Patent Publication Number: US-8973480-B2

Title: Missile canister

Description:
This invention relates to a missile canister. 
     Missile canisters are used to accommodate missiles during transit to provide protection. Missiles can also be deployed in missile canisters ready for launch and may be stacked together in a multi-canister missile system. 
     The current state of the art for launching missiles is generally divided into two categories, namely hard launch and cold launch. 
     In a hard launch system, the missile motor is ignited while the missile is in a missile launch canister. This approach requires significant efflux management due to the forces and debris produced as a consequence of allowing the primary missile launch motor to be ignited within the launch tube. In such a launch system the missile accelerates rapidly and conducts turnover with a high vertical velocity component. 
     In a cold launch system, the missile rocket motor is ignited only after it has been “pushed” out of its canister and in some instances orientated towards its intended flight path. Cold launch systems include apparatus in the launch tube to eject a missile from the tube. 
     Hard and cold launch systems require missile canisters for accommodating missiles during transit and prior to launch. In multi-canister systems, a plurality of canisters are stacked together one adjacent to another. Such multi-canister systems can be employed to launch multiple missiles in a relatively short period. 
     Typically, a missile canister  100  is formed of a cylindrical vessel  102  having a circular cross-section which accommodates a missile  104  along its longitudinal axis, as shown in  FIG. 11 . A circular cross-section is well suited for withstanding the forces caused by the high gas pressures generated during launch of a missile. In this regard, the high pressure generally gives rise to hoop stress in the circular vessel and the load is generally distributed evenly about the circumference without causing significant stress points. Missile canisters can therefore be fabricated from metallic material which are relatively easy to manufacture and have a high tensile strength to resist the circumferencial stress field. 
     However, since a missile canister must accommodate not only the body of a missile but also the wings or fins  106  of the missile, a circular canister must have a radius which is sufficiently large to accommodate the most radially outward portion of the missile, which typically means the wings or fins. Consequently, there is a relatively large internal volume V 1  which is unoccupied when a missile is accommodated within the canister causing inefficient use of space. 
     Further, as shown in  FIG. 12 , circular missile canisters are inherently unsuited for stacking for transport and deployment, and are relatively unstable. When stacked together, there is a relatively large volume V 2  left unused between the canisters meaning that the stacked canisters have an unnecessarily high foot-print. It will also be appreciated that transport containers  108  are typically rectilinear and therefore the volume V 3  may also cause inefficient use of space. 
     The present invention provides an improved missile canister. 
     Therefore, the present invention provides a missile canister for accommodating a missile along a longitudinal axis of the canister, the canister comprising a plurality of generally planar longitudinal wall portions connected together to form a tubular vessel having a polygonal cross-section, the interconnecting portions between wall sections are generally flexible so that when a missile is launched the bending moment at the interconnecting portions generated by the increase of pressure in the vessel is substantially less than the bending moment generated at the wall portions. 
     In this way, the corners behave similarly to a pivot point about which bending moment is reduced so that stress on the canister is resisted by walls rather than the corners. 
    
    
     
       In order that the present invention may be well understood, embodiments thereof, which are given by way of example only, will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  shows a rectangular missile canister; 
         FIG. 2  shows a cross-section of the missile canister accommodating a missile; 
         FIG. 3  shows a plurality of such missile canisters stacked together; 
         FIG. 4  shows a typical loading distribution along one wall of a square missile canister; 
         FIG. 5  shows a simplified bending moment diagram for one canister wall shown in  FIG. 4 ; 
         FIG. 6A  shows a typical loading distribution for a canister wall embodying the invention; 
         FIGS. 6B and 6C  show bending moment diagrams for a canister wall embodying the invention; 
         FIG. 7  shows a cross-section through the canister; 
         FIG. 8  shows part of the canister in more detail; 
         FIG. 9  shows a material construction of the canister; 
         FIGS. 10A and 10B  show a further missile canister; 
         FIG. 11  shows a missile accommodated in a circular missile canister; and 
         FIG. 12  shows a plurality of circular missile canisters stacked together. 
     
    
    
     Referring to  FIG. 1 , a missile canister  10  is shown for accommodating a missile along a longitudinal axis of the canister. The canister comprises a plurality of generally planar rectilinear longitudinal wall portions  14  connected together to form a tubular vessel  12  having a polygonal cross-section. As shown in this example a generally square cross-section is formed although depending on the configuration of the missile, other cross-sectional shapes may be preferred, such as triangular, rectangular or pentagonal. Interconnections  16  are provided between wall sections. As described in greater detail below the interconnections  16  are generally flexible so that when a missile is launched the bending moment at the interconnections generated by the increase of pressure in the vessel is substantially less than the bending moment generated at the wall portions  14 . 
       FIG. 2  shows a missile  20  accommodated in the missile canister  10 . As the canister has a square cross-section, the four fins  22  of the missile are received in the corners formed by the interconnections  16  between the wall portions  14 . In this regard, the lateral distance of the vessel from the central longitudinal axis L of the canister is greatest at the corner which is coincident with the most radially outermost portions of the missile. The wall portions  14  of the vessel are closer to the axis L and therefore the unoccupied or void volume V 4  of the canister is less than the void V 1  shown in  FIG. 11 . Accordingly, the canister utilises space more efficiently. Other missile configurations, for example, with say three or five fins, would require a canister having a triangular or pentagonal cross-section. 
     Additionally, as shown in  FIG. 3 , the canisters  10  can be more efficiently stacked together for transport and deployment thereby efficiently utilising space inside a transport container  24  or when deployed on for example a vehicle or ship. In this regard, the volume V 5  between the canisters is close to zero and the volume V 6  between the stack and a container may also be relatively small. 
     A cross-sectional view of part of a typical square canister is shown in  FIG. 4 . A corner C is shown interconnecting two adjacent wall portions W. During use of a missile accommodated in the canister, high gas pressures are generated within the canister which cause significant deflection of the wall portions in a outward direction D. The corners C are stiff and therefore the deflections cause a high bending moment and consequent stress at the corners. The highest bending stresses are generated in regions R in the interconnecting portions proximate the corners. 
       FIG. 5  approximates the bending moments in a canister wall portion W extending between two stiff corners C. The force applied by the gas pressure is shown by a uniformly distributed load L. It will be appreciated that the exact loading on the canister is somewhat more complicated than represented in  FIG. 5  but the Figure is sufficient for explaining the behaviour of the canister in use. 
     The bending moment Bw at the centre of the wall portion W is less than the bending moment −Bc at the corners. The bending moment at the centre of the wall portion is positive whereas the bending moment at the corners is negative, the inflection occurring where bending moment is zero at B 0 . This bending moment distribution is caused because the corners are stiff and resist relative angular movement of the adjacent wall portions at the corners. The high bending stress at the corners of the canister can be resisted by strengthening the corners, either by increasing the thickness of the canister or by providing reinforcing struts extending between adjacent wall portions at the corner. Both these solutions complicate the construction of the canister and increase cost. Further, reinforcing struts occupy space which could otherwise be occupied by the fins of missile and therefore require an increase in the size of the canister. 
     Embodiments of the present invention overcome the significant stresses which occur at the corners of the missile canister not by increasing the strength of the corners portions, but rather the interconnections between the wall portions are weakened. The weakened corners are flexible and allow movement between adjacent wall portions at the corner. Therefore, the bending moment at the corners is reduced such that it is substantially less than the bending moment in the wall portion. 
     An approximation of the bending moments generated in embodiments of the invention is shown in  FIG. 6 .  FIG. 6A  is a wall portion  14  extending between interconnections  16 . The forces provided by the high gas pressure generated in use of the canister are shown by uniformly distributed load L. The interconnections  16  are represented by simple supports which by definition are perfect pivot points about which bending moment is zero. In this configuration, the distribution of bending moments is shown in  FIG. 6B , in which bending moment B 16  at the interconnections is zero and the bending moment B 14  at the centre of the wall portion  15  is relatively large. Therefore bending stress at the corners is significantly reduced compared to a typical square canister. 
     The theoretical bending moment diagram shown in  FIG. 6B  may not be achievable in practice because of the additional requirements of a missile canister. For example, an interconnection  16  may be formed by a hinge functioning as a simple support. However, there is also a requirement that the canister contains high pressure gas without allowing gas to escape. The configuration of a hinge may not be suited therefore for use in a missile canister. 
     In one preferred embodiment of the present invention as shown in  FIGS. 7 and 8 , thin wall portions  26  form the interconnections  16  between wall portions  14 . The thin wall portions  26  are configured to decrease bending moment at the interconnection  16  so that it is substantially less than the bending moment at the wall portions. The bending moment at the interconnections is not zero because the thin wall portion has some stiffness. However, the thickness of the thin wall portions is selected so that relatively little radial, or lateral, compressive force is generated in the thin wall portion which would otherwise resist relative movement between adjacent wall portions  14 . In this regard, the ratio of the thickness t of the thin wall portion to the radial or lateral distance R between the longitudinal axis and the corner is preferably equal to or less than 1:10 and more preferably less than 1:20. 
     The corner thickness ‘t’ and wall thickness ‘T’ depend on the specific size and demands imposed by the system requirements, i.e. available space &amp; missile calibre. An exemplary aspect ratio of t:T is 5/18 (i.e. 0.28) but this could vary according to the working pressure for example between 0.28+/−0.5. 
     In this way, the bending moment diagram for a wall portion  14  of the missile canister shown in  FIGS. 7 and 8  is as shown in  FIG. 6C . In this latter Figure, the bending moment −B 16  at the interconnections  16  is substantially less than the bending moment B 14  at the centre of the wall portions  14 . However, as the thin wall portions  26  have some internal stiffness, an inflection occurs at B 0  where the bending moment in the wall portion is zero. However, compared to  FIG. 5 , the inflection points occur relatively close to the corner, and the bending moment B 16  is substantially less than the bending moment B 14 . 
     The wall portions are configured to resist compressive and tensile loads and shear stresses through the wall. In one arrangement shown in  FIG. 9 , the vessel is constructed from a composite material which behaves similarly to an I-beam. The wall portions  14  comprise a skin  28  covering a core  30 . The tensile and compressive loads are carried by the skin, similarly the flange of an I-beam, whilst shear stresses are carried by the core, like the web of an I-beam. The skin may be formed from carbon fibre reinforced plastics whilst the core may comprise a tessellated configuration, such as a honeycomb, which has high compressive strength. The interconnecting portions  16  may comprise a high tensile skin  28  covering a low compression flexible core  32 , which may be a low density foam material. The skin  28  may extend around the entire periphery of the wall portions  14  and interconnecting portions  16  of the canister. Additional reinforcing elements or inserts may be provided for attaching items such as a breech, arrestors, or missile connections. The reinforcing elements provide additional strength by spreading the applied load over the composite material. 
     The materials of the wall portions may not be homogenous throughout the longitudinal extent of the canister. As shown in  FIG. 10A , the breech end portion  34  may be formed from different materials having different properties than the materials forming the muzzle end portion  36 . In use, the breech end portion  34  accommodates the means for propelling a missile from the canister whereas the muzzle end portion  36  accommodates the forward part of the missile. In a hard launch system, the breech end portion  34  accommodates a rocket motor which is ignited to eject a missile from the canister. As shown in  FIG. 10B , in a cold launch system, the breech end portion  34  accommodates a piston  38  and an energetic material  40  which is ignited to propel the piston along the tube. This movement of the piston ejects a missile from the canister. Piston arresters  42  are provided for retaining the piston in the canister after launch. 
     It will be appreciated that in either hard or cold systems, on launch greater gas pressure is generated in the breech end portion  34  of the canister than the muzzle end portion  36 . Accordingly, the material properties of the breech end portion  34  are designed to withstand greater stresses that those of the muzzle end portion. If the canister is made from a composite material, the core of the breech end portion has greater compressive strength than that of the core of the muzzle end portion. For example the core of the breech end portion may be formed of a high density foam, whereas the core of the muzzle end portion may be formed of a low density foam. 
     The invention also includes any novel features or combinations of features herein disclosed whether or not specifically claimed. The abstract of the disclosure is repeated here as part of the specification.