Abstract:
A supercavitating water-entry projectile having empennage on the aft end which provides both aerodynamic and hydrodynamic stability and a supercavitating nose section is provided. A representative projectile is a subcaliber munition adapted for use in a 25 mm weapon using a sabot currently in use with the M919 round. The projectile has circumferential grooves around its center section to match these sabots. A key feature in the invention is the size and shape of the nose section. The projectile has a novel high strength extended blunt nose section followed by a truncated conical section which angles towards the body of the projectile in the range of five degrees. During underwater trajectory, the entire projectile is contained within the cavitation bubble formed by the blunt nose tip. The projectile&#39;s aft empennage, which provides both aerodynamic and hydrodynamic stability, fits within the bore of the weapon.

Description:
This is a divisional of Ser. No. 09/014,688 filed on Jan. 28, 1998, U.S. Pat. No. 5,955,698. 
    
    
     ORIGIN OF THE INVENTION 
     The invention described herein was made in the performance of official duties by an employee of the Department of the Navy and may be manufactured, used, licensed by or for the Government for any government purpose without payment of royalties thereon. 
    
    
     FIELD OF THE INVENTION 
     The invention described herein relates to underwater projectiles and in particular to long-rod projectiles used for destroying underwater objects such as obstacles, torpedoes, and mines. 
     BACKGROUND OF THE INVENTION 
     Development of penetrating projectiles as currently used in anti-armor applications has addressed numerous technological difficulties in order to produce effective weapons. The basic requirements of a long-rod penetrator includes the use of high density projectiles having a long length-to-diameter ratio and having very high impact velocities. The presently available projectiles are generally used for maximum target penetration of a hardened structure. The invention adapts long-rod penetrators with the capability of traveling both in air and water where the object is to achieve low-drag water penetration for the purpose of delivering high kinetic energy to underwater targets. This requires that the hydroballistic projectile maintain stability and low drag both in air and water so that sufficient kinetic energy can be delivered to the underwater target to assure its destruction. 
     Current projectiles do not exhibit the capability to travel in both air and water and deliver high kinetic energy to defeat targets at any significant depth below the water surface. An operational need exists for a projectile having the capability of launch above the water surface and providing effective water travel after impact with the water surface. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an air-launchable, penetrating projectile which is ballistically stable both aerodynamically and hydrodynamically. 
     It is another object of the invention to provide a projectile having a supercavitating nose which provides a cavitation bubble of sufficient size to encompass the body of the projectile which reduces hydrodynamic drag. 
     It is a yet another object of the invention to provide a projectile launchable by existing gun systems and having sufficient strength to withstand high speed water impact loads while maintaining sufficient strength and ductility to withstand gun launch and hydrodynamic loads during water travel. 
     In accordance with these and other objects, the invention is a supercavitating water entry projectile having aft mounted empennage which provides stabilization in both air and water and a supercavitating nose section. The projectile is a subcaliber, gun launched munitions using an appropriate sabot assembly to provide full caliber integrity. The projectile has circumferential grooves around its center section to match grooves in the sabot assembly. A key feature in the invention is the size and shape of the nose section. The projectile has a novel high strength extended blunt nose section followed by a truncated conical section which angles towards the body of the projectile in the range of five degrees. During underwater trajectory, the entire projectile is contained within the cavitation bubble formed by the blunt nose tip. The projectile&#39;s empennage, which provides both aerodynamic and hydrodynamic stability, fits within the bore of the weapon. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing objects and other advantages of the present invention will be more fully understood from the following detailed description of representative components of a representative 25 mm projectile and reference to the appended drawings wherein: 
     FIG. 1 is a side view of the projectile with a partial cutaway of the aft end. 
     FIG. 2 is a partial side view of the supercavitating nose section of the projectile. 
     FIG. 3 is a partial side view of a cutaway depicting a variation of the supercavitating nose section of the projectile. 
     FIG. 4 is a side view of the projectile with the sabot installed. 
     FIG. 5 is a graphical representation of the cavitation bubble with a profile of the projectile included. 
     FIG. 6 is a graphical representation depicting decreasing underwater velocity for increasing underwater range. 
     While the aforementioned figures apply to the 25 mm projectile, the invention can be scaled to any caliber weapon. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, the projectile  10  of the present invention is shown with its major sections depicted. The projectile has a cylindrical body comprising three major sections, the cylindrical aft section  12 , the cylindrical center section  22 , and the nose section  32 . The steel aft section  12  is configured with suitable stabilizing empennage. This empennage is in the form of a plurality of fins  14 . In the preferred embodiment, four equally spaced fins are circumferentially located around the aft section  12  and are sized to fit within the gun bore of a selected existing weapon. For aerodynamic stability, the center of gravity of the projectile  10  must be located forward of the center of pressure. The long body design of the projectile  10  with fins  14  as stabilizing empennage on the aft section  12  produces restoring force sufficient to provide good stability in both air and water. Gyroscopic-induced stability, such as used by spinning bullets in air, cannot be achieved because of the difference in the medium density of water versus air. 
     A payload cavity  16  is located inside the aft section  12  of the projectile  10  suitable for containing tracer material or other desired payload. The threaded aperture  18  of the main body  24  attaches into a bored aperture of the aft section  12 . A threaded aperture  18  provides an attachment point for fixing the aft section  12  to the main body  24  of the projectile  10 . The main body  24  is a tungsten or similar heavy metal rod comprising the center section  22  and the nose section  32  of projectile  10 . On the center section  22 , circumferential grooves  26  are machined to provide a matching interface for a sabot assembly (hereinafter described). 
     The forward or nose section  32  of the projectile  10  includes the tapered portion  34  and the supercavitating blunt nose tip  36 . The nose taper angle  38  of the tapered portion  34  forms a shoulder  42  integral with the center section  22 . As previously described, it is necessary to generate a water cavity such that the entire projectile  10  travels within the cavity. This cavity is produced by the supercavitating blunt nose tip  36 . The supercavitating blunt nose tip  36  is cylindrical about the axis of the projectile  10  and has a flat circular face  40  which generates the water cavity as the projectile  10  travels through water. 
     It is imperative to the hydrodynamic stability of the projectile  10  and, thus to success of the invention, that the diametrical size of the flat circular face  40 , the nose taper angle  38 , and the length of the supercavitating blunt nose tip  36  be designed such that the shoulder  42  does not touch the water cavity wall before the fins  14  of the stabilizing empennage touch the water cavity wall. It is also important to minimize the hydrodynamic drag of the projectile  10  by reducing the diametrical size of flat circular face  40  as much as possible without producing a resultant increase in hydrodynamic drag as a result of the fins  14  of the stabilizing empennage contacting the water cavity wall in an excessive manner beyond what is necessary to provide hydrodynamic stabilization. 
     Referring now to FIG. 2, a detailed view of the nose section  32  is shown. The aforementioned supercavitating blunt nose tip  36  is illustrated in more detail. For the subcaliber 25 mm design having a nominal center section  22  (the center section  22  is depicted in FIG. 1) diameter of 0.327 inches, the preferred diameter of the flat circular face  40  is in the range of 0.10 inches in diameter. The preferred length of the supercavitating blunt nose tip  36  is 0.07 inches. The preferred nose taper angle  38  is five degrees. 
     A variation on the supercavitating blunt nose tip  36  is depicted in FIG.  3 . In some applications where the water impact loads are higher than the strength of the material of the projectile  10  (the projectile  10  is depicted in FIG. l), the supercavitating blunt nose tip  36  (as shown in FIGS. 1 and 2) is replaced by a very high strength supercavitating insert  46 . The supercavitating insert  46  is made from very high strength material sufficient to withstand the loads generated by the combination of high speed water impact and impact obliquities approaching perpendicular to the water surface. The supercavitating insert  46  is cylindrical with a flat circular face  50  which generates the water cavity. The supercavitating insert  46  is placed in the insert bore  47  of the projectile  10 . In this variation of the supercavitating invention, the alternative nose section  33  of the projectile  10  includes the tapered portion  44  having a nose taper angle  48  and forms a shoulder  52  with the center section  22  (the center section  22  is depicted in FIG.  1 ). The tapered portion  44  is terminated on the end by the lip  45 . The means of securing the supercavitating insert  46  into the insert bore  47  may be secured by an interference fit, taper fit, threaded fit, or suitable bonding material. 
     It is imperative to the hydrodynamic stability of the projectile  10  (as shown in FIG.  1 ), and thus to success of the invention, that the diameter of the flat circular face  50 , the length that the supercavitating insert  46  protrudes from the projectile  10 , and the nose taper angle  48  be designed such that the shoulder  52  and the lip  45  do not touch the water cavity wall before the fins  14  (the fins  14  are depicted in FIG. 1) touch the water cavity wall. However, in order to minimize the hydrodynamic drag of the projectile  10 , the diametrical size of flat circular face  50  must be reduced as much as possible without producing a resultant increase in hydrodynamic drag which results when the fins  14  of the stabilizing empennage protrude into the water cavity wall to an excessive depth beyond what is necessary to provide hydrodynamic stabilization. 
     For the subcaliber 25 mm design having a nominal center section  22  (the center section  22  is depicted in FIG. 1) diameter of 0.327 inches, the preferred diameter of the flat circular face  50  of the supercavitating insert  46  is on the order of 0.10 inches in diameter. The preferred protrusion distance of the supercavitating insert  46  from the face of the lip  45  is 0.20 inches. The preferred diameter of the lip  45  is 0.136 inches. The preferred nose taper angle  48  is five degrees. 
     The overall configuration of the projectile  10  with the three sabot petals  62  is shown in FIG.  4 . The sabot petal  62  is formed in a 120 degree segment and the three sabot petals form a complete 360 degree fit over the center section  22  of projectile  10 . The circumferential grooves  26  (as shown in FIG. 1) of the projectile  10  match with the circumferential sabot grooves  56 . The sabots are held in place by the obturation band  64 . The obturation band  64  provides a gas seal during cartridge actuation in the weapon. During firing, the sabot petals  62  and the obturation band  64  separate from the projectile  10  shortly after muzzle exit from the weapon. Stabilizing fins  14  are shown for reference. 
     Operation of the Invention 
     FIG. 5 is a two-dimensional, graphical representation of the cavitation bubble formed by travel of the blunt nose through the water. The cavity radius in units of inches, along the ordinate of the graph, is shown with respect to length of cavity in units of inches, along the abscissa. The water cavity wall  72  stands off from the nose section  32  of the projectile and off the entire projectile  10 . By this means, the entire projectile  10  travels inside of the cavitation bubble as it travels through the water. In this illustration, it can be seen that if the projectile  10  is disturbed about its longitudinal axis, the tip of the fins  14  will contact the water cavity wall prior to the shoulder  42 . 
     One embodiment of the invention is adapted to a subcaliber projectile launched from a 25 mm caliber cannon. The cartridge used to launch the projectile utilized existing parts from the standard M919 cartridge, including the 25 mm sabot assembly, the obturator, and the propelling charge. The invention, however, can be applied in similar fashion to other long-rod projectiles. FIG. 6 illustrates the hydroballistic capabilities of several calibers of long-rod projectiles with the invention incorporated into the round. The projectile velocity in units of feet per second, along the ordinate of the graph, is shown with respect to range of water travel in units of feet, along the abscissa. The 25 mm hydroballistic potential  81  shows the exponential decay typical of velocity degradation while traveling through a fluid medium. The 30 mm hydroballistic potential  82 , the 35 mm hydroballistic potential  83 , the 40 mm hydroballistic potential  84 , the 76 mm hydroballistic potential  85 , and the 105 mm hydroballistic potential  86  are also shown in this graph. The water entry velocities of each caliber represented in the graph are decayed from the muzzle velocity by 1000 feet of air flight. The velocity potential of each caliber cartridge is driven by the particular design. 
     The features and advantages of the present invention are numerous. The invention&#39;s unique supercavitating nose section allows current long-rod ammunition designs to have hydroballistic potential. The supercavitating nose, which is designed to be incorporated as part of a subcaliber projectile such as the M919&#39;s subcaliber projectile, is based upon a truncated cone with an extended tip whose diameter in this particular projectile is 0.10 inch. The base diameter of the truncated cone is the diameter of the cylindrical center section of the subcaliber projectile body. The angle of the truncated cone determines the majority of the length of the supercavitating nose. The length of the extended tip, expressed in terms of the diameter of the tip, can be as short as 0.2 times the diameter to as long as 2.0 times the diameter. The extended tip diameter, the length of the extended tip, and the cone angle are critical to the stability during water entry and subsequent travel to the underwater target. The nose diameter also controls the diameter of the water cavity such that the water cavity wall clears the shoulder of the nose cone at the joint with the projectile&#39;s cylindrical center section. However, the stabilizing empennage on the aft end of the projectile can contact the water cavity wall providing stability before the unstable situation of the cone shoulder contacting the water cavity wall can occur. 
     The supercavitating nose tip diameter is made as small as possible to reduce the hydrodynamic drag which results in high kinetic energy delivered to the target. The supercavitating nose tip diameter and cone angle are designed to optimize drag reduction while maintaining the required shape and mass distribution to promote stability not only in water but also in air. 
     The material chosen for the projectile has a number of properties critical to the design. The tungsten alloy or heavy metal equivalent must withstand the high impact loads due to high velocity water impact, particularly for the nose tip. The projectile material must also maintain strength and ductility to withstand gun launch and hydrodynamic loads during underwater travel. To achieve high kinetic energy at the target, the density of the material must be high. The materials of the present invention achieved but are not limited to water entry velocities up to 4300 feet per second. 
     The use of the aft empennage for stabilization in both air and water gives further advantage to the invention. Using the empennage for both fluid mediums gives the invention robustness with simplicity. The empennage is preferably in the form of fins, but it may be of a flared nature (flared designs have strength and mass property disadvantages). 
     The projectile&#39;s basic construction is based on a long-rod projectile design which incorporates empennage on the aft end of the projectile that provides both aerodynamic and hydrodynamic stability. The front end, or nose, of the projectile is shaped in such a way that the water is displaced by the nose tip, creating a cavitation bubble which is large enough for the rest of the projectile to travel in. The 25 mm M919 cartridge was chosen as the basis for proof of the invention, but would apply to other calibers as well. The shape of the existing M919 projectile was modified to incorporate the supercavitating nose. 
     Although the invention has been described relative to a specific embodiment thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in the light of the above teachings. Variations in nose tip design may improve the capability slightly. To achieve underwater stability, the water cavity formed by the tip must clear the forward part of the body such that the fins can stabilize the projectile. Nose tip designs including smaller diameter flats, flared, conical, and power law shapes can be adapted to the projectile to optimize drag. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other that as specifically described.