Abstract:
A variable drag projectile stabilizer is utilized by a training projectile to match the trajectory of a tactical projectile for up to 3 km while having a range limitation of 8 km. The stabilizer applies supersonic flow phenomena to alter the aerodynamic characteristics of a training projectile while in free flight to fulfill this requirement. The stabilizer uses a cowling supported by struts to provide tail lift and ensure a stable flight path. Supersonic flow is established through ducts formed by the cowling and struts when launched from a weapon. The flow remains supersonic until the projectile reaches the desired range but then quickly becomes subsonic (choked) due to shock waves emanating from interior angles in the ducts. The geometry of the ducts can be designed to create different shock wave patterns within the ducts. The variance of leading edge location, leading edge angle, cowling intake angle, and flight Mach number influences the shock patterns within the ducts and consequently, the range of the projectile.

Description:
FEDERAL RESEARCH STATEMENT 
     The inventions described herein may be manufactured, used and licensed by or for the U.S. Government for U.S. Government purposes. 
    
    
     BACKGROUND OF INVENTION 
     Field of the Invention 
     The present invention relates to a tank training projectile. More particularly this invention pertains to a training projectile with an effective range that can be regulated by means of a variable drag projectile stabilizer. In specific, the present invention utilizes supersonic airflow to change the aerodynamics of the training projectile during flight, thus matching the flight characteristics of a corresponding service ammunition during the initial part of the flight while not exceeding a predetermined range of the training projectile. 
     BACKGROUND OF THE INVENTION 
     The Army has an on-going need for long-range kinetic energy projectiles for use in artillery and tank training. For effective training, ballistic characteristics of a training munition should match that of a corresponding battlefield or service ammunition as closely as possible. An example of service ammunition for which a training projectile is used is an armor piercing discarding sabot (APDS) kinetic energy projectile. For maximum effectiveness, the trajectory of the training projectile should closely resemble the trajectory of the armor piercing discarding sabot (APDS) kinetic energy projectile for ranges up to 3 km. Further, the maximum range of the training projectile should be no more than 8 km to confine the training projectile to the boundaries of the training range. While current technology is able to match trajectories at shorter distances (up to 2 km), a primary difficulty is in matching the trajectory of the armor piercing discarding sabot (APDS) kinetic energy projectile at longer distances (up to 3 km) while limiting the range to 8 km. 
     A conventional long range kinetic energy training projec–tile used by the U.S. Army is the Cartridge 120 mm, TPCSDS-T M865 (Target Practice Cone Stabilized Discarding Sabot). A series of slots cut along the top of the flare at an angle to the projectile&#39;s longitudinal axis imparts a roll torque to the projectile. While not required for aerodynamic stability, this spin improves the projectile&#39;s flight accuracy. Although this technology has proven to be useful, it would be desirable to present additional improvements. 
     The M865 has a high aerodynamic drag. Consequently, the M865 is launched at a greater muzzle velocity to match the trajectory of a tactical armor piercing discarding sabot (APDS) kinetic energy. This greater initial velocity causes the trajectory of the M865 in an initial 2 km of flight to be slightly higher than the trajectory of the armor piercing discarding sabot (APDS) kinetic energy projectile over the same range. This small deviation or mismatch in trajectory by the training projectile compared to the service ammunition is within acceptable bounds. However, the high aerodynamic drag of the M865 causes significant deceleration beyond 2 km. Consequently, the flight path of the M865 is well below the trajectory of an armor piercing discarding sabot (APDS) kinetic energy projectile at ranges beyond 2 km. At ranges beyond 3 km, the mismatch in trajectory becomes undesirably large. 
     A self-destructing training projectile for the armor piercing discarding sabot (APDS) kinetic energy projectile uses aerodynamic heating to melt a portion of the self-destructing training projectile, causing the self-destructing training projectile to disintegrate in flight prior to reaching the maximum allowed range. Reference is made here to U.S. Pat. No. 4,413,566, which is incorporated by reference. 
     Although this technology has proven to be useful, it would be desirable to present additional improvements. Accurate range limitation for the self-destructing training projectile is difficult to obtain due to the temperature dependency of the self-destruction mechanism. At lower temperatures, melting of the part of the self-destructing training portion is delayed. Consequently, the self-destructing training projectile may not disintegrate within the desired 8 km maximum range. 
     A mechanically adjusting training projectile employs moving mechanical parts to alter the mass distribution of the mechanically adjusting projectile in flight. Reference is made here to U.S. Pat. No. 4,596,191 which is incorporated by reference. As the center of gravity of the mechanically adjusting training projectile shifts, the mechanically adjusting training projectile becomes statically unstable, resulting in a high angle of attack motion. Although this technology has proven to be useful, it would be desirable to present additional improvements. The mechanically adjusting training projectile is expensive. In addition, a failure in the moving mechanical parts allows the projectile to travel well beyond the maximum desired range. 
     The range of a dynamically unstable training projectile can be limited by launching from a smooth bore weapon, creating a dynamic instability. Reference is made here to U.S. Pat. Nos. 5,125,344 and 6,123,289 that are incorporated by reference. The dynamic instability creates a spin near the natural pitching frequency of the dynamically unstable training projectile, causing an amplification of the trim vector and subsequently causing a high angle of attack motion. The high angle of attack limits the range of the dynamically unstable training projectile. Although this technology has proven to be useful, it would be desirable to present additional improvements. To be effective, the dynamically unstable projectile must have a very large trim amplification factor and a relatively large aerodynamic trim angle that can be amplified by a resonant motion. If the trim angle is insufficient, the dynamically unstable projectile is not driven to a high angle of attack and the dynamically unstable projectile flies beyond the maximum desired range. 
     What is needed is a training projectile that accurately matches the trajectory of a service ammunition such as, for example, a tactile armor piercing discarding sabot (APDS) kinetic energy projectile for an initial 3 km of flight. Further, range of the training projectile should be limited to 8 km to minimize the possibility of the flight of the training projectile exceeding the training range boundaries and subsequently causing the training projectile to pose a danger to non-military personnel. The training projectile should be cost effective and easily manufactured. The need for such a training projectile has heretofore remained unsatisfied. 
     SUMMARY OF INVENTION 
     The present invention satisfies this need, and presents a limited range training projectile stabilizer for a kinetic energy training projectile. The variable drag projectile stabilizer is a passive device that applies supersonic flow phenomena to alter the aerodynamic characteristics of a projectile while in free flight. The variable drag projectile stabilizer enables a training projectile to follow the trajectory path of an armor piercing discarding sabot (APDS) kinetic energy projectile for an initial 3 km of flight while limiting the range of the training projectile to 8 km. 
     The variable drag projectile stabilizer uses a cowling supported by struts to provide tail lift and ensure a stable flight path. The struts extend beyond the aft end of the cowling to carry the setback load of the cowling during acceleration in the gun tube. The cowling and struts form tubular ducts in parallel with a longitudinal axis of the training projectile. 
     When the training projectile is launched, supersonic flow is established through the ducts. The flow through the ducts remains supersonic until the training projectile reaches the target location. The supersonic flow through the ducts ensures that the training projectile flies downrange with a relatively low aerodynamic drag. The low aerodynamic drag enables the trajectory of the training projectile to closely match the flight trajectory of the service ammunition that the training projectile is designed to emulate. 
     As the training projectile decelerates during flight, the supersonic flow through the ducts approaches subsonic flow. To limit the maximum possible range of the training projectile, the variable drag projectile stabilizer is designed to experience a transition to subsonic (choked) flow through the ducts slightly beyond a location of a target. The ensuing rapid increase in aerodynamic drag severely limits further flight. Design details of the strut and cowling control the Mach number at which the high drag phenomenon begins, and thus the range of the training projectile. 
     After the training projectile is launched from a weapon, the approaching supersonic airflow passes over shallow angles in the cowling and strut configuration, forming oblique shock waves. The angle of obliquity of the shock waves is dependent upon the Mach number and the surface incidence angle of the airflow. At high Mach numbers, the oblique shock angles are shallow. Consequently, the shocks emanating off the leading edges of the struts and cowling do not intersect, maintaining supersonic flow through the ducts. 
     As the training projectile flies down range, aerodynamic drag decelerates the training projectile, decreasing the Mach number. As the Mach number decreases, the air pressure entering the ducts decreases and the oblique shock angles increase. The shocks emanating off the leading edges of the struts and cowling intersect, further increasing the aerodynamic drag. As the training projectile further decelerates, the speed of the training projectile becomes too slow to maintain supersonic flow through the ducts. Consequently, the airflow through the ducts becomes subsonic (choked) and the aerodynamic drag acting upon the tail increases substantially. 
     The geometry of the duct can be designed to create different shock wave patterns within the duct. The variance of leading edge location, leading edge angle, cowling intake angle, and flight Mach number influences the shock patterns within the ducts. 
     Target accuracy is enhanced by creating spin along the longitudinal axis of the projectile. In an embodiment, spin is induced by manipulating the geometry of the struts. In another embodiment, spin is induced by placing angled strakes around the periphery of the cowling. Strakes provide a roll torque to spin the projectile as well as act as a bore rider, protecting the cowling from balloting in the gun tube. 
     When the projectile is launched, gun gases flow forward through the ducts creating a significantly higher pressure inside the cowling than outside the cowling. To equalize pressure, the outside diameter of the cowling is designed smaller than the gun bore, allowing the gun gases to flow outside the cowling. In an embodiment, the trailing edges of the cowling are scalloped to allow the gun gases to escape more rapidly to the outside of the cowling. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The various features of the present invention and the manner of attaining them is described in greater detail with reference to the following description, claims, and drawings, wherein reference numerals are reused, where appropriate, to indicate a correspondence between the referenced items, and wherein: 
         FIG. 1  is diagram of an example kinetic energy training projectile in which a variable drag projectile stabilizer of the present invention is used; 
         FIG. 2  is an end view of the cowling and interior struts of the variable drag projectile stabilizer of  FIG. 1 ; 
         FIG. 3  is an oblique view of a leading edge of the cowling, the interior struts, and ducts of the variable drag projectile stabilizer of  FIG. 1 ; 
         FIG. 4A  is a cut away view of the cowling of the variable drag projectile stabilizer of  FIG. 1  showing struts extending beyond the aft end of the cowling; 
         FIG. 4B  is a sectional view of the cowling of the variable drag projectile stabilizer of  FIG. 1  illustrating various design elements of the cowling; 
         FIG. 5  is comprised of  FIGS. 5A ,  5 B, and  5 C and represents an end view of shock wave distribution in the variable drag projectile stabilizer of  FIG. 1  operating at Mach 5.0, Mach 4.0, and Mach 3.0, respectively; 
         FIG. 6  is comprised of  FIGS. 6A ,  6 B,  6 C, and  6 C and represents cut away views of the variable drag projectile stabilizer of  FIG. 1  illustrating various embodiments of configurations of the struts; 
         FIG. 7  is comprised of  FIGS. 7A and 7B  and shows the stabilizer with angled strakes placed around the periphery of the cowling to induce spin during flight; and 
         FIG. 8  is a cut away view of the training projectile exiting a gun tube with an embodiment of the variable drag projectile stabilizer of  FIG. 1  utilizing a cowling with scalloped trailing edges. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary training projectile  100  comprising a variable drag projectile stabilizer  10  that utilizes supersonic airflow to change the aerodynamics of the training projectile  100  during flight. The variable drag projectile stabilizer  10  (also referenced herein as stabilizer  10 ) is mounted on a tail end of a cone-tipped cylindrical rod  15 . Stabilizer  10  is cylindrical with respect to axis  20 . Stabilizer  10  comprises a cowling  25  supported by struts  30 . The cowling  25  and the struts  30  provide tail lift and ensure a stable flight path of the training projectile  100 . 
     Struts  30  extend beyond the trailing edge  37  of cowling  25  to support a setback load or force experienced by cowling  25  during a gun launch of the training projectile  100 . Cowling  25  comprises a trailing edge bevel  35 , a leading edge bevel  40  and an angled interior surface  415 . The cowling  25  and struts  30  are typically made of a lightweight metal, such as aluminum or titanium. However, composite materials may also be used. The length L,  45 , of the cowling  25  is approximately 2.5 inches. The diameter D,  50 , of the cowling  25  is approximately 3.75 inches. In an embodiment, the length L,  45 , of the cowling  25  may range from approximately 1.0 inch to approximately 4.0 inches. In a further embodiment, the diameter D,  50 , of the cowling  25  may range from approximately 3.0 inches to approximately 5.0 inches. 
       FIG. 2  illustrates an end view of stabilizer  10  showing the relative position of cowling  25  and struts  30 . The cowling  25  and struts  30  form ducts  205 . Ducts  205  are roughly tubular in shape; a longitudinal axis of each of the ducts  205  and the longitudinal axis  20  are parallel.  FIG. 3  is an oblique view of the stabilizer  10  illustrating leading edges  305  of struts  30  and further illustrating the leading edge bevel  40  of the cowling  25 . The leading edges  305  of struts  30  are recessed with respect to the leading edge  42  of cowling  25 . 
     With reference to  FIGS. 4A and 4B , struts  30  extend beyond the trailing edge  37  of cowling  25  to carry the force (also known as the setback load) applied to cowling  25  during acceleration of the training projectile  100  in a gun tube. In an embodiment, the leading edges  305  of struts  30  are even with the leading edge  42  of cowling  25 . In another embodiment, the leading edges  305  of struts  30  are located forward of the leading edge  42  of cowling  25 . 
     Each of the struts  30  comprises angled surfaces  405 . Each of the angled surfaces  405  is inclined at a strut surface angle  410  with respect to the longitudinal axis  20  of the training projectile  100 . An angled interior surface  415  of cowling  25  is inclined at an interior surface angle  420  with respect to the longitudinal axis  20  of the training projectile  100 . The angled surfaces  405  of struts  30  and the interior surface  415  of cowling  25  form converging ducts  205 . The airflow through the ducts  205  is affected by the converging strut surface angle surfaces  405  and the interior cowling surface  415 . 
     Stabilizer  10  comprises three struts  30 . The strut surface angle  410  for each of the struts  30  relative to the longitudinal axis  20  is 2 degrees. The total included angle between the surfaces  405  on each strut  30  is approximately 4 degrees. In one embodiment, the strut surface angle  410  ranges from approximately 1.0 degree to approximately 5.0 degrees. In a further embodiment, stabilizer  10  may comprise 2 to 8 struts  30 . 
     Stabilizer  10  comprises one annular cowling  25 . The cowling leading edge bevel angle  41  relative to the longitudinal axis  20  is 5 degrees. In one embodiment, the leading edge bevel angle  41  ranges from approximately 1.0 to 10.0 degrees. The cowling trailing edge bevel angle  36  relative to the longitudinal axis  20  is 40 degrees. The trailing edge bevel angle  36  ranges from 10 to 90 degrees. The interior surface angle  420  relative to the longitudinal axis  20  is 2 degrees. The interior surface angle  420  ranges from approximately 0 to 5 degrees. 
     After launch from a gun tube, stabilizer  10  encounters supersonic airflow. The approaching supersonic airflow passes over the angled surfaces  405  of the struts  30  and the interior surface  415  of the cowling  25 , creating oblique shock waves. The angle of the oblique shock wave formed from the angled surfaces  405  of the struts  30  is dependent upon the Mach number of the supersonic airflow and the angle of incidence of the angled surfaces  405 , the strut surface angle  410 . The angle of the oblique shock wave formed from the interior surface  415  of cowling  25  is dependent upon the Mach number of the supersonic airflow and the angle of incidence of the interior surface  415 , the interior surface angle  420 . The Mach number of the supersonic airflow varies from approximately 5.0 at launch of the training projectile  100  from the gun tube to less than 3.0 at the target location. 
     Performance of an exemplary stabilizer  10  during flight of the training projectile  100  is illustrated by a set of shock wave diagrams shown in  FIG. 5  ( FIGS. 5A ,  5 B,  5 C), viewed from the aft end of stabilizer  10 .  FIG. 5A  illustrates a shock wave distribution of airflow as the airflow exits stabilizer  10  at Mach 5, an approximate speed of the training projectile  100  at muzzle exit after launch from a gun tube. Shock waves  505  emanate off the cowling leading edge  42 . Shock waves  510  emanate off the leading edges  305  of struts  30 . Supersonic region  515  is a region in ducts  205  at Mach 5.0 in which supersonic airflow is unimpeded and free of shock waves. 
     As the training projectile  100  flies down range, the speed of the training projectile  100  decreases and the Mach number of the supersonic airflow through stabilizer  10  decreases.  FIG. 5B  illustrates a shock wave distribution of airflow as the airflow exits stabilizer  10  at Mach 4. Supersonic region  520  is a region in ducts  205  at Mach 4.0 in which supersonic airflow is unimpeded and free of shock waves. As illustrated by comparing supersonic region  515  at Mach 5.0 with supersonic region  520  at Mach 4.0, the decrease of Mach number has increased the area of interference of shock waves  505  and  510  and decreased the area available for supersonic air flow to that of supersonic region  520 . 
     As the training projectile  100  reaches the desired down range location, the Mach number of the supersonic airflow through stabilizer  10  decreases to Mach 3.  FIG. 5C  illustrates a shock wave distribution of airflow as the airflow exits stabilizer  10  at Mach  3 . Shock waves  505  emanating from the leading edge  42  of cowling  25  and shock waves  510  emanating from the leading edge  305  of struts  30  have filled the interior area of ducts  205  such that supersonic flow is no longer present. The transition from supersonic flow to subsonic flow (also known as “choking”) in ducts  205  causes a large increase in aerodynamic drag, limiting the maximum range of the training projectile  100 . 
       FIG. 6  ( FIGS. 6A ,  6 B,  6 C) illustrates various configurations for the angled surfaces  405  of struts  30 . Stabilizer  10  ( FIG. 1 ) utilizes a configuration of struts  30  that is symmetric about a longitudinal axis  20  of the stabilizer  10 . It is often desirable to induce spin in a training projectile during flight, enhancing target accuracy of the training projectile. In an embodiment illustrated by a cut away view of stabilizer  10 A shown in  FIG. 6A , struts  30 A of stabilizer  10 A utilize asymmetrically angled surfaces  405 A as a method of inducing spin. The asymmetric configuration of struts  30 A causes a higher pressure on one side of struts  30 A, resulting in a roll torque about the longitudinal axis  605  of the stabilizer  10 A. Angled surfaces  405 A are configured asymmetrically with respect to longitudinal axis  605 ; for example, angle  610  is greater than angle  615 . Conversely, angle  615  may be greater than angle  610 . 
     In a further embodiment illustrated by a cut away view of stabilizer  10 B shown in  FIG. 6B , asymmetry of struts  30 B is introduced in a trailing edge  620  of one of the angled surfaces  405 B of each of the struts  30 B. In yet another embodiment illustrated by a cut away view of stabilizer  10 C shown in  FIG. 6C , asymmetry of struts  30 C is introduced in a leading edge  620  of one of the angled surfaces  405 C of each of the struts  30 C. 
     In an embodiment illustrated by a diagram of stabilizer  10 D shown in  FIG. 7A  and  FIG. 7B , spin is introduced during flight of a training projectile by utilizing angled strakes  705  placed around the periphery of cowling  25 D. The strakes  705  also provide structural support to the cowling  25  during setback load during acceleration and act as bore riding surfaces as the projectile travels along the gun tube. The angle  707  of the strakes  705  relative to the axis  20  is approximately 5 degrees. In an embodiment, the strake angle  707  ranges from approximately 2.0 degrees to approximately 10.0 degrees. The height  709  of the strakes  705  above the surface of the cowling  25  is approximately 0.10 inch. In an embodiment the strake height  709  varies from approximately 0.03 inch to approximately 0.15 inch. The width  711  of the strakes is approximately 0.15 inch. In one embodiment the strake width  711  varies from approximately 0.06 inch to approximately 0.25 inch. In a further embodiment, stabilizer  10  may contain 3 to 12 strakes  705 . 
     When the training projectile  100  is launched from a gun, gun gases flow forward through ducts  205  creating a pressure differential between the inside and outside of cowling  25  in which the pressure inside cowling  25  is significantly higher than outside cowling  25 . In an embodiment, the outside diameter D,  50 , of cowling  25  is designed smaller than the gun bore, allowing the gun gases to flow outside the cowling  25 , thus reducing the pressure differential. 
     An embodiment for further reducing the pressure differential between the inside and outside of a cowling is illustrated by the diagram of  FIG. 8 .  FIG. 8  is a cut away view of a training projectile  805  exiting a gun barrel  810 . The training projectile  805  comprises a stabilizer  815 . The stabilizer  815  comprises a cowling  820 . Cowling  820  comprises a trailing edge  825  that is scalloped to allow the gun gases to escape more rapidly to the outside of cowling  820 , further reducing the pressure differential between the inside and outside of cowling  820 . 
     It is to be understood that the specific embodiments of the invention that have been described are merely illustrative of certain applications of the principle of the present invention. Numerous modifications may be made to the variable drag projectile stabilizer limiting a flight range of a training projectile described herein without departing from the spirit and scope of the present invention. Moreover, while the present invention is described for illustration purpose only in relation to a training projectile, it should be clear that the invention is applicable as well to, for example, any projectile for which a method of limiting flight range may be used.