CH709673A2 - Aerodynamically improved bullet tail. - Google Patents

Abstract

The invention relates to two methods for reducing the base resistance of projectiles (29), in particular supersonic projectiles, which are stabilized by spin, and constructive form elements at the rear of such projectiles for implementing the method. The invention is also intended to improve the precision of projectiles. A first method increases the pressure in the dead water of the projectile base by an additional wall-resistant shock system (32), which is produced by concave functional surfaces at the rear of the projectile. A second method reduces the turbulence in the wake of the projectile by the rear generated axially oriented vortex filaments, which twist in the wake of a common vortex braid. For this purpose, the tail is provided with axially oriented transport grooves whose longitudinal edges induce vortex filaments. These transport grooves simultaneously reduce the base resistance of the basement floor. Both methods can be combined in one bullet.

Description

The invention relates to two methods for reducing the base resistance of projectiles, in particular of supersonic projectiles, which are stabilized by spin, and constructive form elements at the rear of such projectiles for implementing the method. The invention is also intended to improve the precision of projectiles.

All bullets or projectiles experience in flight a resistance from the wall friction and from a pressure drop at the bullet base. This is the base resistance caused by the bottom suction from the flow around the bullet tail. Supersonic missiles fly faster than the sound and form according to the laws of gas dynamics shock fronts at the bow and stern, which change with decreasing airspeed and cause the wave resistance. The total resistance is then composed of the characteristic impedance, the frictional resistance and the base resistance. All three resistance components change with the bullet velocity.

For a long range, the resistance of a projectile must be as low as possible. To lower the base resistance, the bullet tail is retracted and executed as a truncated cone (boat tail), which reduces the floor of the floor. As a result, the force effect of the soil suction decreases due to the Heckumströmung and the base resistance decreases. Experience has shown that the optimal cone angle for the boat-tail tail is about seven degrees, and its optimal length is about 0.75 caliber (Beat Kneubuehl: projectiles, complete edition, publisher Stocker-Schmidt, Zurich-Dietikon, 2013, page 114 - hereinafter "KNEU, Page 114 »).

A disadvantage of conventional projectiles is the bottom suction from the Heckumströmung. Another disadvantage is the instability of the cone tail from the mouth ballistics due to the residual pressure of the powder gases in the barrel. As the projectile leaves the barrel, the projectile is overflowed by powder gases from the barrel in an expanding annulus, allowing the projectile to receive a side impact near the port, thereby reducing the precision of the shot. The disadvantage is finally the high turbulence in the wake of the projectile during his flight. The level of turbulence is a measure of the energy loss rate in flight.

The object of the invention is to find methods that reduce the base resistance of projectiles from the bottom suction, increase the precision of a shot, and reduce the turbulence in the wake of the projectile. The object of the invention is also to find structural form elements for projectiles, with which the method can be implemented.

The object is achieved by the method according to claim 1 to 3 and constructive form elements according to claim 4 to 6. The invention will be described with reference to 8 figures: <Tb> FIG. 1 <SEP> Representation of the orifice ballistics and outdoor ballistics of boat-tail projectiles in supersonic as well as change of the impact fronts in the transition from supersonic to subsonic sound. <Tb> FIG. 2 <SEP> Typical course of the drag coefficient of supersonic projectiles with decreasing Mach number. <Tb> FIG. 3 <SEP> Behavior of supersonic flows at convex and concave corners, frictionless and frictional. <Tb> FIG. 4 <SEP> Presentation of a first method to reduce bottom suction on the boat tail, as well as constructive form elements to implement the first method. <Tb> FIG. 5 <SEP> Presentation of a second method to reduce bottom suction on the boat tail, including the reduction of turbulence in the wake, as well as constructive form elements for implementing the second method. <Tb> FIG. 6 <SEP> Derivation of shape elements to combine both processes in a bullet shape. <Tb> FIG. 7 <SEP> Mouth ballistics with an optimized bullet shape. <Tb> FIG. 8 <SEP> Comparison of the shock wave systems of conventional and new projectiles.

Fig. 1.1 shows the flow field of a projectile (12) with nose (7), middle part (8) and retracted boat tail (9) in supersonic flight. On the nose, a detached head shaft (1) with transition into an oblique shock (2). At the projectile base is the dead water area TW (3), which follows the projectile at the same speed and ends with a free stagnation point (4). This free stagnation point is the starting point for the rear shock wave (5) and the turbulent wake (6), in which the kinetic energy of the projectile is dissipated. The height of this turbulence is a measure of the loss rate of the kinetic energy of the projectile in flight.

Fig. 1.2 shows the projectile (12) near the mouth shortly after leaving the barrel (10). Because of the conical shape of the projectile tail, an annular gap opens between the barrel and tail in the mouth, through which the powder gases (11) flow out at high speed due to residual pressure in the barrel and overtake the projectile. This process can lead to side force due to asymmetries, which reduces the precision of the shot.

Fig. 1.3 shows the change in the impact fronts on the projectile at decreasing speed or Mach number according Schlieren shots from KNEU, page 89. The representations A, B, C, D, E refer to the marked points in Fig. 2nd , <tb> A <SEP> Fully trained slate thrust in bow wave and stern wave of the missile at high Mach number, supersonic range. <tb> B <SEP> Detached head wave at the bow of the projectile at the entrance to the transonic area just before Mach 1. Start of the transformation of the stern wave system, beginning of transonic range. <tb> C <SEP> Stern wave with wall contact at the base of the projectile, just below Mach 1, no more head wave present, middle of the transonic area. <tb> D <SEP> Reduction of the rear supersonic area until termination of the rear shock wave, end of the transonic area, transition to the subsonic sound. <Tb> E <September> subsonic flow.

Fig. 2 illustrates the representation of the excellent points A to E on the course of the resistance coefficient (cw value) with decreasing Mach number. The shock system around the floor changes with decreasing speed. <tb> A-B: <SEP> Fully formed supersonic flow with oblique shock in bow and stern (A). The cw value increases with decreasing speed, because the angle of the oblique shock increases and the influence of the detached head wave increases. In (B), the head shaft is completely detached, no more slash in the bow wave. <b> B-C: <SEP> The head wave at the bow disappears, the stern wave moves forward from the free stagnation point and receives wall contact at the projectile base at (C). The pressure jump in the rear thrust increases the base pressure and reduces the bottom suction, so that the cw value decreases. <tb> C-D: <SEP> The supersonic region of the stern wave becomes smaller. The pressure jump in the rear shock increases the base pressure and reduces the sinking, the cw value continues to fall. <tb> D-E: <SEP> Pure subsonic flow. As the Reynolds number decreases, the influence of toughness on wall friction increases and the coefficient of resistance increases again.

From the analysis of the schlieren photos from KNEU, page 89, and the figure on the velocity profile of the cw value, it can be seen that the rear shock increases the pressure at the projectile base, as soon as the shock system, the free stagnation point (4). leaves and sticks to the bullet base (points C and D).

In the first method, this mechanism is now used to raise the base pressure in the speed range A-B and to lower the cw value in the supersonic range A-B. For this purpose, the property of supersonic flow is used at concave corners.

Fig. 3 shows the behavior of supersonic flow on convex and concave forms in frictionless and frictional modeling: <tb> - Figures 3.1 and 3.2 <SEP> show the flow around a convex corner. It forms an expansion fan (Prandl-Meyer-flow), the speed increases and the pressure drops. <tb> - Figures 3.3 and 3.4 <SEP> show the flow around a concave corner. It forms a slanted shock, the speed drops and the pressure grows.

The first method of the invention serves to lower the soil suction at the projectile base by creating a slanted shock at the rear of the projectile, which adheres to the projectile wall and increases the pressure in the vicinity of the projectile floor. This increases the pressure in the dead water and the base resistance of the projectile decreases.

The implementation of this method requires one or more functional surfaces on the projectile tail according to FIG. 4.1 to 4.4: <tb> - Fig. 4.1a <SEP> shows a bullet tail with circumferential concave groove (13) in the truncated cone at the rear. This results in a circumferential functional surface (14) which, according to FIG. 4.1b, induces a shock (15) adhering to the wall of the spot. The functional surface (14) is tapered against the flow inclined and executed unterkalibrig. <tb> - Fig. 4.2a <SEP> shows a bullet tail with circumferential concave groove (13). This results in a circumferential functional surface which, according to FIG. 4.2b, induces a shock (15) adhering to the wall of the stern. The functional surface (16) is cylindrical with full caliber. <tb> - Fig. 4.3a <SEP> shows a bullet tail with surrounding concave shape corner (13). This results in a circumferential functional surface, which induces according to Fig. 4.3b adhering to the wall of the rear shock (15). The functional surface (17) is cylindrical with a subcaliber. <tb> - Fig. 4.4a <SEP> shows a bullet tail with several circumferential concave mold corners (13). This results in a plurality of circumferential functional surfaces (18), which induce according to Fig. 4.4b more adhering to the wall of the rear bumps (19). The functional surfaces (18) are cylindrical with sub-caliber.

The rear design of the projectile with a flow around concave action surfaces generates another wall-fixed shock system before the dead water (3) and the rear shock wave (5), whereby the base resistance of the projectile decreases.

This first method will now be combined with the second method.

The second method is based on the observation of flow processes with induced axial vortex flows. It is known from fluid mechanics that turbulent flow processes can have a high transport rate. From the aerodynamics it is known that aircraft wings induce wake vortices, which are very stable. From the bionics of bird flight is known that fanned wings of birds of prey induce wake vortices whose total resistance is lower than that of a single wing. In this process, wake vortices are induced as separate vortex filaments, which join together in the wake and "twist" into a common vortex system. Such a flow is energetically favorable and is also used by migratory birds in cruising in triangular formation. This principle is used here to reduce the high turbulence in the wake of the projectile and to structure the flow process around the projectile energetically favorable.

The second method will be explained with reference to FIG. 5.

Fig. 5.1 shows a projectile whose tail (9) has a number of outwardly open longitudinal grooves (20) which are uniformly distributed over the circumference. They start at the transition from the cylindrical part of the projectile to the truncated cone at the rear end, they end in the floor of the bullet and they are open to the rear. These grooves are flowed through longitudinally and fill up the dead water in the wake of the projectile. Because they end up within the floor of the floor, they virtually reduce the diameter of the floor and therefore reduce the bottom suction. When the projectile is stabilized by spin, these grooves rotate with the projectile and induce vortex filaments in the wake.

Fig. 5.1a shows an example of the virtual effect of longitudinal transport grooves: If the bullet tail is retracted to 80% of the caliber, then the bottom surface is 64% of the projectile cross-section. If the transport grooves extend into the bottom of the projectile up to 60% of its diameter, then the aerodynamically effective bottom suction surface is reduced to about 36% of the projectile diameter, which reduces the base resistance overall.

In Fig. 5.2, the transport grooves (21) have been drawn into the cylindrical middle part (8) of the projectile.

The effect of the transport grooves illustrates Fig. 5.3a. The longitudinal grooves are sharp-edged and rotate with the bullet in a clockwise direction (26). This results in each groove a pressure edge (22) with higher pressure and a suction edge (23) with reduced pressure, each of which induces a vortex filament (24) and (25) whose direction of rotation is opposite to that of the projectile. In the wake of the projectile thus creates a rotating system of vortex filaments with the same direction of rotation, opposite to that of the projectile, which then in the stream then to a common vortex braid (28) twisted - Fig. 5.3b. This process causes the dead water to fill up faster and the turbulence is reduced in the wake. The projectile "moves smoother through the air" and the base resistance sinks. The grooves therefore fulfill two functions: <tb> 1. <SEP> Transport groove for replenishing the dead water. <tb> 2. <SEP> Turbulator for creating vortex filaments in the wake.

The shape elements for method one and method two can be combined as shown in FIG. 6 and thus obtains optimized bullet shapes, in which both methods are combined. In FIG. 6.2, a projectile (29) emerges by way of example from the combination of the functional surface with solid caliber (16) from FIG. 4.2 and transport groove (21) from FIG. 5.2.

Such a projectile has over conventional boat-tail projectiles improved muzzle ballistics according to FIG. 7. When exiting the orifice (31), powder gases (30) from the barrel (10) can overflow the projectile before it leaves the barrel and loses the centric guide. As a result, the side impulse drops to the projectile ballistic missile and the precision of the shot is improved.

Finally, FIG. 8 shows the change of the shock system in comparison to conventional projectiles in supersonic flight. Fig. 8.1 shows a conventional projectile (12) with bow shaft (2) and stern shaft (5). FIG. 8.2 shows an optimized projectile (29) with an additional impact system (32), which is produced by the concave functional surface at the rear of the projectile. Because of the longitudinal transport grooves, the thrust system (32) is interrupted in a fan-like manner, with the individual radial segments rotating at projectile speed. As a result, the pressure in the transport grooves and the dead water is increased and the base resistance decreases.

The principle can also apply to non-rotating projectiles that are fired from smooth tubes. Although it loses the turbulence reduction in the wake by twisting the vortex filament, the lowering of the base pressure due to the virtually reduced diameter of the floor remains.

Claims (6)

1. A method for reducing the base resistance of projectiles in supersonic flight, characterized in that at the rear (9) of a projectile an additional wall-resistant shock system (15, 19, or 32) is generated, which increases the pressure in the dead water of the projectile. 2. A method for reducing the turbulence in the wake of projectiles, characterized in that at I lick (9) of a projectile axial vortex filaments (24, 25) are generated, which twist in the wake of a common vortex braid (28). 3. A method for the aerodynamic optimization of projectiles, characterized in that both methods are combined according to claim 1 and claim 2 in a projectile and act together. 4. Aerodynamically improved tail for projectiles for implementing the method according to claim 1, characterized in that at the rear of the projectile one or more radially encircling functional surfaces (14, 16, 17, or 18) with a concave surface transition (13) are present, the one induce additional shock system (15, 19, or 32) in front of the rear shock wave (5). 5. Aerodynamically improved tail for projectiles for implementing the method according to claim 2, characterized in that at the rear of the projectile a plurality of axially extending transport grooves (20, 21) distributed uniformly on the circumference are present, which terminate in the floor and to the outside and to the rear are open, wherein the longitudinal edges (22, 23) induce axial vortex filaments (24, 25) in the wake. 6. Aerodynamically improved tail for projectiles, characterized in that form elements are combined according to claim 4 and 5 and used together in a projectile.