Performance considerations for a turbine-based combined-cycle engine inlet are presented using the inlet of the Lockheed SR-71 as a baseline. A numerical model is developed using the axisymmetric method of characteristics to perform full inviscid flow analysis, including any internal shock reflections. Self-starting characteristics are quantified based upon the Kantrowitz limit. The original SR-71 inlet is analyzed throughout the designed self-starting regime, beginning at Mach 1.7 and ending with the shock-on-lip condition at Mach 3.2. The characteristics model is validated using computational fluid dynamics. A series of modifications are then considered for their ability to extend the range of the inlet into the hypersonic flight regime. Self-starting characteristics of these new designs are also characterized; results indicate that two new designs can maintain self-starting capability into the Mach 6-7 range. Full external and internal flow properties of the new designs are determined using the characteristics model. Mach number, total pressure ratio, temperature, pressure and mass flow properties (and their levels of distortion) are quantified at the inlet exit plane for all cases considered.