EP0806623B1 - Spin stabilised carrier projectile - Google Patents

Description

The invention relates to a spin-stabilizable, containing a payload projectile according to the Preamble of claim 1.

Projectiles of this type can carry various types of payloads; you will be not only for military but also for civil, for example meteorological, purposes used, and they can be used for different purposes, i.e. soil / soil, soil / air, Air / soil and air / air can be used.

It is obvious that a projectile with the same total volume and weight all the more is more efficient the larger its payload is. The term 'payload' is intended in the context of present invention, the amount of cargo received in the payload chamber be designated. It is therefore endeavored to be available in the payload chamber use the standing space as well as possible, i.e. the payload as tight as possible Place the pack in the payload chamber. The best use of space is achieved when the payload completely fills the payload chamber, i.e. if the cross section of the Payload is the same as the cross section of the payload chamber. Is the payload, as is often the case Case is divided into pillars, so the best use of space is achieved when the pillars are designed in cross-section so that they can be arranged covering the entire area. This could be achieved with columns whose cross-sections are, for example, rectangular, square, are triangular or regular hexagonal. But since there are other requirements for the Pillars are made, such as simple and inexpensive manufacture and Assembly and, if necessary, the possibility of continuing the flight of the columns by means of swirl stabilization To design controlled, generally columns with round cross sections, if necessary with Approaches for arrow stabilization, used with the exception of the achievable space utilization or packing density are superior to all other cross-sections mentioned.

Traditionally, the payload has been released by firing the profile jacket a burst charge and / or the subprojectiles by igniting an ejection charge were expelled from the profile jacket, both for the bursting charge as a considerable amount of explosive was also required for the discharge charge. By doing but you can incorporate a relatively large amount of explosive into the projectile you were forced to limit the payload accordingly, which of course was not wanted.

In order to get by with the smallest possible amount of explosive and thereby one to be able to provide the largest possible payload was therefore attempted instead of a burst charge to detonate the projectile jacket or to eject instead of an ejection charge to provide the payload with an opening load, as this will be significantly lower must as the burst or ejection charge. In principle, the opening charge is only used for Creation of lateral openings in the projectile jacket along several jacket lines, whereupon the parts of the projectile jacket that are released are tangential relative to the rest of the projectile Remove direction; the payload no longer held by the profile jacket thereby released .. The exit of the payload takes place as follows: on the payload, which the projectile jacket exercises due to the swirl of the projectile around the projectile longitudinal axis a centripetal force before its destruction. This centripetal force falls with the Destruction of the projectile jacket by creating the passages in the projectile jacket away by means of the opening charge, so that the payload under the action of centrifugal force leaves its original location and in the tangential direction from the projectile or from Removed the rest of the projectile. The resulting tangential component of speed the payload adds to the axial component of the speed of the payload, which in The amount and direction are the same as the flight speed of the projectile. Is the payload in divided coaxial columns, so each column flies at a certain angle of departure to the trajectory of the projectile, the trajectories of the pillars generating one Form cones, the axis of which is the trajectory of the projectile and the tip of which is the location of the Release of the payload is.

The way of releasing the payload described above can only be successful if if, before opening the projectile jacket, the spin of the projectile is fully on the columns the payload has passed so that it rotates around the project's longitudinal axis from which, when the columns are released, their tangential speed component results.

At this point it should be noted that the payload coincides with the rotation around the projectile axis also a rotation around itself, i.e. an intrinsic rotation called a twist should be granted. The payload rotates both before and after it is released on its own axis; on the advantageous effect of this self-rotation or this Twists will be discussed in more detail below.

In order for the twist of the projectile to be transferred to the payload and the aforementioned tangential component of the speed to emerge from which it is removed from the payload chamber, the payload must be fixed in the payload chamber in such a way that it does not rotate relative to the projectile jacket is in a previously known projectile according to US-603,525 , in which the payload is divided into coaxial columns, the payload chamber formed so that it has axially extending, approximately semi-cylindrical grooves, the diameter of which is equal to the diameter of the columns and in which the columns are arranged are.

To open the projectile jacket as intended in zones along the longitudinal jacket lines, and to keep the amount of explosives required as low as possible hold, the projectile jacket is formed in known projectiles in such a way that it At least approximately axially extending predetermined breaking zones distributed over the circumference possesses along which he is exposed to under the action of the ignited explosive opens the pillars.

In the previously mentioned known projectile according to US-603,525 , such predetermined breaking zones occur, although they are not expressly designated as such, by the above-mentioned grooves with an approximately semi-cylindrical shape, which, as described above, for tangential fixing of the outer layer of the columns lying on the projectile jacket serve relative to the projectile. As already described, these grooves extend axially along the inner wall of the subprojectile chamber and have the consequence that the projectile jacket has varying wall thicknesses in the circumferential direction, the predetermined breaking zones naturally coinciding with the areas of the smallest wall thickness. The predetermined breaking zones are more efficient the more abruptly the wall thickness changes.

For the continued use of the payload, it is of the utmost importance that it is not damaged when the opening charge explodes in the explosive chamber. It must be noted that the projectile already mentioned according to US-603,525 is unsatisfactory in this regard , since the pressure wave that arises when the opening charge explodes naturally affects the relatively weak base plate, so that there is no guarantee that the effect the opening charge is used exclusively to break open the projectile jacket; there is a risk that the payload will be damaged before it is released.

For this purpose, in the projectile known from EP-A-0 698 774 (basis for the preamble of claim 1), as described at the outset, a damping arrangement is provided between the partition wall connected to the projectile jacket and the explosive, which is provided by air or a gap filled with a damping material is formed.

This projectile is ideal for cases in which the profile is disassembled during the flight, i.e. before hitting a target. However, it has to be expected that in a certain number of cases the projectile will hit a target before it is disassembled, and it is a disadvantage of this known profile that in such cases it is not very efficient because it does not have a penetrator . Although projectiles are known, for example from DE-A-36 17 415 , which have a penetrator, the arrangement of a penetrator means an additional design effort.

The object of the invention is therefore seen in a projectile of the type mentioned To create a way that is efficient even when it hits a destination, but at the same time no additional constructive effort is required.

According to the invention, this object is achieved by the features of the characterizing part of patent claim 1 .

Advantageous further developments and preferred exemplary embodiments of the projectile according to the invention are defined by the dependent further claims 2 to 5 .

As described below, the projectile according to the invention is polyvalent. While A variety of conventional similar projectiles are designed to work optimally only by sub-projectile hits, i.e. when the sub-projectiles are released in flight but by projectile hits, also called direct hits, before the subprojectiles are released Unfold by the projectile, the projectile according to the invention is designed such that also has a good effect on a projectile hit. For this purpose, the area a device in front of the subprojectile chamber, but within the ballistic hood or ogive arranged, which in a way hits a direct hit as a penetrator or Plow works. The ballistic hood is advantageously attached to the projectile jacket in such a way that it has the tendency to press the envelope radially away on impact; this has the cheap one Effect that penetration into the target object not only under the action of the penetrator takes place, but that the subprojectiles can also diverge radially. Especially The axial fixation device, with which the subprojectiles, is optimal because it saves weight be tightened together as such a penetrator or plow.

The partition of the new projectile is stable and integrally formed on the projectile jacket or is fixedly connected to the projectile jacket, and due to an axial damping arrangement The forces arising from the explosion of the opening charge act immediately on the Projectile jacket and mainly result in the opening of the projectile jacket, so that the payload is not damaged. This has the further advantage that no explosives is used uselessly, so that one can get by with the smallest possible amount of explosives and can install the largest possible payload accordingly. In a simple one and very effective design, the damping arrangement can be filled with an air Gap can be realized through which the explosive is spaced from the partition is, while it lies in the radial direction close to the explosive chamber wall. The gap can of course also be filled with a damping mass.

So that the projectile jacket really works as intended in the axially running predetermined breaking zones tears open, its wall thickness can be dimensioned so that it in the axial direction of preferably decreases behind to the front; so that the tearing is not in the place of a reinforcement the wall is stopped; mostly, however, project coats become constant Wall thickness chosen, which are easier to manufacture and with which also satisfactory Results are achieved. Functionally disadvantageous and should therefore be avoided but projectile coats with increasing wall thickness.

The payload can be formed by a single column; in most cases it is divided into several coaxial columns arranged side by side.

In order for the payload to move laterally away from the projectile's trajectory, it must perform a rotational movement about the longitudinal axis of the project before it is released or rotate together with the projectile. For this purpose, it must be fastened in the projectile in such a way that it rotates together with the projectile or does not rotate relative to the projectile. In the projectile described in the multiply mentioned US-603,525 , the payload divided into columns is indeed very well fixed in that the columns lying on the projectile jacket engage with approximately half their circumference in grooves with practically semicircular profiles. However, this arrangement has the disadvantage that the columns of the payload must be dimensionally precise, so that only processes with which the required precision can be achieved can be used for their manufacture, such processes generally being quite expensive; Inexpensive processes, especially in the field of non-cutting production, are not considered. In order to avoid this disadvantage, in a preferred embodiment of the projectile according to the invention, the curvatures of the profiles of the grooves are chosen such that they are in any case smaller than the curvatures of the cross sections of the subprojectiles, even if their radii are within a relatively large tolerance range.

The profiles or cross sections of the grooves can be formed by different curves. For reasons of manufacture, grooves are generally preferred, the profiles of which are circular arc sections are so that the grooves themselves have the shape of cylindrical sectors.

The grooves are preferably dimensioned and arranged so that the packing density the payload is practically optimal, i.e. is as dense as it is for columns with the same circular shape Cross-sections are possible at all, although certain deviations in the column mass of the nominal size should be permissible. At the same time, the grooves should be dimensioned and be arranged that the angle of rotation of the payload relative to the payload chamber is as low as possible. This can be achieved if the columns of the payload are arranged in this way are that their envelope in cross-section is an n-corner, preferably a regular one Hexagon, forms, and that n grooves are provided, which are in n / 2 groups of two grooves each are arranged, the angular distance between the groups 360 degrees / (n / 2) and the mutual The distances between the groups are of course greater than the distances between the grooves of one Group.

Because one and the same projectile is used to carry different payloads leaves, and since also the same payloads differ slightly from one another in the axial direction Dimensions due to manufacturing tolerances, it is advantageous if the axial fixing device for the payload by means of a screw on Projectile jacket is attached as this will adjust the length of the payload chamber to the Length of the payload allowed in the sense of tolerance compensation. A further length adjustment The payload chamber can be reached if the axial fixation device is designed so that it has an approach protruding into the payload chamber.

As already mentioned, the payload is often divided into axial columns. The number of Columns are arbitrary upwards and hang among other things. of the properties and purposes of Payload off. In addition to this division in the axial direction, the columns can be transverse to their longitudinal direction be divided into column sections or consist of column sections, whereby these column sections - like the one-piece columns - are not prismatic or non-cylindrical have to be.

In a preferred embodiment of the projectile according to the invention, the payload exists partially or exclusively from subprojectiles. These can - just like for others Payloads are possible - take up the entire length of one column or several stacked on a column. Under the term 'subprojectile' are intended within the scope of this application not only referred to various types of ammunition but all types of payloads be, of which after their release a specific onward flight on a specified trajectory is expected. While with the previously mentioned payloads only a release intended by the projectile at a certain time or place, and the onward flight of the payload was of subordinate importance is placed on subprojectiles the additional requirement of their flight after being released by the projectile in a to continue individually in a predetermined manner. This is a stabilization of the subprojectiles required. It is possible to train certain, especially longer, sub-projectiles in such a way that they fly at least partially arrow-stabilized; however, preference is given to the Swirl stabilization. The required twist of the subprojectiles, i.e. their rotation around itself, is generated to a certain extent automatically with the projectile according to the invention, as follows: as already mentioned above, the one that forms the payload Pillars not only rotate around the projectile longitudinal axis during their flight in the projectile but also a self rotation or a twist. It only has to be paid attention to that this twist of the subprojectile during and after release by the projectile is preserved, otherwise the subprojectiles will not continue to fly stably, but stagger come. The consequences of staggering essentially consist of a loss of energy of the subprojectile, as well as from the restriction in the choice of the shape of the subprojectile, since this taking into account the exit angle, i.e. of the angle between the trajectories of the projectile and the subprojectile, and the swirl stabilization of the Subprojekrtils must be chosen. So that the required twist of the subprojectiles is obtained remains, it is essential that the subprojectiles do not pass through in the area of the partition the explosion of the opening charge will be damaged and that its release will be trouble-free is going on, which is the case according to the invention, because of the explosion of the opening charge indirect or direct forces on the subprojectiles only to a very limited extent be exercised.

Especially when the projectiles or subprojectiles are used as military ammunition are trained, so the hit picture or subprojectile distribution that can be realized with it of great importance for the efficiency of the weapon system, of which they form a part. It is known and easy to see that there are projectiles in which the subprojectiles are not subdivided in the axial direction, results in a subprojectile distribution in which there are the same subprojectiles on a circle at the same distance from the projectile axis, the subprojectiles originally arranged close to the projectile axis get a circle with the smallest radius, while originally at larger distances Concentrate subprojectiles arranged from the projectile axis to the circle mentioned Circles with larger radii can be found. With a suitable design variant of the projectile according to the invention can now be according to the following explanations achieve a much improved hit pattern. As already mentioned, different types can be used Install subprojectiles in the projectiles according to the invention. Taking advantage the fact that the payload chamber is under the effect of the ignited opening charge essentially along a surface line similar to a zipper from behind opens towards the front, a particularly advantageous hit pattern or a special one can be found achieve good distribution of subprojectiles when using subprojectiles that are too several are stacked one above the other in alignment, thus forming the pillars of the payload. The Opening of the payload chamber begins at the back and continues to the front; during the When the payload chamber is opened, the spin-stabilized projectile continues to rotate about the longitudinal axis of the projectile; this means that the sub-projectiles of a column are not released at the same time but that the rearmost sub-projectile is first out of the payload chamber can emerge, whereupon to a certain extent at regular time or angular intervals the other subprojectiles follow the same column in cycles until the last subprojectile the front subprojectile has left the payload chamber. With the resulting Subprojectile distribution essentially get subprojectiles of one column to at least one approximately circular arc. In contrast, subprojectiles are distributed one Pillar, which are all released at the same time, hardly, so that their hit picture on a very short section of a radial beam and shrinks almost to a point. It is easy to see that thanks to the progressive opening from the back to the front Payload chamber the hit probability without the use of additional subprojectiles is significantly increased. Only as a supplement should it be added that the hit picture or the subprojectile distribution also here generally from the distance of the projectiles depends on the longitudinal axis of the projectile. Subprojectiles of a first pillar, closer to the longitudinal axis of the project lies on a circular arc section, whose radius is smaller than the radius of the circle, on which subprojectile a second Get the column close to the projectile jacket.

It has already been mentioned that the projectile according to the invention is designed such that An undisturbed release of the payload or the subprojectile takes place, so that they result maintaining their swirl swirl stabilized and move in a predeterminable manner. There So if the problem of the spin stabilization of the subsidiary floors is mastered, it is possible Provide daughter floors of various designs, especially those whose Front part has a shape for which the external and / or final ballistic performance is optimal.

Fig. 1

ein erstes erfindungsgemässes Projektil in einem Längsschnitt;

Fig. 2

das in Fig. 1 dargestellte Projektil in einem Schnitt längs der Linie II - II der Fig. 1;

Fig. 3

ein durch die Subprojektile des in Fig. 1 dargestellten Projektils erzeugtes Trefferbild bzw. eine Subprojektilverteilung, in einem Schaubild;

Fig. 4

ein zweites erfindungsgemässes Projektil, ausschnittweise, zur Darstellung einer weiteren Dämpfungsanordnung;

Fig. 5a - 5e

fünf Ausführungsbeispiele von aus Subprojektilen gebildeten Säulen, in einer seitlichen Ansicht; und

Fig. 6a - 6c

drei Ausführungsbeispiele von in einer Säule angeordneten Subprojektilen, ausschnittweise, in einer seitlichen Ansicht.

Further details and advantages of the projectile according to the invention are described in detail below on the basis of preferred exemplary embodiments and with reference to the drawing. It shows:

Fig. 1

a first projectile according to the invention in a longitudinal section;

Fig. 2

the projectile shown in Figure 1 in a section along the line II - II of Fig. 1.

Fig. 3

a hit image generated by the subprojectiles of the projectile shown in FIG. 1 or a subprojectile distribution, in a diagram;

Fig. 4

a second projectile according to the invention, in sections, to illustrate a further damping arrangement;

5a-5e

five exemplary embodiments of columns formed from subprojectiles, in a side view; and

6a-6c

three exemplary embodiments of sub-projectiles arranged in a column, in sections, in a side view.

1 , the spin-stabilizable projectile contains a projectile body with a projectile jacket 1 , preferably made of light metal, a ballistic hood 2 attached to the front of the projectile jacket 1 and an igniter 3 attached to the rear part of the projectile jacket 2 , which in this exemplary embodiment is designed as a programmable timer. However, it is conceivable to use another igniter, for example a distance igniter, the ignition of which is triggered by transmission means. The detonator can also be arranged on the front part of the projectile jacket, with the disadvantage that this results in an axially continuous ignition channel, so that the cross-sectional area available for the payload is smaller. The projectile also has a payload chamber 4 for a payload 5 fixed therein, an igniter chamber 6 arranged behind the payload chamber 4 and partially separated by a web-like partition 7 , a guide band 8 and indentations 9 for attachment to a cartridge case (not shown). An axial fixing device, which is designed as a retaining screw 10 , holds the payload 5 fixed in the axial direction and connects the projectile jacket 1 to the ballistic hood or ogive. 2nd The fixed in the fuse area, a known time fuse 3 includes an igniter housing 11, a data receiver coil 12, a power supply 13, for example with a shock generator, an electronic time fuse module 14, an igniter 15, a detonator 16 and arranged in a explosive chamber Oeffnungsladung 17th

An explosive charge is provided as the opening charge 17 , which in the radial direction is in full contact in the detonator or time fuse 3 and / or in a projectile shell part 1A connected to the projectile jacket 1 and in the axial direction at a distance which forms a damping arrangement 18 from the web-like partition 7 is arranged. As such, the opening charge 17 can be arranged directly in the projectile body part 1A , in which case the ignition chain to the detonator or timer 3 or detonator 16 must be ensured. The damping arrangement 18 can be in the form of an air gap between the web-like partition 7 and the opening charge 17 , as shown in FIG. 1 , or for example in the form of a material 18A arranged between the web-like partition 7 and the opening charge 17 with damping properties, as shown in FIG 4 , be formed.

1 and 2 , in this exemplary embodiment the payload 5 consists of a large number of cylindrical sub-projectiles 20 made of heavy metal, which form a plurality of columns 21 in the payload chamber 4 in a coaxial arrangement parallel to the longitudinal axis of the project. The columns 21 are arranged so that their envelope is a regular hexagon in cross section. Eight subprojectiles 20 arranged one above the other form a column 21 , and nineteen such columns 21 are firmly fixed in the payload chamber 4 by the screwed-in axial fixing device 10 . As will be explained further below, this axial fixing device 10 acts as a kind of plow or penetrator in the event of a projectile hit. The projectile jacket 1 is designed in the region of the payload chamber 4 as a hollow cylinder 22 with additional recesses or grooves 23 running in the longitudinal axis direction of the projectile; 2 , six grooves 23 are provided in three groups of two grooves 23 , the groups being distributed at an angular distance of 120 degrees along the circumference of the payload chamber 4, and the mutual distance between the groups being greater than the distance of the subprojectiles one Group. The grooves 23 are eccentrically arranged cylindrical sector-shaped recesses. These recesses or grooves 23 , on the one hand, secure the sub-projectiles 20 or the columns 21 in cooperation with the axial fixing device against movements relative to the projectile jacket 1 , with a certain amount of latitude in the radial direction for accommodating manufacturing-related tolerances of the sub-projectiles, but the relative angle of rotation as small as possible is; on the other hand are formed by the grooves 23 axially extending frangible zones 24 at the points where the smallest wall thickness of the projectile jacket. 1

The mode of operation of the spin-stabilizable projectile to achieve sub-projectile hits is described below. If the detonator 15 is ignited, the projectile jacket 1 or the payload chamber 4 is opened via the detonator 16 and the opening charge 17 , with the subsequent discharge of the payload 5 or the subprojectile 20 relative to the projectile in the tangential direction. Due to the structural arrangement in the area of the opening charge 17 , the shock waves of the detonation act immediately in the radial direction and with a time delay in the axial direction. As a result, the projectile jacket 1 is torn open laterally from the area of the guide band 8 and the payload chamber 4 is continuously opened along the predetermined breaking zones 24 from the back to the front, for example in the manner in which a zipper opens or a banana is peeled; the parts of the projectile jacket 1 thus freed are flung away under the action of the centrifugal force. Due to the damping arrangement 18 , the payload 4 is only slightly pressurized. The release of the undamaged subprojectile 20 is delayed and practically trouble-free. From now on, the sub-projectiles 20 forming the payload will continue to fly, individually spin-stabilized, at an acute angle of departure.

From the sub-projectile distribution or scatter shown in FIG. 3 of such a projectile with one hundred and fifty-two sub-projectiles 20 , the result of the arrangement of the sub-projectiles 20 in nineteen coaxial columns 21 , each with different distances from the longitudinal axis of the project, and the trouble-free 'tier-wise' or cyclical release of the Subprojectile 20 can be seen. For example, the framed group of points 25 can be traced back to subprojectiles 20 from a first column at the greatest distance from the longitudinal axis of the project, ie adjacent to the projectile jacket, point 26A corresponding to the rearmost and point 26B to the foremost subprojectile of this column.

Instead of the described sub-projectile hits, a projectile hit, also called direct hit, occurs in cases in which no ignition took place arbitrarily or involuntarily before the projectile hit a target object. The axial fixing device 10 , which acts as a penetrator, also gives a good end ballistic effect in these cases.

5a-5e show subprojectiles 20A-20E in different versions, only one column 21A-21E being shown in each case. 5a shows subprojectiles 20A which are similar to the subprojectiles 20 described above. 5b contains a column 21B with very short, practically disk-shaped sub-projectiles 20B , which allow very good swirl stabilization. It can be seen from FIG. 5c that a long subprojectile 20C which forms the entire column 21C is also possible; the illustrated embodiment is a partially projectile-stabilized sub-projectile. FIG. 5d shows a sub-projectile 20D , which is also partially arrow-stabilized, two of which correspond to the length of the column 21D . 5e shows subprojectiles 20e which have a spherical shape.

6a-6c show three examples of subprojectiles 20E, 20F, 20G , which are similar in their proportions to the subprojectiles of FIG. 5a , but have differently designed front parts.

Although the payload is always shown as a sub-projectile in the figures, it is nevertheless other types of payloads possible; for example, payloads in the weapons area to create false targets or to blind a flight destination, CHAFF or FLARE payloads be provided. In a single projectile, sub-projectiles can also be different Type and accommodate with different uses. In other areas of application from projectiles according to the invention, for example to meteorological and other purposes, numerous other payloads are conceivable. The invention is intended not limited by the exemplary information but only by the claims be defined.

Claims (5)

1. Spin-stabilised projectile containing a releasable payload (5), having:

   an elongate projectile casing (1) which laterally delimits a payload chamber (4) and the internal contour of which has a plurality of grooves (23) which are at least approximately parallel to the projectile's longitudinal axis and have at least approximately circular-arc-shaped delimitations, for locking the payload (20), which forms at least one column (21) parallel to the projectile's longitudinal axis, against radial and tangential movements and for constituting nominal rupture zones (24) in the projectile casing (1) which are at least approximately parallel to the projectile's longitudinal axis,

   an axial locking device (10) which delimits the payload chamber (4) to the front and is attached to the projectile casing (1),

   a partition wall (7) which delimits the payload chamber (4) to the rear and separates it from an explosives chamber containing an explosive material which rests circumferentially against the wall of the explosives chamber,

   there being a damping arrangement (18) provided between the partition wall (7), joined to the missile casing (1), and the explosive material,

   characterised in that
   the axial locking device (10) for the payload (5) is in the form of a penetrator.
2. Projectile according to claim 1,
   characterised in that
   the axial locking device (10) is bolted to the projectile casing (1) in order to adapt the axial length of the payload chamber (4) to the payload (5).
3. Projectile according to claim 1,
   characterised in that
   the thickness of the material of the projectile casing (1) is constant or decreases in the direction of the profile front.
4. Projectile according to claim 1,
   characterised in that
   the curvature of the cross-sections of the grooves (23) is smaller than the curvature of the cross-sections of the columns (21).
5. Projectile according to claim 4,
   characterised in that
   the columns (21) are so arranged that in cross-section the generating curve of the columns forms a polygon, preferably a regular hexagon, and that n grooves (23) are provided which are distributed in n/2 groups with angular spacings of 360 degrees/(n/2) and every two grooves along the circumference of the payload chamber (4), the distance between the respective groups being greater than the distance between the grooves within a group.

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