Reducing skin friction drag

The skin friction drag acting on a surface of an article travelling at high speed such as a vehicle at supersonic or particularly hypersonic speed can be reduced by introducing a fuel into the boundary layer under conditions of the fuel introduction to ensure combustion in the boundary layer. The fuel is injected through orifice(s) or a slot provided at the surface so that the fuel enters the passing fluid with a major component of the direction of injection being parallel to the local flow direction. The fuel is injected at supersonic speed, e.g. at a speed of about Mach 1.5 or higher. The invention is applicable to scramjet engines with the fuel being injected around the entire internal circumference of the wall of the scramjet engine, upstream of the commencement of the combustion chamber.