Abstract:
Methods and apparatus related to design and construction of lightweight and mobile rail gun barrels. The barrels comprise a pair of elongated, generally parallel conductive rails extending along opposite sides of the bore and being symmetrical about a longitudinal axis of the bore; a pair of elongated insulators disposed generally coextensively with the rails and circumferentially between them and maintained in a compressed state; a circumferentially sleeve surrounding the insulators; a plurality of Belleville containment disk maintained in a stack that are compressed and surround the circumferential sleeve, each containment disk having a substantially hollow form with an outer surface, and an inner surface; and a plurality of longitudinal tension rods, disposed substantially parallel to the longitudinal axis of they bore and disposed external to the sleeve, the tension rods compress the plurality of Belleville containment disk. A protective sleeve and cooling channels can form part of the barrel.

Description:
FEDERAL RESEARCH STATEMENT 
     The invention described herein may be manufactured, used or licensed by or for U.S. Government purposes without the payment of royalties thereon. 
     BACKGROUND OF INVENTION 
     I. Field of Invention 
     This invention applies to the field of rail guns and more particularly, to a barrel assembly in an electromagnetic rail gun. 
     II. Background of the Invention. 
     Challenges presented by rail gun construction have existed since the early 1920s. Examples of such guns are taught in U.S. Pat. Nos. 1,370,200; 1,421,435; and 1,422,427. Current laminated-type rail gun barrels  10  for applications requiring light weight mobile use typically comprise stacked metallic annular or rectangular laminations in combination with pre-stressed tension elements to add radial and longitudinal stiffness. These laminations  11  have engineered shapes (often an elongated annular shape) to accommodate a plurality of longitudinal rails  12  within the barrel  10 , and the lamination shape is optimized to contain radial forces (tending to spread the rails  12 ) associated with the launching of rail gun projectiles. 
     Longitudinal stiffness of a rail gun barrel  10  can be obtained from tension elements  13  that may be spirally wound around the barrel at a preferred angle with the barrel longitudinal axis as taught in U.S. Pat. No. 5,454,28? entitled “Lightweight High L” Electromagnetic Launcher.” Alternatively, axial tension rods  14  may be used to preload the metallic laminates  11  in compression as shown in FIG. 1 (Prior Art), to provide a high effective bending modulus, provided the preload is not defeated by disturbance loads, Ref. “A High Performance Rail Gun Launcher Design,” Juston, John M. and Bauer, David P., IEEE Trans Mag., v33n1, pp. 566-570, Jan. 1997. 
     Radial stiffness of a rail gun barrel  10  is required in order to contain the rails  12  of the gun. In strong analogy with gas propulsion guns, the electromagnetic forces that propel the projectile out of a rail gun, also apply substantial loads to drive the rails  12  apart. The resulting strain of the rails  12  under the imposed magnetic stress may impair performance of the launcher as it propels projectiles at velocities in excess of 2/km/s. Since the rails  12  must be electrically insulated from each other, current practice is to place an insulator  15  between the two rails  12 . Since it is most challenging to engineer a structure that would allow the insulator material  15  to both bind to the rails, and to provide stiffness in tension, high performance rail gun designs incorporate a substantial compressive preload of the rails  12  against the insulator  15  that separates them. Using this approach, the modulus of the insulator  15  (often a ceramic), may contribute to reducing the dynamic strains of the rails  12  during operation, as long as the stresses do not exceed the preload magnitude. This is in complete analogy with any engineered tension compression system including tires and pre-stressed concrete. 
     Two methods of achieving this desirable pre-stress are depicted in FIGS. 2A (PRIOR ART) and  2 B (PRIOR ART). In FIG. 2A (PRIOR ART), a wedge  16  is driven between a split insulator  15  after the rail gun assembly has been undertaken. In FIG. 2B (PRIOR ART), a “flatjack”  18  is assembled between the main rails  12  and outer tensile containments structure  11 . After assembly, the “flatjack”  18  is pressurized with epoxy driving the rails  12  inward against the insulator (ceramic sidewall  19 ) while pulling the composite overwrap  20  out in tension. The epoxy subsequently cures to a solid state, making the pre-stress permanent. 
     Active cooling channels  21  are also generally required for heat dissipation in rail guns due to the very high ohmic heat loss effects which are on the order of twenty times more heat input to the launcher than a traditional gas gun. Thus active cooling channels  21  are desirable. In the design shown in FIG. 2B, active cooling channels  21  are integrated directly with the conducting rails  12 , see “Cannon-Caliber Electromagnetic Launcher, ” Zielinski, Alex E. and Werst, M. D. . IEEE Trans Mag, v33n1, pp. 630-635, Jan. 1997. With the design shown in FIG. 2A, conduction of the heat from the rails  12  through the containment structure  11  is sought to later dissipate the heat to the environment through the outer layer of the barrel  10 . Both of these approaches have limited success due to the fact of the small heat capacity of the surrounding air, and the low surface area of the outer skin of these types of rail designs. 
     To date, there is a need in this art for robust structural components in a rail gun, wherein these components can be preloaded to provide adequate pre-stressing of the containment structure for applications requiring light-weight mobile use. 
     Accordingly, it is an object of this invention to provide an improved containment barrel for a rail gun that enables increased muzzle energy and accuracy of a projectile. 
     Another object of this invention is to provide a design for a rail gun, wherein the barrel incorporates desired preload stresses in the gun containment structures. 
     Another object is to provide a design for a rail gun, wherein location of the cooling channels are in an area of low structural strain to provide active cooling of the rail gun, yet impart minimal impact to a rail gun design. 
     Finally another object of this invention is to substantially overcome the shortcomings in prior rail gun barrels and methods for making them, relating particularly to rail guns which are sufficiently strong, lightweight and stiff for mobile applications. 
     SUMMARY OF INVENTION 
     It has now been discovered that the above and other objects of the present invention may be accomplished by the following mechanism and in the following manner. 
     The rail gun barrel of the present invention comprises a pair of elongated, generally parallel conductive rails extending along opposite sides of the bore and being symmetrical about a longitudinal axis of the bore; a pair of elongated insulators disposed generally coextensively with the rails and circumferentially between them and maintained in a compressed state; a circumferential sleeve surrounding the insulators; a plurality of Belleville containment disks maintained in a stack that are compressed and surround the circumferential sleeve, each containment disk having a substantially hollow form with an outer surface, and an inner surface; and a plurality of longitudinal tension rods, disposed substantially parallel to the longitudinal axis of the bore and disposed external to the sleeve, the tension rods compresses the plurality of Belville containment disks. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The features of the present invention and the manner of attaining them will become apparent, and the invention itself will be understood by reference to the following description and the accompanying drawings. In these drawings, like numerals refer to the same or similar elements. The sizes of the different components in the figures might not be in exact proportion and are shown for visual clarity and for the purpose of explanation. 
     FIG. 1 (PRIOR ART) is a diagrammatic perspective view of a laminated rail gun containment structure. 
     FIG. 2A and 2B (PRIOR ART) are transverse sectional views of a containment structure using laminated steel and composite overwrap methods. 
     FIGS. 3A,  3 B,  3 C, and  3 D are plots of internal stresses of a cylindrical gun barrel as a function of radius for (a) normal gun firing, (b) during autofrettage, (c) after autofrettage, and (d) autofrettage gun during firing respectively. 
     FIG. 4 shows a longitudinal sectional view of a barrel in accordance with the invention. 
     FIG. 5 is a transverse sectional view a—a of the barrel shown in FIG.  4 . 
     FIGS. 6A,  6 B,  6 C, and  6 D show alternative forms of the Belleville effect disk that can be used by the invention. 
    
    
     DETAILED DESCRIPTION 
     During firing of a cylindrical gas-type gun as in FIG. 1, such a gun exhibits peak stresses at the bore  17 . A radial compressive stress, sigma r, results as the bore  17  is prevented from being pushed out by the outer wall of the barrel. In addition, a tangential or “hoop” stress, sigma t, is formed as the bore  17  resists enlargement itself. Neglecting the axial tension load of recoil as it is small, these two principal stresses combine to form the equivalent (von Mises) stress, sigma e, shown in FIG. 3A. A uniform stress distribution over the entire tube cross section during firing can be attained by means of autofrettage, see “Guns,” Horn, F., in “Handbook on Weaponry,” Second Edition Ed., Rheinmetall Gmbh.Dusseldorf, 1982. 
     Autofrettage is the process of pre-stressing traditional high pressure guns by applying an internal pressure sufficient to cause a plastic radial dilation of the bore. As high pressure guns are thick walled pressure vessels, the strain imposed by the autofrettage pressure may result in decreasing levels of plastic deformation at outer radial portions of the wall. Typically, the autofrettage pressure results in strains that are insufficient to cause plastic deformation of the outer hoop layers of the gun barrel. FIG. 3B depicts that once the material begins plastic deformation, the stress no longer increases, the equivalent stress flat-lines in the plastic zone during autofrettage. Once the autofrettage pressure is relieved, the outer hoop layers are left in a state of tension as they attempt to drive the permanently enlarged bore hoop layers back to their original size. FIG. 3C shows the results that being in the compression of the surface of the bore. Upon firing, a nearly constant equivalent stress is achieved through out the radius of the gun barrel. The propulsion pressure overcomes the compression preload gun as shown in FIG.  3 D. 
     Autofrettage is distinct from the current pre-stressed rail gun construction as typified by the designs shown in FIGS. 2 a  and  2   b  (PRIOR ART). First, the bore  17  of the gas gun (typically constructed of steel) is fully capable of tolerating substantial tension. This is in sharp contrast to the insulators  15  of the rail guns that must be bound in compression to prevent separation of the rails  12  from the insulator  15 . Second, autofrettage achieves a favorable stress distribution throughout the structure. The outer tensile containment structure of these rail guns exhibits the highest strain at the inner layers of the tensile containment structure. 
     Using principles presented by autofrettage in gun construction as discussed above, it has been discovered -that modified designs of rail guns can incorporate principles of this phenomenon. The present invention, embodying a rail gun barrel defined by an elongated bore  31  shown at FIG. 4 for passage of a projectile, is explained as follows. 
     FIGS. 4 and 5 depict a preferred rail gun barrel  22  construction which comprises a pair of elongated, generally parallel, electrically conductive rails  23  and a pair of elongated generally parallel insulators  24 . FIG. 4 shows the gun wherein Belleville containment disk  25  are in a non-compressed state and in a compressed disk  26  when assembled. The insulators  24  are disposed circumferentially between the rails  23 . That is the rails  23  and insulators  24  are disposed alternately about the circumference of the barrel so that the rails  23  do not contact one another. The rails  23  are preferably made of a copper alloy. The compressed insulators  24  herein are made of a ceramic material. The rails  23  are disposed symmetrically about the longitudinal axis of the barrel, as are the insulators  24 . Each rail  23  has a pair of generally planar side surfaces that abut generally side surfaces of the insulators  24  at interfaces that define radial planes. The rails  23  are electrically connected at their respective rearward or breech ends to opposite terminals of a source of direct current(not shown).Means for loading projectiles into the barrel  22  are provided at the breech end. 
     The rails  23  preferably have cooling passages  27  adjacent to them for coolant flow. These cooling passages are located in an area of low structural strain so as to provide active cooling of the rail gun  22 . These passages contained within the compressed containment disks  25 , 26  after assembly enable liquid coolant to effect heat transfer from the compressed containment disks  26  to an external heat exchanger (not shown). The location for these cooling passages  27  are symmetrically disposed about the barrel  22  with the axial tension rods  28 . 
     The rails  23  and insulators  24  herein define a substantially cylindrical bore  31  through which the projectile (not shown) travels. More specifically, the rails  23  and insulators  24  have curved inner surfaces that collectively define the substantially cylindrical bore  31 . The bore may be of circular or may alternatively be of rectangular or other suitable cross section. 
     The rails  23  and insulators  24  are contained within a circumferential insulator sleeve  29  that prevents current flow from rails  23  from passing through containment disks  26 . This sleeve  29  also provides adequate lubrication to enable the Belleville effect containment disk  25  to be compressed during manufacture. 
     A circuit through the rails  23  may be completed either by a conductor or a plasma arc disposed between the rails  23 . Where a plasma arc is used, high fluid pressures are generated within the bore  31  by vaporization of a strip of metal. As current flows through the circuit, magnetic flux is generated between the rails  23 . The magnetic flux cooperates with the current in the conductor or plasma arc to accelerate the conductor or plasma forward between the rails  23 . The projectile may include the conductor or may be positioned forward of the conductor or plasma arc and driven forward thereby. 
     When the rail gun is fired, bursting forces resulting from the interaction of the current in the rails  23  with the magnetic flux generated thereby urge the rails  23 , outwardly. In addition, where a plasma arc is present within the bore, high fluid pressures urge both the rails  23  and insulators  24  radially outward. 
     The bursting forces are not uniform along the length of the barrel  22 , but rather act only on the portion of the barrel  22  behind the projectile. Thus, at any point in time during firing, each of the internal barrel components  23 , 24  has a highly stressed region behind the projectile and a less stressed region ahead of the projectile. 
     A circumferential insulator sleeve  29  prevents the rails  23  and insulators  24  from being displaced radially outward, but it has been found that the inner surfaces of the rails  23  and insulators  24  may be displaced outward by compression of these components. The sleeve provides a thin electrically insulating layer to prevent current flow from the rails  23  from passing through the compressed containment disks  26 . This also functions to provide adequate lubrication to enable the Belleville effect containment disks  25  to become compressed containment disks  26  during the assembly of the rail gun barrel. 
     The above described stress pattern thus may instantaneously compress rearward portions of the rails  23  and insulators  24  more than forward portions thereof, generating bending moments along the inner surfaces of the rails  23  and insulators  24  which define the bore  31 . The tensile stresses attendant to the bending moments in the rails  23  are generally not of sufficient magnitude to damage the rails  23 , but those in the insulators  24 , which are preferably made of a ceramic material having very good electrical insulating properties, may cause cracking because such ceramic materials typically have low tensile strength and are relatively brittle. 
     In accordance with the invention, the insulators  24  are pre-stressed axially so that bursting forces acting thereon during firing which could otherwise produce axial tensile stresses near the inner surfaces thereof instead simply diminish the magnitude of the axial compressive stresses near the inner surfaces. Axial compressive pre-stressing of the insulators  24  maintain spacing of rails  23 . 
     Axial tension rods  28 , which can be hollow, are used to compress the Belleville effect containment disks  25  from their relaxed state to a compressed state shown as disk  26  after assembly of the rail gun barrel  22 . These rods  28  provide sufficient axial preload to achieve high bending modulus, as described above. 
     The Belleville effect containment disk ( 25 -unassembled and  26 -after assembly) are laminated containment plates. Laminations (disk)  25 ,  26  are typically required to reduce unwanted eddy currents when the gun is fired. The disk  25 ,  26  have an out-of-plane distortion that minimizes this eddy current effect. The Belleville effect containment disk  25 ,  26 , having characteristics similar to Belleville washers, thereby enabling improved flexibility in the control of preloads that result within the structure when compressed. These disk may also include perforations to induce discontinuous strain distributions upon their compression. For example, inward facing fingers that are deflected upwards out of plane. FIGS. 6A-D depict various Belleville containment effect disk designs that can be used in this invention. 
     When the Belleville effect containment disk  26  are compressed, the non-compressed disk  25  are subject to axial compression sufficient to defeat any out of plane distortion. As in the case of Belleville washers, as long as the disks  26 ,  27  are assembled with alternating out of plane deformations, the net load required to prevent distortion is the same for either on one disk or as may as a hundred disks. This is due to the fact of an additive superposition of each disk spring effect, as opposed to a parallel addition effects that occurs if the deformations were not alternating. As the disks  25  are compressed, the strain of defeating the out of plane deformations results in stress distributions that vary with both radius and azimuth imposed throughout the laminate. Depending upon the magnitude of deformation of the Belleville effect containment disks  25 ,  26 , plastic deformation can be achieved, if desired. Using FIG. 5, for most configurations, the accompanying radial and azimuthal strain will tend to contract the bore of the laminate, applying a compressive preload through the circumferential insulator sleeve  29  and rails  23  and compressed insulators  24 . 
     For preferred packaging of the gun, FIG. 5 shows a wrapping shroud  30  which provides environmental protection, additional bending stiffness if required, thermal insulation from uneven heating sources, and for minimizing electromagnetic signature emanation for stealth use in the field. 
     Other types of Belleville effect containment disk  25  prior to compression during assembly of the barrel may be used to achieve discontinuous changes in pre-strain. For example, these disks  25  can include perforations as shown in FIGS. 6A,  6 B,  6 C, and  6 D, which are generally referred to as rectangular speed nuts, push nuts for screws and studs, and external-internal respectfully. 
     From the foregoing, it will be appreciated that the invention provides an improved rail gun barrel and an improved method of manufacturing rail gun barrels. However, the embodiments described herein are included for the purposes of illustration, and are not intended to be exclusive; rather, they can be modified within the scope of the invention. Other modifications may be made when implementing the invention for a particular application.