Abstract:
A method of fuze sterilization is provided for a fuze that includes a first component and a second component with a prescribed relationship being defined therebetween. The prescribed relationship is one that is required for proper detonation operation of the fuze. The first and second components are fabricated from materials having different galvanic potentials. An electrolyte is introduced between the first and second components to initiate galvanic corrosion of one of the components. The galvanic corrosion continues for a period of time until the prescribed relationship between the first and second components changes sufficiently to disable the detonation operation of the fuze.

Description:
ORIGIN OF THE INVENTION 
     The invention described herein was made in the performance of official duties by an employee of the Department of the Navy and may be manufactured, used, licensed by or for the Government for any governmental purpose without payment of any royalties thereon. 
     FIELD OF THE INVENTION 
     The invention relates generally to the sterilization of a fuze, and more particularly to a method and system for implementing fuze sterilization using a sacrificial anodic component. 
     BACKGROUND OF THE INVENTION 
     When a munition is deployed, a fuze is used to detonate the munition reliably. However, since fuzes are not reliable 100% of the time, it is possible to have a number of undetonated munitions littering a battle zone. Such undetonated munitions pose a safety hazard to both advancing friendly forces and to.civilians who later reside in or pass through the area. Accordingly, the North American Treaty Organization (NATO) and the U.S. Department of Defense (DoD) have regulations specifying safety criteria for all munition fuzes. For example, the DoD uses Military Standard 1316 which requires all fuzes to provide a sterilization feature, the primary function of which is to disable the fuze so that it can no longer detonate the munition after a specified amount of time. Timing and reliability requirements for fuze sterilization are determined by system safety issues and mission requirements. 
     Undetonated underwater munitions (e.g., underwater mines) are of great concern for several reasons. Since underwater munitions are designed to be deployed in the water, they are inherently invisible to friendly and/or civilian ship traffic. Further, underwater munitions are frequently scattered in an area of anticipated enemy activity and are designed to detonate when such activity is detected. However, if some of the underwater munitions are not in a position to be detonated by the enemy activity, they remain as a safety hazard in the presence of subsequent activity by friendly forces or civilians. Still further, the harsh seawater environment could disable the underwater fuze sterilization system thereby allowing the munition to remain live for long periods of time. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a method of fuze sterilization for a munition. 
     Another object of the present invention is to provide a method of fuze sterilization that is reliable in harsh underwater environments. 
     Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings. 
     In accordance with the present invention, a method of fuze sterilization is provided for a fuze that includes a first component and a second component with a prescribed relationship being defined therebetween. The prescribed relationship is one that is required for proper detonation operation of the fuze. The first component is fabricated from a first material and the second component is fabricated from a second material where the first and second materials have different galvanic potentials, i.e., one of the materials is anodic relative to the other material in the presence of an electrolyte. An electrolyte is introduced between the first and second components. As a result, one of the first and second components undergoes galvanic corrosion. The galvanic corrosion continues for a period of time until the prescribed relationship between the first and second components changes sufficiently to disable the detonation operation of the fuze. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will become apparent upon reference to the following description of the preferred embodiments and to the drawings, wherein corresponding reference characters indicate corresponding parts throughout the several views of the drawings and wherein: 
     FIG. 1 is a side cross-sectional view of a firing pin assembly constructed to fail after a period of time in accordance with the present invention; 
     FIG. 2 is a side view of the firing pin after it has been disabled in accordance with the present invention; 
     FIG. 3 is a side view of a portion of a sealed fuze cavity equipped with a seal constructed to fail after a period of time in accordance with the present invention; and 
     FIG. 4 is a side view of a fuze design having an assembly that disrupts the fuze&#39;s detonation train after a period of time in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the present invention, the goal is to bring about failure of a munition&#39;s fuze device. By way of illustrative example, the present invention will be described for use with fuzes that will be deployed in underwater (i.e., seawater) environments. However, it is to be understood that the method of the present invention could also be adapted for use with fuzes that are not deployed in water. 
     A common unintended failure experienced by equipment used in seawater (i.e., salt water) is failure by galvanic corrosion of a critical component. Seawater, because of its mineral content, is an electrolyte. When two materials with sufficiently different galvanic potentials are placed in contact with an electrolyte, one will act as the anode and the other as the cathode. In this environment, the anode will give up electrons (i.e., oxidation) and the cathode will accept electrons (i.e., reduction). This process is destructive to the anode. 
     It is the intent of the present invention to sterilize a fuze using galvanic corrosion of a critical component. By making a critical component(s) in a fuze the anode and adjacent or surrounding component(s) cathodes, and subsequently introducing an electrolyte therebetween, a galvanic couple is formed that will corrode away the critical (anode) component(s). The present invention can be used for both in-water and out-of-water applications. The in-water applications can use the water environment as the electrolyte. The out-of-water environments can store the electrolyte inside the fuze and introduce it between the anodic and cathodic components when required. 
     The present invention is achieved by intentionally making a critical component of the fuze the anode in a galvanic couple. Any critical component that is exposed to seawater, or some other electrolyte, after deployment can be used as the anode. The electrolyte can be obtained from the environment or stored/released by the fuze. The rate of oxidation at the anode could also be increased by choosing an electrolyte with a lower electrical resistance than that of seawater. However, if the electrolyte is not readily available from the surrounding environment, the electrolyte must be stored with the fuze. Another alternative is to mix dry chemicals with seawater in order to increase the electrical conductivity thereof. For example, sodium chloride could be mixed with seawater. 
     The time it takes to cause a failure of the critical anodic component will primarily depend on the size of the anode relative to the cathode and the potential difference between the anode and cathode. To increase the rate of oxidation of the anode, the anode is chosen to be as small as possible and the cathode is chosen to be as large as possible while maintaining other fuze design constraints. The anode could also be reduced in size by coating or painting it everywhere except where the failure is intended. The effect of the cathode can be increased by coating a surrounding material with a material that is less active. To prevent polarization of the cathode, the cathode should be placed such that water (or other electrolyte) flow over the surface of the cathode is maximized. 
     The present invention can be implemented in a variety of ways, three of which will be described herein. Referring now to the drawings, and more particularly to FIG. 1, the firing pin assembly of a fuze equipped for fuze sterilization in accordance with the present invention is shown and is referenced generally by numeral  10 . Firing pin assembly  10  is typical of what might be used with a stab detonator. Specifically, a housing  12  has a sleeve  14  formed therein for slidingly receiving a two-part firing pin  16 . Firing pin  16  has a shaft  16 A coupled to pin  16 B at, for example, a z-clasp  16 C that resides in sleeve  14  as long as assembly  10  is enabled for operation. That is, for detonation to occur, a detonator (not shown) would be forced into engagement with pin  16 B. Accordingly, pin  16 B must be present and protrude from housing  12  as shown for firing pin assembly  10  to be enabled. To retain firing pin  16  in the enabled configuration, i.e., in its prescribed relationship with housing  12 , an annular flange  16 D on pin  16 B engages an annular seat  12 A in housing  12  while shaft  16 A protruding from the opposite end of housing  12  is engaged by a threaded nut  18 . 
     Disposed between nut  18  and housing  12  are a series of washers  20 ,  22  and  24 . Washers  20  and  24  are made from a dielectric material while washer  22  is made from a material that will serve as a cathode as compared to the necked-down portion  16 E of shaft  16 A that it surrounds. That is, shaft  16 A (or at least portion  16 E) is made from a material that is anodic relative to washer  22  when shaft  16 A and washer  22  are contacted with an electrolytic material. A gap or air space  28  is defined between washer  22  and shaft  16 A. Washer  22  is provided with slots  22 A (or ports) to provide for the introduction of an electrolyte into gap  28 . Note that portion  16 E of shaft  16 A surrounded by washer  22  can be sized to control the amount of time it takes for corrosion failure to occur as will now be explained. 
     It is assumed herein that firing pin assembly  10  will be immersed in a seawater environment during its use such that the area about washer  22  is immersed in seawater. Once this occurs, seawater (not shown) will flow through slots  22 A into gap  28  and initiate galvanic corrosion of portion  16 E of shaft  16 A. Corrosion will continue until failure occurs at portion  16 E whereby a compressed spring  30  (engaging shaft  16 A in sleeve  14 ) can act on firing pin  16 . As shown in FIG. 2, the release of spring  30  causes z-clasp  16 C to exit the radial constraint of sleeve  14  thereby allowing pin  16 B to fall off. Note that spring  30  is sized to maintain the remaining portion of shaft  16 A in sleeve  14 . Thus, the fuze incorporating assembly  10  is disabled since there is no longer any pin to engage an impinging detonator. 
     Another type of fuze that could utilize the fuze sterilization of the present invention is one having a sealed cavity that must remain dry at all times for proper operation. That is, the critical anodic component could be the cavity&#39;s seal while the cathodic component could surround the seal. For example, as shown in FIG. 3, a fuze cavity  40  could be sealed in the following manner. A cathodic sleeve  42  could support therein an anodic sealing disk  44  that is sealingly supported in a cavity hole  46  by a dielectric gasket assembly  48 . This fixed prescribed relationship between sleeve  42  and disk  44  will be maintained as long as no electrolyte is present therebetween. However, when immersed in an electrolyte such as seawater, disk  44  and sleeve  42  are coupled via the seawater and disk  44  corrodes until it fails whereby seawater enters cavity  40  to disable the fuze. 
     In yet another type of fuze design illustrated in FIG. 4, a detonation train is required for proper fuze operation. The detonation train can include a detonation cord  50  coupled to a fuze output charge  52  by means of a booster charge  54 . One way to disable or sterilize the detonation train is to move detonation cord  50  and booster charge  54  out of alignment with fuze output charge  52 . For example, detonation cord  50  could terminate in a cord holder  62  of a pivot plate  60  while booster charge  54  is maintained in pivot plate  60  as shown. Pivot plate  60  is attached to, for example, a munition body  70  by means of a screw  64 . A torsion spring  66  is coupled between pivot plate  60  and screw  64  such that pivot plate  60  is biased to rotate about screw  64 . Such rotation is designed to disrupt the alignment of booster charge  54  and fuze output charge  52 . 
     To prevent such rotation when the detonation train is in alignment, a retaining pin  68  is captured in pivot plate  60  and munition body  70 . In accordance with the present invention, pin  68  is anodic relative to pivot plate  60  and/or munition body  70 . Thus, a gap or air space  69  must be provided between pin  68  and pivot plate  60  and/or munitions body  70 . Further, pin  68  should be electrically isolated from pivot plate  60  and/or munition body  70  and is, therefore retained in dielectric sleeves  61  and  71 , respectively. The prescribed relationship between the cathodic pivot plate  60  (and/or munitions body  70 ) and the anodic pin  68  will be maintained as long as no electrolyte is present therebetween. However, when an electrolyte is introduced into gap  69 , pin  68  undergoes galvanic corrosion until it fails whereby pivot plate  60  rotates under the force of torsion spring  66  to disrupt alignment of the above-described detonation train to disable the fuze. 
     The advantages of the present invention are numerous. A reliable failure mechanism is now available for harsh seawater environments that takes advantage of the seawater&#39;s electrolytic properties. The method can be applied to a variety of underwater fuze designs without requiring any provision for an electrolyte. However, the method can also be adopted for dry-land fuze sterilization as long as provision is made for the timely introduction of an electrolyte between the critical anodic and cathodic components. 
     Although the invention has been described relative to a specific embodiment thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.