Pumpjet

The pumpjet is a propulsor that was first used on torpedoes and adopted for submarines - the UK's Trafalgar-class SSNs and the US's Seawolf SSN. The pumpjet is essentially an axial turbine pump consisting of a duct or shroud surrounding a fixed stator with radial slots that twist the direction of water flow and a rotor with more blades than a conventional propeller. This cylinder arrangement increases propulsive efficiency and lowers noise by reducing tip vortices. The pumpjet on the Navy's Seawolf is both quieter and more efficient than an open propeller.

Torpedoes and submarines are spool shaped vessels that are propelled below or at the surface by means of propeller drive. To avoid the torpedoes from rotating around its longitudinal axis, they normally need some device for recovering the rotation energy created by the propeller. Counter rotating propellers have been used to provide propulsion without generation of torque, see e.g. SE 40408. However, the solution is not always acceptable under military circumstances, where nowadays both high speed and silent propulsion are given high priority.

A propulsion system, designed according to the state of art generally designated "pumpjet" system, for torpedoes or submarines is designed with the shroud supported by a number of supports that extend in different radial directions from the vessel's body. The stator on its part is fixedly mounted in the rear part of the shroud, downstream the rotor. The supports placed in front of the rotor create interferences in the flow to the rotor. These interferences can cause variations in stress on the rotor blades. The variations in stress produce a striking increase in noise radiating out from the pumpjet system arrangement. Besides, the interferences can cause hydraulic cavitation on the rotor blades, which forms a very strong source of noise, and can damage the surfaces of each blade.

Cavitation (generation of noisy water vapor bubbles) was reduced in the pumpjet through pressurizing the propeller blade areas and eliminating the propeller tip vortices, making higher speeds at stealth possible.

Underwater vehicles, such as submarines, are currently pushed through the water using propeller-based propulsion system typically located at the stern of the vehicle. Drag forces acting on the vehicle cause the water in front of, and around the vehicle, to become displaced and turbulent in nature. These drag forces lead to decreased efficiency and a lower overall thrust. Further, such propeller-based propulsion systems cause an increase in the submarine's noise with an associated increase in speed. This may aid others in detecting the submarine, enabling its destruction.

The increase in noise is due largely to cavitation. Cavitation is the formation of water vapor bubbles caused by rapid propeller movement that creates a vacuum-like area in the incompressible water. The vapor pressure of the water forms a bubble. Surrounding water pressure soon violently collapses the bubble creating substantial noise.

As the speed of the submarine increases, a geometrically increasing wave generated by frontal water resistance limits the increase in speed and contributes to increased cavitation. This wave is the main resistance to high speed travel in surface vessels and plays a role in submarine speed, albeit less when the submarine is at a depth of greater than three submarine diameters below the surface.

In addition, as submarine speed increases, surface friction from turbulence-related viscous shear stress creates a boundary layer of retarded fluid along the surface of the moving vessel. In this boundary layer, eddies of high-speed fluid contact the surface, causing deceleration, sapping the watercraft's momentum. This boundary layer turbulence increases in magnitude as flow progresses rearward from the bow. Thus nearly all of the vehicle's surface boundary layer is turbulent. The friction or drag of a turbulent boundary layer is seven to ten times that of a laminar boundary layer, so the possibility of achieving significant reductions in vehicle drag by boundary layer management is attractive.

Stealth is the priority with submarines, it has always been the priority. Submarine surfaces are presently coated with rubber to make it less reflective to sonar and mute submarine noise. Short of eliminating surface resistance and wave generation by other means, stealth submarine speed is slow, and top speed is below that of important surface ships, such as an aircraft carrier.

The primary propulsor systems used to drive large surface ships and submarines generally comprise a fossil fuel or nuclear powered prime mover that powers a propeller located on a shaft that extends through the hull of the vehicle through a water-tight seal. A gear train is typically provided between the output shaft of the prime mover and the shaft connected to the propeller. In many cases the drive train arrangement uses gears to match the speed-torque characteristics of the power source and the propeller. The propeller must be in the water, whereas the rotating power source is mostly dry and seals are used about drive train components to keep water in the propeller area away from the rotating power source area.

There are three major shortcomings associated with such primary propulsor systems that limit their usefulness in military applications. First, the shaft seals necessary to keep water out of the hull of the ship are relatively delicate structures which are highly vulnerable to damage when subjected to the kind of mechanical shocks that may be expected under combat conditions. Second, the use of a gear train generates a relatively high level of noise that may render the vehicle easily detectable by the sonar equipment of hostile nations. Thirdly, the prime mover, gear train, and propeller shaft must all be located in alignment with one another at the rear of the vehicle for the efficient transfer of power, which in turn limits the design options for the vessel designer.

Electric motor type propulsor units for water vehicles may be used for surface vessels, but they find their primary application as secondary drive units for submarines where reliability, control, high thrust coupled with low noise emissions, and shock resistance are at a premium. Such propulsor units have typically comprised a "canned" electric motor having an output shaft that is connected to a propeller. Such propulsor units advantageously eliminate the vulnerable seals and noisy gear trains associated with conventional primary propulsor systems. They also afford the designer of the vehicle some liberality with respect to the design of the vehicle, as such propulsor units may be located at any one of a number of locations along the hull of the vessel, and not necessarily at the rear of the vessel. Unfortunately, such electrically-powered propulsor units also have certain drawbacks. For example, because the "canned" motor must be disposed either directly in front of or behind the flow of water generated by the propeller, the location of the motor creates obstructions to fluid flow that tends to reduce the effective thrust that can be generated by these units while at the same time creating unwanted noise. Of course, the thrust may be increased by increasing the rotational speed of the motor. However, this may create cavitation in the water surrounding the propeller which creates even more noise.

To overcome these shortcomings, the Westinghouse Electric Corporation developed an integral motor propulsor unit that is disclosed and claimed in U.S. Pat. No. 4,831,297 of May 16, 1989. This particular propulsor unit generally resembles a jet engine in structure and comprises a cylindrical shroud having a water inlet and a water outlet, a propeller having a hub rotatably mounted within a shroud on a shaft that is concentrically mounted within the shroud by a plurality of support vanes, and an electric motor for driving the propeller that includes an annular rotor mounted around the periphery of propeller blades, and a stator that is integrated within the shroud of the unit. The advanced design of this particular prior art propulsor unit substantially increases the thrust output for a propulsor for a given weight and size while simultaneously reducing the noise generated by the unit due to the largely unencumbered flow of water that the propeller of the device can force through the fluid-dynamically shaped shroud, and the relatively large-diameter propeller that this design is compatible with. The quietness of the unit is further improved due to the noise-blocking characteristics of the shroud.

While the Westinghouse integral motor propulsor unit represents a substantial advance in the art, the applicants have noted a number of limitations associated with the design of this device which might impair its ability to fulfill certain applications. For example, while the thrust output per unit weight ratio is associated with this particular prior art propulsor is very high, the absolute amount of thrust that can be generated by this propulsor might not be high enough for certain applications. Of course, this propulsor unit could be upscaled in all dimensions to produce more power. However, for certain submarine and military surface ship applications, there are limitations with respect to the width of the propulsor unit which might not allow such an overall upscaling of the device to solve the problem of the need for increased thrust. As the width of the propulsor unit increases, the unit as a whole exposes more and more area to fore and aft shock waves that military submarines and surface ships might be exposed to during combat. Still another limitation associated with such prior art propulsion units is caused by the arrangement of the thrust bearings used in these units. These bearings must be routinely serviced, and the difficult access caused by the manner in which these bearings are arranged necessitates either the complete removal of the propulsor unit from the vessel whenever the bearing assemblies must be repaired or replaced, or the dry docking of the vessel itself which, of course, requires considerable effort and expense. After such removal or dry docking has taken place, a large amount of disassembly of the unit is required to access the bearings.

Clearly, there is a need for a submarine propulsor unit that maintains all of the advantages associated with the latest prior art propulsor units, but which is capable of generating larger amounts of thrust with a mechanism which does not exceed the maximum width limitations associated with submarine applications. It would further be desirable if the bearing assemblies could be easily accessed in the event that a repair or maintenance operation were necessary without the need for removing the unit from the vessel or dry docking the vessel, and without the need for a large amount of disassembly of the unit. Ideally, such a propulsor unit would be even more durable and reliable than prior art units, and would possess even greater shock resistance.