Extendable Fin

The present invention relates to an extendable fin for a fin-stabilized ammunition unit in the form of a shell, projectile, or the like, which may be fired from a gun, mortar, or the like.

Extendable fins on ammunition facilitate firing, at the same time increasing the stability of the shell in its ballistic trajectory, and therefore the range and probability of hitting.

Extendable fins are previously well known. Thus, for example, a narrow groove in a main fin or the body of the unit, is arranged a spring fin which is supported at its front end, and which when spread springs forth with its rear parts above the main fin or the envelope surface of the unit so that it inclines obliquely outwards and rearwards.

The increased stabilizing effect obtained from the spring fin is determined by the design of the spring fin, and it is desirable to have a comparatively large spring fin area over the main fin or the envelope surface, which should also have a certain, predetermined form.

For want of space, however, there are difficulties involved in obtaining the desired configurations with the previously known spring fins. If, for instance, the previously known spring fin supported at its front end is utilized, this fin has an elongate form which involves an increased degree of spreading which does not give any substantial improvement of the stability of the shell.

The extendable fin according to the present invention solves the above-mentioned problems, and a fin is proposed which uses the same space for the extendable fin as previously and substantially increases the stability of the shell.

The feature that characterizes the new extendable fin is that it comprises two fin parts which are supported separately in relation to each other. In their extended position they are joined together to form the fin.

In further developments of the invention more detailed proposals are given for the positioning of the fin parts in the shell and how these should be designed in order that distinct extension positions are obtained, which are substantially independent of the decrease in acceleration and rotation which takes place in the ballistic trajectory of the ammunition unit.

However, the feature that can mainly be considered to be characteristic for an extendable fin according to the invention will be noted from the claims.

Through the invention it is possible, utilizing the same space as has previously been used for the previously known spring fin, to substantially increase the stability of the ammunition unit. The fin in its extended position becomes more effective and provides a substantially increased distance between the centre of gravity Tp of the shell and the centre of pressure Tc compared with what has previously been obtained.

In the aforesaid further developments extendable fins which functionally are obtained, in practice, notwithstanding a simple design. It is also be possible, in a simple way, to obtain distinct spreading positions of the fin in question, although a special spring for the spreading function, which is awkward from the point of handling, can be avoided.

An embodiment according to the invention will be described in the following with reference to the accompanying drawings, in which

FIG. 1 shows a projectile which is provided with the new extendable fin (solid lines), and also with a previously known spring fin (dot-dash lines),

FIG. 2 in cross-section and enlarged in relation to FIG. 1 shows a first embodiment of the new extendable fin in its retracted position.

FIG. 3 shows in cross-section the fin according to FIG. 2 in its extended position,

FIG. 4 in cross-section shows a second embodiment of the new fin in its retracted position, and

FIG. 5 shows in cross-section the fin according to FIG. 4 in its extended position.

FIG. 1 is intended to show a fin-stabilized shell 1, which is known in itself, which can utilize the extendable fin according to the invention. In the figure, the position of the center of gravity of the shell has been indicated by Tp. Further, two positions of the center of pressure of the shell have been indicated by Tc1 and Tc2. Tc1 indicates the position obtained with the previously known spring fin and Tc2 indicating the position that can be obtained with the new extendable fin. The distance between Tp and Tc is essential to the stability of the shell, and the greater the distance, the greater the stability. At tests which have been made, it has been established that the earlier distance A has been approx. 45 mm, while the distance B can now be increased to approx. 70 mm.

The shell is provided with a fin assembly consisting of main fins 2 and in these, in grooves 3 which emerge at the upper edges 2a, extendable auxiliary fins or so-called spring fins 4 are placed. In a way which is known in itself, each main fin is provided with an auxiliary fin 4.

The new extendable fin comprises two fin parts 4' and 4" which are supported separately in relation to each other, each on its journal support 5 and 6, respectively, which are well known in themselves, and comprise a journal fastened in two opposite walls, and on which the respective fin part is rotatably supported.

The fin parts 4' and 4" are shown in their retracted positions--see the fin parts for the upper main fin 2 in the figure--and also in their extended positions--see the fin parts for the lower main fin 2' in FIG. 1.

In FIG. 1, a spring fin 5 of a previously known type has also been indicated.

In their extended positions, the fin parts 4' and 4" give rise to the center of pressure Tc2, while the spring fins 5 in their extended positions give rise to the center of pressure Tc1. The new fin parts 4' and 4" are placed in the same space as a spring fin 5' which for its extending function also requires an extending spring, not shown, and which thus is not needed for the new extendable fin.

FIGS. 2 and 3 are intended to show a first embodiment of the fin parts 4' and 4" in more detail. The fin parts have straight upper edges 4a and 4b, respectively. Also a part of the lower edge 4c and 4d, respectively, is straight, but at the end of the respective fin part which is not supported, the lower edge is curved so that it has a tapering free end.

The first fin part 4' has a convex rear edge 4e which in the retracted position of the fin part is in coaction with a concave corresponding end surface 4f on the second fin part 4". In the lower corner of the second fin part the concave surface is transformed into a pronounced corner section, which will be eccentrically arranged in relation to the supporting axle 6. The fin parts are sheet formed and have a thickness of approx. 2 mm, and the groove 3 in which the fin parts are placed is somewhat wider. The fin parts are made of some appropriate metal alloy. The first fin part can have a mass of approx. 16 kg and the second fin part a mass of approx. 13 g. The location of the centre of gravity in the first fin part is indicated by Tp' and the corresponding position of the centre of gravity of the second fin part by Tp". In the figure, the centrifugal force acting upon the fin part is indicated by Fc' and Fc", which force forms moments with the distance a' and a" respectively. The acceleration of the shell causes an acceleration force on the respective fin part which forms moments with the distance b' and b", respectively. At a rotation of approx. 3500 r.p.m. and an acceleration of approx. 3560 g, said distances can be chosen as follows: a'=24.5 mm, a"=26.8 mm, b'=5.4 mm and b"=6.5 mm, which for the first fin part gives an acceleration moment of 30.5 kpcm and Fc' a moment of 28.2 kpcm, while the corresponding values for the second fin part will be 24.8 kpcm and 22.9 kpcm, respectively.

During the firing in the barrel of a gun, the fin parts are retracted according to FIG. 2, the first fin part then locking the second fin part, because of the convex fin parts and concave rear edges 4e and 4f, respectively. The first fin part is kept pressed in through coaction with the gun barrel.

As soon as the shell has left the barrel, the first fin part is released, and its moment Fc'.times.a' forces the first fin part out to the extended position. The second fin part is then released and is forced out to its extended position by the moment Fc".times.a". Further, the corner section 4g goes into coaction with the surface 4a by the corner being arranged eccentrically in relation to the supporting axle 6. The coaction between the corner 4g and the surface 4a defines the spreading angle in relation to the longitudinal axis 1a of the projectile (see FIG. 1). The thus locked second fin part also determines the position of the first fin part which is freely supported on the supporting journal 5. The supporting journal 5 and the center of gravity Tp' are then chosen in such a way that in its extended position the first fin part, by the acceleration moment, strives to be turned further rearwards from the extended position thus achieved against the influence of the locking function for the second fin part. The two fin parts will thus be in contact with each other via the straight upper end edges 4a and 4b, respectively. The moment chosen for the centrifugal force on the second fin part is then chosen so that it will be certain that the second fin part is held out in spite of the acceleration moments in the first and second fin parts. A condition for the above is, that the supporting point 6 for the second fin part is farther in towards the center line 1a of the projectile than the supporting point 5.

The fin parts thus extended form a configuration above the upper edge 3a of the main fin which is effective for the stabilization of the shell. The wide (2 times the width of the respective fin part, i.e. twice as wide as previously) and the comparatively short fin is entirely superior to the fin configuration above the edge 2a which is obtained with the spring fin 5 (FIG. 1). It has been proved that a greater degree of extension for the fin 5 than shown in FIG. 1 gives only an insignificant increase of the stability of the shell, and it has therefore not been possible to use this way of increasing the stability.

In the embodiment according to FIGS. 4 and 5, gear arcs 4h and 4i, respectively, have been arranged at the rear edges of the fin parts, and are located at the upper, rear corners of the fin parts. When the first fin part 4h, as above, is extended, the teeth on the two fin parts go into coaction with each other, and a co-ordinated extending function for the fin parts is obtained.

In the fully extended position, the teeth are still in mesh with each other, and the spreading positions of the fin parts will be less sensitive to decreases in the acceleration and/or rotation of the projectile.

Also in this embodiment, the positions of the rotating supports 5" and 6", and also the other moments and masses, are chosen in the same way as described above, and the function for the embodiment shown in FIGS. 4 and 5 is the same as described above.

The invention is not limited to the embodiments shown above as examples, but can be subject to modifications without departing from the scope of the invention as defined in the following claims.

The extendable fin proposed through the invention consists of parts of a simple design, which are easy to integrate in the ammunition for efficient ammunition production.