For typical hypersonic cruise missions, the second mode is unstable in the ultrasonic frequency band. Fedorov et al. (2001) have discovered that an ultrasonically absorptive coating (UAC) can stabilize the second mode by absorbing acoustic energy at the wall. This is achieved using porous surface tuned to provide certain acoustic impedance properties in the ultrasonic range. The stability experiment results by Fedorov et al. agree satisfactorily with theoretical predictions and transition experiments by Rasheed et al. (2002) in the Caltech T5 tunnel demonstrated that equally spaced blind micro holes doubles the laminar run on a sharp cone at a freestream Mach numbers between 5 and 6. These efforts validate our theoretical approach and demonstrate the robustness of the UAC concept. The UAC concept is especially appealing because it can be implemented into the TPS with virtually no penalties.