Patent Abstract:
A flechette has a forward body ( 20 ) containing its center of gravity which is connected to a tail section ( 24 ). The tail section has a pair of fins ( 24 A,  24 B) each having a preselected longitudinal angle and radial angle. When the two fins are viewed from the aft of the flechette, the pair of fins demonstrate a S-shaped orientation. The size, shape and orientation of the pair of fins provide aerodynamic stability to the flechette while allowing the flechette to be stacked with like-shaped flechettes. The two-piece assembly of the flechette easily accommodates the use of different density materials for the respective pieces.

Full Description:
This is a continuation-in-part of U.S. Pat. No. 8,375,860 filed on May 4, 2011 and which is incorporated by reference herein. 
    
    
     DEDICATORY CLAUSE 
     The invention described herein may be manufactured, used and licensed by or for the U.S. Government for U.S. Government purposes without payment of any royalties thereon. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention pertains to flechettes or dart-like projectiles. 
     2. Discussion of the Background 
     Conventional flechettes in the 60 grain to 150 grain weight class have been used successfully in weapons but suffer from two drawbacks. The first drawback is that their flight characteristics are suboptimal. High speed film of their flight shows that most of the flechettes dispensed from a warhead pitch and yaw significantly during their flight. 
     It is understood that the pitch and yaw behavior, which slows the flechettes and reduces their lethality, is due to a combination of transverse angular rates induced at dispense, aerodynamic or physical interactions between flechettes in the dispensed population, and manufacturing imperfections in the flechettes themselves. 
     As a result of these effects, flechette patterns are typically extremely elongated along the axis tangent to the flight path, with a significant time lag between the arrival at the target of the first flechettes, (which have the highest velocity and are the most lethal), and the last arriving, slower flechettes (which are the least lethal). The elongated patterns indicate that conventional flechettes lose significant portions of their velocity and lethality attempting to recover a nose-first orientation after experiencing high transverse angular rate perturbations. 
     The second drawback with the conventional flechette design is that packing constraints limit the size of the flechette tailfins to a size smaller than would be ideal to optimize their flight stability. (Flechettes having four tailfins are the conventional design). If the tailfins are made larger for better flight performance, the flechettes do not pack well. If they are made smaller for better packaging, the flechettes lose even more terminal performance due to increased angular rate oscillations. 
     SUMMARY OF THE INVENTION 
     The flechette of the present invention has its concentration of mass centered in a forward section for stability with a center of pressure being located proximate to the root of the tail. In the tail section of the flechette, two tailfins are arranged in a flattened out “Z” or S-shaped formation when viewed from the aft end of the flechette. The flechette of the present invention is designed to allow for effective stacking while maintaining effective flight performance. 
     The flechette body is rectangular with an aspect ratio chosen so that the packing density is maximized, and the tailfins are rotated to an angle relative to the rectangular flechette body so that the tailfins of adjacent flechettes do not interfere with each other. Additionally, the tailfins of the flechette are angled to improve flight characteristics by inducing a spin to the flechette as it flies through the air. The wide separation between the center of gravity of the flechette and its center of pressure ensures that the flechette recovers quickly from any pitch or yaw angle (up to being completely reversed). Inducing a rolling moment to the flechette allows the perturbations caused by manufacturing imperfections to be integrated out of the flight path while the flechette is in flight. 
     The flechette of the present invention experiences low drag while achieving uniform and stable flight characteristics. When multiple flechettes of the present invention are stacked into a packaged unit, each flechette of the packaged unit, upon being dispensed, will achieve similar flight characteristics so as to arrive at a target with greater uniformity and accuracy. 
     The flechette of the present invention is made by a two-part construction, with a two-fin spinning airframe and is manufactured by sheet metal or equivalent by folding and bending operations. 
     When multiple flechettes are stacked, the forebodies of the flechettes stack in parallel and in contact, in rows and columns. The parallel stacking is both on the top and bottom surfaces and on the sides. The canted two tailfins nest without interference when stacked in rows and columns. The flechette has a generally rectangular forebody, with curved sides, that is self clocking for stacking purposes. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained by reference to the following detailed description when considered in connection with the accompanying drawings. 
         FIG. 1  is a perspective drawing of the flechette of the present invention. 
         FIG. 2  is a top or bottom view of the flechette of the present invention. 
         FIG. 3  is an aft view of the two tailfins of the present invention which demonstrates a relatively flat, generally “Z” or S-shaped arrangement of the tailfins. 
         FIG. 4  is an exploded view of the tip and quill of the present invention prior to assembly. 
         FIG. 5  is frontal perspective view of an assembled flechette of the present invention. 
         FIG. 6  is a perspective view of packaged flechettes of the present invention which are stacked in rows and columns. 
         FIG. 7  is a perspective view of packaged flechettes of the present invention which are stacked in a radial arrangement. 
         FIG. 8  is a side, sectional view of a warhead in which flechettes of the present invention are stacked into discrete packages or pucks without interleaving. 
         FIG. 9  is a side view of a typical prior art flechette which illustrates the location of its center of gravity relative to its center of pressure. 
         FIG. 10  is a side view of a flechette according to the present invention which illustrates the location of its center of gravity relative to its center of pressure. 
         FIG. 11  is a cut-away, perspective view of stacked flechettes according to the present invention stacked within a shotgun shell. 
         FIG. 12  is a perspective view illustrating flechettes of the present invention as they would appear exiting the barrel after having been fired from a shotgun. 
         FIG. 13  is an aft view of the two tail fins of the flechette of the present invention demonstrating that the end aft radial edges or points on the undersides of the two tail fins are approximately 180 degrees apart. 
         FIG. 14  is a side-view of the flechette of the present invention with axis provided for relational location appreciation of the various points and parts of the flechette. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , the flechette  10  of the present invention has a forward body  20  which has a substantially rectangular box-like shape, with the forward body  20  having a front tip or nose  22 . The forward body  20  is connected to a tail section  24  with the tail section  24  having two integrally connected tailfins or fins  24 A,  24 B located at the aft of the flechette  10 . Both fins  24 A,  24 B are arranged so as to form a compound angularity which is represented by a longitudinal angle θ and a radial angle Φ ( FIGS. 2 and 3 ). 
     In  FIG. 2 , angle θ is understood as being that angle formed by dotted lines AA and BB. Line AA represents the bend axis where the tailfin  24 A adjoins the flat portion of the tail section  24  and line BB represents the longitudinal center line of the flechette  10 . In a flight-tested prototype of the present invention, the angle θ measured 4.5 degrees. 
     With reference to  FIG. 3 , a radial angle Φ is formed by axis line CC and line DD. Line DD is colinear with the underside edge of fin  24 A. Line EE is normal to line CC. Lines DD and EE form angle α. As  FIG. 3  further demonstrates, fins  24 A and  24 B have a Z-shaped or S-shaped orientation. As is portrayed by arrow  18  of  FIG. 3 , the shape and angular orientation of fins  24 A and  24 B cause flechette  10  to spin or rotate in flight. 
     In a successfully tested prototype of the present invention, the angle θ measured 4.5 degrees, the radial angle Φ measured 57 degrees and angle α formed by lines EE and DD measured 33 degrees. Also, in the successfully tested prototype of the present invention, the total length of the flechette measured approximately two inches long. The tail section was approximately 0.5 inches long, with the forward body being about 1.5 inches long. The forward body was approximately 0.2 inches wide and 0.1 inches thick. The width of the tail section at its widest point was approximately 0.4 inches. The teachings of the present invention can be utilized in a flechette of other dimensions and angularities; thus the given dimensions of the successfully tested prototype are in no way to be considered limiting as to the invention claimed. 
     To further appreciate the angular relationship of tailfins  24 A and  24 B, in the successfully tested prototype of the present invention an extreme aft point M located on the topside of tail fin  24 A and an extreme aft point N located on the underside of tail fin  24 B were located approximately 180 degrees apart (see  FIG. 13 ). As such, in the prototype tested, the extreme aft point M and extreme aft point N could be thought as being in a substantially half-circle orientation to one another. 
     In  FIG. 4 , a flechette  10  of the present invention includes forward section  20 F having sides  25 A,  25 B which define and are integrally connected to a bottom or trough  29  of the forward section  20 F. A quill section  30 , is integrally connected to tail section  24 , and extends from tip  35  to the roots  35 A,  35 B of tail section  24 . 
     Quill section  30  slides into the trough  29  of the forward section  29 F until the front tip  35  of the quill section  30  is located at the nose  22  of the forward section  20 F. Serrated barbs, such as barbs  32 A,  32 B,  32 C are positioned on the sides of the quill section  30  so as to secure contact with the sides  25 A,  25 B of forward section  20 F upon assembly. 
     Upon insertion into the trough  29  of the front section  20 F, the tip  35  of quill section comes to rest at the nose  22  of the forward section  29 F. When press-fit and stamped during the assembly process, the quill section  30  and the front section  20 F become forward body  20 . 
     The flechette  10  of the present invention can be made of carbon steel sheet or strip or virtually any appropriate material. It is not required that the quill section  30  and the front section  20 F be made from the same material. 
     The nose of the flechette is tapered as is the rear  28  of the forward body  20 . This tapering can be done before or after the assembly process. The nose  22  can be further machined to give a desired shape, such as a sharp or pointed nose, but the tapered nose shown in  FIGS. 2 and 5  has performed well in tests. 
     Once the flechette  10  of the present invention is manufactured and assembled, the flechette becomes a one-piece aerodynamic body of symmetrical shape. (Thus, the terms top or bottom can be used interchangeably in respect to flechette  10 ). The quill section  30  can be cut from steel or aluminum sheet or strips with a material composition and thickness suitable to common sheet metal for manufacturing and forming processes. The front section  20 F can be made from similar or higher density materials to that of quill section  30  and can be formed from metal tubing, metal sheet, strip material or other suitable material. 
       FIG. 6  demonstrates the stacking capability of the flechette of the present invention, where a stacked rectangular array of flechettes  100  according to the present invention has three columns and four rows of flechettes with flechettes  10 A,  10 B and  10 C forming one row of flechettes and flechettes  10 C,  10 D,  10 E and  10 F form one column of flechettes. Dotted circle  75  highlights how the “Z” or S-shaped fins of the flechettes of the present invention allow effective stacking without detrimental interference between the flechettes. 
     In  FIG. 7 , a radially stacked arrangement or puck  40  of flechettes according to the present invention is shown which demonstrates four radially oriented rows or circles of flechettes. Dotted circle  759  highlights that the “Z” or S-shaped fins of the flechette  10  of the present invention allow multiple flechettes of the present invention to be radially packaged without interference between adjacent flechettes within the same radially row and without interference between the flechettes in adjacent radial rows. 
     In  FIG. 8 , a warhead  55 , such as, for example, the warhead of a Hydra  70  rocket, is provided with a hollow cylindrical casing in which discrete pucks of flechettes are stacked unlike the prior art where the flechettes are longitudinally interleaved to achieve the necessary packing density. Pucks  40 A,  40 B, etc., of flechettes according to the present invention are stacked within the casing in the orientation demonstrated in  FIG. 7 . 
     The discrete packaging arrangement is shown as the areas  45 A,  45 B,  45 C, etc., where the tails of the flechettes in the preceding puck are in contact with the nose of the flechettes in the subsequent puck. A pusher charge  47  burns to shear the warhead nose off thereby expelling the flechettes out of the front of the casing. 
     In  FIG. 9 , the center of gravity C g  and the center of pressure C p  of a typical, conventional, prior art flechette  66  is shown. 
     In  FIG. 10 , a side view of the flechette  10  according to the present invention demonstrates the location of the center of gravity Cg′ and the center of pressure Cp′ on the flechette of the present invention. One will notice that the center of gravity is further forward and the center of pressure is further backward than in the typical prior art flechette which indicates greater aerodynamic stability. 
     In  FIG. 11 , a shotgun shell  60  according to the present invention has a stacked configuration of flechettes  109  arranged within the shell. As an alternative to the arrangement of  FIG. 12 , the flechettes of the present invention could be arranged in a radial orientation so as to be radially stacked within the shotgun shell&#39;s wadding. 
       FIG. 12  shows a stacked configuration of flechettes  109  as they would appear after being fired from a shotgun as the conformal plastic sabots  61  housing the flechettes in the shotgun shell are aerodynamically discarded upon exiting the gun&#39;s barrel. 
     With reference to  FIG. 14 , the flechette  10  of the present invention has a most forward point F and a most rear point R. The line KK is the horizontal axis of flechette  10  and extends through the center of gravity Cg′ of flechette  10 . Line GG extends through the center of gravity Cg′ with line GG intersecting and being normal to line KK. Line PP extends through the center of pressure Cp′ with line PP intersecting and being normal to line KK. Line MM extends through the most forward point F and line NN extends through most rear point R. Lines MM, GG, PP and NN are parallel to each other. Line LL is parallel to line KK. The distance from point A to point B on line LL is equal to the distance between the most forward point F and the center of gravity Cg′. The distance from point A to point C on line LL is equal to the distance between the most forward point F and the center of pressure Cp′. The distance from point F to point R is equal to the distance between point A and point D on line LL. 
     Still with reference to  FIG. 14 , in the present invention, the center of gravity is designed to be closer to point F than to point R, i.e., the center of gravity is located in the front portion of the flechette at a location which is less than half the length of the flechette as measured from point F. In other words line segment AB divided by line segment AD is less than 50%. In a protoype of the present invention, AB/AD was equal to 45.8%. Ideally the center of gravity Cg′ should be as close to the front of the flechette, i.e., as close to forward point F as possible. 
     The radial distance of line LL from the horizontal axis KK is a further radial distance than from the horizontal axis than is the radial distance from the horizontal axis to any point on the flechette. Line LL is normal to line NN and Line LL is normal to line MM. Accordingly in that line MM intersects line LL at point A and line NN intersects point D on line LL, the distance from line segment AD on line LL is equal to the distance between the most forward point F and most rear point R. 
     The pragmatic features of the present invention include the fact that when the pucks  40  of flechettes are stacked within a warhead such stacking can be done without the increased cost and complexity and without the longitudinal interleaving of flechettes which occurs in the prior art. Further, the flechettes of the present invention remove the need to turn the flechettes to a particular clocking angle (to improve packing density) as is done in the prior art. 
     The rectangular cross section of the flechettes (see,  FIG. 13 ) of the present invention ensures the flechettes have consistent clocking orientations and that the radial angle of the fins  24 A,  24 B is oriented at an angle that allows adjacent fins to nest without interference. 
     The transition from dispense to stable flight is a critical event in the flight of a flechette. When a shotgun shell containing the flechettes according to the present invention is fired or when the flechettes of the present invention are dispensed from a warhead, the flechettes are ejected with high translational velocity, moderate roll rate and moderate to high transverse angular pitch and yaw rates and attitudes into the air. 
     The location of the center of gravity of the flechette  10  of the present invention when combined with the relatively large tailfin region and its angled “Z” or S-shaped oriented, rotation-inducing fins  24 A,  24 B ensure optimal performance. Upon dispense, the flechettes of the present invention quickly weathervane into a nose-first flight orientation even when the fins are aerodynamically stalled due to high angles of attack. 
     As the flechettes of the present invention assume a nose-first orientation they begin to spin around the longitudinal axis as demonstrated by arrow  18  in  FIG. 3 . This spinning is accomplished by the offset separation and small incidence angle θ ( FIG. 2 ) of the fins  24 A,  24 B ( FIG. 4 ). The spinning serves the purpose of further enhancing the aerodynamic stability of the flechettes and mitigating the negative effects of high volume production tolerances and misalignments on their flight path. 
     As a result of the improved aerodynamic properties of the flechette of the present invention, the dispensed flechettes are able to arrive at a target area with greater accuracy and at higher and more consistent velocity. Thus, the size and number of gaps in the dispersion pattern of the flechettes is reduced and target effects are improved. 
     The flechette of the present invention combines simple and inexpensive manufacturing techniques with improvements in flight performance and packaging. The result is that manufacturing costs of the present invention are competitive with prior art designs; however, the effectiveness of the flechettes is much improved compared to the prior art. 
     Since the flechettes of the present invention are designed to be self-correcting and self-orienting, an acceptable packing density can be achieved in a warhead or shotgun shell without undue effort and expense. 
     After the flechettes of the present invention are released from their packaging, their forward placed center of gravity and fin dimensions and orientations ensure that the flechettes are quickly directed toward their intended flight path. 
     For flechettes which are dispensed from a shotgun shell, the velocity improvements translate into increased range while increasing accuracy. 
     The flechettes of the present invention allow for rectangular stacking with virtually any number of desired rows or columns of flechettes and allow for radial stacking with virtually any number of radial rows. 
     Various modifications are possible without deviating from the spirit of the present invention. Accordingly the scope of the invention is limited only by the claim language which follows hereafter.

Technology Classification (CPC): 5