Open loop minesweeping system

An open loop magnetic field minesweeping system, with a small and light weight body to be towed through seawater by a helicopter or other vehicle, hydrodynamic control surfaces on the body, a single sweep cable extending a substantial distance from the body with a first electrode in cable, sleeve or sock form attached to the end of the sweep cable, and a second electrode provided on the body as part of its skin. A rectifier and transformer on the body convert AC power fed to the towed body from the towing vehicle, to DC power applied to the first electrode. The rectifier and transformer are encapsulated by a thin waterproof layer and directly exposed to the sea water for cooling through the layer. The body may have a winch to deploy and retrieve the first electrode. Acoustic transducers may be mounted on or within the body.

FIELD OF THE INVENTION
 The present invention relates to minesweeping equipment, and more
 particularly to equipment that will clear a shallow body of water of mines
 that can be set off by influence signatures.
 BACKGROUND OF THE INVENTION
 A minesweeping system that creates influence signatures generally must
 provide a large enough influence field to be effective while still
 minimizing the size and weight of the equipment to make the system
 practical from the standpoint of the platform which controls and/or tows
 the system. This platform may be a ship, a helicopter, a remote controlled
 vehicle operating above or below the water surface, or a slow moving
 aircraft. Minesweeping systems to date have therefore involved a trade-off
 of performance vis-a-vis size and weight.
 Prior art systems to date have included sweep systems using open loop
 magnetic technology, wherein electrical current is distributed between two
 or more towed electrodes and the intervening seawater between the multiple
 electrodes is used as the electrical return. One such system, the Mk-105,
 utilizes a hydrofoil vehicle towed by a helicopter with a gas turbine
 power plant on the hydrofoil to generate electricity for the open loop
 electrodes. The Mk-105 system is powerful, but also quite large and heavy,
 thus requiring the hydrofoil vehicle. In general, however, the most
 efficient means to achieve a large magnetic field is to use the open loop
 means of generating the field. Thus, a ship or helicopter-hydrofoil system
 has generally been required for the towing. Further, such open loop
 systems require sufficient physical handling equipment to handle the two
 or more electrodes, including the appropriate deployment and retrieval of
 the multiple electrodes as well as maintaining the multiple electrodes
 separated from one another for proper functioning and to avoid tangle.
 An alternative prior art sweep system, for example the SWIMS system,
 generates the magnetic influence field utilizing conventional dipole
 technology with large magnetic cores. Because of the size and weight
 associated with this technology, however, the magnetic field is limited by
 the size and weight of a practical towed body in which the system is
 housed.
 Still further prior art minesweeping systems have involved various coils or
 permanent magnet solutions which also have size and weight problems that
 result in limited field strength.
 Various prior art minesweeping and other marine systems are also
 illustrated in U.S. Pat. Nos. 2,393,466; 2,937,611; 3,060,883; 3,273,110;
 3,938,459; 3,940,732; 4,562,789; 4,627,891; 4,697,522; 5,001,485;
 5,063,850; 5,323,726; and 5,941,744.
 SUMMARY OF THE INVENTION
 The present minesweeping invention is intended to utilize the open loop
 means of generating the magnetic field to obtain a powerful field, while
 overcoming the deficiencies of the prior art to provide a smaller system,
 a lightweight system, and a system that simplifies electrode handling. The
 present invention is sufficiently small and stable that it can be utilized
 with and towed by smaller helicopters, smaller water vehicles or remotely
 operated vehicles. The invention is particular adapted to littoral
 operation, for example to clear mined ports or offshore areas or off a
 beachhead where it is desirable to minesweep the shallow waters in
 preparation for landing craft.
 The present invention includes a body to be towed in the water, the body
 containing hydrodynamic control surfaces and designed to provide a
 high-speed and stable tow. The body provides the means to generate the
 magnetic influence signatures, and the body may also include transducers
 to generate acoustic influence signatures. A significant aspect of the
 present invention is that the towed body does not tow multiple electrodes
 to generate magnetic signatures, but rather only tows one (the first)
 electrode while still using an open loop means of generating the magnetic
 field. This is accomplished by having the towed body function as the other
 (the second) electrode, either by making the skin of the body the
 electrode, or by having removable electrode panels on the skin of the
 towed body, or perhaps by incorporating pieces of standard electrode
 designs into the body. Thus the towed body only has one cable which
 contains the first electrode extending behind the towed body. The physical
 handling equipment for the single cable is thus considerably simplified as
 contrasted with what is needed for open loop systems handling and towing
 multiple cables, each with electrodes.
 Open loop power and control systems generally provide an input AC power
 which is then rectified to DC power and controlled to either continuous
 level or to relatively low frequency (pulsed) waveforms. This
 rectification and conditioning generally are done on the primary towing
 platform, i.e., the helicopter or ship, which requires weight and space,
 and requires large diameter cables to handle and pass the large DC
 currents associated with open loop sweeps. Particularly when the primary
 towing vehicle is a helicopter, the cable with DC power from the
 helicopter to the towed body is in air and thus presents difficulties in
 cooling absent such a large diameter cable. Accordingly, in a further
 aspect of the present invention, AC input power of low amperage and high
 voltage is passed from the primary towing platform to the towed body,
 enabling the use of a lower weight cable of small diameter that can be
 handled by a small helicopter. The AC power is then transformed and
 rectified at the towed body.
 Although the transformer and rectifier components would normally generate
 excessive and damaging heat at the towed body, the heat is dissipated in
 the present invention by exposing the transformer and rectifier components
 at the towed body directly to the sea water. These components are not
 retained within a watertight enclosure with cooling mechanisms, but are
 encapsulated within a thin waterproof coating directly exposed to the sea
 water, the coating protecting the components from the conductive sea water
 but otherwise cooling the components by passing heat through the thin
 coating directly to the sea water. Maximum cooling is obtained, and the
 components can be of significantly reduced size and weight from that which
 would be required by alternative forms of cooling at the towed body.
 The body to be towed also may contain a winch to deploy and return the
 first electrode. The first electrode also may take alternative forms, such
 as a cable, a rigid sleeve, or a flexible sock.
 Other features and advantages of the present invention will be apparent
 from the following description, drawings and claims.

DETAILED DESCRIPTION OF EMBODIMENTS
 Referring to FIG. 1, towed body 10 is illustrated which is generally shaped
 in a torpedo-like, streamlined fashion for smooth, fast and stable passage
 through the seawater 11. Body 10 when towed may be submerged, and includes
 rear hydrodynamic fins 12 and possibly hydrodynamic wings 13 to control
 the orientation, depth and direction of the towed body. As illustrated,
 tow cable 14 is connected at one end to the towed body 10 at connector
 mechanism 15, and the other end of tow cable 14 may be connected to a
 winch mechanism on the towing platform (for example on a towing
 helicopter, not shown). The towing platform also will have means to cradle
 and carry the towed body 10 when not in minesweeping use from one location
 to another. The towing platform additionally will have power means to
 provide AC power of low amperage and high voltage down tow cable 14 to the
 towed body 10. As previously noted, the providing of AC power of low
 amperage to the towed body allows the power cable along tow cable 14 to be
 of small diameter and light weight as compared to cables providing high DC
 current from the towing platform to the towed body.
 Extending rearwardly from towed body 10 when it is in minesweeping
 operation is an insulated and waterproof, sweep separation cable 16 and
 the aft (first) anode electrode 17 in cable form. Cable 16 and electrode
 17 may be non-buoyant to minimize size and drag, and are of standard known
 design. The open loop magnetic method of minesweeping requires a second
 electrode, but in the present invention, there is no towed second
 electrode. Rather, a cathode electrode 18 is shown in FIG. 1 as part of
 the outer skin of the towed body. Cathode 18 may be constructed of metal
 plate, or alternatively as wires or metal braid, or sections of cable
 electrode, connected on the outside surface of the towed body 10, for
 example. Cathode electrode 18 is of course insulated from electrode 17,
 and the return path from electrode 17 to electrode 18 is through the
 intervening sea water 11. It will be apparent that there are not two towed
 cables to be separately handled and maintained in a tangle-proof state.
 DC electrical power as noted is provided across electrodes 17 and 18 for
 the open loop magnetic method of minesweeping. Since AC power of low
 current and high voltage is provided to towed body 10 along tow cable 14,
 the high voltage, low current AC is transformed to low voltage, high
 current AC at the towed body 10 by transformer 19, and is then rectified
 by rectifier 20 to provide the constant level or pulsed DC power required.
 The power conversion electrical elements are shown schematically at
 cut-out 21 in FIG. 1, and as transformer 19 and rectifier 20 in FIG. 2.
 Additionally illustrated schematically in FIG. 1 at cut-out 22 is an
 electrodynamic acoustic device that may take various well-known forms such
 as an electrodynamic moving coil transducer. One or more such transducers
 may be located in towed body 10. Accordingly, towed body 10 provides
 complementary magnetic and acoustic influence signatures for minesweeping.
 The acoustic source generally will produce a sweep path width that equals
 or exceeds the magnetic sweep path width, in order to deal with dual
 influence mines typically found in shallow water.
 The sweep cable 16 and aft electrode 17 may be stowed on a small winch 23
 contained within an open and hollow rear end of towed body 10, cable 16
 and electrode 17 being deployed therefrom to the FIG. 1 position during
 minesweeping and reeled back into towed body 10 after use prior to
 retrieval of towed body 10. The winch 23 may be controlled from control
 signals from the towing platform.
 Alternative forms to aft electrode 17 are illustrated in FIGS. 3 and 4.
 FIG. 3 illustrates aft electrode 30 configured as a rigid sleeve of larger
 diameter and shorter length than electrode 17. The shorter length is a
 function of having more surface area by virtue of the larger diameter of
 the electrode. From the perspective of system resistance of the water
 interface of the aft electrode, a primary factor is the wetted surface
 area of the electrode. Thus the larger diameter and shorter sleeve
 electrode obtains the same result as a smaller diameter and longer cable
 electrode. Electrode 30 may assume the same dimensions as the forward skin
 electrode 18 of FIG. 1 for example, and may also be retrieved by winch 23
 into an open and hollow end of towed body 10.
 FIG. 4 is an alternative form to FIG. 3, wherein electrode 40 is similar in
 deployed dimension to electrode 30 of FIG. 3 but is flexible like a
 windsock so that it can flatten and be easily rolled up into towed body 10
 by winch 23 on retrieval.
 Referring back to FIG. 2, transformer 19 and rectifier stack 20 generate
 considerable heat in operation. Rather than utilizing enclosed waterproof
 boxes and cooling plates aboard towed body 10, the transformer 19 and
 rectifier 20 are each completely encapsulated within very thin and
 conformal waterproof coatings 24, 25 respectively of material which may
 for example be a moldable polymer. The sealed transformer 19 and rectifier
 20 are in turn mounted on towed body 10 so that the encapsulation layers
 24, 25 are directly exposed to the sea water, thereby allowing heat
 conduction directly through the thin layers 24, 25 to the sea water. The
 transformer 19 and rectifier 20 may for example be mounted in an internal
 cavity of body 10, which cavity is flooded with sea water. Alternatively,
 they may be mounted in a pocket in the side wall of towed body 10 exposed
 to the sea water. Alternatively a tunnel may pass through a portion of
 towed body 10 through which sea water passes, the transformer 19 and
 rectifier 20 then being mounted within or on the side wall of said tunnel.
 Waterproof pigtails 26 shown schematically in FIG. 2 in turn pass between
 transformer 19 and rectifier 20 respectively and the power connections
 internal to towed body 10. This cooling aspect of the present invention
 provides for very efficient cooling and component design to minimize size
 and weight on the towed body 10.
 Solely as an exemplary embodiment of one form of the present invention, the
 following parameters may apply:

Length of towed body 10 10 feet
 Diameter of towed body 10 18 inches
 Length of sweep cable 16 250 feet
 Length of anode electrode 17 150 feet
 Diameter of cable 16 and .65 inches
 electrode 17
 Length of cathode electrode 5 feet
 AC power along towing cable 14 19 kilowatts
 DC current to anode electrode 17 400 amps
 DC power to anode electrode 17 16 kilowatts
 Weight (in air) of towed body 1000 pounds
 Tow speed of system Up to 50 knots
 Field strength 4 MGauss
 Weight (in air) of cable 230 pounds
 16 and electrode 17
 It will be seen from the above parameters that a very light weight, small
 size open loop magnetic field system is provided, including simplified
 electrode handling and efficient cooling.
 It will be appreciated by persons skilled in the act that numerous
 variations and/or modifications may be made to the invention without
 departing from the spirit and scope of the invention. The present
 embodiments are, therefore, to be considered as illustrative and not
 restrictive.