In the past year there bas been a lot or speculation about the use or the pod atop the sternpost or the Soviet VICTOR III attack submarine. Several periodicals have leapt to the conclusion that the pod houses some sort or silent propulsion system. In the foreward to Jane’s Fighting Ships, 1985-1986, its editor implies that the pod may hold an auxiliary propulsor “of the MHO variety.”
Magnetohydrodynamics (MHO) and Electromagnetic Thrust (EMT) have been actively explored for underwater propulsion since the early 1960s. MHD for this propulsion mode would use a magnetic field as a means to convert electrical energy to hydrodynamic energy. Basically, if an electric current and a magnetic field are maintained normal to each other, the result is a force normal to the plane or action or the current and the magnetic field. See Figure 2.
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The distinction between this type of energy conversion and EMT is blurred, however EMT distinctly uses electrodes to generate the necessary electrical currents, while some MHD advocates have postulated systems which do not require current-generating electrodes to create the propulsive forces for driving the submarine through the water.
Electromagnetic Thrust
Of the two proposed systems, latter has received the most researchers in the 1980s. There MHD and EHT, the attention by are two possible ways to build a system to fit the pod: the EHT internal duct propulsion using direct current, and the d.c. external field propulsion. The internal duct propulsion system uses a hole through the center or the pod through which water is thrust to propel the submarine. The force which thrusts the water is generated by the interaction of a powerful superconducting-coil-generated magnetic field and current flowing between two electrodes. Because salt water- is a relatively poor conductor of electricityt to get the necessary current flow between the electrodes requires a high amperage flow — resulting in large expenditure of electricity to achieve a significant thrust.
The application of this type of propulsor has been established by the Japanese. Two ship models have been constructed using the Japanese designed ducted propulsor shown in Figure 3.
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Several problems however prevent tbis type of propulsion from being used in the VICTOR III pod. First and foremost, there is evidently no water intake in the VICTOR III’s pod (as clearly shown in the recent Jane’s publication The Soyiet Submarine Fleet; a PhotograPhic Suryey.) However, even if such an intake existed, for a pod 9 meters long and 2.4 meters in diameter, the large magnetic forces and enormous power density involved in pushing a 5,800 ton submarine through the water, appear to be unreasonable. Extrapolating from the research done by Dr. Hummert of Westinghouse in a 1979 report to ONR, the pod would require greater than 1.5 megawatts of electrical power and a magnetic field of 5 Tesla (50,000 gauss) to move the submarine at 5 knots. Even with a more powerful magnetic field across the duct, for example, using a 10 Tesla field, at least 1 megawatt of electrical power would still be necessary to drive the submarine at 5 knots.
Since there is apparently no intake, the explanation above becomes somewhat academic. But there remains the possibility of an EHT d.c. external field propulsion system. In this type of propulsion, electrodes are mounted externally along the length of the pod, and an external magnetic field is generated, so that the interaction of the field and the electrode current will produce pressure gradients along the centerline of the pod. This pressure pushes the water between the electrodes, creating the propulsive thrust aft.
The problem is much more complex with the internal duct system. In the internal duct, having the high amperage current flow intersect the magnetic field at a right angle — for the maximum, most efficient thrust force — is not difficult. But in the external system it is impossible. The resulting magnetic field will not be uniform between the electrodes due to the curvature of the pod and, as the water velocity will vary with the strength of the local magnetic field, turbulence will be created. Using the multiple coil system shown in Figure 4, with a field strength of 5 Tesla and electrodes raised several inches from the pod’s surface, a 5,800 ton submarine would still require nearly 4 megawatts of power.
There is, however, good visual evidence that this type of raised electrode system is not used on the VICTOR III.
In both cases of EMT, the power estimates are very optimistic. They do not take into account extra drag for the pod, magnetic losses due to the faired surfaces of the pod, or any parasitic power consumed by the cryogenic cooling system, necessary to provide magnetic fields of the strengths required to move the VICTOR III.
Magnetobydrodvnamics
Like EMT, there are two types of MHD propulsion concepts possible for the pod: internal duct HHD and free field MHD. An example of internal duct MHD is shown in Figure 5. This is the so-called “traveling wave” pump. A.C. electric current is used to create a magnetic field of varying intensity in the coils surrounding the sea water duct in the pod. This generates a traveling wave in a flexible membrane which encloses a ferromagnetic fluid. The pulsations, as they move down the length of the pod, squeeze out the water at the stern of the pod, providing submarine thrust. The ferromagnetic fluid is used to translate the magnetic field energy to hydrostatic energy and pushes the water. This system is plausible, though with drawbacks of its own, but is discarded for lack of visual evidence of an inlet for the pod.
The free field HHD propulsion system examined here was proposed originally by Owen Phillips 23 years ago. His system has coils generating a magnetic field radially outward from the pod. This magnetic field flows continually back towards the stern of the pod, as shown in Figure 6. The movement of the magnetic field (traveling wave), creates circumferential eddy currents which react with the magnetic field, t.o create propulsion forces on the surrounding waters.
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This system, if proven feasible, would require more than 2 megawatts at 5 Tesla to move the VICTOR III at 5 knots. This is, however, the only system of the four presented which has the external appearance which agrees with that of the VICTOR III, i.e. no intakes and no external electrodes.
There is a common thread through all of these proposed propulsion systems of EHT and MHO; they consume considerable amounts of power to move a submarine at only a slow speed. To supply such an auxiliary propulsion system of the MHD variety would probably require extra SSTGs or an allelectric main propulsion system for the VICTOR III.
Further practical problems plague the concept of using MHD or EMT auxiliary propulsion plants in the pod. First, the location of the pod makes it vulnerable in under-ice operations, as it is the first part of the submarine which would encounter ice on surfacing. Also, the structural strength required for the pod is at odds with the open interior needed for effective cryogenic cooling of the electromagnetic coils. This cooling is necessary to provide the strong magnetic fields of a propulsor. Second, a cryogenic support system will require room inside the submarine hull as there will be no room in the pod for compressors, pumps, condensers and liquid helium and nitrogen storage tanks. The cryogenic equipment will also provide a noise burden to the submarine.
Lastly, such auxiliary propulsion systems will generate a large external magnetic field. This has two disadvantages for a quiet submarine maneuvering on the auxiliary propulsor. With such a large magnetic field being generated, the submarine would be exceptionally vulnerable to detection by a Magnetic Anomaly Detection system. The large magnetic field will also attract all sorts of magnetic debris which will cling to the pod and cause disturbances in the pod’s magnetic field, additional flow noise and drag on the submarine.
There are many other problems which the designer who wants to put an MHD propulsor in a pod only 9 meters long meters in diameter.
lt is clear that the technology exists to create an auxiliary propulsor of the MHD or EHT variety, but that the attendant drawbacks particularly power consumption, location of the pod and noise of the cryogenic support system make such a system questionable for submarines. As for an EHT/MHD propulsion system in the VICTOR III pod — it seems unlikely.
David Brady and Jobn EdyYane
