This work investigates how the vehicle-to-tube suspension gap governs compressible flow physics and operating margins in Hyperloop-class transport at 10 kPa. To our knowledge, this is the first study to apply adjoint aerodynamic optimization to mitigate gap-induced choking and shock formation in a full pod–tube configuration. Using a steady, pressure-based Reynolds-averaged Navier-Stokes (RANS) framework with the GEnerlaized K-Omega (GEKO) turbulence model, a simulation for the cruise conditions was performed at M = 0.5–0.7 with a mesh-verified analysis (medium grid within 0.59% of fine) to quantify gap effects on forces and wave propagation. For small gaps, the baseline pod triggers oblique shocks and a near-Kantrowitz condition with elevated drag and lift. An adjoint shape update—primarily refining the aft geometry under a thrust-equilibrium constraint—achieves 27.5% drag reduction, delays the onset of choking by ~70%, and reduces the critical gap from d/D ≈ 0.025 to ≈0.008 at M = 0.7. The optimized configuration restores a largely subcritical passage, suppressing normal-shock formation and improving gap tolerance. Because propulsive power at fixed cruise scales with drag, these aerodynamic gains directly translate into operating-power reductions while enabling smaller gaps that can relax tube-diameter and suspension mass requirements. The results provide a gap-aware optimization pathway for Hyperloop pods and a compact design rule-of-thumb to avoid choking while minimizing power.

Confined evacuated-tube aerodynamics can choke at subsonic vehicle Mach number when a high blockage ratio forces the displaced gas through a narrow annulus. We developed an in-house MATLAB axisymmetric finite-volume compressible-flow solver and a 30-variable discrete-adjoint free-form-deformation (FFD) framework for a $\beta =0.36$ vehicle at $p_{\infty }=10$ kPa, $T_{\infty }=300$ K and $M_{\infty }=0.6$–0.8. The inviscid discretisation was verified by uniform-flow, smooth-wave, manufactured-solution and shock-tube tests, while the complete FFD gradient agrees with central finite differences to $7.18\times {10}^{-6}$. A matched-grid reassessment at $M_{\infty }=0.6$ gives pressure-drag reductions of 51.75%, 52.42% and 51.77% with first-order Rusanov, first-order HLLC and second-order HLLC/MUSCL, respectively. The integrated drag benefit is therefore insensitive to the tested flux/order choice, whereas the local supersonic topology is not: the dissipative Rusanov calculation gives a subsonic optimised peak, while HLLC-based calculations retain local supersonic flow. We therefore withdraw any claim that optimisation removes choking and report only a robust reduction in the integrated pressure load. The saved FFD geometry has $R_{v,\max }=1.2939$ m ($\beta =0.2679$), with the same fixed reference area used for all coefficients. By contrast, the previously reported 11–20% fixed-radius length-only benefit is not grid-robust: on the $\mathrm{36,504}$-cell refinement it becomes −0.01%, 2.95% and 3.20% at $M_{\infty }=0.6$, 0.7 and 0.8. A pressure sweep from 10 kPa to 10 Pa reproduces identical $C_{D,p}$ and peak Mach to nine significant figures while dimensional drag scales linearly with $p_{\infty }$; at $M_{\infty }=0.6$ the matched Rusanov pair gives $45.78\to 22.09$ kN and $9.54\to 4.60$ MW at 10 kPa. These are Euler pressure-drag quantities, not total vehicle drag or total system energy.