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In recent plasma injection experiments, counter ow jet interaction has been identi ed as one of the mechanisms that signi cantly reduces the drag. There is a sudden change of the dynamic state from an oscillatory to nearly steady motion of this jet-shock interaction which depends on the relative magnitude of injection and the stagnation pressure. The shock-wavebifurcation associated with a counter ow jet from a hemispherical cylinder is investigated by a side-by-side experimental and computationaleffort. Shock-wave bifurcation has been discovered by the present experiments over the entire range of tested conditionsin the Mach 6 wind tunnel. This oscillatory motionis sustained by the upstream propagationof selectively ampli ed frequencies from the free-shear layer to the Mach disk through the embedded subsonic domain. The breakdown of this feedback loop occurs when a higher injecting jet pressure creates a supersonic zone separating the interconnecting embedded subsonic domains. Nomenclature D = total drag p = air pressure Q = conservative variables q = primitive variable r = radius of hemispheric nose T = air temperature u = velocity vector, u, v, w D = standoff distance of bow shock q = air density