Supersonic Skin-Friction Drag with Tangential Wall Slot Fuel Injection and Combustion

The large viscous drag of hypersonic vehicles is a major obstacle to the successful development of vehicles for prolonged atmospheric travel at high Mach numbers. On such vehicles, this skin-friction drag can be of similar magnitude to that of the inviscid drag on the body. To improve the vehicle's performance, the net thrust of the propulsion system has to be enhanced, either by increasing the thrust force or by reducing the vehicle drag. Here, reductions in viscous drag can offer a large margin for improvement of the net thrust level. One of the methods for the reduction of supersonic skin friction is the injection of low-density gas into the boundary layer (film cooling). In a recent development, film cooling by slot injection of hydrogen was combined with its combustion within the boundary layer. Analysis and experiments indicated substantial reductions of supersonic skin-friction drag. This paper provides further evidence for substantial skin-friction reductions by presenting shock-tunnel data of direct drag measurements in a circular supersonic combustion chamber with and without pressure gradients and with and without combustion of hydrogen in the boundary layer. The measurements are compared with theories, and the influence of entropy layers and combustor size is investigated. Measurements show a reduction of viscous drag of up to 77%.