Patent Publication Number: US-9410779-B1

Title: Breakaway fin ring for projectile

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The inventions described herein may be manufactured, used and licensed by or for the United States Government. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates in general to gun-launched projectiles and in particular to stabilizing fins for gun-launched projectiles. 
     Some gun-launched projectiles are stabilized in flight by fins. Fins come in many designs. In some designs, a fin ring is attached to the projectile body and the fins are attached to the fin ring with pins and springs. Known fin rings survive gun launch, projectile flight and projectile impact with a target. 
     Projectiles must have large amounts of kinetic energy to penetrate targets such as triple brick walls and double reinforced concrete. The fin rings and fins on a projectile consume a portion of the available target-penetrating kinetic energy in the process of breaking the fin ring and/or fins during target penetration by the projectile. The fin ring may also have a step up in diameter compared to the warhead body, which impedes penetration. In addition, the fins that are attached to the fin ring may have a hooking effect when they contact the target surface. Projectiles with a relatively low velocity, such as those fired from multi-target shoulder fired weapons, have less kinetic energy than higher velocity projectiles. The consumption of kinetic energy by the fin ring and fins on a low kinetic energy projectile can inhibit the complete penetration of a target. 
     A need exists for a fin ring that survives gun tube launch and projectile flight but fails upon target impact to enable a low velocity projectile to penetrate a target with less resistance. 
     SUMMARY OF INVENTION 
     One aspect of the invention is a fin ring for a gun-launched projectile. The fin ring includes a circular base centered on a central longitudinal axis of the fin ring. A plurality of trunnions are equally circumferentially spaced around the circular base. Each trunnion extends radially outward from the circular base and includes fin mounting holes therein. A plurality of axially extending rear notches are formed in the circular base. The rear notches begin at a rear perimeter of the circular base and extend axially forward. A plurality of axially extending front notches are formed in the circular base. The front notches begin at a front perimeter of the circular base and extend axially rearward. A plurality of circular openings are formed in the circular base. 
     The fin ring may include a plurality of fins. Each fin may be mounted to a respective one of the plurality of trunnions. 
     The plurality of rear notches may be circumferentially equally spaced around the circular base. The plurality of front notches may be circumferentially equally spaced around the circular base. The plurality of circular openings may be circumferentially equally spaced around the circular base. 
     Each rear notch may be axially aligned with a front notch. One of the plurality of circular openings may be disposed between each rear notch and front notch. The plurality of rear notches may be circumferentially equally spaced between the plurality of trunnions. 
     Another aspect of the invention is a projectile having a novel fin ring. 
     In another aspect, the invention includes a method. The method includes launching a projectile having a novel fin ring. Upon impact of the projectile with a target, the fin ring is broken into a plurality of pieces. 
     The invention will be better understood, and further objects, features and advantages of the invention will become more apparent from the following description, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, which are not necessarily to scale, like or corresponding parts are denoted by like or corresponding reference numerals. 
         FIG. 1  is a schematic of a projectile. 
         FIG. 2  is a perspective view of one embodiment of a fin ring. 
         FIG. 3  is a front end view of the fin ring of  FIG. 2 . 
         FIG. 4  is a sectional view taken along the line  4 - 4  of  FIG. 3 . 
         FIG. 5  is a rear end view of the fin ring of  FIG. 2 . 
         FIG. 6  is an enlarged view of a portion of  FIG. 4 . 
         FIG. 7  is a perspective view of a fin. 
     
    
    
     DETAILED DESCRIPTION 
     A novel fin ring with fins is fixed to a projectile. When the projectile impacts a target, the fin ring breaks up. When the fin rings breaks up, it snaps off the projectile body and eliminates the stepped up diameter transition that hampers penetration of the projectile into the target. Further, the grappling hook effect of the fins is eliminated because the fins are discarded when the fin ring breaks off the projectile. 
       FIG. 1  is a schematic of a gun-launched projectile  10  having a front end  12  and a rear end  14 . A fin ring having deployable fins may be fixed to the rear end  14  of projectile  10  to stabilize its flight. The caliber of projectile  10  may vary, for example, from about 40 mm to 100 mm or larger. When fitted with the novel fin ring disclosed herein, projectile  10  may be launched from a variety of gun tubes and weapons. For example, projectile  10  may be launched from a single use, shoulder-fired munition. The novel fin ring is advantageous for use with low velocity projectiles because the fin ring will break apart upon impact of its carrier projectile with a target. The breaking apart of the fin ring eliminates the loss of kinetic energy associated with interaction of the fin ring and fins with the target. 
       FIGS. 2-7  show one embodiment of a fin ring  16  for a gun-launched projectile. Ring  16  may be made of, for example, an aluminum alloy. Ring  16  has a central longitudinal axis A and a circular base  18  centered on axis A. A plurality of trunnions  20  are equally circumferentially spaced around the circular base  18 . Each trunnion  20  extends radially outward from the circular base  18  and includes fin mounting holes  22  therein. A fin  30  ( FIG. 7 ) is mounted to each trunnion  20 . The number of fins  30  may vary, with an odd number of fins preferred. 
     A plurality of axially extending rear notches  24  are formed in the circular base  18 . The rear notches  24  begin at a rear perimeter of the circular base  18  and extend axially forward. A plurality of axially extending front notches  26  are formed in the circular base  18 . The front notches  26  begin at a front perimeter of the circular base  18  and extending axially rearward. A plurality of circular openings  28  are also formed in the circular base  18 . The rear perimeter of the circular base  18  may include a chamfer  32 . 
     The plurality of rear notches  24  may be circumferentially equally spaced around the circular base  18 . The plurality of front notches  26  may be circumferentially equally spaced around the circular base  18 . The plurality of circular openings  28  may be circumferentially equally spaced around the circular base  18 . Each rear notch  24  may be axially aligned with a front notch  26 . Each of the plurality of circular openings  28  may be disposed between a rear notch  24  and a front notch  26 . Preferably, the number of rear notches  24 , the number of front notches  26  and the number of openings  28  equal the number of trunnions  20  or fins  30 . The rear notches  24  may be circumferentially equally spaced between the plurality of trunnions  20 . 
     In one embodiment, the axial length B ( FIG. 6 ) of each rear notch  24  is more than twice the axial length C of each front notch  26 . In some embodiments, the combined axial length (B plus C plus E) of a rear notch  24  a circular opening  28  and a front notch  26  is more than one half of the axial width D of the circular base  18 . 
     To mount fin ring  16  to the rear of projectile  10 , pins (not shown) may be inserted through circular openings  28  in base  18  and press fit into openings (not shown) in projectile  10 . In addition to being part of the mounting assembly, openings  28  function as stress risers that facilitate the breakup of ring  16  along the longitudinal axis of aligned notches  24 ,  26 . 
     Upon impact of projectile  10  with a target, notches  24 ,  26  create a moment in the target impact direction. The larger rear notch  24  allows more deformation of material. The moment of inertia of the trunnion  20 , fin  30  and fin mounting spring (not shown) creates forces to break the ring  16 . The fin ring  16  conserves kinetic energy because the impact of fins  30  is not used the break the fin ring  16 . In addition, breakup of the fin ring  16  removes the stepped up diameter of the fin ring, which results in a smoother, more continuous projectile surface for target penetration. The fin ring  16  remains robust and structurally sound during gun launch and fin opening. 
     During testing, the fin ring  16  was intact after gun launch and remained together during fin deployment and projectile flight. After target impact, fin ring  16  broke into a plurality of pieces at respective aligned pairs of notches  24 ,  26 . 
     While the invention has been described with reference to certain embodiments, numerous changes, alterations and modifications to the described embodiments are possible without departing from the spirit and scope of the invention as defined in the appended claims, and equivalents thereof