Abstract:
The invention relates to safety priming device for an explosive charge, notably a shaped charge, comprising a pyrotechnic igniter and at least one igniting relay placed between the igniter and an explosive load. 
     This priming device is characterized in that the igniting relay comprises means enabling the detonation wave produced by the igniter or igniters to be re-centered along the charge axis, said means comprising a confinement block having a bore converging between an external face positioned beside the igniter or igniters and an inner face positioned beside the explosive load, said bore filled with a relay explosive, the confinement block comprising means to prevent the propagation of a shock wave axially through the confinement block between the igniter or igniters and the explosive load.

Description:
BACKGROUND OF THE INVENTION 
     The technical scope of the invention is that of priming devices for an explosive charge, and notably for a shaped charge. 
     Known priming devices generally comprise at least one pyrotechnic igniter and at least one igniting relay placed between the igniter and an explosive load. 
     One of the problems encountered with known priming devices is the difficulty of ensuring the accurate centering of the pyrotechnic igniter with respect to the charge body. 
     More particularly, in the case of shaped charges, the detonation wave that is propagated in the charge must be perfectly symmetrical with respect to the charge axis. 
     Such a symmetry enables the optimal displacing or deformation of the shaped charge liner (slug or hollow charge). Even slight asymmetry (for example of around a few tenths of a millimeter) risks causing a reduction in effectiveness of the shaped charge. 
     Moreover, in the field of self-destruct charges for ballistic missiles, it is customary to include back-up priming means so as to reduce this risk of failure of the self-destruct system. 
     The multiplication of priming means thus raises the problem of producing a priming wave that is symmetrical and this whatever the position of the igniter being activated. 
     SUMMARY OF THE INVENTION 
     The aim of the invention is to propose a priming device that overcomes such problems and does not suffer from the drawbacks of known devices. 
     Thus, the priming device according to the invention ensures the ignition of an explosive charge along the charge axis whatever the position of the igniter or igniters with respect to said axis. 
     The invention thus makes it possible to obtain priming symmetry, using simple means. 
     Thus, the invention relates to a safety priming device for an explosive charge, notably a shaped charge, comprising a pyrotechnic igniter and at least one igniting relay placed between the igniter and an explosive load of the charge, wherein the igniting relay comprises means enabling the detonation wave produced by the igniter or igniters to be re-centered along the charge axis, said means comprising a confinement block having a bore converging between an external face positioned on the said having the igniter or igniters and an inner face positioned beside the explosive load, said bore filled with a relay explosive, the confinement block comprising means to prevent the propagation of a shock wave axially through the confinement block between the igniter or igniters and the explosive load. 
     According to a first embodiment of the invention, the confinement block may be made of an organic material having acoustic impedance that is less than 15.10 6  kg/m 2 s, this material constituting means to prevent the propagation of a shock wave through the confinement block. 
     According to a second embodiment of the invention, the confinement block may incorporate at least one collar that will be placed in the vicinity of the igniter or igniters and which will be followed by a free space surrounding the block, said free space constituting means to prevent the propagation of a shock wave axially through the confinement block. 
     This free space may be formed by a cylindrical groove delimited by two collars. 
     The bore in the confinement block may incorporate at least one conical part having a half-angle at the apex of between 10 and 25°, the small diameter of the cone being of between 2 and 5 mm and the large diameter of the cone being of between 13 and 30 mm. 
     The igniting relay may comprise a first layer of relay explosive applied to the confinement block and placed between the igniter or igniters and the confinement block. 
     The confinement block will be generally cylindrically shaped and arranged in a body. 
     The first relay layer may be of a thickness of at least 2 mm. 
     The first layer of relay explosive may be ring-shaped or else may be in the shape of a substantially rectangular tongue. 
     Advantageously, the device may comprise at least two pyrotechnic igniters placed at a distance from the charge axis. 
     A further subject of the invention is a shaped charge incorporating a safety priming device having at least two igniters and having the same performances whichever igniter is activated. 
     Such a charge may be used notably to ensure the destruction function during the trajectory for ballistic projectiles or for their payload. 
     In this case, this charge may advantageously be an explosively-formed charge. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become more apparent after reading the following description of the different embodiments, such description being made with reference to the appended drawings, in which: 
     FIG. 1 is a longitudinal section view of a shaped charge fitted with a priming device according to a first embodiment of the invention, 
     FIG. 2 is a longitudinal section view of a shaped charge fitted with a priming device according to a second embodiment of the invention, 
     FIG. 3 is a transversal section view of a shaped charge according to a third embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a shaped charge  1  (in this case an explosively-formed charge) comprises an explosive load  2  placed in a cylindrical casing  3  screwed to a body  9  having fastening lugs  19   a ,  19   b.    
     A cup-shaped liner  4  is applied to the explosive load  2 . A priming device  5  allows the explosive load  2  to be ignited. 
     In accordance with the invention, the priming device  5  comprises a confinement block  6  placed in a cylindrical cavity  10  in a body  9 . 
     According to this first embodiment, the block  6  is made of an organic material having acoustic impedance less than 15.10 6  kg/m 2 s. The block  6  may, for example, be made of polyacetal. 
     The block  6  has a bore  7  that comprises a conical part  7   a  extended by two cylindrical parts  7   b  and  7   c.    
     The bore  7  is filled with a relay explosive  8 . 
     The priming device  5  also comprises a first layer  11  and a second layer  12  of relay explosive. 
     These two relay layers  11  and  12  are arranged on the upper and lower faces of the confinement block  6 . 
     The second relay layer  12  is, in this case, housed in a cavity made in the explosive load  2 . It might also be simply applied to an upper surface of the explosive load  2 . It would also be possible for no second relay layer to be provided and for the block  6  to be applied directly onto the load  2 . 
     The first relay layer  11  is arranged at the bottom of the cavity  10  in the body  9 . It communicates with two pyrotechnic igniters  13   a ,  13   b  placed symmetrically on either side of the charge axis  14 . 
     Here these igniters are electrically-ignited primers and are controlled by an ignition device  20  placed at a distance and connected to the igniters  13   a ,  13   b  by conductors  15   a ,  15   b . The igniters may also be formed by pyrotechnic transmission lines, for example detonating lines. 
     The conical part  7   a  of the bore  7  converges between an outer face of the block  6  positioned beside the igniters  13   a ,  13   b  and an inner face of the bock positioned beside the explosive load  2 . This conical part  7  has a half-angle at the apex that is of between 10 and 25°, the small diameter of the cone being of between 2 and 5 mm and the large diameter of the cone being of between 13 and 30 mm. 
     Relay layers  11  and  12  may be made of composite explosive cut out of plates. 
     The relay composition  8  filling the bore  7  will be cyclonite, for example. This composition  8  will be put in place by compression. 
     This priming device operates as follows. 
     When the charge  1  is required to be fired, the ignition device  20  sends a firing order to both igniters  13   a ,  13   b  simultaneously. These ignite the first relay layer  11  which in turn ignites the relay composition  8  placed in the bore  7  of the bore  6 . 
     Because of the convergence of the conical part  7   a  of this bore, the shock wave that is propagating in the composition  8  also converges towards the second relay layer  12  which is ignited practically along the axis  14  of the charge  1 . 
     The second relay layer in turn ignites the explosive load  2 , causing the projectile to be formed by the liner  4 . 
     If only one of the igniters  13   a ,  13   b  functions, the other presenting a failure, the first layer  11  is ignited out-of-line with the axis. It nevertheless ignites the relay composition  8  and the convergence of the conical bore  7   a  ensures the re-centering of the shock wave and thus the faultless ignition of the second relay layer  12 , and thus of the explosive load  2 . 
     So as to avoid the inadvertent ignition of the second relay layer  12  or of the load  2  directly by the shock wave through the material of the confinement block  6 , means must be provided to prevent such a propagation. 
     According to this first embodiment, the block  6  is made of a material enabling such a propagation to be absorbed. The block  6  will thus be made of an organic material having an acoustic impedance of less than 15.10 6  kg/m 2 s. 
     Other means can be used to prevent the direct ignition of the relay layer  12  or the explosive  2  by the propagation of the shock wave through the material of the confinement block  6 . 
     FIG. 2 thus shows a second embodiment of the invention that differs from the first one in that the confinement block  6  incorporates a collar  16  placed at the upper face of the block and onto which the first relay layer  11  is applied. This collar  16  is followed by a free space  17  surrounding the block  6 . 
     A second collar  18  allows the block  6  to be positioned in the bore  10 . Thus, the free space  17  is formed by a cylindrical groove arranged in the block  6  and delimited by the two collars  16  and  18 . 
     The free space  17  constitutes means to prevent the propagation of a shock wave axially through the confinement block  6 . Indeed, the shock received by the collar  16  further to the ignition of the first relay layer  11  is not able to propagate directly to the second collar  18 . 
     The relay composition  8  is ignited as in the previous embodiment and the convergent profile of the bore  7   a  ensures the centering of the shock wave and the axial ignition of the second relay layer  12  and thus of the explosive load. 
     Once again, this axial ignition is ensured even if only one of the igniters  13   a ,  13   b  functions. 
     Thanks to the presence of the free space  17 , it is possible for the confinement block  6  to be made of metal, for example an aluminum alloy. 
     The first layer  11  of relay explosive shown in FIGS. 1 and 2 has revolving symmetry. 
     It is possible for a first relay layer of a different shape to be implemented. 
     FIG. 3 thus shows a top view and section view of a priming device according to a variant embodiment in which the first layer  11  is in the shape of a substantially rectangular tongue passing through the axis  14  of the charge. 
     This view is a section made along a plane referenced AA in FIG.  1 . The latter Figure has been described previously with reference to an embodiment in which the first relay layer  11  is ring-shaped. This Figure may also be associated with this third embodiment where the first layer is a tongue. 
     The igniters  13   a ,  13   b  (the position of only one of which is shown) are arranged on either side of axis  14 , each at one end of the relay layer  11 . 
     The relay composition  8  arranged in the confinement block  6  is ignited by means of the relay layer  11  whichever igniter is primed. 
     As in the previous example, the convergent profile of the bore  7   a  ensures the centering of the shock wave and the axial ignition of the second relay layer  12  and of the explosive load. 
     The block  6  can be either structured according to FIG. 1 (organic material) or to FIG. 2 (peripheral groove) regardless. 
     Other variants are possible without departing from the scope of the invention. 
     Thus, the device according to the invention may implement only one igniter that is out-of-line with respect to axis  14  of the charge. Such a configuration makes it easier to integrate a charge in a given projectile. Indeed, thanks to the invention, it is no longer necessary for the igniter to be positioned axially with respect to the charge. 
     It is also possible for a first relay layer  11  and the relay explosive placed in the confinement block to be made in the form of a single mass of explosive, implemented for example by compression. The explosive mass will comprise a conical lower part and a disk or tongue-shaped upper part. In this case, the confinement block will be given a suitably shaped upper face enabling it to receive the disk or tongue-shaped relay explosive part. 
     It is naturally possible for the priming device according to the invention to be implemented with other types of explosive charges: hollow charges, splinter-generating charges, etc. 
     The shaped charge proposed by the invention is fitted with at least two igniters. Greater reliability is thereby ensured in the event of using the charge for the function of destroying a ballistic projectile such as a rocket or missile during its trajectory or else for the destruction of the charge carried on-board this projectile. This improved reliability is due to the backed-up igniters, of which there may be more than two. This is coupled thanks to the invention to an effectiveness that is the same whatever the number and position of the igniters primed, the priming device ensuring in any case the ignition of the explosive load along the axis of symmetry  14  of the charge. The igniters are shown in the Figures having orientations substantially parallel to one another and to the charge axis. These igniters may also be placed at a different orientation making an angle with the charge axis.