INNOVATIVE CONCEPTS FOR HIGH-SPEED UNDERWATER PROPULSION

Concepts and research on high-speed jet-based underwater propulsion are summarized. It is shown that certain marine propulsion concepts apply similar principles used in their aeronautical counterparts, while employing water instead of air as the surrounding and main working medium. The propulsor types analyzed are solid rocket motors, two-phase bubbly ramjets, and water-breathing ducted rockets (ram-rockets). They are characterized by their energetic performance (specific impulse, density specific impulse, and specific fuel consumption), as well as by their specific thrust reflecting the boost and high-speed capabilities, indicated by the thrust per unit cross section and the thrust over drag ratio. It is revealed that the specific thrust of the two-phase ramjet marginally overcomes the vehicle drag and may be used for cruise at medium-high speeds. However, it exhibits the highest energetic performance if air is supplied by an external compressor; in such a case, it needs to get access to air via snorkel and may not be used for deep underwater propulsion. Its variant using liquefied gas stored on board can operate fully under water, but its energetic performance shows only some superiority over rockets. Rockets show the lowest specific impulse, independent of speed, but high thrust over drag values, which decrease with speed. Marine ducted rockets can yield theoretical specific impulses two- or threefold larger than solid rockets and their thrust over drag values may exceed those of solid rockets at the high-speed end. Original plots display the performance characteristics of the different propulsion methods over a broad speed range.