Contrarotating Turbine Aerodesign for an Advanced Hypersonic Propulsion System

Contrarotating turbomachinery is part of a novel airbreathing system to power a vehicle from takeoff to hypersonic speeds. The turbine-based combined cycle precools the inlet air before compression, with the hydrogen fuel as the heat-rejection sink. To ensure a safe engine operation, a helium loop is introduced between the air and hydrogen cycles; the helium drives the turbines of the main turbocompressor. The high speed of sound and low compressibility of the helium results in subsonic blading with low divergence of the endwalls. This paper describes in detail the aerodesign procedure of contrarotating helium turbines, which allows a net reduction in turbine size while augmenting the turbine efficiency. The design methodology is reviewed starting from the preliminary velocity triangle analysis. Studies were conducted to determine the effect over efficiency, outlet swirl and periodicity of the blade height, degree of reaction, and work distribution. The three-dimensional optimizer demands efficient parameterization techniques to account for the lean/sweep/twist geometry. The present multidisciplinary design includes the calculation of the centrifugal stresses. Three different helium turbine architectures have been designed and optimized, with special care to ensure an adequate performance at offdesign operation, and particular attention was devoted to minimize the secondary flows.