Abstract:
A projectile ( 10 ) with extensible fins ( 24 ) is designed to be lightweight by removing material unnecessary to structural strength and filling the resulting voids with a non-metallic filler material ( 26 ). Although particularly suited where the extensible fins ( 24 ) are mounted behind the obturator ( 22 ) and that are therefore subjected to turbulent, destructive shock waves when the projectile is accelerated down the gun barrel, the method and apparatus can be used elsewhere as well. The filler material ( 26 ) may be high temperature grease, an epoxy, a silicone or other similar materials. The filler material ( 26 ) may be designed to fall away as soon as the projectile ( 10 ) exits the gun barrel, or it may be permanently adhered to the material of the projectile. The filler may be surrounded by a frangible boot ( 66, 66 ′) to protect the filler during storage, shipment and loading into the gun.

Description:
FIELD OF THE INVENTION 
     The present invention relates to projectiles and particularly to an apparatus and method to protect extensible fins mounted in the rear portion of a projectile as the projectile is fired from a gun. 
     BACKGROUND OF THE INVENTION 
     An aim of projectile design is to deliver a payload farther from the gun that fires it. This goal of longer range can be met by reducing the weight of the projectile and by increasing the size of the charge used to propel it. These two factors are not independent of one another, and increasing the propellant charge may damage the projectile because of increased gas pressure in the firing chamber and barrel. Reducing the weight may reduce the strength of the projectile to sustain the increased charge and may also reduce the apogee of the ballistic flight path, thereby reducing the range. 
     Projectiles have had control surfaces such as fins that fold inward to fit inside the projectile when it is fired from a gun and then fold outward once the projectile has cleared the gun barrel. Smaller fins create less drag and so allow a longer range. However, the fins must provide enough surface area to control the projectile in its flight. The required fin size can be reduced by minimizing the aftward mass of the projectile, shifting its center of gravity forward. 
     Past attempts at reducing the weight at the tail or aft end of a projectile by removing material from around the folded-in fins have resulted in a desirable weight reduction, but the exposed fins have been unable to withstand the concussion of being fired from a gun, especially as the charge used has been increased to increase range. 
     In the past, projectile fins have been mounted by a pivot pin to bosses that extend outward from the base of the projectile. Because the pressure retaining obturator has been mounted in front of the fins, the fins in their folded-in positions have been exposed to destructive pressure forces from the charge that fires the projectile. 
     Accordingly, there is a need for a projectile with fins pivotably mounted and able to survive the shock of being fired from a gun and that is also free of unnecessary mass at its aft end. 
     SUMMARY OF THE INVENTION 
     The present invention provides a projectile (or round) that has reduced weight in its aft end and has retractably mounted fins that are packaged to survive the shock of being shot from a gun. 
     A projectile utilizing the present invention has a base that forms the rearmost portion of the round. The base supports the fins and in some projectiles may form a nozzle for directing rocket exhaust to propel the projectile. An obturator is located at the front end of the base, in front of the fins. 
     We have discovered that the pressures inside a gun barrel behind the obturator when the charge is fired are not isostatic, but rather dynamic and turbulent. Resonances may occur in open volumes, and such resonances may cause destructive pressure waves to course through these spaces as the charge is ignited and the projectile accelerates through the barrel. The present invention limits the development of such destructive pressures while allowing a light-weight base. This is accomplished first by reducing the weight of the base as much as possible and then filling any voids in the base with an incompressible material that is lighter in weight than the metal it replaces. 
     The present invention may be carried into practice using a light-weight, non-metallic, substantially incompressible filler material to surround and support the fins while the projectile is in the barrel. Once clear of the barrel, the filler material may fall away, allowing the fins to extend so as to guide the projectile. 
     The invention may be carried out with or without using a separable, frangible sleeve or boot that surrounds the aft portion of the projectile. If such a boot is used, the volume it encloses, including the cavities housing the in-folded fins and other cavities within the boot, is filled with a filler of the kind described. The filler material may be any of a variety of materials that meet the performance specifications including high temperature grease, GE&#39;s RTV, a wax material or any other substantially incompressible material. For ease of installation, the filler material should be flowable and, if intended, it should break away cleanly and completely from the base of the projectile. The material may also be similar in all respects to the above, but remain completely in place. With this sort of material the base may be designed as a composite structure, with the filler material bonded to the metal of the base to provide a light weight yet structurally strong base. In addition, combinations of the two types of fillers (fall away and permanently adhered) may be used. 
     As the projectile emerges from the barrel, the boot, if used, falls away. If the filler is designed to fall away, it too falls away. If the filler is designed to remain, then it does so. The result is that the fins are protected from concussive resonances during the first moments of acceleration as the charge speeds the projectile down the barrel. 
     The fins are mounted to the base of the projectile. The use of a nonmetallic, incompressible filler material allows the base to be designed to be as light-weight as possible consistent with providing the necessary strength. This results in a base with fin mounting bosses that also has many hollowed out cavities where metal not necessary for strength purposes has been removed. These cavities and the spaces around the outside of the projectile and within the envelope of its outside cylindrical shape are filled with one or more of the filler materials described above. Because the filler eliminates any voids or cavities where resonances could occur, the base of the projectile is not subjected to destructive pressure waves. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the present invention will become clear to those skilled in the art from the following description of preferred embodiments when taken together with the accompanying Figures in which: 
     FIG. 1 is a perspective illustration of a projectile having a reduced weight base, a boot surrounding the base and a filler material between the inside of the boot and the base as well as in voids within the base; 
     FIG. 2 is a vertical cross-section view through the projectile of FIG. 1 with the filler material omitted; 
     FIG. 3 is a view to looking in the direction of arrows  3 — 3  of FIG. 2; 
     FIG. 4 is an exploded view of the aft end components of the projectile of FIG. 1 with the filler material omitted; 
     FIG. 5 is a perspective illustration of the projectile of FIG. 1 showing its fins in the extended position; 
     FIG. 6 is a cross sectional view looking in the direction of arrows  6 — 6  in FIG.  2  and showing the base, fins, fin inserts, and filler material; and 
     FIG. 7 is an illustration of an alternative boot that may be used with the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The projectile  10  illustrated in FIG. 1 is representative of projectiles that may benefit from use of the present invention. The projectile  10  is suitable for use, for example, in a 155 mm gun. The projectile  10  has a nose  12  that may carry guidance systems as well as the payload to be delivered. The body  14  of the projectile  10  may include fuel that burns to propel the projectile to its intended target. As loaded into a gun the base  16  of the projectile  10  includes an outwardly tapering nozzle  18  (FIG. 2) and an igniter  20  located in the nozzle. When the igniter  20  is activated after leaving the gun barrel, it incinerates itself to ignite the fuel in the body, which then bums, forcing exhaust gasses through the nozzle  18  to propel the projectile  10 . Although shown and described in connection with a projectile  10  that has a nozzle  18  and therefore a rocket motor, it will be appreciated that the present invention is equally applicable to projectiles that do not have a nozzle  18  or a rocket motor. 
     An obturator  22  (FIGS. 1 and 2) is located at the forward end of the base  16 . Fins  24  are mounted for pivoting movement at the rear of the base  16 , with their free ends just behind the obturator  22 . Accordingly, all of the base  16  including the fins  24  mounted to it, is subject to the concussive pressures generated by the charge (not shown) used to drive the projectile  10  out of the gun barrel. 
     Although the present invention has particular application to that part of a projectile behind the obturator and so exposed to high, turbulent pressures as the charge is set off, it may find use in other locations where components must be protected temporarily from vibration or shock. Therefore, as used in the claims and specification of this application, the term base includes not only structures such as that shown in the Figures but also any other structure supporting or surrounding components that are to be temporarily protected from shock and vibration. 
     All the space in the base  16 , not otherwise occupied, is filled with a substantially incompressible material  26  (FIG.  6 ). This reduces or eliminates entirely empty chambers that can generate destructive transient pressure waves. 
     The base  16  is generally circular (FIGS. 1,  2  and  3 ) and is machined from a solid billet of metal, preferably titanium or an alloy of titanium. The base  10  can also be machined from a casting to further minimize weight and expense. The base (FIG. 2) has a central opening  28  that includes the nozzle  18 . A passage  30  leads from the upstream end of the nozzle  18  to a propellant supply in the body  14  of the projectile  10 . The nozzle  18  is a conical passage opening at the rear to direct the burning propellant gases. Before firing the projectile an igniter fuse  20  fills the nozzle cavity. 
     The base  16  has eight-fold symmetry, and the various parts of the base and related components are identified with reference numerals where they appear most clearly in the Figures. Not every identical component is identified with a reference numeral in order to leave the Figures clear of excess reference numerals. The base  16  (FIGS. 5 and 6) includes eight fin slots  32  each having a closed bottom  34  (FIG. 6) extending parallel to the axis of the nozzle. The fins  24 , the slots  32  they are held in, and their mountings are all alike and only one is described in detail. 
     The fin  24  has an aerodynamic shape that tapers from a narrow leading edge  36  to a broader midsection  38  and then tapers down to a narrow trailing edge  40 . During loading and while the round  10  is in the barrel of the gun, the leading edge  36  of the fin  24  is folded inward and is close to the bottom  34  of its slot  32 . The slot  32  extends from outside of the base radially inward toward (but not meeting) the nozzle  18 . The fin  24  is rotatably mounted on a pin  44  (FIGS. 2,  4  and  5 ) that extends across the aft end of the slot  32 . When ready for firing, the fin  24  is folded inward as shown in FIG.  1 . As soon as the round exits the barrel, the fin  24  folds out as shown in FIG.  5 . 
     Support walls  48 ,  50  (FIG. 6) form each side of the slot  32 . Coaxial bores  52  (FIG. 4) through support walls  48 ,  50  receive and hold the pin  44  about which the fin  24  pivots. As illustrated in FIGS. 4 and 6, inserts  56  may be placed on each side of the fin  24 . The fin inserts  56  complement the tapers of the fin  24  and help fill the space between the fin and the inside walls  48 ,  50  of the slot  32 . 
     As the round  10  emerges from the end of the barrel, gas pressure is released from around the outside of the fins almost instantaneously as the obturator  22  clears the end of the barrel. However some pressure that was acting behind the fins and fin inserts takes a moment longer, and during that moment the pressure difference drives the fins  24  and fin inserts  56  outward, whereupon aerodynamic drag forces the fins to their fully extended position. The fin inserts  56  help to capture and use this momentary pressure difference. However they are not necessary to practicing the invention as it has been found that the drag on the fins  24  alone is enough to open them, or mechanical means such as springs may be used to push the fins out far enough that the drag can take over and move them to their fully extended position. Although the present invention is described in connection with aft deploying fins  24 , the fins may also be mounted to pivot about a pin at the forward end of the slot  32 , in which case an actuating mechanism must be provided to fold them outward. 
     The base  16  also includes axially extending, closed cavities  60 . (FIG. 3) These cavities are formed between each pair of pin receiving slots to reduce the weight of the base. The cavities are closed or blind in that there is only a single opening  62  into each. Indeed, the entire base  16  is made with the thinnest sections possible consistent with reliable operation of the projectile, and the cavities  60  are formed to eliminate unnecessary metal. 
     As noted the entire volume inside the gun barrel and behind the obturator  22  is pressurized when the charge is set off to launch the projectile  10 . Upon firing, the pressures in this space rise rapidly to approximately 50 kpsi, and the volumes between the fins  24  and within the base  16 , would resonate as the shock waves compress the air within if they were left empty. These resonances have proven destructive of the fins  24  and the base  16  itself. To prevent this from happening, the spaces within the base and surrounding the fins are filled with a lightweight, incompressible filler material  26  (FIG.  6 ). Preferably all of such spaces are filled, but on some designs some cavities may not resonate in a way that is harmful and so need not be filled. 
     Any of a range of filler materials may be used satisfactorily, so long as they are environmentally safe, non-corrosive, not destructive of surrounding materials, and stable under a wide variety of shipping and storage conditions for as long as 20 years or more. While no material is completely incompressible, it is important that the filler be free of significant change in volume from sub-atmospheric pressures up to about 50 kpsi. Some filler materials may be chosen that will adhere permanently to fill the cavities  60  and the spaces between the fins  24 . These filler materials may be permanently bonded to the surfaces of the cavities  60  and may contribute to the structural strength of the base  16 . Such filler materials may include epoxies, fiber reinforced epoxies, or other adhering compounds. 
     Other fillers may be used that are intended to separate from the projectile as soon as it exits the barrel. Suitable materials may include high-temperature grease such as Kendall Super Blu High Temp E. P. L-427 grease, wax, epoxies, or General Electric&#39;s RTV. 
     It is important that whatever filler is used, it operates as intended and that it either adhere completely or separate completely. The projectile  10  may become unbalanced and so uncontrollable if some part of the filler material remains while another part of the filler material separates. 
     A sleeve called a boot  66  (FIGS. 1,  2  and  6 ) may also be used in carrying out the invention. The boot  66  completely surrounds the base  16 , extending axially from the plane of the outlet of the nozzle  18  forward as far as the forward edge of the obturator  22 . To accommodate the obturator  22 , the boot  66  is formed with a reduced diameter portion  68  and a shoulder  70 . The obturator  22 , which is of conventional design, fits around the reduced diameter portion  68  of the boot, and its trailing face rests against the shoulder  70 . The leading face of the obturator  22  is pressed against the rear face  72  of the body  14  of the projectile when the base  16  is fastened to the body  14 . 
     The boot  66  may be used with certain types of filler materials  24 , especially those, such as grease, that are not self-supporting. The boot  66  supports the filler material  26  and protects it against becoming dislodged during shipment, storage, or loading into the gun breach. With other filler materials  66 , such as hard waxes or some epoxies, no boot may be required. If storage, shipping and handling are not a concern, then the boot may prove unnecessary, even with semi-solid fillers such as grease. 
     The boot  66  is frangible designed to self-destruct upon exiting the barrel. This can be accomplished in any manner, but it has proven workable to take advantage of the pressure generated by the charge to begin the destruction process. In one embodiment, the walls of the boot  66  are thin enough that hoop stresses upon leaving the barrel are so large that the boot  66  ruptures. In another embodiment (FIG.  7 ), the boot  66   1  has multiple axial grooves  74  equally spaced about the boot and extending from the rear of the boot  66  up to the shoulder  70 . The grooves  74  are proportioned so that once the charge is ignited in the gun breach, the boot  66  is crushed while still inside the barrel. The boot  66  then falls away immediately upon exiting the barrel. 
     The rear end face of the boot  66  is closed by a circular end plate  74  (FIGS.  1  and  2 ). This plate is welded around its perimeter to the trailing edge of the boot  66 . The plate  76  includes holes  78  aligned with the cavities inside the base. The holes  78  are used to fill the cavities  60  (FIG. 3) with filler material, and they are then plugged with set- screws or the like. The end plate  76  falls away with the rest of the boot  66 , exposing the end of the nozzle  18  through which exhaust gases from the propellant carried in body of the projectile may escape. 
     Thus it is clear that the present invention provides, in a projectile  10  with extensible fins  24  that is to be shot from a gun, a method and apparatus for packaging and protecting the fins  24  from destructive shock waves produced by the charge that fires the projectile, and for making the mounting structure for the fins as light as possible while protecting it from the same destructive shock waves.