MK 56 / MK 57

The MK56 ASW mine (the oldest still in use) was developed in 1966. The Mine Mk 50 series includes the Mine MK 56, a 2000-pound aircraft-laid moored mine containing an explosive charge of 360 lbs. of HBX-3, designed specifically for effectiveness against high-speed and deep-operating submarines. The MK 57 is similar to the MK 56 in its mission, however, it is a submarine laid moored mine.

The Mk 56 has a magnetic firing mechanism that uses a total-field magnetometer as its detector. Total-field magnetometers are three dimensional target sensors that respond to changes in the earth's magnetic field as caused by the presence of a ship. Consisting of a nonmagnetic stainless-steel case and a cast-steel anchor, the Mk 56 is equipped with flight gear for launching from aircraft. When laid, the mine sinks to the bottom where case and anchor separation take place. Should the mine become embedded in bottom sediment before case/anchor separation and mooring take place, a slow burning propellant in the anchor is ignited which frees the mine from any mud it may be buried in. As the case rises, a hydrostat, which clamps to an 18-foot loose bight in the mooring cable, senses the preset mooring depth and falls free to release the loose bight, thus permitting the tension on the cable to relax and cause a pawling mechanism in the anchor to lock and stop further cable payout. Should the mooring mechanism allow the mine to rise to a depth which is too shallow, the case will scuttle. This feature reduces the possibility of compromise and eliminates a navigational hazard. Scuttling will also occur on sterilization or if the mooring cable breaks.

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