GB2365952A - Drag brake for a munition - Google Patents

Abstract

A drag brake for use with a munition in flight so as to effect course correction thereof comprises a first portion 20 for connection to the munition and a second portion comprising a plurality of flaps 4 hinged at 18 to the first portion, the first and second portions being formed from a single piece of material. In an undeployed configuration the flaps lie against an outer surface 14 of a fuze 2. When a signal is received from a control means the flaps pivot outwardly to the position shown in fig.6 and are held in this position by tapes 16. The drag brake may be formed of metal or a composite material. The control means may comprise an GPS receiver.

Description

1 2365952 Drag Brak This invention relates to a means of increasing the

drag on a munition in order to correct the course of the munition, in particular to a drag device 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 use an area target approach for the destruction of multiple distributed targets. Although the increased cost and complexity of "smart" guidance systems is unjustified for such generally low cost munitions there is considerable scope for reducing the munition's dispersion "footprint" particularly at larger ranges.

The dispersion of projectiles and unguided bombs is generally greater in the longitudinal direction due to the nature of the system errors, e.g. discrepancies in launch velocity, than the dispersion in the transverse direction (Longitudinal dispersion also increases with the range of the target). Thus the accuracy of such munitions can be greatly increased solely by reducing 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. Such "course corrected" munitions are deliberately fired "long" and the munition is then corrected from its overshoot trajectory during flight.

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 in the flight of the munition. The braking means consists either of protruding braking flaps or of nos e 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 volume which is available for the payload.

Projectiles and the like often have external constraints on size and 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 yet robust.

PCT International Application W098/01719 discloses a course correction system for a munition where the drag device comprises a set of coplanar interlocking pairs of drag plates which are laterally extendable from an undeployed configuration, contained substantially within the munition, to a deployed configuration. The system of interlocking plates is, however, complex. Furthermore, the retaining pins which prevent premature deployment of the drag flaps are subjected to high shear stresses which result from the munition's rotation and are therefore either at risk from breaking or from failure to release the drag plates.

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 its flight to the target.

According to the present invention therefore, there is provided a drag brake for use with a munition in flight so as to effect course correction thereof said brake comprising a first portion of a drag means adapted for connection of the brake to the munition-, a second portion of a drag means located generally radially outwards of the first portion and comprising a plurality of drag flaps each of which is connected to the first portion by hinge means, the drag flaps being capable of moving between an undeployed configuration wherein the flaps lie on the outer surface of the munition between the nose of the munition and the hinge and a deployed configuration wherein the flaps project outwardly from the munition and at a predetermined angle thereto; deployment means adapted to retain the flaps in the undeployed configuration until receipt of an appropriate signal and thereat to effect deployment of the drag flaps; and locking means which is activated by said deployment means and which is adapted to retain the drag flaps in the deployed configuration wherein the first and second portions of the drag means are formed from a single piece of material.

Constructing the drag means from a single piece of material reduces the number of separate components and complexity of the drag brake. There are therefore sizeable cost savings associated with forming the drag brake in this way. The drag means could be constructed ftom a thin sheet of pressed metal. or alternatively from a single piece of composite material.

Composite materials are macroscopic combinations of two or more distinct materials. Many, such as wood, are naturally occurring but the term "composite material" will be taken here to mean synthetic, man-made materials possessing high strength and/or stiffness relative to weight. These modem structural composites are a blend of two or more components, one of which is made up of stiff, long fibres (hereinafter referred to as "composite fibres") and the other of which is a binder or matrix (hereinafter referred to as the "composite matrix") which holds the fibres in place.

The drag brake of the invention can therefore be formed by blending a mixture of composite fibres and a composite matrix into a single piece of composite material which can then be suitably shaped to form the drag means.

Preferably, composite materials are chosen because they are simple to manipulate and also will not interfere with any radio control systems that may be mounted within the munition. Therefore, preferably, the drag means is constructed from a composite material which has been stiffened in the region of the drag flaps and arranged to be left substantially unstiffened in the region of the hinge.

Using drag flaps which lie on the outer edge of the munition, between the nose of the munition and the hinge when in the undeployed configuration, minimises the volume of the drag means actually located within the munition and also allows the flaps to have a large surface area. The fact that the drag flaps are hinged upon the outer surface of the munition means that all the of the drag flaps' surface area can be deployed as a drag device. Preferably the drag flaps should be stowed, when in the undeployed configuration, within a complementary recess in the outer surface of the munition in order to reduce the aerodynamic drag that the munition experiences before the flaps are deployed.

Once deployed the drag flaps experience forces due to air resistance. Without the facility to lock the flaps in place these forces would cause the drag flaps to pivot about the hinge until they assumed the most aerodynamic configuration, i.e. until the aerodynamic and rotational forces acting on the flaps were in equilibrium (For non-rotating munitions this would result in the flaps lying along the outer surface of the munition between the hinge and the tail; for rotating munitions the flaps would still point away from the direction of flight but would be held away from the munition's surface by the above mentioned forces). Such an "inversion" would obviously reduce the efficiency of the drag flaps and so it is essential to include locking means to lock the drag flaps in the deployed configuration. This can be achieved by, for example, extending a strip of unstiffened composite fibre from the tip of the drag flap and then anchoring this material strip forward of the hinge. By choosing an appropriate length of strip it can be arranged to become taut as the drag flaps project at a particular angle from the longitudinal axis of the munition (as measured from the munition's nose) thereby locking the drag flaps in that position. In an alternative embodiment of the invention adjacent drag flaps could be connected to each other by one or more strips of unstiffened composite fibre. Providing that the material is not elastic this would also, for the correct length of interconnecting strip, lock the drag flaps in the deployed configuration and prevent inversion thereof The main purpose of the composite fibre is to carry the loads experienced by the drag flaps when the munition is in use and the drag flaps have been deployed. There are many fibre composites that could perform this function but preferably the fibre material is chosen from Kevlar fibre, glass fibre, Dynema, or carbon fibre, the most common composite fibres.

Preferably the composite fibre should be Kevlar fibre. Although carbon fibre is stiffer than Kevlar (or glass fibre) it is more brittle. Glass fibre on the other hand is less stiff than Kevlar and is more susceptible to abrasion. Therefore, Kevlar is preferably chosen since it has reasonable stiffness (Young's Modulus 130 GPa) and abrasion resistance, a factor that is important when considering the requirements a material must satisfy for use as the hinge.

In the deployed configuration a number of different forces will act on the drag flaps. Aerodynamic braking loads are caused by air resistance and, for spin stabilised munitions, the munition will also experience loads due to its rotation. For spinning munitions the rotational loads are much areater than the aerodynamic loads.

i= Spin stabilised munitions rotate about the longitudinal axis of the munition. This rotation gives rise to forces acting radially from the rotation axis and also forces which act tangentially to the radial forces. Once in the deployed configuration the drag flaps experience both of these forces. The fibres within the composite fabric can be isotropic so that the structure has the same strength in every direction. However, it is preferable to maximise the strength of the composite fabric in the radial and tangential directions by ensuring that the fibres in the cloth are arranged to lie substantially along two perpendicular axes. It is further preferable to ensure that one of these fibre axes is aligned with the radial rotational loading and that, consequently, the other axis is aligned with the tangential rotational loading. This particular fibre alignment means that, in the undeployed configuration, one of the fibre axes will be aligned with the longitudinal axis of the munition.

Preferably the angle 0 subtended by the drag flaps and the longitudinal axis in the direction of motion should be close to but not actually 90 degrees. At 0 = 90 degrees there would be significant instabilities which would cause the flaps to wobble thereby reducing the stability of the munition and the effectiveness of the drag device. A suitable range of values would be 80:50<90".

In order to improve the effectiveness of the drag flaps the composite fibre needs to be stiffened. This stiffening is achieved by using a matrix to support the fibres. There are two main matrix materials, these being thermosets and thermoplastics, and either could be used to provide the structural stiffness required. Once fabricated the stiffened drag flaps will consist of a core of composite fabric surrounded by the chosen matrix material. There are in general two ways of fabricating such an arrangement. Either the matrix material element can be fabricated separately and then bonded to the composite fabric or the matrix material and composite fabric could be formed in a single process. The first method of fabrication could utilise either a thermoplastic or thermoset material. However, in order to have a simpler and therefore cheaper method of fabrication it is preferable to exploit the ability of thermoplastic materials to be injection moulded and to produce the stiffened drag flap in one process. In this method of fabrication the composite material is placed into a mould which is then injected or compressed with thermoplastic material. Conveniently the composite fabric can be laid out in a desired configuration prior to moulding so that a solid disc of stiffened material results.

Composite fabrics are difficult to cut accurately with conventional knife tools and so the desired profile of drag flaps is preferably cut either with a laser or water jet cutter. This can be achieved by using a CNC (computer numerically controlled) cutter.

Once the drag flaps have been fabricated the hinges need to be formed. This is conveniently done by placing the stiffened drag flaps between heated knife edges and then exerting pressure. This squeezes out the thermoplastic material from the hinge region transforming the stiff board into a flexible joint. This process can conveniently be incorporated into the fabrication process to reduce the overall number of stages required to make the drag flaps.

In the event that the device is made from a metallic material instead of a composite material, the material can be reduced in thickness in the region of the hinge by stamping, pressing or other methods which are well known in the art.

The number of components involved in the drag brake is greatly reduced over prior systems by virtue of the fact that the drag flaps, flexible hinge and locking means are constructed from a single sheet of material. Since the stiffening process is preferably incorporated within the fabrication process the drag brake is simple to construct and has few separate parts.

Conveniently there is further provided deployment means adapted to retain the drag flaps in the undeployed configuration until receipt of a control signal. Such deployment means can be achieved in a number of ways. In one embodiment the drag flaps are secured in the undeployed configuration by internal locking pins. A locking pin removal means is further provided for removing the locking pins from the drag flaps on receipt of the control signal such that, in use, the removal of the locking pins allows the rotational and/or aerodynamic forces acting on the munition to cause the drag flaps to move to the deployed configuration. In an alternative embodiment the drag flaps are secured in the undeployed configuration by means of a clip which encircles them. Once again removal means are provided such that, on receipt of the control signal, the retaining clip is removed and the flaps are free to move to the deployed configuration.

The deployment of the drag flaps should preferably be symmetrical about the longitudinal axis of the munition so as to prevent any significant off axis forces which would result in an increase in the dispersion in the transverse direction in the fall of the munition. Therefore, in order to ensure a symmetrical deployment there should be an even number of drag flaps mounted about the periphery of the munition.

According to a further aspect of the invention a munition comprises a payload volume, a fuze having associated control means and a drag brake according to the invention. The control means detects deviations of the trajectory of the munition from a nominal trajectory and, at an appropriate time, generates a control signal to effect deployment of the drag device, 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. Range corrections are effected by deploying the drag flaps at appropriate times as required.

The control means does not have to comprise a GPS system and other control means are possible. For example, the munition could be tracked from a ground station which would then transmit a signal to the munition. Alternatively, the munition could carry a GPS receiver and transmitter only. In this embodiment the GPS data is fed back to a ground based processor which calculates the position of the munition based on that data, works out the course correction solution and then transmits this back to the munition. An inertial guidance based system could also be employed. Other methods of arranging for activation of the drag brake means within the scope of this invention will be readily apparent to the skilled person.

Further advantages and embodiments will be shown by way of example only with reference to Z the following drawings in which-, Figure I shows a projectile having a drag brake according to the invention flaps in undeployed position, Figure 2 shows the projectile of Figure I with the flaps in deployed position, Figures 3 to 6, show an embodiment of the invention, in which:

Figure 3 shows in section the fuze and drag flaps prior to assembly.

Figure 4 shows the assembled arrangement with drag flaps in the undeployed configuration; Figure 5 shows in plan the drag means only with flaps in the deployed configuration; Figure 6 shows the assembled fuze with the drag flaps in the deployed configuration; Figures 7 and 8 show a further embodiment of the invention, in which the drag brake is carried in a different manner in the fuze of the munition:

Figure 7 shows in section the arrangement prior to complete assembly; Figure 8 shows the drag device only in plan with the flaps in the deployed position, Referring now to figure I a spin stabilised projectile, generally indicated 1, has a fuze 2 located forward of the payload volume 3, About the periphery of the fuze 2 is a drag means consisting of eight drag flaps 4. Figure I shows the drag flaps in the undeployed configuration. Figure 2 again shows the spin stabilised projectile I but with the drag flaps 4 now in the deployed configuration.

According to this embodiment of the invention, the drag flaps will be mounted within a standard NATO fuze. The external profile of standard NATO fuzes is described in STANAG 2916 Annex A and a slightly modified version of this type of fuze is shown in Figure 3. The fuze 2 is shown in section in Figure 3 and has a substantially conical body within which is contained, amongst other things, guidance systems, on-board power supplies and fuze electronics (a GPS antenna 6, processor 8 and battery 10 are depicted). The fuze 2 is conveniently screwed into the main payload volume (not shown in Figure 3) and is therefore provided with a screw threaded portion 12 in the base 13 of the fuze. The fuze shown has been modified such that the outer surface of the fuze 2 is recessed in the region 14.

Figure 3 also shows the drag means which consists of drag flaps 4 connected via flexible hinges 18 to a radially inner portion 20. Locking tapes 16 are attached to the radially outward ends of the drag flaps 4. The drag means is shown prior to its attachment to the base 13 of the fuze.

Figure 4 shows the undeployed configuration of the drag means wherein the drag flaps 4 are now stowed within the recess 14. The recess 14 is designed to complement the shape of the drag flaps 4 and to accommodate the drag flaps and locking tapes 16 such that in this undeployed configuration the drag means is contained within the aerodynamic profile of the fuze.

A removable cover (not shown) may be provided around the drag flaps in the undeployed configuration to protect the drag means during storage. 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 flaps 4 but which would not interfere with-their deployment.

Figure 5 shows a plan view of the drag means of Figure 3. The drag flaps 4, which comprise the outer portion of the drag means, are connected by hinges 18 to the inner portion 20 of the drag means. This inner portion has a hole 22 whose diameter is equal to that of the screw thread 12 (shown in Figure 3).

The drag flaps 4 can conveniently be made by laying down a Kevlar tape to form equally spaced flaps. In Figure 5 there is shown an arrangement (for example only) in the form of a double cross, with one cross being rotated by 45' with respect to the other, to form 8 equally spaced flaps. This arrangement can then be placed in a mould and injected or compressed with thermoplastic material to form a solid disc. The disc can then be trimmed using a CNC controlled laser to produce the individual profile for each drag flap 4.

The hinges 18 can be formed by placing the trimmed flaps 4 in turn between heated knife edges as described above. The locking means which are adapted to keep the drag flaps 4 in the deployed configuration can be formed as part of the fabrication process. Kevlar tape can be added to the double cross arrangement described above to project from the end of each flap and being of a suitable length for connection to the nose of the fuze 2. This is shown as a projecting tape 16 in figure 5. This tape 16 is then excluded from the injection or compression moulding process to ensure it remains unstiffened.

For the NATO fuze reference above the flaps will typically have a maximum length of 43 mm and a width of 23 mm tapering to 18 mm towards their tips. For flaps of thickness 2mm, and allowing a suitable design safety margin, the Kevlar hinge will be 0. 1 mm thick (this is assuming that the composite fibres are arranged along two perpendicular axes as specified above).

Figure 6 shows the drag means and fuze arrangement of Figure 3 once it has been assembled. The inner portion 20 is now in contact with the base 13 of the fuze (This can either be done by attaching the inner portion 20 to the base 13 with a suitable adhesive or the inner portion can be held in place by clamping it between the fuze 2 and the projectile I - not shown in Figure 6). The locking tapes 16 have also been attached to the fuze in the region of the recess 14 which is closest to the nose of the fuze, The arrangement shown is equivalent to the deployed configuration of the device.

Figure 7 shows a second embodiment of the invention which differs from the first embodiment only in the way the drag means is located with respect to the fuze. Figure 7 shows a view in section of this alternative configuration wherein the solid disc of composite material that comprises the drag flaps 4, hinges 18 and inner portion 20 is mounted within the body of the fuze as opposed to being attached to the base 13 of the fuze 2. The fuze 2 is divided into two sections as shown in figure 7. The solid disc of composite material has a number of small holes 24 (each considerably smaller than the hole 22 above) punched into its central section as shown in Figure 8. The holes 24 are arranged so as to allow the structural connections 26 to link the two sections of the fuze 2 and also to allow any electrical connections to be made between the two sections. This is the preferred of the two assembly embodiments since the large hole 22 defined in the first embodiment could potentially compromise the structural strength of the composite material.

The drag flaps 4 of this second embodiment will be stored and operated in a manner which is analogous to that depicted in Figures 3 and 6.

In use (both embodiments above) the projectile is launched with the drag flaps 4 retained in the undeployed configuration until receipt of a control signal. The GPS antenna 6 in the front of the fuze 2 receives GPS signals and a processor 8, powered by battery 10 tracks the projectile's 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 as described above, 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 and the drag flaps are released.

The deployment means which controls the deployment of the drag flaps could consist of internal locking pins to hold the flaps until deployment or alternatively the drag flaps could be surrounded by a restraining sheath of material. If this material is provided with some means of severing itself, e.g. a pyrotechnic strip, then the flaps can be deployed by causing the release control signal to effect detonation of the pyrotechnic strip thereby allowing the restraining sheath to fall away and the drag flaps to deploy. One advantage of using a sheath of material to restrain the drag flaps is that the sheath can easily be designed to produce a smooth aerodynamic fuze profile.

Once the retaining mechanism has been released the centrifugal forces due to the projectile's spin cause the drag flaps 4 to move outwards. When the drag flaps reach the deployed configuration the strips of Kevlar 16 become taut thereby locking the drag flaps in place as shown in Figure 6.

The aerodynamic and rotational forces that act on the flaps will cause them to deploy very quickly and the Kevlar strips will therefore be put under a large amount of strain. In order to reduce the sudden loading on the strips and to mitigate the possibility of the strips 16 and/or 28 breaking on deployment lower modulus fibres could be incorporated into the Kevlar material. These lower modulus fibres, e.g. Nylon, would be shorter than the main Kevlar fibres to allow the braking loads to be initially carried in the Nylon fibres. The Nylon fibres would then act as a brake allowing the flaps to be gradually brought to rest. The skilled man would appreciate that any material with a large strain to failure and a visco-elastic response would be suitable materials to use instead of Nylon.

Both of the embodiments shown above depict locking tapes 16 which hold the drag flaps in the deployed configuration. An alternative way of forming the locking means for the drag flaps which is applicable to both of the above embodiments would be to use Kevlar tape to link adjacent flaps together. This tape would be excluded from the moulding process in order to ensure it is unstiffened. It will be obvious to the skilled man that this alternative locking means arrangement could be combined with the arrangement depicted above (Figures 5 and 8) within one device.

Other means of incorporating the drag device of the invention into the fuze will be readily apparent to the skilled person.

Claims (1)

1) A drag brake for use with a munition in flight so as to effect course correction thereof said brake comprising i) a first portion of a drag means adapted for connection of the brake to the munition; ii) a second portion of a drag means located generally radially outwards of the first portion and comprising a plurality of drag flaps each of which is connected to the first portion by hinge means, the drag flaps being capable of moving between an undeployed configuration wherein the flaps lie on the outer surface of the munition between the nose of the munition and the hinge and a deployed configuration wherein the flaps project outwardly from the munition and at a pre-determined angle thereto; iii) deployment means adapted to retain the flaps in the undeployed configuration until receipt of an appropriate signal and thereat to effect deployment of the drag flaps and iv) locking means which is activated by said deployment means and which - is adapted to retain the drag flaps in the deployed configuration wherein the first and second portions of the drag means are formed from a single piece of material 2) A drag brake for use with a munition as claimed in claim I wherein the drag means is formed from a metal sheet.

3) A drag brake for use with a munition as claimed in claim I wherein the drag means is constructed from composite material which has been stiffened in the region of the drag flaps and arranged to be left substantially unstiffened in the region of the hinge 4) A drag brake for use with a munition as claimed in claim 3 wherein the composite fibre is chosen from Kevlar fibre, glass fibre, Dynema or carbon fibre.

5) A drag brake for use with a munition as claimed in claims 3 or 4 wherein the composite fibre is Kevlar cloth.

6) A drag brake for use with a munition as claimed in claim 5 wherein the fibres in the cloth are arranged to lie substantially along two perpendicular axes.

7) A drag brake for use with a munition as claimed in claim 6 wherein, in the undeployed configuration, one of the fibre axes is aligned with the longitudinal axis of the munition.

8) A drag brake for use with a munition as claimed in any one of claims 3 to 7 wherein the locking means comprises unstiffened composite fibre which is connected to each drag flap and is anchored to the munition forward of the hinge to hold said drag flap in place in the deployed configuration 9) A drag brake for use with a munition as claimed in claims 3 to 7 wherein the locking means comprises unstiffened composite fibre which connects adjacent drag flaps to hold each drag flap in place in the deployed configuration 10) A drag brake for use with a munition as claimed in any preceding claim wherein in the undeployed configuration the drag flaps are stowed within a complementary recess in the outer surface of the munition.

11) A drag brake for use with a munition as claimed in any preceding claim wherein the angle 0 subtended by the drag flaps and the longitudinal axis in the direction of motion is 80'<0<90.

12) A drag brake for use with a munition as claimed in any one of claims 3 to I I wherein the stiffened drag flap comprises a core of composite fibre surrounded by either a thermoplastic material or a thermoset material.

13) A drag brake for use with a munition as claimed in any preceding claim wherein the deployment means comprises internal locking pins adapted to secure the drag device in the undeployed configuration until receipt by the deployment means of a control signal.

14) A drag brake for use with a munition as claimed in any preceding claim wherein the number of drag flaps is an even number.

15) A munition having on-board course correction device means and comprising a payload volume, a fuze having associated control means and a drag brake according to any preceding claim wherein the control means detects deviations of the trajectory of the munition from a nominal trajectory and, at an appropriate time, generates a control signal to the deployment means to effect deployment of the drag device.

16) A munition having a course correction device according to claim 16 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.