Patent Publication Number: US-4318344-A

Title: Spinning tubular projectile combustible sabot

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to ammunition and firearms. More particularly, it relates to sabots for spinning tubular projectiles. Still more particularly, it relates to a combustible sabot for a tubular projectile. And, still further it relates to a novel composition and method for making a combustible sabot wherein it is consumed at a rate that substantially coincides with the projectile exit time from the weapon barrel. 
     2. Description of the Prior Art 
     Spinning tubular projectiles offer advantages over conventional non-tubular projectiles, among which are flatter trajectory, longer range, shorter flight time and superior penetration of the target. 
     When a tubular projectile is fired from a gun, it is preferable to plug the opening in the tube with a sabot. Because, the sabot provides surface area against which weapon gases can expand to impart momentum to the projectile. However, once the projectile leaves the barrel, the sabot must be removed in some manner. Removal is usually accomplished either by the sabot being installed in a tubular projectile in such a way that it drops out when the projectile leaves the weapon barrel, or that it be fabricated so that it disintegrates when the projectile leaves the weapon barrel. 
     A sabot that drops out or disintegrates upon exit from a weapon barrel has a real disadvantage when fired from an aircraft weapon, namely, the drop out or disintegrating sabot may be ingested into the aircraft engine. However, a sabot which completely combusts after having completed its job of providing surface area against which weapon barrel gases can expand leaves no debris to be ingested into an aircraft engine. Thus, combustible sabots are considered essential when the ammunition is to be fired from an aircraft weapon. 
     The combustible type sabots heretofore were either too weak to maintain the pressure in the weapon barrel, or burn too slowly at the pressure in the breech of the propellant powder, or too difficult or impossible to fabricate wherein the ultimate product is castable after being cured. 
     SUMMARY OF THE INVENTION 
     The combustible sabot, according to this invention, overcomes these problems. The combustible sabot of this invention is accomplished through the use of an epoxy anhydride binder compatible with and filled with energetic solid particles consisting of ammonium perchlorate, magnesium or aluminum, amorphous boron and molybdenum trioxide. The blended composition is castable and cures over the temperature range from about 60° C. to about 125° C. 
     The boron and molybdenum trioxide are preconsolidated into a blend to maintain intimate contact and sensitizes the composition to compressive ignition in the presence of finely divided air filled voids which are formed through controlled vacuum applications after mixing in air or by addition of a small quantity of phenolic or glass microballoons, that is, up to about four percent. 
     The sensitivity of the composition to compressive ignition is increased by the substitution of magnesium particles for aluminum particles. The ability of the cured composition to hold the pressure generated by propellant gases depends upon the sensitivity to the composition to ignition by compression, and the diameter of the sabot, that is, its form and the length of the sabot. It is useful in many tailored or customized diameters and lengths. 
     The installation of a sabot as disclosed by this invention is brought about by plugging the forward end of the projectile and depositing the above ingredients as a mixture in the tube behind the plug to a depth sufficient to fill about two thirds of the plugged portion of the tube. The tube is then placed in a vacuum chamber with its plugged (forward) end down and subjected to repeated evacuations and releases. The evacuation is carried out to a degree such that the mixture rises to a level about even with the upper or aft end of the tube and then released. The alternate evacuation and release is carried out from about ten to about twenty times. The remainder of the tube is then filled with the above ingredients, and they are then compacted and cured. This process yields a sabot having the proper number and size of voids that gives excellent combustion when the projectile is fired from a weapon. Hollow microspheres, of either phenolic or glass, may also be utilized to assist in rendering the sabot of this invention more readily combustible. 
     It is an object of this invention to make a new novel combustible sabot for a spinning tubular projectile. Another object is to make a combustible sabot in such a way and of such materials that it combusts spontaneously upon exiting a weapon barrel. Still another object is to make a combustible sabot that is utilized by ordnance or combat aircraft wherein substantially all risk of ingestion of sabot material by an engine is avoided. Other objects and advantages of the instant invention will become more apparent as the description proceeds hereinafter. The following tables and examples are further illustrative of the present invention and, it will be understood, however, that the invention is not limited thereto. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The best mode for practicing the invention resides in fabricating a combustible sabot in a tubular projectile from a mixture of an anhydride curable epoxy resin, an anhydride curing agent, powdered boron, powdered molybdenum trioxide, powdered ammonium perchlorate oxidizer, and powdered magnesium fuel. Aluminum powder may also be used in lieu of the magnesium. 
     The various ingredients of the invention are defined and characterized in Tables 1, 2, 3, 4, and 5. The abbreviations used therein, such as, ERL-4221, NMA, HHPA, and so forth, are used hereinafter in lieu of the chemical name, formula, etc. The abbreviation BD is 1,4-butanedoil and, AP is the abbreviation for ammonium perchlorate. 
     
                                           TABLE 1                                 
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Typical Properties and Applications                                       
            ERL-4221       ERL-4289                ERR-4205               
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            3,4-Epoxycyclohexylmethyl-                                    
Chemical Name                                                             
            3,4-Epoxycyclohexane                                          
                           bis(3,4-Epoxy-6-methylcyclohexyl-              
                                                   bis(2,3-Epoxy-         
            carboxylate    methyl adipate          cyclopentyl)ether      
Structural Formula                                                        
             ##STR1##                                                     
                            ##STR2##                                      
                                                    ##STR3##              
                                                   Used mainly as a       
                                                   reactive diluent or    
            General purpose casting                in high performance    
            resin. Filament winding                                       
                           For flexibilized products.                     
                                                   reinforced systems.    
Applications                                                              
            Acid scavenger.                        Higher reactivity;     
                                                   high                   
            Plasticizer.                           exotherm; amine        
                                                   hardeners              
Viscosity, cps.                                                           
            350 to 450 (25° C.)                                    
                           500 to 1,000 (25° C.)                   
                                                   &lt; 100 (45° C.)  
Apparent Specific                                                         
Gravity at 25°/25° C.                                       
            1.175          1.124                   1.16 to 1.18           
Color 1933 Gardner                                                        
            1              1                       2                      
maximum                                                                   
Epoxy                                                                     
Equivalent Weight,                                                        
grams/gram mol                                                            
            131 to 143     205 to 216              91 to 102              
oxirane oxygen                                                            
Boiling Point at                                                          
760 mm. Hg. °C.                                                    
            354            258 (10 mm.)            --                     
Vapor Pressure                                                            
at 20° C., mm. Hg                                                  
            &lt;0.1           &lt;0.1                    --                     
Freezing Point °C..sup.(a)                                         
            -20            9                       38 to 42               
Solubility,                                                               
% by wt. at 25° C.                                                 
            0.03           0.01                    --                     
In Water                                                                  
Water In    2.8            1.8                     --                     
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 .sup.(a) Sets to glass below this temperature                            
 
    
     
                                           TABLE 2                                 
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NADIC® METHYL ANHYDRIDE (NMA)                                         
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(Methylbicyclo [2.2.1]heptene-2,3-dicarboxylic anhydride isomers)         
FORMULA: C.sub.10 H.sub.10 O.sub.3                                        
        ##STR4##  The positions of the double bond and the methyl group   
                  of the individual isomers comprising this mixture are   
                  unknown. The methyl group in this formula is drawn as   
                  being attached to the center of one ring to indicate    
                  that it replaces one of the hydrogens shown in the      
                  formula.                                                
PHYSICAL PROPERTIES:                                                      
Appearance              Clear, colorless to light yellow                  
Molecular Weight        178.2                                             
Neutralization Equivalent                                                 
                        89.1                                              
Viscosity, 25° C., cps.                                            
                        175-225                                           
Refractive Index, n.sub.D.sup.20                                          
                        1.500-1.506                                       
Specific Gravity, d.sub.20.sup.20                                         
                        1.200-1.250                                       
Flash Point (open cup), °C.                                        
                        140                                               
Distillation Range, °C., 10mm. Hg                                  
                        135-143                                           
Solidification Point, °C.                                          
                        See footnote*                                     
Solubility:             Miscible in all proportions at                    
                        room temperatures with ace-                       
                        tone, benzene, naphtha, and                       
                        xylene.                                           
Vapor Pressure:                                                           
               Vapor Pressure Temp.                                       
               1.5 mm         102° C.                              
               22 mm          164° C.                              
               50 mm          181° C.                              
               95 mm          196° C.                              
               470 mm         243° C.                              
__________________________________________________________________________
 *NADIC Methyl Anhydride has no definite freezing point. The only effect o
 decrease in temperature is that it becomes more viscous. No special      
 handling or storage is needed in cold weather.                           
 
    
     
                       TABLE 3                                                     
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HEXAHYDRO- PHTHALIC ANHYDRIDE (HHPA)                                      
             ##STR5##                                                     
(cis-1,2-Cyclohexanedicarboxylic Anhydride                                
PHYSICAL    Appearance: A glassy solid,                                   
PROPERTIES  which on melting gives a clear,                               
            colorless viscous liquid.                                     
            Molecular Weight: 154.1                                       
            Solidification Point (as is), °C.: 35-36               
            Boiling Point, °C., 16.2 mm. abs.: 160.6               
            Density, 40° C., g./ml.: 1.18                          
            Solubility: Miscible with benzene,                            
            toluene, acetone, carbon                                      
            tetrachloride, chloroform, ethanol and                        
            ethyl acetate.                                                
            Only slightly soluble in petroleum ether.                     
            Infrared Curve: See FIG. 1, pp. 4-5.                          
STRENGTH    Total acidity as hexahydrophthalic                            
            anhydride, 99% minimum.                                       
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                                           TABLE 4                                 
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DIMER ACIDS                                                               
Hystrene                                                                  
Humko Sheffield&#39;s developing technology brings to                         
market a range of Hystrene dimer acids to cover a                         
variety of applications. There is, of course, the                         
standard tall oil derived series. In addition, a series                   
of dimer acids from other fatty acid sources offers a                     
wide range of use. In many cases these new products                       
(the X and S types) can be substituted for the tall oil                   
dimers with little or no reformulation. Dimer acids                       
impart flexibility into polymeric systems which has led                   
to their use in polyesters, polyamides, polyurethanes,                    
polyureas and epoxy systems. Dimer acids and their                        
derivatives have found a myriad of end uses in such                       
applications as corrosion inhibitors, metal-working                       
lubricants, adhesives, inks and surface coatings.                         
 ##STR6##                                                                 
         Specification                                                    
                   Color             Typical                              
         Acid Sap  Gardner                                                
                         Neutral                                          
                               Monomer                                    
                                     Viscosity   Composition              
Product  Value                                                            
              Value                                                       
                   (1963)                                                 
                          Equivalent                                      
                               Acid  at 25° C. (cSt)               
                                             Unsap                        
                                                 Monomer                  
                                                       Dimer              
                                                           Trimer         
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Hystrene 3695                                                             
95% Dimer Acid                                                            
         194-198                                                          
              198-202                                                     
                   5 Max 283-289                                          
                               1.5 Max                                    
                                     6,800   0.5 1     95  4              
Hystrene 3695S                                                            
95% Dimer Acid                                                            
         197-202                                                          
              198-203                                                     
                   7 Max 278-285                                          
                               1.5 Max                                    
                                     11,000  1.0 1     95  4              
Hystrene 3695X                                                            
95% Dimer Acid                                                            
         195-199                                                          
              196-200                                                     
                   7 Max 282-288                                          
                               1.5 Max                                    
                                     7,200   1.0 1     95  4              
Hystrene 3680                                                             
80% Dimer Acid                                                            
         190-197                                                          
              191-199                                                     
                   8 Max 285-295                                          
                               1 Max 8,000   1.0 Tr    83  17             
Hystrene 3680S                                                            
80% Dimer Acid                                                            
         194-201                                                          
              196-203                                                     
                   8 Max 279-289                                          
                               1,5 Max                                    
                                     14,000  1.0 1     84  15             
Hystrene 3680X                                                            
80% Dimer Acid                                                            
         194-201                                                          
              196-203                                                     
                   8 Max 279-289                                          
                               1.5 Max                                    
                                     8,300   1.0 1     85  14             
Hystrene 3675                                                             
75% Dimer Acid                                                            
         189-197                                                          
              191-199                                                     
                   9 Max 285-297                                          
                               1 Max 9,000   1.0 Tr    75  25             
Hystrene 3675X                                                            
75% Dimer Acid                                                            
         192-200                                                          
              193-201                                                     
                   9 max 281-292                                          
                               1 Max 9,300   1.0 1     87  12             
Hystrene 3675C                                                            
75% Dimer Acid                                                            
3% Monomer                                                                
         189-197                                                          
              191-199                                                     
                   9 Max 285-297                                          
                               3-4 Max                                    
                                     7,500   1.0 3     75  22             
Hystrene 3675CS                                                           
75% Dimer Acid                                                            
3% Monomer                                                                
         194-201                                                          
              196-203                                                     
                   8 Max 279-289                                          
                               4 Max 12,000  1.0 3     85  12             
Hystrene 3675CX                                                           
75% Dimer Acid                                                            
3% Monomer                                                                
         192-200                                                          
              193-201                                                     
                   9 Max 281-292                                          
                               4 Max 8,000   1.0 3     86  11             
Hystrene 5460                                                             
Trimer Acid                                                               
         182-190                                                          
              190-198    295-308                                          
                               Tr    30,000  1.0 Tr    40  60             
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DIMER AMINES                                                              
Kemamines                                                                 
The dimer derivatives represent a marriage of Humko                       
Sheffield dimer technology and fatty nitrogen chemistry.                  
These high-molecular-weight fatty nitrogen chemicals                      
have found use as corrosion inhibitors for petroleum-                     
processing equipment and as intermediates, extenders                      
and cross-linking agents in high-polymer systems. -                       
 ##STR7##                                                                 
                                               Color %                    
                                 Amine Value, Min                         
                                               Gardner                    
                                                     Water                
Product   Description            Primary                                  
                                     Secondary                            
                                           Total                          
                                               Max (1963)                 
                                                     Max                  
__________________________________________________________________________
Kemamine DP-3680                                                          
          Dimer Diprimary Amine (3680)                                    
                                 105       175 14    1.0                  
Kemamine DC-3680                                                          
          Dicyanoethylated Dimer Diprimary Amine (3680)                   
                                     135   140 14    1.0                  
Kemamine DD-3680                                                          
          Di-N-Aminopropyl Diprimary Amine (3680)                         
                                 135 135   280 14    1.0                  
Kemamine DP-3695                                                          
          Dimer Diprimary Amine (3695)                                    
                                 175       185 14    1.0                  
Kemamine DC-3695                                                          
          Dicyanoethylated Dimer Diprimary Amine (3695)                   
                                     135   140 14    1.0                  
Kemamine DD-3695                                                          
          Di-N-Aminopropyl Diprimary Amine (3695)                         
                                 135 135   280 14    1.0                  
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                       TABLE 5                                                     
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Glass Microballoon Data                                                   
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No. 1G25 Eccospheres                                                      
Emerson &amp; Cumings, Inc.                                                   
Canton, Massachusetts                                                     
Gardena, California                                                       
Bulk density, lb/ft.sup.3    9.0                                          
g/cc                         0.145                                        
True particle density                                                     
lb/ft.sup.3                  14.8                                         
g/cc                         0.237                                        
Particle size, (mu) %                                                     
______________________________________                                    
&gt;175                0                                                     
149-175             6                                                     
125-149             6                                                     
100-125             13                                                    
 62-100             42                                                    
 44-62              12                                                    
&lt; 44                21                                                    
Packing factor      0.614                                                 
Average wall thickness, (mu)                                              
                    1.5                                                   
Softening temp °C.                                                 
                    482                                                   
Strength-hydrostatic pressure                                             
                    44                                                    
[volume % survivors at 1500 psi (110kg/cm.sup.2)]                         
______________________________________                                    
 Note                                                                     
 ##STR8##                                                                 
 -                                                                        
 
    
     A sabot must meet two requirements to be useful. First, it must have enough compressive strength to withstand the pressure exerted on it by the expanding weapon gases. And, second, it must have properties which cause it to spontaneously combust due to all the interactions it undergoes when it is fired from a weapon. It has been found that a sabot fabricated from the above enunciated ingredients meet these requirements. 
     The preferred binders are (a) ERL-4289/HHPA type and (b) sixty five percent Dimer Acid blend with ERL-4221 and HHPA. 
     The preferred binders, compared to an ERL-4221/NMA system, yield the following improvements: 
     (a) HHPA-increases strength and heat resistance to deformation; 
     (b) ERL-4289-increases elongation of propellent; 
     (c) Dimer Acid-increases elongation of propellent; 
     (d) BD-required to provide hydroxyls for systems without acid; 
     (e) Sn (Octoate) 2  -catalyst for all systems, and 
     (f) solvent of ethylacetate or butyl acetate 
     The preferred curing conditions occur at 65° C. for 24 hours and 120° C. for 48 hours. The blending of basic binder formulations are made to adjust strength of propellants, as desired, for sabot diameter and lengths. 
     EXAMPLES 
     
         ______________________________________                                    
 Preferred Propellent No. 1                                               
______________________________________                                    
4289/HHPA Type                                                            
ERL-4289                     19.10                                        
HHPA                         6.70                                         
BD                           0.24                                         
Sn(Oct).sub.2                0.20                                         
Boron (amorphous)            3.54                                         
                     Blend                                                
MoO.sub.3                    20.06                                        
AP (90mu)                    36.50                                        
Al (5mu) or Mg               13.66                                        
Compressive strength, psl    11,391                                       
Elongation at maximum strength, %                                         
                             12                                           
Elongation at break, %       15                                           
Compressive modulus, psl     849,200                                      
 Preferred Propellent No. 2                                               
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                 65% Dimer Acid formulation                               
Blend                                                                     
                 35% HHPA                                                 
ERL-4221                     12.91                                        
HHPA                         3.28                                         
Dimer Acid                   8.53                                         
BD                           0.12                                         
Sn(Oct).sub.2                0.16                                         
Boron (amorphous)            2.25                                         
                     Blend                                                
MoO.sub.3                    12.75                                        
AP (90mu)                    30.00                                        
Al (5mu) or Mg               30.00                                        
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     Tubes observed after experimental firings using the composition of this invention were found to be clean in comparison with other tubes utilizing ERL-4221/NMA system. 
     It is understood that the invention is not limited to the specific embodiments thereof except as set forth in appended claims, as many variations within the spirit and scope of the invention will occur to those skilled in the art.