The system (10) provides a modulator ring (54) secured adjacent an exhaust end (56) of an inner wall of a by-pass fan air duct (40) and secured adjacent a convergent nozzle section (38) of the engine wherein flow of by-pass fan air (42) and core engine air (18) converge within an exhaust nozzle (34), The modulator ring (54) may be actuated to extend beyond the exhaust end (56) of the duct (40) to decrease air passage (62) forcing more air into the core engine (14) for greater thrust from the core engine (14) with minimal loss of thrust from the by-pass fan air (42). The modulator ring (54) stay alternatively rotate to uncover trailing-edge slots in the exhaust end (56) of the duct to also modulate flow rates of by-pass fan air (42) into the convergent nozzle section (38).