Analysis and testing are used to show the feasibility of an innovative shaft driven lift fan propulsion system for supersonic STOVL aircraft. Dual cycle operation of the cruise engine makes it possible to convert some of the jet thrust to shaft horsepower for driving the lift fan. Operation of the propulsion system is described and it is analytically shown that the designs of the engine, drive shaft, and clutch are within the state of the art. A demonstrator engine and lift fan were assembled from available components and operated for almost 200 hours in a full size airframe model. Testing proved the feasibility of changing the engine cycle to drive the lift fan, and of rapidly transferring thrust back and forth between the engine and lift fan to provide pitch control. The durability of the mechanical drive system and flight weight gearbox were also demonstrated.