Direct Numerical Simulations (DNS) were carried out to investigate the laminar-turbulent boundary-layer transition process for a 7◦ half-angle straight (right) cone at Mach 5.2 and zero angle of attack. The flow conditions used in the simulations matched the conditions at different time instances during the ascent trajectory of the Hypersonic International Flight Research Experimentation (HIFiRE-1) flight tests as closely as possible. The wall was considered to be isothermal with a temperature distribution considering the heating of the nose region of the cone. Primary and secondary instability investigations were carried out which have indicated that the so-called fundamental breakdown is may be viable path to transition. A strong fundamental resonance was found for a wide range of azimuthal wavenumbers and the various unit Reynolds numbers considered here. A highly-resolved “controlled” fundamental breakdown DNS confirmed that fundamental resonance is a viable nonlinear mechanisms that can lead to laminar-turbulent transition for the flight conditions investigated here.