A proposed modification of the test section in the Boeing/AFOSR Mach 6 Quiet Tunnel is evaluated using CFD. The new design incorporates a section of increased diameter with the intention of enabling the tunnel to start in the presence of larger blunt models. Cone models with fixed base diameter (and hence fixed blockage ratio) are selected for this study. Cone half-angles from 15° to 75° are examined to ascertain the effect of the strength of the test model shock wave on the tunnel startup. The unsteady, laminar, compressible Navier—Stokes equations are solved. The resulting flowfields are examined to see what affect the shocks and shear layers would have on the quiet test section flow. This study indicates that cone angles ≤20° allow the tunnel to start.