Abstract:
The illustrative embodiment of the present invention is a launch system that includes a missile canister cover that, in use, is attached to a missile canister. The cover is capable of being blown off of the canister before there is any contact between the nose of the missile and the cover and is further capable of withstanding a higher ambient pressure than internal canister pressure. These capabilities are achieved based on an attention to material mechanics and the prevailing geometry of the system.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under contract DAAH01-03-C-0035 awarded by the US Army. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to canistered missile, and more particularly to covers for missile canisters. 
     BACKGROUND OF THE INVENTION 
     It is well known in the art to launch a missile from a canister. The canisters are typically round or square tubes that contain a missile, missile-launch hardware such as rails and/or sabots, and electronics for initiating launch. In addition to functioning as a launch system, the canisters provide environmental protection for the missile, simplify missile-handling issues, and provide an efficient and long-term solution for missile storage. 
     In order to provide full environmental protection for the missile and other components within the canister, the canister must be sealed. This is typically done using a cap or cover. The cover is ideally able to protect or isolate the missile from a variety of environmental factors and must provide unimpeded passage of the missile upon launch. 
     A variety of canister covers are known. Some covers tear or shatter upon contact with the missile during launch. This is acceptable for some but not all types of missiles. In particular, some missiles (e.g., LAM, PAM, etc.) include fragile mechanisms in the nose that could be damaged on impact with the cover. For these types of missiles, the canister cover must blow off without making contact with the missile. 
     Non-contact covers are typically more elaborate than tear-through designs since they must be actuated to release. This usually equates to increased weight, complexity, and expense. 
     SUMMARY OF THE INVENTION 
     The present invention provides a cover for a missile canister that avoids some of the costs and disadvantages of the prior art. 
     The illustrative embodiment of the present invention is a launch system that includes a canister and a missile, in addition to the canister cover. 
     A canister cover in accordance with the present invention is capable of:
         Being blown off of the canister before there is any contact between the nose of the missile and the cover; and   Withstanding a higher ambient pressure than internal canister pressure.
 
These capabilities are provided without any moving parts; rather, they are realized based on an attention to material mechanics and the prevailing geometry of the system.
       

     More particularly, in accordance with the illustrative embodiment, the capabilities described above are obtained by:
         Segregating the canister into two regions—a main region and a marginal regional—that have different structural properties or characteristics, wherein the marginal region is disposed near the outer edge of the cover and the main region is disposed inward of the marginal regional.   Attaching the marginal region to the wall of the missile canister.       

     In the illustrative embodiment, the structural characteristic that differs between the regions of the cover is the thickness of the region. In particular, the marginal region is thinner than the main region. 
     Since the marginal region is thinner than the main region, and by virtue of the way in which the cover is attached to the canister, the cover breaks directly over the canister wall when exposed to an elevated internal canister pressure. The break occurs in the marginal region, or at the interface between the marginal region and the main region. As a consequence, a single large piece representing the main region of the cover is blown off of the canister. 
     When the cover is exposed to an external pressure, there is a relatively limited build-up of stress/strain at the interface of the marginal region and the main region. This is because the marginal region is attached to and supported by the forward end of the missile canister. In fact, on exposure to external pressure, the cover might fail at the main region, which is unsupported by the canister, before a failure occurs at the marginal region. On the other hand, to the extent that an internal pressure acts on the cover, high levels of stress/stain will build at the interface of the marginal region and the main region since this region of the cover is not supported against forces that are applied from the inside of the canister. As a consequence, the cover fractures at a relatively lower internal pressure than external pressure. 
     If an imperfection exists in the cover at some location along the marginal region, a gap or fracture might occur at that location when exposed to elevated internal pressure. Were that to occur, pressure would dissipate such that the cover would not blow off of the canister. To that end, the cover is physically adapted to equalize the response of the marginal region to internal pressure, regardless of location-to-location variations in the marginal region. In the illustrative embodiment, that adaptation is a ridge of material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a missile launcher including a missile canister and a missile-canister cover in accordance with the illustrative embodiment of the present invention. 
         FIG. 2  depicts a bottom view of the missile-canister cover. 
         FIG. 3  depicts a side view of the missile-canister cover. 
         FIG. 4  depicts a cross-sectional view of an embodiment of the missile-canister cover in use, coupled to the missile canister. 
         FIG. 5  depicts a top view of a missile-canister cover in accordance with the illustrative embodiment of the present invention, wherein the cover is bolted to a missile canister. 
         FIG. 6  depicts a top view of the missile-canister cover of  FIG. 5  after it has ruptured due to exposure to an internal canister pressure that exceeds the pressure tolerance of the cover. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts missile launcher  100  in accordance with the illustrative embodiment of the present invention. Launcher  100  includes missile canister  102 , canister cover  106 , and missile  108 . Canister cover  106  is coupled to forward end  104  of canister  102 . Typically, cover  106  is bolted to canister  102 . Missile  108  resides within canister  102 . Side  105  of cover  106 , which is exposed to the interior of canister  102  when the cover is attached to the canister, is referred to herein as the “inside surface” of cover  106 . Side  107  of cover  106 , which is exposed to the ambient environment when the cover is attached to missile canister  102 , is referred to herein as the “outside surface” of cover  106 . 
     Canister  102  usually contains any one or more of a variety of internal elements or mechanisms, as is well known to those skilled in the art. Since these internals are not germane to an understanding of the present invention, they will not be described in this specification. 
       FIG. 2  depicts further detail of the inside surface of canister cover  106 , in accordance with the illustrative embodiment of present invention. In the illustrative embodiment, canister cover  106  has a polygonal shape; in particular, cover  106  has a substantially square shape. A cover having this shape is typically used in conjunction with a canister that has a square cross section. In some other embodiments, canister cover  106  has a different polygonal shape. In some further embodiments, canister cover  106  has a circular shape, which would be used in conjunction with a canister having a circular cross section. 
     With continuing reference to  FIG. 2 , canister cover  106  includes main region  210  and marginal regional  214 . Main region  210  includes all the area within dashed line  209  and marginal region  214  encompasses all portions of the cover outside of dashed line  209 . It is understood that line  209  is not a feature of canister cover  106 ; it appears simply for pedagogical purposes. Marginal region  214  is distinguished from main region  210  by its tendency to break or fragment before main region  210 . As described in further detail later in this specification, this behavior is due to a physical/structural difference between main region  210  and marginal region  214 . 
     Marginal region  214  includes holes  216  for receiving a bolt, etc., by which canister cover  106  is attached to canister  102 . Main region  210  includes a physical adaptation that enables cover  106  to resist pressure equally at all locations along the marginal region (when cover  106  is attached to canister  102 ). In the illustrative embodiment depicted in  FIG. 2 , the physical adaptation is ridge  212  that depends from the inside surface of cover  106 . 
     As depicted in  FIG. 2 , ridge  212  forms a continuous ring within main region  212 . As shown in  FIG. 3 , which is a cross-section through cover  106  at A-A in  FIG. 2 , ridge  212  extends downward (into the interior of canister  102  when the cover is coupled to canister  102 ). In some embodiments, ridge  212  is formed of the same material as the rest of main region  210 . In some of those embodiments, main region  210  is monolithically formed; that is, ridge  212  is simply formed (e.g., moulded, cut, etc.) from the piece of material that serves as main region  210 . In some other of those embodiments, ridge  212  is attached to the surface of main region  210 . In some further embodiments, ridge  212  is formed from a different material than the rest of main region  210  and is attached to its surface. 
       FIG. 4  depicts further detail of missile launcher  100 . In  FIG. 4 , missile cover  106  is coupled to missile canister  102 . 
     It was previously disclosed that marginal region  214  is distinguished from main region  210  by a tendency to break or fragment before main region  210 . In the embodiment that is depicted in  FIG. 4 , this is due to the fact that marginal region  214  is thinner than main region  210 . In some other embodiments, this behavior results from materials selection, wherein a different material is selected for main region  210  than marginal region  214 . That is, the material selected for marginal region  214  has a tendency to break or fragment under pressure at a lower pressure than the material selected for main region  210 . In some additional embodiments, the interface between main region  210  and marginal region  214  is pre-stressed or pre-strained, with the result that the interface fractures before either main region  210  or marginal region  214 . The stress/strain can result from using dissimilar materials in the two regions, or due differences in crystal structure of the same material, as can develop due to processing conditions, etc. 
     Cover  106  is coupled to missile canister  102  via bolts  322 . In particular, bolt  322  extends through hole  216  in marginal region  214  and engages bolt-receiving hole  320  in forward portion  318  of canister  102 . Gasket  324  is disposed between the bottom surface of cover  106  at marginal region  214  and the upper surface of forward portion  318  of canister  102 . Gasket  324  provides a pressure-tight seal. 
     It accordance with the illustrative embodiment, cover  106  is released from canister  102  and blown away from canister  102  during launch but before the nose of missile  108  has a chance to contact the cover. The cover releases due to the pressure generated from exhaust gases on when the missile fires. The design of cover  106  is, therefore, a function of the internal pressure that is developed when the missile ignites and materials composition of cover  106 . The internal pressure that develops depends on missile type and the dimensions of the canister. Those skilled in the art will be able to design and build cover  106 , after reading the present disclosure, as a function of desired materials of construction, missile type and canister dimensions. Suitable materials for cover  106  include for example, aluminum or glass-filled nylon. More generally suitable materials include any isotropic or quasi-isotropic material with predictable mechanical properties. 
     As previously described, marginal region  214  is less able to withstand pressure than main region  210 . As a consequence, cover  106  fractures at marginal region  214  due to launch pressure. 
     Launch system  100  must be able to withstand elevated external pressure. In particular, it is important that when cover  106  is exposed to such elevated external pressures, it does not fracture. In fact, it is possible that cover  106  will be exposed to external pressures that are as high as the internal pressure that is developed by the missile plume (which causes cover  106  to blow off of canister  102 ). As a consequence, cover  106  must be able to withstand a higher level of external pressure (i.e., pressure against its outside surface  107 ) than internal pressure (i.e., pressure against its inside surface  105 ). In other words, assume that cover  106  fractures when the internal canister pressure reaches magnitude P 1 , wherein the elevated pressure results from the release into canister  102 , on ignition, of missile exhaust gases. Cover  106  will not fracture, however, when exposed to an external pressure of the same magnitude, P 1 . 
     In accordance with the illustrative embodiment, this differential response to pressure is achieved by the way cover  106  is supported. In particular, in the illustrative embodiment, inside surface  105  of cover  106  is supported at marginal region  214 , but outside surface  107  is not supported. 
     Specifically, to the extent an external pressure is applied, marginal region  214 , the interface of the marginal region and the main region, and peripheral portion  430  of main region  210  deflect only a minimal distance (i.e., the thickness of gasket  324 ) until they abut upper surface  432  of forward end  318  of canister  102 . They are, therefore, prevented from deflecting to any substantial degree. As a consequence, there is a relatively limited build-up of stress/strain in the interface of main region  210  and marginal region  214 . As a result, a break does not occur. To the extent that an internal pressure acts on cover  106 , high levels of stress/stain will build at the interface of marginal region  214  and main region  210 . This occurs since cover  106  is not supported against forces that are applied against it from the inside of the canister. As a consequence, cover  106  has a diminished ability to resist internal pressure as compared to its ability to resist external pressure. 
     As previously described, ridge  212  equalizes the response of marginal region  214  to internal canister pressure. In particular, ridge  212  prevents cover  106  from fracturing at a single location along marginal region  214 , as might otherwise occur if marginal region  214 , at that location, were structurally compromised relative to other locations along the marginal region. If marginal region  214  were to fracture at a single location, then pressure would rapidly dissipate at that fracture and cover  106  would not blow off, as desired. 
       FIGS. 5 and 6  depict a top view of launch system  100  before and during launch. 
       FIG. 5  depicts launch system  100  in a pre-launch state. Cover  106  is bolted to canister  102  (not depicted in  FIG. 5 ) via bolts  322  in marginal region  214 .  FIG. 6  depicts launch system  100  after cover  106  has blown off canister  102 . As depicted in  FIG. 6 , cover  106  fractures at  626  near interface of marginal region  214  and main region  210 . The main region flies off of canister  102  in a substantially single piece, while marginal region  214  remains attached to upper surface of forward region  318  of canister  102 . Missile  108  is visible within canister  102 . Egress is now unimpeded such that there will be no contact between the nose of missile  108  and missile cover  106 . 
     It is to be understood that the above-described embodiments are merely illustrative of the present invention and that many variations of the above-described embodiments can be devised by those skilled in the art without departing from the scope of the invention. It is therefore intended that such variations, and others that will occur to those skilled in the art in view of the present disclosure, be included within the scope of the following claims and their equivalents.