An air intake for a supersonic air-breathing flight vehicle introduces or injects fluid in order to create predetermined regions of separated flow and an attendant fluid shear layer. This shear layer forms an aerodynamic boundary for the capture flow with a profile determined and optimised by appropriate injection of fluid. The aerodynamic boundary so generated replaces mechanically moveable solid surfaces used to vary the geometry of prior-art intakes. Use of an introduced or injected fluid can provide an advantage of a variable geometry, but with reduced weight and reduced mechanical complexity. In some embodiments, the injection of fluid has the effect of re-energizing any ingested boundary layer, thus obviating the conventional requirement for a bleed system to provide boundary layer control.