Abstract:
This invention relates to a combustible sabot and process for its  preparan for a spinning tubular projectile. This combustible sabot is prepared in such a way and of such materials that it combusts spontaneously while exiting the gun barrel. The sabot is fabricated from an anhydride cured epoxy binder, boron, molybdenum trioxide, ammonium perchlorate and a metallic fuel selected from either aluminum or magnesium in the presence of a catalyst.

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__________________________________________________________________________Typical Properties and Applications       ERL-4221       ERL-4289                ERR-4205__________________________________________________________________________       3,4-Epoxycyclohexylmethyl-Chemical Name       3,4-Epoxycyclohexane                      bis(3,4-Epoxy-6-methylcyclohexyl-                                              bis(2,3-Epoxy-       carboxylate    methyl adipate          cyclopentyl)etherStructural 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                                              hardenersViscosity, cps.       350 to 450 (25° C.)                      500 to 1,000 (25° C.)                                              &lt; 100 (45° C.)Apparent SpecificGravity at 25°/25° C.       1.175          1.124                   1.16 to 1.18Color 1933 Gardner       1              1                       2maximumEpoxyEquivalent Weight,grams/gram mol       131 to 143     205 to 216              91 to 102oxirane oxygenBoiling Point at760 mm. Hg. °C.       354            258 (10 mm.)            --Vapor Pressureat 20° C., mm. Hg       &lt;0.1           &lt;0.1                    --Freezing Point °C..sup.(a)       -20            9                       38 to 42Solubility,% by wt. at 25° C.       0.03           0.01                    --In WaterWater In    2.8            1.8                     --__________________________________________________________________________ .sup.(a) Sets to glass below this temperature 
    
     
                                           TABLE 2__________________________________________________________________________NADIC® METHYL ANHYDRIDE (NMA)__________________________________________________________________________(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 yellowMolecular Weight        178.2Neutralization Equivalent                   89.1Viscosity, 25° C., cps.                   175-225Refractive Index, n.sub.D.sup.20                   1.500-1.506Specific Gravity, d.sub.20.sup.20                   1.200-1.250Flash Point (open cup), °C.                   140Distillation Range, °C., 10mm. Hg                   135-143Solidification 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______________________________________HEXAHYDRO- PHTHALIC ANHYDRIDE (HHPA)        ##STR5##(cis-1,2-Cyclohexanedicarboxylic AnhydridePHYSICAL    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.______________________________________ 
    
     
                                           TABLE 4__________________________________________________________________________DIMER ACIDSHystreneHumko Sheffield&#39;s developing technology brings tomarket a range of Hystrene dimer acids to cover avariety of applications. There is, of course, thestandard tall oil derived series. In addition, a seriesof dimer acids from other fatty acid sources offers awide range of use. In many cases these new products(the X and S types) can be substituted for the tall oildimers with little or no reformulation. Dimer acidsimpart flexibility into polymeric systems which has ledto their use in polyesters, polyamides, polyurethanes,polyureas and epoxy systems. Dimer acids and theirderivatives have found a myriad of end uses in suchapplications as corrosion inhibitors, metal-workinglubricants, adhesives, inks and surface coatings. ##STR6##    Specification              Color             Typical    Acid Sap  Gardner                    Neutral                          Monomer                                Viscosity   CompositionProduct  Value         Value              (1963)                     Equivalent                          Acid  at 25° C. (cSt)                                        Unsap                                            Monomer                                                  Dimer                                                      Trimer__________________________________________________________________________Hystrene 369595% Dimer Acid    194-198         198-202              5 Max 283-289                          1.5 Max                                6,800   0.5 1     95  4Hystrene 3695S95% Dimer Acid    197-202         198-203              7 Max 278-285                          1.5 Max                                11,000  1.0 1     95  4Hystrene 3695X95% Dimer Acid    195-199         196-200              7 Max 282-288                          1.5 Max                                7,200   1.0 1     95  4Hystrene 368080% Dimer Acid    190-197         191-199              8 Max 285-295                          1 Max 8,000   1.0 Tr    83  17Hystrene 3680S80% Dimer Acid    194-201         196-203              8 Max 279-289                          1,5 Max                                14,000  1.0 1     84  15Hystrene 3680X80% Dimer Acid    194-201         196-203              8 Max 279-289                          1.5 Max                                8,300   1.0 1     85  14Hystrene 367575% Dimer Acid    189-197         191-199              9 Max 285-297                          1 Max 9,000   1.0 Tr    75  25Hystrene 3675X75% Dimer Acid    192-200         193-201              9 max 281-292                          1 Max 9,300   1.0 1     87  12Hystrene 3675C75% Dimer Acid3% Monomer    189-197         191-199              9 Max 285-297                          3-4 Max                                7,500   1.0 3     75  22Hystrene 3675CS75% Dimer Acid3% Monomer    194-201         196-203              8 Max 279-289                          4 Max 12,000  1.0 3     85  12Hystrene 3675CX75% Dimer Acid3% Monomer    192-200         193-201              9 Max 281-292                          4 Max 8,000   1.0 3     86  11Hystrene 5460Trimer Acid    182-190         190-198    295-308                          Tr    30,000  1.0 Tr    40  60__________________________________________________________________________ 
    
     
         __________________________________________________________________________DIMER AMINESKemaminesThe dimer derivatives represent a marriage of HumkoSheffield dimer technology and fatty nitrogen chemistry.These high-molecular-weight fatty nitrogen chemicalshave found use as corrosion inhibitors for petroleum-processing equipment and as intermediates, extendersand cross-linking agents in high-polymer systems. - ##STR7##                                          Color %                            Amine Value, Min                                          Gardner                                                WaterProduct   Description            Primary                                Secondary                                      Total                                          Max (1963)                                                Max__________________________________________________________________________Kemamine DP-3680     Dimer Diprimary Amine (3680)                            105       175 14    1.0Kemamine DC-3680     Dicyanoethylated Dimer Diprimary Amine (3680)                                135   140 14    1.0Kemamine DD-3680     Di-N-Aminopropyl Diprimary Amine (3680)                            135 135   280 14    1.0Kemamine DP-3695     Dimer Diprimary Amine (3695)                            175       185 14    1.0Kemamine DC-3695     Dicyanoethylated Dimer Diprimary Amine (3695)                                135   140 14    1.0Kemamine DD-3695     Di-N-Aminopropyl Diprimary Amine (3695)                            135 135   280 14    1.0__________________________________________________________________________ 
    
     
                       TABLE 5______________________________________Glass Microballoon Data______________________________________No. 1G25 EccospheresEmerson &amp; Cumings, Inc.Canton, MassachusettsGardena, CaliforniaBulk density, lb/ft.sup.3    9.0g/cc                         0.145True particle densitylb/ft.sup.3                  14.8g/cc                         0.237Particle size, (mu) %______________________________________&gt;175                0149-175             6125-149             6100-125             13 62-100             42 44-62              12&lt; 44                21Packing factor      0.614Average wall thickness, (mu)               1.5Softening temp °C.               482Strength-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 TypeERL-4289                     19.10HHPA                         6.70BD                           0.24Sn(Oct).sub.2                0.20Boron (amorphous)            3.54                BlendMoO.sub.3                    20.06AP (90mu)                    36.50Al (5mu) or Mg               13.66Compressive strength, psl    11,391Elongation at maximum strength, %                        12Elongation at break, %       15Compressive modulus, psl     849,200 Preferred Propellent No. 2______________________________________            65% Dimer Acid formulationBlend            35% HHPAERL-4221                     12.91HHPA                         3.28Dimer Acid                   8.53BD                           0.12Sn(Oct).sub.2                0.16Boron (amorphous)            2.25                BlendMoO.sub.3                    12.75AP (90mu)                    30.00Al (5mu) or Mg               30.00______________________________________ 
    
     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.