Controlled fragmentation warhead
BACKGROUND OF THE INVENTION 
     This invention relates to controlled fragmentation explosive devices. More particularly the invention relates to explosive devices having control over the size and shape of fragments produced by the device. 
     To avoid random distribution of fragments propelled by exploding anti-property and personnel devices, it is necessary to control the size, shape, and weight of the fragments. Small fragments have low mass and will not possess optimum amount of kinetic energy against a desired target compared to a larger mass fragment traveling at the same velocity. Large fragments, and in particular, bar, plate, and diamond shapes, however, offer more atmospheric drag causing the fragment velocity to slow down rapidly, resulting in a reduced kinetic energy on the target. It can be appreciated that inconsistant fragment size, shape and weight are undesirable. 
     Heretofore, fragmentation control has included providing grooves on either the external or internal surfaces of the wall of the case or a liner inserted into the case. The grooves create stress concentrations that cause the case to fracture along the grooves forming fragments. Generally these grooves are longitudinal, circumferential, or both, or constitute a series of intersecting helical grooves designed to produce diamond shape fragments. While these devices have demonstrated the ability to create fragments, they are not completely satisfactory for several reasons. 
     First, the fragments are often much smaller than they ordinarily should be due to fragment weight loss during the fragmentation process. Allowance for weight loss requires that the device be designed to produce larger fragments than will actually result. This reduces the number of fragments available for a given warhead. 
     Second, the prior art devices produce fragments of a variety of weights and do eliminate the variations in kinetic energy resulting therefrom. Additionally, diamond shaped fragments have high drag coefficients, which as stated, result in rapid decay of fragment velocity. 
     Casings that are relatively thick are susceptible to producing fragments of varying shapes and weights. The helical grooves heretofore utilized are ineffective in controlling these fragment variations. 
     Finally, during the fragmentation process much energy is wasted on metal deformation. Frequently, the corners of the fragments are turned up which further increases drag. It is desirable to provide the device with means for increasing the amount of energy directed to fragmentation rather than being wasted in fragment deformation. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide for a warhead having a high degree of fragmentation control for impacting a target with fragments of a uniform size and shape 
     It is another object of the invention to provide for a fragmentation explosive device yielding fragments of uniform size and shape 
     Another object of the invention is to provide for a fragmentation explosive device having an increased level of explosive force directed to producing fragments of a desired shape and size. 
     Another object of the invention is to provide for a fragmentation device that produces fragments having minimum drag characteristics 
     A still further object of the invention is to provide for a fragmentation explosive device that maximizes the number of fragments produced in a specific weight group. 
     A further object of the invention is to provide for a fragmentation explosive device that maximizes the kinetic energy available from each fragment produced. 
     The objects are achieved and the limitations of the prior art are overcome by providing both the inner and outer surfaces of a cylindrical case with longitudinal and circumferential &#34;v&#34; grooves having specific dimensional relationships. The inner and outer grooves are preferably aligned with each other. The outer grooves are filled with a material for improving the acoustic impedance mismatch between the case and the air within the grooves thereon. 
     Other objects and attendent advantages of the invention will become apparent to those skilled in the art from reading the following detailed description of the preferred embodiment in conjunction with the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary longitudinal section of the preferred embodiment showing the inner and outer circumferential grooves. 
     FIG. 2 is an end view of the preferred embodiment showing the inner and outer longitudinal grooves. 
     FIG. 3 is a fragmentary partial longitudinal cross section of the preferred embodiment showing the inner and outer surface grid patterns. 
     FIG. 4 is an enlarged view of B in FIG. 2 showing details of the inner and outer longitudinal grooves. 
     FIG. 5 is an enlarged view of A on FIG. 1 showing details of the inner and outer circumferential grooves. 
    
    
     Referring to FIG. 1, there is shown a fragmentation explosive device 10 including a cylindrical case 12 for holding an explosive, not shown. Case 12 is normally of steel construction and includes circumferential grooves 14 on its outer surface and circumferential grooves 16 on its inner surface. Circumferential grooves 14, 16 are preferably radially aligned with each other forming individual circumferential groove pairs. As best shown in FIG. 2, cylinder case 12 is also provided with outer longitudinal grooves 18 and inner longitudinal grooves 20 which are also radially aligned with each other forming individual longitudinal groove pairs. Longitudinal grooves 18, 20, intersect circumferential grooves 14, 16, to form the grid patterns shown in FIG. 3. While the preferred embodiment has longitudinal grooves 18, 20, parallel to the longitudinal axis of case 12 they may be skewed therefrom to change the pattern of ejection of the fragments. The inner and outer circumferential grooves have an included angle falling within 30° to 60° and are preferably 45°. 
     Likewise, the inner and outer longitudinal grooves have an included angle falling within 30° to 60° and are preferably 45°. 
     It has been found that to achieve uniform fragment size and shape, and to assure that substantially all of the fragments fall within the same desired weight group, a relationship exists between the depths of the various grooves. During detonation of case 12, strain is greatest in the circumferential direction and fracture of longitudinal grooves 18, 20, will occur more readily than along circumferential grooves 14, 16. Therefore, circumferential grooves 14, 16, are made deeper than the longitudinal grooves. It has been found that a high degree of fragmentation size, shape and weight control is achieved by making outside circumferential grooves 14 deeper than inside circumferential grooves 16 by a ratio of 2:1. The relationship between inside and outside longitudinal grooves 20, 18, is less critical; however, it has been found that improved fragmentation control is achieved by making the inside longitudinal grooves deeper than the outside longitudinal grooves by a ratio of 3:2. Additionally, the ratio of the total depth of any circumferential groove pair to the total depth of any longitudinal groove pair, also referred to as the groove depth ratio, must be greater than 2:1. As the data presented below shows, as the groove depth ratio falls below 2:1 less than optimum fragmentation control takes place. 
     The above specific depth ratios are applicable to warhead casings made of low carbon steel which is readily available, inexpensive and easily machined. When other materials are used the same fragmentation control technique and general relationships between the various groove depths as disclosed herein are applicable thereto. Only the specific numerical values of the depths of the grooves applicable to the specific material used must be determined. Those skilled in the field of controlled fragmentation devices will readily be able to determine the specific depths of the various grooves for other materials having the benefit of the general relationships therebetween as taught in this disclosure. 
     While the exact mechanism is not conclusively known, it has been determined that by filling the external grooves of the case with a material 22, see FIGS. 4 and 5, as disclosed herein, control over the size, shape and weight of the fragments is improved. It is known that as the device detonates, shockwaves travel through case 12. Because the acoustic impedance of the air within the groove and the steel case are substantially different, the shockwaves impinge upon and are reflected from the interface of the case wall outer surface with circumferential grooves 14 and outer longitudinal grooves 18. The impingement and reflection causes the grooves to collapse and deform creating fragments with turned up edges as hereinabove mentioned. Additionally, reflected shockwaves causes spalling of the metal case resulting in fragments having uneven, rough, and non-uniform size and weight. By filling the grooves with a material having an acoustic impedance substantially matching that of the case, the acoustic impedance mismatch between the material in the grooves and case is reduced which diminishes the reflected shockwaves and reduces spalling of the metal. The material in the grooves helps prevent groove collapse, deformation and metal spalling, leaving smooth, uniform shaped and weight fragments. Any material that has an acoustic impedance substantially matching that of the case, or at least being between that of air and the case, and which is preferably in a fluid or semi-fluid state for easy filling of the grooves, can be used. Representative materials are epoxy, iron filled epoxy, or a urethane. These materials are representative only and are not to be considered all inclusive. 
     The test data presented herein shows the effectiveness of the present invention. The warheads tested had relatively thick, low carbon, steel wall cases ranging from 0.35 to 0.40 inches. The cases were loaded with high explosive and initiated from the center of one end. The warhead was placed vertically in an area of CELOTEX bundles located 20 feet from the warhead to catch the fragments. 
     Referring to Table I, tests 1 and 2 substantiate the conclusion that making the outer circumferential groove deeper than the inner circumferential groove by approximately 2:1 produces a considerably larger percentage of fragments in the desired weight range. 
     As shown in Table II, tests 3, 4, and 5 substantiate the conclusion that an increased percentage of fragments fall within the desired weight range by filling the exterior circumferential and longitudinal grooves with either urethane or iron filled epoxy. Additionally, visual inspection of the fragments from filled and unfilled grooves showed that those from the warhead having unfilled exterior grooves had considerable plastic metal flow and irregular surfaces as compared to the fragments from the warhead having its exterior grooves filled. 
     Referring to Table III tests 6-10 substantiate the conclusion that substantially all of the fragments produced by the warhead will fall within the desired weight group by making the groove depth ratio, as defined hereinabove, greater than approximately 2:1. As shown in the data for tests 6 and 7, when the groove depth ratio falls substantially below 2:1, multiple fragments are formed and less than 50% of the total fragments produced fall in the desired weight group. Multiple fragments are those that occur when a complete fracture of a longitudinal or circumferential groove between adjacent columns or rows does not take place. The failure of the grooves to fracture when the warhead is exploded results in a larger fragment made up of 2, 3, or more smaller fragments of the desired size but which failed to separate. Test 8 again substantiates the effectiveness of filling the exterior grooves as evidenced by the increased number of fragments falling in the desired weight group even though the groove depth ratio is less than the preferred ratio of 2:1. Finally, as shown in tests 9 and 10, when the groove depth ratio is substantially close to the preferred ratio of 2:1, effective fragmentation control occurs as evidenced by more than 95% of the fragments falling in the desired weight group. 
     Having described the preferred embodiment of the invention, other embodiments and modifications will readily come to the mind of one skilled in the art of controlled fragmentation devices. It is therefore to be understood that this invention is not limited thereto and that said modifications and embodiments are to be included within the scope of the appended claims. 
     
                                           TABLE 1                                 
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            Total Weight                                                  
   Circumferential                                                        
            of Recovered                                                  
                    % of Fragment                                         
                            Fragment                                      
                                  Wall                                    
   Notch Depth, in.                                                       
            Fragments, gm                                                 
                    Weight in                                             
                            Design                                        
                                  Thickness                               
Test                                                                      
   Inside/Outside                                                         
            Group   13.4-17.5 gm                                          
                            Weight, gm                                    
                                  in.                                     
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1. .070 .150                                                              
            389.8   79.1    14.5  .35                                     
2. .150 .070                                                              
            368.7   32.0    14.5  .35                                     
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 Inside and Outside Longitudinal Notch Depth: 0.100 in                    
 
    
     
                                           TABLE 2                                 
__________________________________________________________________________
                % of Recovered                                            
          Average                                                         
                Fragment Weight                                           
                         Fragment                                         
                               Circumferential                            
                                        Longitudinal                      
Notch     Fragment                                                        
                in 5.8-8.4 gm                                             
                         Design                                           
                               Notch Depth, in.                           
                                        Notch Depth, in.                  
Test                                                                      
   Filler Weight, gm                                                      
                group    Weight, gm                                       
                               Inside                                     
                                   Outside                                
                                        Inside                            
                                            Outside                       
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3  Urethane                                                               
          7.15  57.8     7.8   .075                                       
                                   .160 .060                              
                                            .100                          
4  50% Iron                                                               
          6.40  59.3     7.8   .075                                       
                                   .160 .060                              
                                            .100                          
   Filled Epoxy                                                           
5  Unfilled                                                               
          5.62  40.0     7.8   .075                                       
                                   .160 .060                              
                                            .100                          
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                                           TABLE 3                                 
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                             % of Fragment                                
                                     % of  Fragment                       
                                                 Circumferential          
Circumferential                                                           
            Longitudinal                                                  
                     Total Weight                                         
                             Weight in                                    
                                     Multiple                             
                                           Design                         
                                                 to Longitudinal          
Notch Depth, in.                                                          
            Notch Depth, in.                                              
                     of Recovered                                         
                             13.5-17.5                                    
                                     Fragment                             
                                           Weight                         
                                                 Notch Depth              
Test                                                                      
   Inside                                                                 
       Outside                                                            
            Inside                                                        
                Outside                                                   
                     Fragments, gm                                        
                             gm group                                     
                                     by weight                            
                                           gm    Ratio                    
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6  .072                                                                   
       .189 .099                                                          
                .059 1902.2  43.6    25.0  15.5  1.652                    
7  .072                                                                   
       .187 .110                                                          
                .070 1626.0  42.5    29.3  15.5  1.439                    
 8*                                                                       
   .079                                                                   
       .175 .083                                                          
                .059 762.8   68.8    0     14.5  1.789                    
9  .106                                                                   
       .208 .105                                                          
                .052 864.4   95.4    0     15.2  2.000                    
10 .093                                                                   
       .194 .091                                                          
                .054 746.6   98.3    0     15.2  1.979                    
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 *0.375 in. wall thickness; all others 0.400 in.                          
 NOTE: Exterior notches filled with epoxy.