<p>Hypersonic vehicles experience high drag and aerodynamic heating owing to their high Mach numbers. When aerodynamic heating occurs, the vehicle surface temperature can increase to 2200&#xa0;K, causing severe damage to the vehicle surface and electronic components. Aerodynamic heating varies with the state of the boundary layer on the vehicle surface. A laminar boundary layer has a low heat transfer rate and minimizes damage. However, turbulence significantly increases the heat transfer rate, causing thermal damage. This study aims to observe the development of the Mack second mode and boundary-layer instability on a cone model in hypersonic flow, using an impulse-type hypersonic shock tunnel. It was observed that boundary-layer transition occurs further upstream with increasing Reynolds number. Furthermore, in the early stages of the transition process, the presence of an unstable developing boundary layer was observed. It was also found that the development time of this unstable state varies depending on the Reynolds number.</p>