MEANS FOR INCREASING THE DRAG ON A MUNITION

Abstract: A means for increasing the drag of a munition to effect a course correction having coplanar interlocking pairs of drag plates (24, 26) being laterally extendable from an undeployed configuration, substantially contained within the munition, to a deployed configuration. The drag plates (101, 103) are provided with projections (34, 38) and slots (36, 40), the projections (34, 38) of one of a pair of drag plates being adapted to locate in the slots (34, 40) of the other drag plate in the undeployed configuration and being adapted such that they extend beyond halfway into the munition. A deployment means (60, 62, 70, 72) retains the drag plates in the undeployed configuration until receipt of a control signal and then effects deployment of the drag plates, the distance travelled between the deployed and undeployed configurations being determined by the extent of the projections (34, 38).

Description: Means for Increasing the Drag on a Munition This invention relates to a means of increasing the drag on a munition in order to correct the course of the munition, in particular a drag means for correcting the course of a projectile or unguided bomb. There is a constant military requirement to enhance the accuracy of munitions in order to increase effectiveness and minimise collateral damage. For the attack of high value targets this has led to the development of 'smart' guidance systems which are capable of discriminating between target types and selecting and engaging the appropriate target. However, such an approach is inappropriate for use with munitions such as conventional artillery or unguided bombs, which are used against a wide variety of targets and may use an area target approach for the destruction of multiple low value targets The increased cost and complexity of 'smart' guidance systems is unjustified for such generally low cost munitions and can lead to a loss of flexibility of the weapon system if the munitions are programmed to select only one target type For projectiles and unguided bombs the accuracy requirement is that the munition shows only a small deviation from the target impact point and that several munitions launched at the same target point have an acceptable dispersion The dispersion of projectiles and unguided bombs is often greater in the longitudinal direction, due to discrepancies in launch velocity, than the dispersion in the transverse direction. Thus, the accuracy of such munitions can be greatly increased by correcting the deviances in range. A simple means of correcting these deviances is to provide the munition with a means of altering its air resistance in flight in response to a deviation from some measured trajectory parameter European Patent specification No 0 138 942 discloses a course correction system for gun launched ballistic munitions which measures the launch velocity of the munition, predicts the impact point and relays a signal to the munition to activate a braking means at an appropriate point. The braking means consists either of protruding braking flaps or of nose sections which can be ejected to leave a substantially flat forward face. However, due to the relatively small area presented by the protruding braking flaps or flat nose section, the braking effect produced is limited and the amount of range correction achievable is correspondingly limited. Also, the volume contained within the ejectable nose segments is redundant and reduces the available volume for the payload. Projectiles and the like often have external constraints on size and or weight, for example gun bore sizes or maximum breech pressures and therefore to maximise efficiency of the munition any course correction means should be as compact as possible. Also, the low cost of such munitions and the stresses encountered during launch of projectiles dictate that the system should be inexpensive and fairly simple. It is therefore an object of the present invention to provide an inexpensive, compact and non complex means of creating a large consistent increase in the drag of a munition at a point during flight. Thus, according to the present invention there is provided a means for increasing the drag on a munition to effect a course correction comprising at least one pair of coplanar drag plates being laterally slidable in opposite directions between an undeployed configuration wherein the drag plates are adapted to interlock and be substantially contained within the body of a munition, and a deployed configuration and a deployment means for retaining the drag plates in the undeployed configuration until receipt of a signal from a control means, wherein each drag plate has at least one projection and at least one slot, the extent of the projections being adapted such that the distance from the tip of the projection to the outer edge of the drag plate measured along a line parallel to the sliding direction of the drag plate is greater than half the width of the interlocked drag plate pair along that line and the slots being adapted such that in the undeployed configuration each projection is accommodated in a complementary slot, the extent of the drag plate deployment being determined by the extent of the projections. Using co-planar interlocking drag plates allows the drag plates to have a large surface area whilst minimising the volume of the drag means in the munition. As the distance from the tip of the projection to the outer edge of the drag plate is greater than half the width of the interlocked drag plate, in use the projections extend beyond halfway into the munition. This means that the allowable extension of the drag plates to the deployed configuration is high and virtually all of the surface area of the drag plate can be deployed outside the body of the munition whilst the ends of the projections are retained within. The amount of extension is therefore determined by the extent of the projections as clearly the longer the projection then the greater extension possible. Pairs of plates are used as the projections of one plate have to be able to slide out of the slots in the other plate which, to maximise the surface area of the plates, requires two plates sliding in opposite directions. In order to maximise the increase in drag that the plates provide, the drag plates are preferably adapted such that in the undeployed configuration the circumference of the interlocked drag plates is substantially the same as the circumference of the munition at the point from which the drag plates extend. This ensures that the maximum possible surface area is being presented by the plates when in the deployed configuration and yet the plates can be entirely contained within the munition in the undeployed configuration. Preferably there are two pairs of drag plates, a first pair of drag plates being laterally extendable in opposite directions along a first axis and a second pair of drag plates being laterally extendable in opposite directions along a second axis wherein the first axis is substantially perpendicular to the second axis. Four drag plates extended in perpendicular lateral directions present a large surface area thus giving a large increase in drag, whilst only requiring two layers of plates. Additional plates would take up greater space in the munition and would result in overlap of the plates when extended. Overlap of plates would reduce the weight/space to drag effect efficiency and would not greatly increase the drag performance. Conveniently the drag plates are adapted such that both drag plates of a pair are identical. This requires that the drag plates be asymmetrical and that the interlocked drag plate pair be rotationally symmetric. Use of a single plate design in the drag means offers obvious advantages in terms of cost of manufacture and ease of assembly and repair. According to an embodiment of the invention a munition comprises a payload volume and a fuze having a control means and a drag means The control means determines deviations of the trajectory of the munition from a nominal trajectory and, at a determined time, generates a control signal to effect deployment of the drag means. Preferably the control means comprises a GPS receiver and a logic unit The GPS (Global Positioning System) receiver and logic unit can locate the position of the munition by triangulation with GPS satellites in known orbits around the earth as is well known in the art and can compare the evolving trajectory of the munition against a stored nominal trajectory. This nominal trajectory can be based on a standard range and need not be programmed for separate firing locations although specific target locations could be chosen if required. Range corrections are effected by deploying the drag plates at differing times. According to a further embodiment of the invention the munition is a spin stabilised munition and the deployment means comprises at least two retaining pins, the retaining pins being arranged so that at least one retaining pin passes through each drag plate in the undeployed configuration. A retaining pin removal means is provided for removing the retaining pins from the drag plates on receipt of a control signal and the drag plates are freely slidable between the undeployed and deployed configurations such that, in use, removal of the retaining pins allows the centrifugal forces on the munition to cause the drag plates to slide to the deployed configuration. Deployment of the drag plates under the action of centrifugal forces increases the simplicity of the device and removes the need for internal pyrotechnic devices to push the plates outward. The centrifugal forces experienced by a spin stabilised munition are quite sufficient to effect deployment of the plates and retain them in the deployed configuration during flight. Removable pins are again simple and inexpensive and can be made robust. Preferably there are two retaining pins to reduce costs and complexity. Further advantages and embodiments will be shown by way of example only with reference to the following drawing in which; Figure 1 shows a projectile having a drag means according to one embodiment of the invention, Figure 2 shows an exploded view of the drag means of figure 1, Figure 3 shows the top of the projectile of figure 1 with the drag plates extended, Figure 4 shows an alternative design of drag plate Figure 5 shows an embodiment of the drag means using the drag plate design of figure 4 Referring now to figure 1 a spin stabilised projectile, generally indicated 1, has a fuze 2 located forward of the payload volume 3 At the base of the fuze 2 is a drag means consisting of a forward base plate 20, two pairs of drag plates 22, 24 and a rear base plate 26. The drag means is shown more clearly in figure 2. Rear base plate 26 is connected to the forward base plate 20 by circular connecting member 30 (connection not shown in exploded view). Located between the base plates 26 and 20 are rearward drag plate pair 24, consisting of drag plates 101 and 103, and forward drag plate pair 22, consisting of drag plates 105 and 107. Each of the drag plates is generally semi-circular, having a diameter the same as that of the munition and a semi-circular recess 32 to enable the plate to fit around connecting member 30 when in the undeployed configuration. Base plate 101 has projections 34 and slots 36 and base plate 103 has projections 38 and slots 40, the drag plates being arranged such that in the undeployed configuration projections 34 are accommodated in slots 40 and projections 38 are accommodated in slots 36. Similarly projections 42 of plate 105 fit into slots 44 of plate 107 and projections 46 of plate 107 fit into slots 48 of plate 105 when in the undeployed configuration. Projections 34 and 38 of plates 101 and 103 respectively are provided with rearward projecting studs 50 which locate into tracks 52 in rear base plate 26. Similarly forward facing studs 54 on projections 42 and 46 locate into tracks (not shown) in the forward base plate 20 which run perpendicular to the tracks 52 in the rear base plate 26. Each drag plate is also provided with a hole 56, the holes 56 being arranged so that when all the plates are in the undeployed configuration then the holes 56 on plates 101 and 105 line up, as do holes 56 on plates 103 and 107. Two retaining pins 60, 62 pass through the holes 56 in the drag plates in the undeployed configuration and locate in recesses 64, 66 in the rear base plate 26. The retaining pins 60, 62 also pass through holes (not shown) in base plate 20 and are held by gas actuators 70, 72. The recesses 64, 66 and tracks 52 in the rear base plate do not extend through the rear base plate 26 so that rear base plate 26 acts as a complete seal between the drag plates and the rest of the projectile to prevent entry of moisture or dirt Similarly the gas actuators and retaining pins are sealed to prevent moisture entering the rest of the fuze through the holes in forward base plate 20 A removable cover may be provided around the drag plates in the undeployed configuration to protect the drag means during storage This cover would be removed prior to loading similarly to the removable covers provided around the driving band of shells Alternatively a wax coating could be applied to the perimeter of the projectile around the drag means which would prevent dirt from entering and jamming the drag plates but which would not interfere with the deployment of the drag plates The plates may be conveniently made from either aluminium or steel The thickness would typically be 3mm for steel plates and 5mm for aluminium plates Thus the drag means can offer a substantial increase in drag whilst only taking up approximately 20mm in the projectile The plates may conveniently be made by stamping out the shape and friction welding the studs onto the projecting members Stamping offers an inexpensive and non complex method of manufacture Alternatively the plates could be machined to the desired shape, but thicker than required, then the top layer could be removed to leave the required studs The base plates are structural parts of the fuze and are constructed from the same material as the fuze casing In use the projectile is launched with the drag plates held in the undeployed configuration by retaining pins 60, 62 A GPS antenna 4 in the front of the fuze receives GPS signals and a processor 5, powered by battery 6 tracks the projectiles trajectory and compares it to the desired trajectory, however, it will be apparent to one skilled in the art that other sensors could be used to monitor the trajectory of the projectile An algorithm is used to compare the actual with the required trajectory and calculate a deployment time for the drag device The solution is continually refined throughout the flight until the deployment time is reached, at which point a control signal is generated The control signal causes discharge of capacitors which activate gas actuators 70, 72 to pull the retaining pins 60, 62 forward The pins are pulled forward through the forward base plate rather than through the rear base plate 26 to prevent the possibility of launch stresses from falsely actuating the drag means Once the pins 60, 62 have been removed the centrifugal forces due to the projectiles spin cause the drag plates to move outwards Studs 50 located in tracks 52 allow the plates 101 and 103 to move in a lateral direction only so that the plates slide outward until the studs reach the end of the tracks 52 Similarly studs 54 in plates 105, 107 slide along the tracks in the forward base plate which are perpendicular to tracks 52 It will be apparent that the length of the tracks in the base plates and the positioning of the studs on the projections define the amount of extension of the plates The top of a projectile with the drag plates in the deployed configuration is shown in figure 3 The extension of the plates is such that when fully deployed the corners of the plates are overlapping The corners overlap as the studs on the projections are not secured in the tracks in the base plates and so without being sandwiched between the other pair of plates and the base plate the drag plates could come completely away from the projectile An alternative design of drag plate are shown in figures 4 The drag plate 201 has projections 203, 205 which are adapted to locate in slots 213 and 215 respectively of another identical drag plate whilst leaving space for a central connecting member The plate 201 has two holes 220 located such that one design of plate can be used and the holes of layers of plates will align A stud 230 is located on projection 203 to locate in a track in a base plate As only one stud is used per plate the stud 230 is elongated in the direction of movement of the plate to prevent rotation of the plate about the stud. The projection 205 is present to aid in deployment of the plates and could be omitted. Figure 5 shows a drag means using the drag plate of figure 4 with the plates in the deployed configuration. The extension of the plates is such that the plates do not overlap at all when fully deployed. Therefore to retain the plates in position a middle base plate 90 is provided. A further alternative design of drag plate is shown in figure 6. Here the drag plate 301 has only one projection 303 and one slot 3 13 Again the drag plate is asymmetrical but rotationally symmetric when interlocked with another identical plate. The projection 303 is thicker than the drag plate 301 , being raised above one surface by a small amount, typically not greater than the thickness of the plate 301 itself. The increased thickness of the projection 303 extends to the outer edge of the drag plate. The raised part of the projection 303 locates into a groove in a base plate, which aids in deployment of the plates and gives rotational rigidity A stud 330 is located on the raised surface of projection 303 and locates into a track in the groove of the base plate to define the amount of extension of the drag plates The projection 303 is only raised on one side of the plate 301, the other side being flat. This keeps manufacture of the plates as simple and inexpensive as possible and is necessary when there is no middle base plate, to enable the plates to slide over one another. It will be apparent however, that where a middle base plate is used this could incorporate grooves to locate raised parts of the drag plates or tracks to accommodate studs on the drag plates, either in addition, or as an alternative, to those for location in the front or rear base plates. The flat surface area presented by the extended drag plates is approximately twice the area of the cross section of the munition. Deployment of the plates increases the drag on the munition by about two and a half times which allows for a significant correction in range to be made. The increase in drag effected by the drag plates is therefore approximately double that available from known braking means.

Claims: CLAIMS A means for increasing the drag on a munition to effect a course correction comprising at least one pair of co-planar drag plates being laterally slidable in opposite directions between an undeployed configuration wherein the drag plates are adapted to interlock and be substantially contained within the body of a munition, and a deployed configuration and a deployment means for retaining the drag plates in an undeployed configuration until receipt of a signal from a control means, wherein each drag plate has at least one projection and at least one slot, the extent of the projections being adapted such that the distance from the tip of the projection to the outer edge of the drag plate measured along a line parallel to the sliding direction of the drag plate is greater than half the width of the interlocked drag plate pair along that line and the slots being adapted such that in the undeployed configuration each projection is accommodated in a complementary slot, the extent of the drag plate deployment being determined by the extent of the projections A means for increasing the drag on a munition according to claim 1 wherein the drag plates are adapted such that, in use in the undeployed configuration the circumference of the interlocked drag plates is substantially the same as the circumference of the munition into which the drag means is fitted A means for increasing the drag on a munition according to claim 1 or claim 2 wherein there are two pairs of drag plates, a first pair of drag plates being laterally extendable in opposite directions along a first axis and a second pair of drag plates being laterally extendable in opposite directions along a second axis wherein the first axis is substantially perpendicular to the second axis A means for increasing the drag on a munition according to any preceding claim wherein the drag plates are adapted such that both drag plates of a pair are identical 5. A means for increasing the drag on a munition substantially as hereinbefore described with reference to the accompanying drawings of figures 1-5. 6. A munition having a course correction device comprising a payload volume and a fuze having a control means and a drag means according to any preceding claim wherein the control means determines deviations of the trajectory of the munition from a nominal trajectory and, at a determined time, generates a control signal to effect deployment of the drag means. 7. A munition having a course correction device according to claim 8 wherein the control means comprises a GPS receiver for determining the location of the munition during flight and a logic unit capable of determining deviations of the munition from a nominal trajectory from the GPS data and generating a control signal to effect deployment of the drag plates in order to correct the course of the munition. 8. A spin stabilised munition incorporating a means for increasing the drag according to any of the claims 1-5 wherein the munition is a spin stabilised munition and the deployment means comprises at least two retaining pins, the retaining pins being arranged so that at least one retaining pin passes through each drag plate in the undeployed configuration, and a retaining pin removal means for removing the retaining pins from the drag plates on receipt of a control signal wherein the drag plates are freely slidable between the undeployed and deployed configurations such that, in use, removal of the retaining pins allows the centrifugal forces on the munition to cause the drag plates to slide to the deployed configuration. 9. A spin stabilised munition according to claim 8 wherein the number of retaining pins is two 10. A munition substantially as herein before described with reference to figures 1-6 of the accompanying drawings.

Inventors: SIDFORD RUFUS WILLIAM, SIDFORD, RUFUS, WILLIAM
Applicants: SECR DEFENCE, SIDFORD RUFUS WILLIAM, SIDFORD, RUFUS, WILLIAM, THE SECRETARY OF STATE FOR DEFENCE
Application year: 1997
Main classification: F42B10
IPC codes: F42B10