An underwater two-phase ramjet engine propulsion unit, includes an inlet for receiving a flow of water, compressed gas injector for injecting compressed gas into the flow of water, a mixing chamber for mixing the compressed gas with the flow of water to provide a two-phase flow of working fluid and a nozzle for accelerating the two-phase flow of working fluid so as to generate a two-phase jet. The propulsion unit can be implemented with a fixed geometry or with a variable geometry. The propulsion unit includes a supersonic gas injector as well as a subsonic gas injector. The propulsion unit includes a control system the controlling the compressor, supersonic gas injector, the subsonic gas injector, the geometry of the propulsion unit, and the direction of the thrust vector.
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