The present invention relates to a device for exerting an aerodynamic drag force particularly, though not exclusively, on a ballistic shell whilst in flight. 
     It is advantageous to be able to improve the accuracy of ballistic shells fired from artillery pieces, for example, so that there is greater probability of hitting the intended target and lower probability of so-called collateral damage. The accuracy of such shells is much greater in the azimuth direction than in the longitudinal direction. Thus, an error zone of generally elliptical shape results where the long axis of the ellipse is in the longitudinal direction. 
     It is possible to alter the range of an artillery shell in flight by increasing its drag coefficient. 
     There has been a proposal for an artillery shell which has a course correction applied to it during flight. The shell is initially aimed to overshoot the target in the longitudinal direction and, whilst in flight, applying an aerodynamic brake to cause it to fall short of the original overshoot and much closer to the target than would otherwise have been the case. In this way it is thought that an error zone of significantly smaller area may be achieved. 
     However, there are problems associated with applying drag increasing brakes in that the brake must be applied as symmetrically as possible about the projectile axis so as to minimise the possibility of the spinning projectile becoming unstable in its trajectory. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to make possible the provision of a device able to exert a substantially symmetrical drag force about the axis of spin of a projectile so as to increase its drag coefficient during flight. 
     According to a first aspect of the present invention there is provided a braking device for increasing the drag coefficient of an associated projectile whilst in flight, the device comprising: at least two braking vane means which, when released, extend substantially symmetrically into a surrounding airstream whilst said projectile is in flight; retaining means for maintaining said at least two vane means in a retracted first position out of said airstream during an initial portion of said flight; releasing means to allow said at least two vanes to extend to a second position into said airstream at a desired point during said flight; and, said at least two vane means further including co-operating means to ensure substantially symmetrical deployment into said airstream. 
     The braking vane means may be extensible by centrifugal force due to rotation of the associated projectile about its axis. 
     The device is preferably positioned on the nose of the projectile, which may be an artillery shell. Shells sometimes achieve supersonic speed in flight and positioning the device on the nose of the shell ensures that the braking vane means can extend into the surrounding airstream per se. 
     The device may be incorporated in a fuzing device positioned on a forward part of the shell and which fuzing device arms the shell and causes it to function when required. 
     The braking vane means may comprise braking vane members which extend substantially normal to the projectile axis into the surrounding airstream. The braking vane members may be pivoted about an inner end such that the centrifugal force generated by the projectile spinning about its axis causes the braking vane members to extend into the airstream. 
     Pivoted braking vane members are advantageous over vane members which are, for example, arranged to slide out into the airstream in guide members under the action of centrifugal force. Such sliding vanes have limited area available to extend into the airsteam due to the need to maintain adequate support of the vanes within the device to counteract the stresses imposed on them by the airstream. Furthermore, unless such sliding systems are very accurately made, they have a tendency to jam due to any misalignment which may be present. Thus, such sliding systems are inherently more expensive to make and less efficient in operation. 
     Pivoted vane members are advantageous under spin conditions because the distance between the pivot point and the centre of gravity of the vane members provides the mechanical advantage of allowing the pivoted vane members to deploy under less force than said sliding vanes, due to the turning moment generated during deployment. Pivot vane members also have the advantage of not requiring guide members, and so the misalignment of vane members and their guide members does not create a problem. 
     The retaining means may be a cover member which surrounds the braking vane members during an initial part of the flight so as to prevent them extending until desired. 
     The retaining means may be one or more straps. 
     The retaining means may be latches or hooks positioned on a support or base member in a way which prevents the braking vane members extending until desired. 
     The retaining means may be one or more pins which may extend into or through at least one braking vane member and a support or base member. 
     The releasing means may be explosive releasing means such as a small explosive charge or explosive cord for example, or may comprise a gas motor device. The releasing means may be detonated, for example, by a remote radio signal at the appropriate time so as to cause the retaining means to release the braking vane means to deploy by extending out into the airstream. The releasing means may cause fracture of the retaining means. The releasing means may alternatively cause the retaining means to move to a position which allows the braking vane means to deploy. 
     The releasing means may achieve its object by causing a retaining cover member to fracture and/or be jettisoned from the shell. 
     The releasing means may alternatively cause frangible fingers which interlock the braking vane means together to break and allow them to deploy through slots, for example, in a nose cover member. 
     The releasing means must be actuated at the appropriate time in order to provide the desired course correction. The releasing means may be activated as stated above by a remote radio signal. The device of the present invention itself may comprise a radio receiver device to receive the remote radio signal and to cause activation of the releasing means. Alternatively, any such radio receiver device may be associated with a fuzing device or with the shell itself, the radio receiver merely being operatively connected to the releasing means. The remote radio signal may come from a ground control station or a reconnaissance aircraft, for example. 
     Alternatively, the releasing means may be actuated by use of the Global Positioning System (GPS) as follows. At a given. point in its trajectory, an on-board processor compares the predicted position of the projectile with its actual position as determined through remotely accessing the GPS. The processor then calculates the appropriate time delay at which the braking vane means need to be deployed, in order to provide the proper course correction, to bring the projectile on course for its intended target. The processor then sets an on-board timer accordingly, and the timer actuates the releasing means after the said appropriate time delay. 
     The braking vane means also employs co-operating means to ensure that, in use, they deploy substantially symmetrically about the axis of the shell. Such means may comprise control areas of the braking vane members, the control areas being arranged such that any asymmetric extension of radially adjacent vane members would result in mechanical interference between the control area of one vane member and an adjacent part of the other vane member. Thus, if one vane were to jam or stick in the closed or partially extended position, the control area of the adjacent vane would prevent the adjacent vane from extending further and substantially preventing asymmetrical deployment from occurring. 
     Alternatively, intermeshing gear teeth may be employed on curved portions of the braking vane members which ensure that they are deployed symmetrically. 
     The device may comprise pairs of braking vane members, each pair being disposed axially adjacent another. 
     The device may include means for preventing the braking vane members from extending further than desired into the airstream. 
     The device may comprise twin-bladed pairs of braking vane members where the twin blades are axially adjacent each other. Both blades may be pivoted about an inner end such that the centrifugal force generated by the associated shell spinning about its axis causes both blades of the braking vane members to extend into the airstream. One of the twin-blades may be prevented from extending as far into the airstream as the second of the twin-blades. The second of the twin-blades may be prevented from extending further than desired into the airstream by restraining means which may be carried by the first blade. The second of the twin-blades may overlie the first blade such that support is provided for the second blade by the first blade when both blades are fully extended. The second blade may advantageously provide an increased area extending into the airstream and therefore an increased drag coefficient for the shell in flight. 
     According to a second aspect of the present invention, there is provided a fuzing device incorporating the braking device of the first aspect of the present invention. 
     According to a third aspect of the present invention, there is provided a projectile incorporating the braking device of the first aspect or the fuzing device of the second aspect of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the present invention may be more fully understood, examples will now be described by way of illustration only with reference to the accompanying drawings, of which: 
     FIG. 1 shows a general cross sectional view of a typical shell; 
     FIG. 2 shows a schematic cross section through a device according to the present invention; 
     FIG. 3 shows an explanatory view of means to ensure symmetrical deployment of braking vane means; 
     FIG. 4 shows a front view along a shell axis of the braking vanes of FIG. 3 deploying normally; 
     FIG. 5 shows a view similar to that of FIG. 4 but where one vane has failed to deploy normally; and 
     FIG. 6 which shows a front view along a shell axis of a device having two pairs of braking vane members. 
     FIG. 7 a  shows the form of single-blade braking vane member which is used in FIG. 6, and can be used in place of the vanes shown in FIGS. 1-4. 
     FIG. 7 b  shows an alternative form of single-blade braking vane member together with limiting means for preventing the braking vane member from extending further than desired into the airstream. 
     FIG. 8 a  shows a front view along a projectile axis of a device having two pairs of single-blade braking vanes, each being of the kind shown in FIG. 7 b.    
     FIG. 8 b  shows a front view along a projectile axis of the device shown in FIG. 8 a,  wherein the two pairs of single-blade braking vanes are fully deployed. 
     FIG. 9 a  shows the two blades of a twin-blade braking vane member which can be used in place of the single-blade versions shown in FIGS. 7 a,    7   b,    8   a  and  8   b.    
     FIG. 9 b  shows a front view along a projectile axis of a device having two pairs of twin-blade braking vanes as shown in FIG. 9 a,  wherein the vanes are fully deployed. 
     Referring now to the drawings and where the same features are denoted by common reference numerals. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a cross section through a shell indicated generally at  10  and incorporating a braking device according to the present invention. The shell includes a body casing  12 , a fuzing device  14  and a braking device  16  according to the present invention at the nose end of the shell. The braking device  16  as shown in the schematic cross section of FIG. 2 comprises a base member  20  which is used to attach the device  16  to the fuze  14  which has a central column  15  onto which the base plate  20  is fixed. The base plate  20  provides support for pivots  46 ,  48  for the rearward pair of braking vane members  22 . Support for pivots  36 ,  38  for the forward pair  24  of braking vane members is provided by a second support plate  39  attached to the central column  15 . A cover member  26  provides an aerodynamic nose to the shell  10  and also constitutes the retaining means which retain and prevent the two pairs of braking vane members  22 ,  24  from deploying until desired. The nose portion  28  of the cover  26  houses a small explosive charge  30  which is detonated by an electrical impulse via a wire  32  to an igniter  34  in the charge  30 . The electrical impulse comes from radio receiver means (not shown) associated with the fuzing device  14 , the radio receiver means being itself activated by a remote radio signal. The cover  26  may be made from a plastics material and may have various formations (not shown) such as grooves for example, which cause it to fracture along preferred paths to achieve a desired mode of fracture and separation from the shell whilst in flight. 
     Pair  22  of braking vane members is shown in FIG. 3, pair  24  is similar to pair  22  but displaced by 90° from pair  22  about the shell axis  18 . Each pair of braking vane members  22 ,  24  comprises two distinct braking vane members  40 ,  42 , each member having a respective pivot  44 ,  46  about which it is able to rotate under the influence of centrifugal force from the spinning shell whilst in flight and when the retaining cover member  26  is jettisoned by the explosive charge  30 . Each member has a control area  48 ,  50  on the opposite side of the pivots  44 ,  46  to the drag or braking area  52 ,  54  of each member. The effect of the control area is to create a potential overlapping area indicated by the shaded area at  56 . However, since the two members  40 ,  42  lie in the same plane and are of significant thickness, it is not possible for them to overlap. Therefore, it is only possible for both braking vane members to deploy simultaneously. As shown in FIG. 5, if braking vane member  40 , for example, jams for any reason, the tip  60  of control area  50  abuts the edge  62  of braking or drag area  52  and prevents braking vane member  42  from extending further thus, maintaining a substantially symmetrical drag force about the shell axis  18 . Similarly, if member  42  were to jam, tip  64  of control area  48  would abut edge  66  of braking or drag area  54  of member  42  and prevent member  40  from deploying further. When both members  40 ,  42  deploy normally as shown in FIG. 4, the tips  60 ,  64  and edges  62 ,  66  move along each other to give substantially equal and simultaneous deployment of the braking vane members thus exerting and maintaining a symmetrical force about the shell axis  18 . 
     FIG. 6 shows a schematic front view of the device  16 . Pairs of braking vane members  22 ,  24  are shown deployed, together with pair  22  also shown still in the retracted position ( 22 ) in order to show the difference between the extended and retracted positions of the vane members. 
     In FIG. 3, a point indicated at  70  is where the tip  60  eventually clears the path of the edge  62  during the course of deployment of the braking vane members. Similarly, there will be a corresponding point (not shown) where the tip  64  clears the path of edge  66 . Once the tips  60 ,  64  have moved past these points, neither member  40  nor member  42  can exert any influence over the other with regard to deployment thereof. However, this is not important since it is in the initial phases of braking vane member deployment that jamming or sticking is most likely to occur. 
     FIG. 7 a  shows a single blade braking vane member  40  having a pivot  46  about which it is able to rotate under the influence of centrifugal force from the spinning shell whilst in flight. The member  40  is able to be positioned about a central column  15  (shown in FIG.  6 ). Support for the pivot is provided by the base plate  20  or the support plate  39  (both shown in FIG.  2 ). The member  40  is profiled to interact with other braking vane members as described in FIG.  3 . 
     FIG. 7 b  shows a single-blade braking vane member  72  similar to member  40  (shown in FIG. 7 a ) having a pivot  46  but also a having a groove  74  and a lip  76 . A pivot or pin  78  fixed to an axially adjacent plate such as the base plate  20  or support plate  39  (both shown in FIG. 2) limits rotational movement of the braking vane member  72  about the pivot  46 . As the member  72  moves into the airstream the groove  74  moves with it, the member  72  being restrained when the lip  76  at the end of the groove  74  comes against the pin  78 . 
     FIG. 8 a  shows one pair  80  of single-blade braking vane members axially adjacent to another pair  90 . The single-blade braking vane members  72  and  82  are pivoted at points  46  and  84  respectively, the pivots  46  and  84  being fixed to a base plate  20  or support plate  39  (as shown in FIG.  2 ). Member  72  is prevented from extending too far into the airstream by the pin  78  meeting the lip  76  of the groove  74 . Members  72  and  82  are only able to deploy symmetrically, as described in FIG.  3 . 
     FIG. 8 b  shows both pairs  80  and  90  of single blade braking vane members fully deployed. Member  72  has rotated about pivot  46  and is constrained from rotating further by the lip  76  of the groove  74  reaching the pin  78 . The pins  78  and  86 , which restrict the movement of the members  72  and  82  as previously described, also act as pivots for the pair of braking vane members  90  axially adjacent to the pair  80 . Member  94  rotates about the pivot  86  and member  92  rotates about the pivot  78 . Similarly, the pivots  46  and  84 , around which the members  72  and  82  rotate, also act as the pins which constrain the rotation of the members  94  and  92  respectively. The pivots  46 ,  78 ,  84  and  86  are fixed to a base plate  20  and a support plate  39  as shown in FIG.  2 . 
     FIG. 9 a  shows two blades  102  and  104  which together form a twin-blade braking vane assembly  100 . The blade  102  is similar to the member  72  shown in FIG. 7 b.  The blade  102  rotates about a pivot  46  which is fixed to a base plate  20  or support plate  39  (shown in FIG.  2 ). This rotational movement is limited by the lip  76  of the groove  74  reaching a pivot or pin such as  78  (shown in FIG. 7 b ), which is fixed to a base plate  20  or a support plate  39  (shown in FIG.  2 ). The blade  102  also has fixed to it a pin  106 . The blade  104  is designed to fit axially adjacent to the blade  102  when not deployed such that both blades  102  and  104  are able to rotate about the pivot  46 . The blade  104  has a groove  108  which is axially adjacent to the groove  74  when the blades  102  and  104  are not deployed. The groove  108  does not have a lip. The blade  104  has a second closed groove  110  which receives the pin  106  fixed to the blade  102 . The movement of the blade  104  relative to the blade  102  is restricted by the pin  106  coming against the ends of the groove  110 . 
     FIG. 9 b  shows two axially adjacent pairs of twin-blade braking vane assemblies  110  and  112  fully deployed. Blades  102  and  104  rotate about the pivot  46  which is fixed to a base plate or support member  20 . Blade  102  is deployed as far as possible and is restrained from further rotation by the pivot  78  meeting the lip  76  (shown in FIG. 9 a ) of the groove  74 . The blade  104  does not have a lip on the groove  108  and is therefore able to rotate further into the airstream than the blade  102 . Blade  104  is prevented from rotating further than desired by the pin  106  fixed to the blade  102  within the groove  110 . Blade  102  advantageously provides support to blade  104 , which in the deployed position is otherwise only supported by the pivot  46 . This twin-blade arrangement shown in FIG. 9 b  provides a larger braking surface area than that of the single-blade arrangement shown in FIG. 8 b.
A braking device for increasing the drag coefficient of an associated shell at a desired point while in flight is described. The device comprises: at least two braking vane means which, when released, extend substantially symmetrically into a surrounding airstream while said shell is in flight; retaining means for maintaining said at least two vane means in a retracted first position out of said airstream during an initial portion of said flight; releasing means to allow said at least two vanes to extend to a second position into said airstream at a desired point during said flight; said at least two vane means being extended by centrifugal force due to rotation of said associated shell about its axis; and, said at least two vane means further including co-operating means to ensure substantially symmetrical extension into said airstream.