Axisymmetric Inlet Design for Combined-Cycle Engines

Abstract

Performance considerations for a turbine-based combined-cycle engine inlet are presented using the inlet of the Lockheed SR-71 as a baseline. The methodology incorporated is to modify the SR-71 inlet to expand its operational envelope to higher speeds. A numerical model is developed using the axisymmetric method of characteristics to perform full inviscid flow analysis, including any internal shock reflections. Starting characteristics are quantified based on the Kantrowitz limit. The original SR-71 inlet is analyzed throughout the designed flight regime, beginning at Mach 1.7 and ending with the shock-on-lip condition at Mach 3.2. A series of modifications are then considered for their ability to extend the range of the inlet into the hypersonic flight regime. The results show that two modifications, the widened shoulder centerbody and the variable cone with reextension, have the ability to remain started into the Mach 6‐7 range and have similar compressive performance to each other across the entire Mach spectrum. Nomenclature A = area M = Mach number m = mass flow P = pressure r = radial location T = temperature x = axial location γ = ratio of specific heats χ = flowfield property