, with uncertainties of a few percent at most.