Abstract:
A blast armor including a base plate made of a ballistic material and having an inner and outer surface. The cover plate is made of ballistic material and spaced from the inner surface of the base plate to form a space therebetween. The armor comprises at least one energy absorbing module disposed within the space between the base and cover plate. The module has front and rear surfaces, and side surfaces extending therebetween. The module is positioned with at least one of its surfaces facing the base plate and at least another surface thereof facing the cover plate. The base and cover plate are each made of a material tougher than that of the module. The module is configured to progressively deform between the base and cover plate under the application of a force to the outer surface of the base plate at least partially directed towards its inner surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Israel Patent Application No. 207241 filed on 26 Jul. 2010, the contents of which are incorporated herein, in their entirety, by this reference. 
     TECHNICAL FIELD 
     Embodiments of the invention relate to belly armor and, in particular, armor constructions adapted for articulation to the belly of a vehicle. 
     BACKGROUND 
     It is well known to provide vehicles with add-on armor, in order to protect the occupants of the vehicle from different threats, for example, incoming projectiles or nearby explosions. Adding armor is usually performed for combat and logistics vehicles taking part in military operations or stationed in hostile environments. 
     In particular, one major threat to combat and logistics vehicles is explosive devices usually buried or concealed along roads on which the combat vehicle travels, and designed to explode (under a belly of the vehicle), when the vehicle comes to the location of the explosive device, or passes thereover. 
     Such an explosion, can cause structural damage to the belly of the vehicle (e.g. rupture, penetration etc.), hurting its occupants (e.g. soldiers). For this purpose, armor is commonly attached to the belly of the vehicle (also referred to as ‘belly armor’), and adapted to shield the belly from the explosion hazards. 
     Various constructions for protecting a vehicle belly are known, for example, such constructions as described in U.S. Pat. No. 7,712,823, WO2010/090661, EP2267400, U.S. Pat. No. 5,533,781 and US2011/0079134. 
     SUMMARY 
     According to one aspect of the subject matter of the present application, there is provided a blast armor for a belly of a vehicle, comprising:
         a base plate made of a ballistic material and having an inner surface and an outer surface;   a cover plate made of ballistic material and spaced from the inner surface of the base plate to form a space therebetween; and   at least one energy absorbing module disposed within the space between the base plate and the cover plate, the module having:
           a front surface;   a rear surface; and   side surfaces extending therebetween;   the energy absorbing module being positioned so that at least one of its surfaces faces the base plate and at least one other surface thereof faces the cover plate;   
               

     wherein:
         the base plate and the cover plate are each made of a material having a toughness greater than that of the energy absorbing module/s; and   the module is configured, to progressively collapse/deform between the base plate and the cover plate, under the application of a force to the outer surface of the base plate at least partially directed towards its inner surface.       

     The module can occupy an area constituting a minority of the area of the base plate and of the cover plate, and it can be provided with a frame confining the module at least at two sides thereof, and attached to the inner surface of the base plate. The arrangement may be such that a plurality of module can be used to cover the majority of the inner surface of the base plate. 
     The frame of the module can be in the form of a cell formed at the inner surface of the base plate and having at least two opposing side walls. The cell can be formed by at least one grid beam attached to the inner surface of the base plate and projecting therefrom towards the cover plate. 
     According to one example, the length of the grid beams can be sufficiently short so as to allow a plurality of such beams to cover the inner surface of any desired base plate. Alternatively, at least one of the grid beams can be curved so as to form at least one cell configured for accommodating an energy absorbing module, and can even be sufficiently long and convoluted so that it forms several such cells. 
     The energy absorbing module can be positioned within the cell such that the module is confined by the cell&#39;s walls, and by the base and the cover plates. In particular, the module can be confined:
         at its front face by the base plate;   along its side faces by the side walls of the cell; and   at its rear face by the cover plate.       

     According to one example, both the base plate and the cover plate can be made of the same material, which has a toughness greater than that of the energy absorbing module. Alternatively, the base plate and the cover plate can be made of different materials, each of which has a toughness greater than that of the energy absorbing module. 
     The base plate can be made of a single continuous piece of ballistic material, e.g. rolled homogenous armor (RHA). Alternatively, it can be made of several units constituting together the base plate. In any case, the base plate can be such that withstands the impact of a level 3 explosion on the STANAG 4569 scale (NATO Standardization Agreement covering the standards for the “ Protection Levels for Occupants of Logistic and Light Armored Vehicles ”). 
     In particular, the arrangement may be such that while the armor may be adapted to withstand the above level 3 explosion, the same armor without the energy absorbing modules may be configured for withstanding a level 2 on the STANAG 4569 scale. 
     For example, both the base plate and the cover plate can have a nominal hardness ranging between 220 and 650 BHN. However, since the cover plate is positioned behind the base plate (with respect to an explosion under the belly of the vehicle), and the base plate is the first to absorb the energy of the explosion, the cover plate can have a thickness which is lower than that of base plate. For example, the thickness of the cover plate may be between about 25-50% of the thickness of the base plate. According to a particular example, the base plate can have thickness of 12.7 mm while the cover plate can have a thickness of only 4.5 mm. 
     The armor can further comprise a plurality of grid beams at the inner surface of the base plate so that a grid of cells is formed along the inner surface, configured for holding therein a plurality of energy absorbing modules. The number of energy absorbing modules can be equal to the number of cells formed by the grid beams, so that each cell is occupied by one energy absorbing module, adjacent modules being separated from each other by the cells&#39; walls. Alternatively, only some of the cells can be occupied by the energy absorbing modules. 
     There can be several types of grid beams forming the cells grid, each type being configured for attachment to a different area of the inner surface of the base plate according to the design and geometry of the latter. For example, the base plate can have a planar portion and at least one curved portion, and correspondingly, one type of the grid beams can be straight to fit the planar portion, while another type of the grid beams can be curved to fit the curved portion. 
     The grid beams are configured so as to provide the armor with a required structural integrity, i.e. to allow both the base plate and the cover plate to absorb at least the majority of the energy of the explosion without deforming to an extent affecting the occupants of the vehicle. Thus, the grid beams do not take up more than 30% of the overall area of the inner surface, more particularly no more than 25%, and even more particularly no more than 20% of the overall inner surface. It follows from this, that the majority of the inner surface of the base plate is covered by the energy absorbing modules. 
     The grid beams can be made of materials, which, on the one hand provide the required structural integrity for the base plate, and on the other hand, are sufficiently deformable (under an explosion loading), to allow the base plate and the cover plate to absorb part of the energy of the explosion against which the armor is designed and transmit this energy to the energy absorbing modules. In other words, the grid beams should allow the base plate to deform to an extent which is, on the one hand sufficient to apply pressure to the energy absorbing modules so as to cause them to collapse and absorb the energy of the explosion, and on the other hand, do not allow the base plate to deform to an extent endangering the passengers of the vehicle. For example, the grid beams can be made of metal, for example iron, steel or steel/titanium/aluminum alloy. 
     For construction purposes, the base plate can be pre-formed with attachment ports for the attachment of the grid beams thereto, and the grid beams can be correspondingly sized and shaped, and have corresponding attachment ports so as to be articulated to the base plate. Articulation of the grid beams to the base plate can be detachable, e.g. using bolts, or can be a fixed attachment, e.g. using welding. 
     The cover plate can also be pre-formed with attachment ports for attachment to the grid beams, and can be sized and shaped so as to confine, when attached to the beams, the grid beams between the base plate and the cover plate. 
     Each of the energy absorbing modules can be in the form of a tile, configured for being laid within the cell, and can be adapted for undergoing progressive collapse/deformation under application of a sufficient load thereto (e.g. in the case of deformation of the base plate as a result of an explosion). 
     In particular, the energy absorbing module can have a density which is in the range of 5%-35% of the density of the material comprising the majority of the volume of the module. In other words, if the majority of the volume of the module is constituted by material A, the density of the module may be in the range of 5%-35% of the density of A. Alternatively, the energy absorbing module can be designed such that the density thereof does not exceed 2.8 g/cm^3. 
     In addition, the specific weight of the energy absorbing module can be lower than the specific weight of the base plate and of the cover plate. In other words, the arrangement can be such that, in comparison with an initial armor, reducing the thickness/size of the base plate and/or cover plate on account of adding an energy absorbing module/s, while providing the same ballistic protection, yields an armor with an overall weight which is lower than that of the initial armor. 
     Furthermore, the arrangement can be such that, compared to a reference armor having a similar design but without energy absorbing modules, a thickness X of the base plate and cover plate combined, and configured for withstanding the same level of explosion, the thickness of the base plate of the present armor (on account of the energy absorbing modules) can be reduced to about 0.65×. In other words, the existence of the modules compensates for about 0.35× of the thickness of the base plate. 
     One of the advantages to the above described arrangement is that most of the area of the inner surface of the base plate, and consequently, most of the volume of the armor is constituted by low-density, light-weight modules. The overall reduction in the weight of the armor can allow increasing the thickness of the base plate (and consequently the weight of the armor) on account of the overall low weight of the armor. 
     The energy absorbing module can be made of a low-density porous material. One example of such a porous material can be metallic foam, in particular, Aluminum foam. Alternatively, the module may be in the form of a low density structure. One example of such a structure can be a honey-comb structure. In any of the cases, the majority of the volume of the energy absorbing module can be constituted by the spaces/pores. 
     The energy absorbing module can be encapsulated by a covering layer, so as to protect the module, so that the module does not undergo collapse/deformation as a result of shocks and vibrations inflicted thereon which are not caused by an explosion or an impacting projectile. Thus, the covering layer can be, on the one hand, tough and robust enough to securely shield the module, and, on the other hand, can be deformable enough to allow the module to undergo the desired deformation during the explosion/impact. 
     In particular, the covering layer can have an elongation coefficient of about 20%, more particularly, at least 15%, and even more particularly, at least 10%. The covering layer can be made of resin, polyurethane, polyurea, rubber types etc. 
     It should also be appreciated that armor is usually designed corresponding to the size and shape of the body it is configured to protect, so that different shaped bodies are usually fitted with different shaped armors (and consequently, different shaped base plates). The arrangement can be such that the energy absorbing modules are of a shape and size allowing for a plurality of such modules to cover the inner surface of base plates of various designs. In other words, the arrangement is modular, in the sense that the same energy absorbing modules can be fitted in the cells of different base plates of different belly armors. 
     In assembly, the grid beams are attached to the base plate so as to form the cells, each of which is in the shape of a 3D space delimited by the inner surface of the base plate and the side walls of the grid beams. The 3D space is sized and shaped for accommodating therein an energy absorbing module. 
     The modules are then fitted into the respective cells, such that the front face thereof is facing the base plate, and can even come in contact therewith, and the side faces thereof are facing the side walls and can even come in contact therewith. In this position, the energy absorbing module is securely confined inside the cell and is positioned therein without the need for any articulation means. 
     It should be noted that according to a particular design, the energy absorbing module is only held securely between the inner surface of the base plate, and the cover plate, i.e. the side faces of the module do not come in contact with the grid beams defining its respective cell so that there exists a gap between the side faces of the module and the grid beams. One advantage which can arise from such a design is that, during explosion loading, the energy absorbing module is pressed between the base plate and the cover plate, and as a result deforms and expands sideways. Thus, the gap between the grid beams and the side faces of the module provides it with just enough space to expand during deformation. 
     Once the energy absorbing modules are positioned within the cells, the cover plate is attached to the grid beams so as to encapsulate them, such that the rear face of the energy absorbing modules is facing the cover plate, and can even come in contact therewith. The term ‘encapsulate’ is used herein to define that the energy absorbing module is confined on all sides, in particular, by the inner surface of the base plate at the front face thereon, by the cover plate at the rear face thereof, and by the side walls of the grid beams at the side faces thereof. 
     In attachment to the vehicle, the armor is mounted onto the body of the vehicle so as to be oriented such that the inner surface of the cover plate is facing a belly of the vehicle while the outer surface of the base plate is facing away from the vehicle. 
     The base plate (and consequently the armor) can extend along a direction defined between the front and the rear of the vehicle, and have a central portion extending along the direction, and two peripheral portions also extending along the first direction on both sides of the central portion. In particular, the central portion can have a ballistic resistance which is greater than that of the peripheral portions. 
     According to one example, the central portion of the base plate can be fitted with one or more additional elements configured for providing the central portion with an increased ballistic resistance, e.g. and additional armor member mounted onto the central portion. According to another example, the central portion of the base plate can be of increased thickness compared to the peripheral portions, in order to provide it with the increase ballistic resistance. It should be noted that both examples can be implemented together, i.e. a central portion of increased thickness and also provided with an additional armor member. 
     The armor can be an add-on armor, adapted to be attached in a removable manner to the body to be extra protected. In particular, the armor can be formed with attachment ports configured for attachment thereof to the body to be protected. 
     According to a specific example, the base plate of the armor can be fitted or integrally provided with at least two extensions being formed with the attachment ports. In particular, the armor can be can be sized and shaped for mounting, externally, onto the belly of a vehicle comprising a hull having the belly and side walls extending therefrom, such that, when mounted, the attachment ports of the extensions can engage corresponding ports formed in the hull. 
     The arrangement can be such that the extensions constitute part of the peripheral portions of the base plate. 
     According to one example, the arrangement can be such that the armor attaches directly to the hull, through its side walls or through the belly itself. Alternatively, according to another example, the side walls or belly of the vehicle can be fitted with one or more intermediary members fixedly attached thereto, the intermediary member being configured for attachment thereto of the extensions of the base plate. 
     Each intermediary member can extend in a direction parallel to the belly of the hull (i.e. a direction extending between a front of the vehicle and a rear thereof), and can have, in a cross-section taken along a plane perpendicular to this direction, at least one side configured for attachment to the hull and another side configured for attachment thereto of the base plate of the armor. According to a specific example, the intermediary member can extend along the side walls of the hull of the vehicle. 
     The base plate of the armor can be V-shaped in a cross-section taken along a plane perpendicular to the front-rear direction of the vehicle (when mounted thereto), such that peripheral portions thereof formed with the attachment ports are angled to the side walls of the vehicle hull. For this purpose, the intermediary member, in cross section taken along the same plane, can be formed with a first side oriented parallel to the side wall of the hull, and a second side wall angled to the first side wall, and configured to be oriented parallel to the peripheral portion of the base plate. 
     According to another aspect of the subject matter of the present application, there is provided an armored vehicle comprising:
         a hull having:
           a front end and a rear end defining therebetween a first direction of the vehicle:   a belly extending along the first direction; and   side walls extending along the first direction transverse to the belly.   
           a belly armor according to the previous aspect of the subject matter of the present application mounted onto the vehicle hull;   so that when mounted, the cover plate, grid beams and energy absorbing modules are disposed between the base plate and the belly of the vehicle.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to understand the invention and to see how it can be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
         FIGS. 1A and 1B  are schematic and respective isometric and front views of an armored vehicle hull comprising a belly armor according to the subject matter of the present application; 
         FIG. 1C  is a schematic isometric view of the hull shown in  FIG. 1A , with some of the components removed; 
         FIGS. 2A and 2B  are schematic and respective top and bottom isometric views of the belly armor shown in  FIGS. 1A and 1B ; 
         FIG. 2C  is a schematic isometric view of the belly armor shown in  FIG. 1A , with a cover plate thereof being removed; 
         FIG. 2D  is a schematic enlarged view of detail A shown in  FIG. 2C ; 
         FIG. 2E  is a schematic isometric view of the belly armor shown in  FIG. 2C , with some of the energy absorbing modules thereof being removed; 
         FIG. 2F  is a schematic isometric view of the belly armor shown in  FIG. 2E , with all the energy absorbing modules removed; 
         FIG. 2G  is a schematic enlarged view of detail B shown in  FIG. 2F ; 
         FIG. 3A  is a schematic front view of the belly armor shown in  FIG. 2A , when attached to an intermediary attachment members; 
         FIG. 3B  is an enlarged isometric view of a detail C shown in  FIG. 3A ; 
         FIG. 3C  is a schematic isometric view of the belly armor shown in  FIG. 3A , when attached to a side wall of the hull of the vehicle shown in  FIG. 1A ; 
         FIG. 3D  is a schematic isometric view of a detail D shown in  FIG. 3A ; and 
         FIG. 3E  is a schematic enlarged view of detail D shown in  FIG. 3A . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     With reference to  FIGS. 1A to 1C , there is shown an armored hull of a vehicle generally designated as  1 , having side walls  2  and a belly (not shown), the armored hull being fitted with a belly armor, generally designated as  10 . 
     Turning now to  FIGS. 2A to 2G , the belly armor  10  comprises a base plate  22 , a cover plate  30  and an energy absorbing arrangement  40  disposed between the base plate  22  and the cover plate  30 . 
     The base plate  22  has an inner surface  22   i  and an outer surface  22   o , in cross-section taken along a plane perpendicular to the direction extending between the front and the rear of the vehicle  1 , a concave shape, with a central portion  22 C and two peripheral portions  22 P extending on both sides of the central portion  22 C. The base plate is made of HH or UHH steel and has a thickness of about 0.5″. 
     Thus, it is appreciated that when the belly armor  10  is mounted onto the vehicle  1 , and the vehicle  1  is positioned on the ground, the central portion  22 C is closer to the ground than the peripheral portions  22 P. Therefore, in the event of an explosion under the belly of the vehicle  1 , the central portion  22 C is the first to experience the force of the explosion, and is configured, due to the concave shape, to deflect the blast to the peripheral portions  22 P. 
     For this reason, the central portion  22 C of the base plate  22  is fitted at its outer surface  22   o  with a reinforcement plate  24 , so as to thicken it, and make it more blast resistant. The reinforcement plate  24  is of a thickness smaller than that of the base plate  22  (e.g. 0.25″), and is made of the same HH or UHH steel. Adding of the reinforcement plate  24  provides an increased thickness of central portion of the base plate assembly  20  to about 0.75″. 
     Between the base plate  22  and the reinforcement plate  24 , there is lined a layer of reinforced glass fibers (GRF). 
     With particular reference being made to  FIG. 2D to 2G , the belly armor  10  is shown with the cover plate  30  removed, so as to expose the energy absorbing arrangement  40 . The energy absorbing arrangement  40  comprises a plurality of longitudinal grid beams  42 ,  44 , and transverse grid beams  46 ,  48  disposed on the inner surface of the inner plate  22 B, so as to form a grid of cells  43 , configured for structurally reinforcing the base plate assembly  20 . 
     The arrangement of the beams  42 ,  44 ,  46  and  48  is such that, besides structurally reinforcing the base plate assembly  20 , there are formed spaces between the beams referred herein as cells. Along the central portion  22 C, the beams  42 ,  44  and  48  form rectangular cells  43 , while at the peripheral portions  22 P, the beams  42  and  46  form together cells  45 . 
     Transverse beams  48  are disposed between the longitudinal beams  42 ,  44  so as to form generally rectangular cells  45 , in which the energy absorbing modules are located. The beams  48  are generally thin, and are used merely to confine each of the modules  50  within a four-wall cell. 
     The beams  46  are disposed on both sides of the central portion  22 C of the base plate  20 , bridging the central portion  22 C and the peripheral portion  22 P. Each such beam  46  has a generally triangular cross-section, such that one side of the triangle is supported by the central portion  22 C, while the other side of the triangle is supported by the peripheral portion  22 P (the third side of the triangle is facing the cover plate  30 ). 
     The transverse beams  46  defined between each two neighboring beams a cell  47  in which an energy absorbing modules can be confined. It is noted that, unlike in the central portion  22 C, this cell  47  is not four-walled, but rather two-walled, open at both ends. 
     The longitudinal beams  42 ,  44  extend along the central portion of the base plate  22 , the longitudinal beam  44  being thinner, and disposed between the two longitudinal beams  42  which are of greater width. 
     The arrangement can be such that the two longitudinal beams  42  are disposed, when the belly armor  10  is mounted onto the hull  1 , under the chassis of the vehicle (see  FIG. 1B , as denoted CH in  FIG. 1B , thus also using the chassis to provide additional deformation resistance to the armor  10  during an explosion. It should be noted however, that when mounted, there does not have to be direct contact between the belly armor  10  and the chassis CH. 
     Reverting to  FIG. 2A , the cover plate is formed with a central portion  31 C and two peripheral portions  31 P disposed on both sides thereof, similar to the base plate  22 , but at a different angle. When attached over the energy absorbing arrangement  40 , the central portion  31 C is attached to the longitudinal beams  42 ,  44  via ports  34  formed in the central portion  31 C of the cover plate  30 , and the peripheral portions  31 P are attached to the transverse beams  46  via ports  32  formed in the peripheral portion  31 P of the cover plate  30 . 
     When the cover plate  30  is attached over the grid beams  42 ,  44 ,  46 ,  48 —the modules received within the cells  43 ,  45  and are confined there by the base plate  22 , cover plate  30  and grid beams  42 ,  44 ,  46 ,  48 . 
     Each energy absorbing module  50  is configured for performing progressive deformation under application of a load thereto. In particular, the energy absorbing module  50  can either be a structure adapted to collapse under the load (e.g. honeycomb), or can be made of a material adapted to collapse under the load (e.g. aluminum foam). 
     Each such module  50  is confined within a wrapping  52  adapted to provide the energy absorbing module with structural stability, so that it only collapses/deforms under the application of a load caused by an explosion, rather than by shocks and vibrations occurring during regular operation of the vehicle. It is also noted that certain materials such as aluminum foam tend to disintegrate under vibrations, and so the wrapping provides protection against this undesired phenomena. The wrapping  52  can be made of a resilient material, e.g. Polyurea (PU) or polyethylene. 
     Turning to  FIGS. 3A to 3E , in assembly, the side walls of the vehicle hull  1  are fitted with an attachment beam  60 , having, in cross-section, a polygonal shape with four sides  62 ,  64 ,  66  and  68 . The design is such that the side  68  is parallel to the side  64 , while the side  62  is angled to the side  64  at an angle corresponding to that of the peripheral portions  22 P of the base plate  22 . 
     Thus, in assembly, the side  64  is configured for attachment to the side walls of the vehicle hull  1  via steel spacers  63  (passing also through side  68 ), and the side  62  is configured for attachment to the peripheral portions  22 P of the base plate assembly  22  using bolts  65 . The peripheral portions  22 P, in turn, are formed with attachment ports at the ends thereof remote from the central portion  22 C, configured for attachment to side  62  of the attachment beam  60 . 
     It is noted that under the above design, the only direct contact between the belly armor  10  and the vehicle hull  1  is through the attachment beam  60 , so that the belly armor  10  ‘hangs’ from the attachment beam  60 . Under this design, there is no direct contact (when the vehicle is at rest), between the belly armor  10  and the belly of the vehicle  1  and/or the chassis CH. 
     In an assembled position, when the vehicle is positioned on the ground on its wheels, the belly armor  10  extends between the belly of the vehicle and the ground, such that the base plate assembly  20  faces the ground, while the cover plate faces the vehicle. 
     In operation, at the event of an explosion under the belly armor  10 , the force of the explosion will first impact the deflector plate  24  and be dispersed to the sides (towards the peripheral portions  22 P) owing to the V-shape design of the belly armor  10 . 
     The loads still applied to the base plate  22  by the explosion will be dispersed over the base plate  22  being partially absorbed thereby, causing the base plate  22  to deform in an upward direction (i.e. towards the belly of the vehicle  1 ). However, due to the grid beams, the base plate is prevented from deforming to an extent which may affect the passengers occupying the vehicle. 
     On the other hand, the base plate  22  is configured for undergoing deformation to an extent sufficient to allow it to transfer the energy of the explosion to the energy absorbing modules  50 . In other words, the base plate  22  will deform so as to depress the energy absorbing modules  50  arranged between the base plate assembly  20  and the cover plate  30 . Owing to the longitudinal beams  42 ,  44  and transverse beams  46 ,  48 , the majority of the energy of the explosion is designed to be absorbed by the collapse/deformation of the modules  50  rather than by structural deformation of the base plate assembly  20 . 
     In addition, during an explosion, the loads applied to the base plate  20  of the belly armor  10  tend to apply to the base plate assembly  20 , a torque T (shown  FIG. 3E ) which operates to detach the base plate assembly  20  from the hull  1 . However, since the belly armor  10  is attached to the attachment beam  60  and not directly to the hull  1 , the torque operates against the angled side  62  of the attachment beam  60 , and not directly on the side walls of the hull  1 . 
     It should thus be understood that by adding the attachment beam  60 , the entire area of attachment between the hull  1  and the belly armor  10  is more robust and reinforced, and also prevents direct operation of the belly armor  10  on the hull  1  during explosion. 
     Those skilled in the art to which this invention pertains will readily appreciate that numerous changes, variations, and modification can be made without departing from the scope of the invention, mutatis mutandis.