Abstract:
A pressure release mechanism for a sonic rarefaction wave-type low-recoil gun system employing controlled plastic venting that exhibits adiabatic shear banding to effect a delayed pressure release.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This invention claims the benefit of U.S. Provisional Application No. 60/889,562 filed 13-Feb. 2007 the entire file wrapper of which is incorporated by reference as if set forth at length herein. 
    
    
     UNITED STATES GOVERNMENT INTEREST 
     The inventions described herein may be manufactured, used and licensed by the United States Government for United States Government purposes. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the field of low recoil gun systems employing a delayed pressure release mechanism. More specifically, it pertains to a delayed pressure release mechanism exhibiting a controlled plastically deforming, adiabatic shear failure mechanism. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 6,460,446—which is incorporated herein by reference as if set forth at length—is directed to a Sonic Rarefaction Wave Recoilless Gun System and issued on 8-Oct.-2002 to Eric L. Kathe and assigned to the assignee of the instant application. That patent describes a low-recoil and low-bore heat gun system that employs a delayed pressure release mechanism for fired propellant charges in the rear of a gun barrel. According to the patent, a delayed pressure release of exhaust gases causes a sonic rarefaction along the length of the gun barrel bore to arrive at an exit end of the gun barrel at a predetermined time, generally coincident with the fired projectile. As a result, such a gun system exhibits lower recoil without an appreciable loss of projectile velocity. 
     As can be readily appreciated, the delayed pressure release mechanism is a critical component of the Sonic Rarefaction Wave Recoilless Gun System, and essential to its operation. As described in the Sonic Rarefaction Wave Recoilless Gun System patent, the delayed pressure release mechanism comprises a physically heavy, bulky, and mechanically complex inertial breech. 
     SUMMARY OF THE INVENTION 
     An advance is made in the art according to the principles of the present invention directed to a pressure release mechanism for a sonic rarefaction wave-type low-recoil gun system employing controlled plastic deformation venting that exhibits adiabatic shear banding phenomena to effect the delayed pressure release. 
     In sharp contrast to the prior art, a pressure release mechanism according to the present invention uses a controlled plastic deformation based venting exhibiting adiabatic shear banding that advantageously does not employ any inertial breech mechanisms having moving parts, thereby saving space, weight, and complexity. 
     Instead, in an exemplary embodiment, a pressure release mechanism according to the present invention allows the rear face of a round itself to vent in a controlled and deliberate manner. Advantageously, since the pressure release mechanism is a combination of structural round design and material selection—coupled with a high energy material failure mechanism—each round can be designed for any particular chamber geometry and peak pressure. Since the mechanically complex and heavy inertial breech mechanism is eliminated—a low recoil gun system employing the present invention may advantageously be fired from a shoulder or other lightweight emplacement. 
     Additionally, a pressure release mechanism according to the present invention—which employs adiabatic shear banding plasticity to effect the venting—is scalable to any caliber gun system and adaptable to any given design criteria including caliber, chamber pressure, chamber profile, and burn profile(s) of a given propellant. 
     Finally, a pressure release mechanism according to the present invention vents after the system reaches a peak pressure, and therefore allows the use of a variety of materials for its implementation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Further features and advantages of the present invention will be understood with reference to the drawing in which: 
         FIG. 1  shows in schematic form a representative cut-away view of a sonic rarefaction wave-type low recoil gun system according to the present invention; 
         FIG. 2(A)  . . .  FIG. 2(E)  show in schematic form a sequence of events within a Sonic Rarefaction Wave-Type Recoilless Gun System according to the present invention; 
         FIG. 3  shows a number of views of a sever disk according to the present invention in which:  FIG. 3(A)  is a top view;  FIG. 3(B)  is a right side cutaway view;  FIG. 3(C)  is a left side cutaway view;  FIG. 3(D)  is a perspective view from the front (top of sever disk); and  FIG. 3(E)  is a perspective view from the rear (bottom of the sever disk); and 
         FIG. 4  shows the mitigation of rotational momentum of a low recoil gun system according to the present invention in which:  FIG. 4(A)  is a side view of a gun system and  FIG. 4(B)  (inset) shows side vents for pressure release which provide rotational mitigation. 
     
    
    
     DETAILED DESCRIPTION 
     The following merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. 
     Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. 
     Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. 
       FIG. 1  shows a cutaway view of an exemplary low recoil gun system  100  according to an aspect of the present invention. The gun system  100  includes a barrel  110  positioned within a support cradle  120  which may be part of a turret, carriage, shoulder fired arrangement, or fixed emplacement. As shown, barrel  110  may be movably positioned within the support cradle  120  and utilize dampers or recoil cylinders  122  to dampen the movement of the barrel  110  relative to the support cradle  120  due to the recoil after a firing. 
     For the purposes of this discussion, the barrel  110  may be viewed as having a forward end  112  which terminates at an open muzzle  116  and a rear, or breech end  114 . Shown within the breech end  114  is an integrated breech vent nozzle  140  through which internal pressure(s) will be vented during operation. 
     Shown positioned within the barrel  110  is a self venting round  150  generally comprising a cylindrical casing  156  with a projectile  152  inserted into one end of the casing  156  and a sever disk  158  inserted or otherwise positioned at another end of the casing  156  so as to form a closed cylinder (round). Loaded into the casing is a propellant charge  154 . 
     As should be apparent to those skilled in the art, when the round  150  is discharged, the rapidly burning propellant will generate a quantity of expanding gases which in turn propel the projectile  152  along the barrel  110  towards its forward end  112  until it exits the open muzzle  116 . 
     As the propellant  154  burns resulting in the translation of the projectile  152  along the central axis, or bore of the barrel  110 , the pressure resulting from the burning propellant  154  builds. In a conventional closed breech gun system the pressure will reach a maximum and is then reduced to atmospheric pressure upon projectile exiting from the muzzle  116 . 
     The maximum rarefaction wave phenomena recoil mitigation results when the breech of the gun system is vented approximately 10% after peak pressure and the resulting rarefaction wave meets the projectile at muzzle exit  116 . Consequently, maximum venting time past peak pressure and associated recoil mitigation are realized while not diminishing the kinetic energy of the projectile. 
     According to the present invention, the sever disk  158  will undergo an adiabatic shear band plastic deformation which will vent the gun system at an appropriate time. Significantly—and according to an aspect of the present invention—the adiabatic shear band produced in the sever disk  158  will survive the peak pressure of the chamber and fail after peak pressure thereby supporting a proper rarefaction delay. 
     Turning now to  FIG. 2(A)-FIG .  2 (E), there is shown a series of diagrams which generally depict the operation of the present invention. With simultaneous reference to those figures, it may be seen in  FIG. 2(A)  that burning propellant produces an expanding gas  158  which—as in a closed breech firing—causes a chamber pressure within the barrel  110  to build behind the projectile  152 . 
     Due to the expanding gas  156  the projectile  152  is translated along the bore of the barrel  110  towards the muzzle  116  ( FIG. 2(B) ). As a result of the internal pressure and heat generated, and strain rate, the sever disk  158  begins to initiate controlled complex plastic deformation due to the generated heat which cannot diffuse. The sever disk begins to undergo controlled dynamic plastic deformation as the pressure increases. 
     At maximum pressure (FIG.  2 (C)), the projectile  152  has translated along the bore of the barrel  110 . Although the pressure is still contained, adiabatic shear band is initiated within the sever disk  158 . 
     In  FIG. 2(D) , peak pressure has already occurred and the projectile  152  has translated along a substantial length of the bore of the barrel  110 . As the internal pressure continues dropping, the controlled adiabatic shear band propagates within sever disk  158  allowing venting after the peak pressure is reached within barrel. While not specifically shown in great, detail in this  FIG. 2(D) , a rarefaction wave  159  is created upon venting. The rarefaction wave  159 , moving faster than the propellant gases propelling the projectile  152 , “chases” the projectile  152  down the bore of the barrel  110 . 
     As the projectile reaches the muzzle  116  of the barrel  110 , shear band induced catastrophic load bearing capacity collapse of the sever disk  158  permits complete venting of gases  156  which are shown in  FIG. 2(E)  as venting through integral breech nozzle. As can be now appreciated, the rarefaction wave  159  has met the projectile  152  at muzzle exit  116 , which maximizes the rarefaction wave recoil mitigation benefits, and the recoil of the gun system is appreciably reduced. 
       FIG. 3(A)-FIG .  3 (E) show a number of views of a representative sever disk  300 .  FIG. 3(A)  shows a top view of the sever disk and its annular, v-shaped groove  310  cut into its bottom surface.  FIG. 3(B)  and  FIG. 3(C)  are right and left side cut-away views of the sever disk  300  showing the v-shaped groove  310  and trapezoidal shaped annular “dovetail”  320  cut into its top surface. This “6.5/3.5/2.5” geometry shown in  FIG. 3  was determined to be preferable and exhibits a 6.5 mm notch to notch thickness, a 3.5 mm lower (bottom) edge radius, a notch to notch axial offset of 2.5 mm. Note that these dimensions are particular to an AL6061-T651 material for a 30 mm low-recoil gun system (cannon). 
     More particularly, the particular sever disk in  FIG. 3  was purchased according to ASTM B221 standard for aerospace grade 6061-T651 aluminum. The material was Aluminum extruded rod 3″ diameter stock. As received, the material exhibited a Yield stress property of 42.5K psi and an ultimate tensile strength property of 48.3 Kpsi. A standard ASTM tensile strength specimen was created to verify these. As those skilled in the art may already be aware, there are minimum strength values for such material certifications, but no specified upper limits. Accordingly, a tensile test and a shear test were undertaken which showed that the material purchased was above the designed value of 35,000 psi for yield stress (and the corresponding shear strength). The materials were annealed to a value of 35,000 psi by a custom annealing process comprising submitting in a HOMO #3 Furnace at 175 C. (374 F.) for approximately 100 hours. 
     Dimensionally, the exemplary sever disk  300  shown in  FIG. 3  may be understood by the following relationships. For a disk substantially 15.2 mm thick  301 , if it is given that the notch to notch thickness  330  as parameter A (6.5 mm), then the lower curved radius  332  must be approximately 0.54 A. The bottom notch depth must be approximately 0.83 A (about ⅞&#39;s A). The offset of the top notch to the bottom notch  333  (radial offset) is approximately 0.385 A (about ⅜&#39;s A). 
     As already noted, such a sever disk  300  will advantageously vent a gun system after peak pressure has been reached in the barrel. That is to say, peak pressure is reached, it begins to fall and then—sometime after peak pressure begins to fall—the sever disk will plastically fail due to adiabatic shear banding thereby venting the pressurized barrel to the atmosphere. When performed in this manner, the recoil is advantageously reduced 
     Of further advantage, the present invention may be used to arrest or otherwise mitigate rotational momentum associated with the firing of a gun system. With reference now to  FIG. 4 , there is shown a cross section of a gun system  4 (A) and a breech end view  FIG. 4(B)  of that same gun system. As can be appreciated, for a rifled gun system, the spinning projectile imparts a substantial rotational momentum to the gun barrel upon firing. To counteract this rotational momentum, a series of directed “jets” or nozzles  140  are disposed around the perimeter of the breech. Upon firing, the sever disk will vent some of the pressure within the barrel through the nozzles  140 . When the nozzles are angled sufficiently, the gases vented through the nozzles will itself impart a rotational momentum to the barrel—which may advantageously be directed to counteract that momentum imparted by the spinning projectile. The number, shape and size of the nozzles  140  may advantageously be varied such that a desirable amount of counteraction is imparted. 
     At this point, while we have discussed and described the invention using some specific examples, those skilled in the art will recognize that our teachings are not so limited. For example, the preferred embodiments of the invention have been provided for the purpose of explaining the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention. Various embodiments and various modifications are contemplated. Accordingly, the invention should be only limited by the scope of the claims attached hereto.