A BALISTIC BODY STABILIZED BY FLEXIBLE FINS

Abstract: A fin stabilized projectile (1) in which each fin (2) is con­ nected to the body by a twistable connecting member (3), the location of connection of each fin being rearward of the aerodynamic centre thereof.

Description: A balistic body stabilized by flexible fins ¯¯¯¯¯¯¯¯¯¯ ¯¯ ¯ ¯¯¯¯¯¯¯ ¯ The present invention concerns the stabilization of flying ballistic bodies such as nissifes, rockets and bombs, whether self-propelled or not. In the following all such flying ballistic bodies will be referred to for short as projectlles", it being understood that this term refers to fin-stabilized projectiles. More specifically the invention concerns an improvement in the aerodynamic stabilization of projectiles. The aerodynamic stability is one of the main considerations in the design of projectiles, whether launched from landborne, seaborne or airborne launchers or, whether dropped from an aircraft. Conventionally 4erodynamic stability is achieved by means of stabilizing fins and the larger the surface of the stabilizing fins the better the aerodynamic stability. However, there exist geometric constraints which restrict the space available for launching. For example, in the arming of fighter aircraft, the geometry of the aircraft has to be taken into account; or in case of landborne or seaborne launchers the design of the latter must be considered. In consequence of this, restrictions are put on the span of the stabilizing fins, which restrictions are incompatible with considerations of stability. It thus occurs quite often that projectiles are developed which are insufficiently stabilized with the result that the ballistic performance of such pro jectiles is highly sensitive to atmospheric interfer ences and geometric tolerances which in turn results in inaccurate performance expressed in terms of high dis persion. It has already been proposed to overcome these defects by means of popout stabilizing fins which emerge out of the body of the projectile after its launching but this solution turned out to be complicated and unreliable and to cause an undesired increase of the body weight of the projectile. Another proposed solution for improving the aerodynamic stability of projectiles is to shift the centre of gravity forward. However this solution requires the addition of considerable dead weight at the front part of the projectile which reduces the payload for a given propulsion. A further drawback inherent in large size stabilizing fins, whether static or of the popout type, is the fact that the larger the diameter of a flying projectile the easier becomes its detection by radar. It is accordingly the object of the present invention to overcome the above-described deficiencies by providing a new type of fin stabilization for projectiles. In accordance with the present invention there is provided a fin stabilized projectile in which each fin is connected to the body by a twistable connecting member, the location of connection of each fin being rearward of the aerodynamic centre thereof. It has been found in accordance with the present invention that projectiles with twistably connected fins have a significantly larger stabilizing effect than rigidly connected fins of the same size and geometry. In flight, when the projectile oscillates about its theoretical trajectory in consequence of atmospheric interferences, the twistably connected fins are deflected out of their rest position whereby the oscillation of the projectile is automatically attenuated. The extent to which the connecting members can be twisted can be determined empirically for each specific design. The connecting members may, for example, be in form of torsion bars or helical springs and may be made of metal or of suitable plastic material. The invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a fragmentary diagrammatic elevation of a projectile with twistably connected stabilizing fins according to the invention; Fig. 2 shows in a diagrammatic manner the flight stabilizing effect of twistably connected fin according to the invention; and Fig. 3 is a comparative oscillation attenuation diagram. The projectile diagrammatically illustrated in Fig. 1 comprises a body 1 having at its rear a number of stabilizing fins 2 each connected to body 1 by means of a helical spring 3. In Fig. 2 the fraction of the body 1 shown in Fig. 1 is signified by the longitudinal axis 1'. The point CGB shown on line 1' signifies the centre of gravity of the body. The point CGF on fin 2 signifies the centre of gravity of the fin and the point AC the aerodynamic centre thereof. The projectile moves at an angular velocity q and at a translatory velocity v with an angle of attack cx formed between the trajectory and the axis 1' which means that the projectile oscillates about its theoretical trajectory. In consequence the angle of attack a,each stabilizing fin 2 is deflected by being turned around the spiral spring 3 by which it is connected to body 1 forming with the axis of the latter an angle 6. As a result of the deflection of the stabilizing fins 2 the oscillation of the projectile about the trajectory is gradually attenuated which means that the angle of attack a is diminished, whereby in turn the deflection angle 6 of the fins is equally diminished In Fig. 3 there are shown two attenuation curves of the oscillations of a fin stabilized projectile. The drawn out curve shows the attentuation of a projectile with twistably connected fins according to the invention, and the dashed curve shows the attenuation of a projectile having the same geometry and flying under the same conditions with rigidly connected fins. The curves show the reduction of the angle of attack o as a function of time and while the drawnout curve shows that with a projectile according to the invention the angle of attack cx diminishes rapidly and after three seconds is reduced to less than half the original, the attenuation of the angle of attack a with a conventional missile with rigidly connected stabilizing fins of the same size and geometry, is much slower. The invention is further illustrated by the following example: For imparting a satisfactory aerodynamic stability to a projectile weighing 200 kg. having a diameter of 40 cms. and being 4 m. long and flying at a low altitude at a velocity of 250 m/sec., conventional rigidly connected fins are required having a span of 70 cm. if, however, the fins are twistably mounted in accordance with the invention using a torsion bar of a m2 hardness of 2,000 kg m---7 and each mounted 10 cm. rec behind the aerodynamic centre of the stabilizer, the surface of the fins can be reduced by 20 as compared to rigidly connected stabilizers of the same size and geometry. Such a reduction is of great value, for example, for air to ground bombs mounted on and launched from the lower face of the wings of an aircraft. For further comparison, if the surface area of rigidly connected stabilizing fins were reduced by 20t the stability would be reduced four times and such a missile would perform inaccurately. The features disclosed in the foregoing description, in the following claims and/or in the accompanying drawings may, both separately and in any combination thereof, be material for realising the invention in diverse forms thereof.

Claims: CLAIMS 1. A fin stabilized proiectile in which each fin (2) is connected to the body by a twistable connecting member (3), the location of connection of each fin being rearward of the aerodynamic centre thereof. 2. A projectile according to Claim 1 wherein said twistable connecting member (3) is a torsion bar. 3. A projectile according to Claim 1 wherein said twistable connecting member is a helical spring. 4. A projectile according to any one of Claims 1 to 3 wherein said twistable connecting member (3) is of metal. 5. A projectile according to any one of Claims 1 to 3 wherein said twistable connecting member (3) is of plastic material.

Inventors: MISHNE, DAVID
Applicants: THE STATE OF ISRAEL MINISTRY OF DEFENCE  RAFAEL - ARMAMENT DEVELOPMENT AUTHORITY
Application year: 1986
Main classification: F42B10
IPC codes: F42B10