Bow-shock instability in entry, descent, and landing vehicles under high-enthalpy conditions

Laminar--turbulent transition remains a major uncertainty in the aerothermal design of entry, descent, and landing (EDL) vehicles. We show that, under high-enthalpy Mars-entry conditions, the detached bow shock and shock-generated shear--entropy layer can become unstable under freestream disturbances, leading to nonlinear breakdown and enhanced wall heating. The analysis spans freestream Mach numbers ($M_\infty$) up to 30 for both Earth and Mars at high altitude, with Mars being more susceptible. The receptivity analysis shows that disturbance amplification occurs through a three-step mechanism: (i) transmission and amplification of acoustic and entropic freestream components across the bow shock; (ii) further convective amplification within the post-shock shear--entropy layer; and (iii) bow-shock corrugation driven by the downstream pressure field, which reinforces the instability. The dominant response is localized in the shock layer, with no classical boundary-layer mode required. The total optimal energy gain scales as $\overline{G}_T^{\rm opt}\sim \gamma_2^*M_\infty^2 \exp[(\rho_2/\rho_1)/C-B/\sqrt{Re_\infty}]$, where $\gamma_2^*$ is an effective specific-heat ratio, $\rho_1$ and $\rho_2$ the pre- and post-shock densities, $Re_\infty$ the freestream Reynolds number, and $B$, $C$ geometry-dependent constants. For a representative EDL vehicle during Mars entry, amplification factors reach order $10^6$. Flight measurements from the Mars Science Laboratory (MSL) and Mars 2020/Perseverance capsules are consistent with these results, as are wall-modeled large-eddy simulations of MSL under representative Mars-entry conditions. These results suggest that bow-shock instabilities may constitute a transition mechanism for blunt hypersonic entry vehicles, either alone or combined with others.