Abstract:
An elongated electromagnetic railgun ( 1 ) adapted to propel a moving armature ( 30 ) through a bore ( 11 ) along the length of the railgun ( 1 ) from its breech end ( 21 ) to its muzzle end ( 22 ). The railgun ( 1 ) comprises two elongated mechanically rigid electrically conductive barrel sections ( 13 ), said sections ( 13 ) being spaced apart from each other along the length of the railgun ( 1 ). Mechanically coupled via a dielectric ( 18 ) to each barrel section ( 13 ) is an elongated current carrying rail ( 14 ) for providing electromagnetic propulsive force to the armature ( 30 ). The two rails ( 14 ) face each other across an elongated open channel, defining the bore ( 11 ). The two barrel sections ( 13 ) are electrically connected to each other at a maximum of one location of the railgun ( 1 ).

Description:
RELATED APPLICATIONS 
     This patent application claims the benefit of commonly-owned U.S. provisional patent applications 61/475,414 filed Apr. 14, 2011; 61/488,614 filed May 20, 2011; 61/513,729 filed Aug. 1, 2011; 61/525,303 filed Aug. 19, 2011; 61/549,928 filed Oct. 21, 2011; 61/567,070 filed Dec. 5, 2011; 61/588,498 filed Jan. 19, 2012; and 61/595,110 filed Feb. 5, 2012; all eight of which previously-filed patent applications are hereby incorporated by reference in their entireties into the present patent application. 
    
    
     TECHNICAL FIELD 
     This patent application pertains generally to the field of electromagnetic launchers, and specifically to railguns. 
     BACKGROUND ART 
     Background references include the following references, all of which are hereby incorporated in their entireties into the present patent application:
         1. “Electrical and Thermal Modeling of Railguns”, Kerrisk, Jerry F.,  IEEE Transactions on Magnetics , Vol. Mag-20, No. 2, March 1984, pp. 399-402, U.S.A.   2. “Loss of Propulsive Force in Railguns with Laminated Containment”, Parker, Jerald V., and Levinson, Scott,  IEEE Transactions on Magnetics , Vol. 35, No. 1, January 1999, U.S.A.   3. “Eddy Current Effects in the Laminated Containment Structure of Railguns”, Landen, Dwight and Satapathy, Sikhanda,  IEEE Transactions on Magnetics , Vol. 43, No. 1, January 2007, U.S.A.   4. “Phenomenological Electromagnetic Modeling of Laminated-Containment Launchers”, Mallick, John,  IEEE Transactions on Magnetics , Vol. 43, No. 1, January 2007, U.S.A.   5. “Enhancement of the Compressive Strength of Kevlar-29/Epoxy Resin Unidirectional Composites”, D&#39;Aloia et. al,  High Performance Polymers , Vol. 20, pp. 357-364, June 2008, first published December 11, 2007.   6. Quickfield Version 5.7, Finite Analysis System, Tera Analysis, Ltd., Svendborg, Denmark, 2009, http://quickfield.com (last downloaded Nov. 1, 2010)       

     Kerrisk [Reference 1] taught that a gun barrel electrically conductive along the major gun axis could not be brought into close proximity to the current carrying rails of a railgun without significantly reducing rail inductance. Given the barrel geometry, which was fully enclosing of the rails, and the other boundary conditions used, the conclusions arrived at were correct. 
     However, consider the following. The gas law is represented by a scalar equation and hot gas produces an isotropic pressure. Consequentially, the barrel for a standard gun must be everywhere continuous in theta and z to prevent gas escape and force loss on the back projectile surface. On the other hand, the magnetic field is defined by Maxwell&#39;s equations, and the magnetic field is a vector quantity. It follows that the magnetic pressure is a vector quantity. The barrel design for a magnetic gun can take advantage of this fundamental difference between these two cases. It is not necessarily required that the barrel be continuous in theta and z for full magnetic pressure containment and for the magnetic pressure to be properly applied to the back armature surface. That is, the barrel need not be fully enclosing of the rails. 
     If the electrically conducting gun barrel: (1) is split open top and bottom from the breech to the muzzle, and (2) the two new barrel sections make contact with each other only at the gun base (i.e., the gun breech), the condition for completing the image current circuit in the armature region can no longer occur, as discussed by Kerrisk [Reference 1]. This represents the case where each of the two independent barrel sections is mechanically anchored to the gun base with direct metal-to-metal mechanical contact. Therefore, the barrel sections are electrically connected to each other at the base. However, the two barrel sections remain electrically isolated from each other everywhere else along the length of the gun barrel. This new barrel configuration is described herein. 
     DISCLOSURE OF INVENTION 
     An elongated electromagnetic railgun ( 1 ) adapted to propel a moving armature ( 30 ) through a bore ( 11 ) along the length of the railgun ( 1 ) from its breech end ( 21 ) to its muzzle end ( 22 ). The railgun ( 1 ) comprises two elongated mechanically rigid electrically conductive barrel sections ( 13 ), said sections ( 13 ) being spaced apart from each other along the length of the railgun ( 1 ). Mechanically coupled via a dielectric ( 18 ) to each barrel section ( 13 ) is an elongated current carrying rail ( 14 ) for providing electromagnetic propulsive force to the armature ( 30 ). The two rails ( 14 ) face each other across an elongated open channel, defining the bore ( 11 ). The two barrel sections ( 13 ) are electrically connected to each other at a maximum of one location of the railgun ( 1 ). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other more detailed and specific objects and features of the present invention are more fully disclosed in the following specification, reference being had to the accompanying drawings, in which: 
         FIG. 1  is an exploded isometric view of one embodiment of railgun  1  of the present invention. 
         FIG. 2  is a cross-sectional view of the  FIG. 1  embodiment. 
         FIG. 3  is an isometric view of the embodiment of  FIG. 1  in which retention frames  32  are used. 
         FIG. 4  is an isometric view of a retention frame  32 . 
         FIG. 5  is a magnetic field line plot for the embodiment of  FIG. 1 . 
         FIG. 6  is a closeup of a portion of the magnetic field line plot of  FIG. 5 . 
         FIG. 7  is an isometric view of the  FIG. 1  embodiment showing a first means for electrically coupling the barrel sections  13  to each other at the base  23 . 
         FIG. 8  is an isometric view of the  FIG. 1  embodiment showing a second means for electrically coupling the barrel sections  13  together at the base  23 . 
         FIG. 9  is an isometric view of the embodiment of  FIG. 8  in which a dielectric shell  10  has been added. 
         FIG. 10  is a modification of the  FIG. 1  embodiment in which the barrel sections  13  are tapered, and the cross-section of each barrel section  13  is a non-square rectangle. 
         FIG. 11  is a top view of the  FIG. 10  embodiment. 
         FIG. 12  is an isometric view of a second embodiment of the railgun  1  of the present invention. 
         FIG. 13  is a cross-sectional view of the barrel section  13  of  FIG. 12 . 
         FIG. 14  is an isometric view of the  FIG. 12  embodiment showing the use of retention frames  32 . 
         FIG. 15  is a magnetic field line plot for the  FIG. 12  embodiment. 
         FIG. 16  is a closeup of a portion of the magnetic field line plot of  FIG. 15 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     By electrically isolating two specially designed electrically conductive gun barrel sections  13  along their lengths from breech  21  to muzzle  22 , the present invention changes the boundary conditions used in the 1984 paper by Kerrisk [Reference 1]. The result allows a metal gun barrel  13  to be located close to the current carrying rails  14  while still maintaining high inductance per unit length (L′). The following is a description of a two-rail  14  open air railgun  1  that uses this principle to achieve high efficiency. Each current carrying rail  14  is mechanically supported by its own barrel section  13 . The rails  14  are normally identical to each other, and are spaced apart from each other along the entire length of the railgun  1 . The space between the rails  14  defines the gun bore  11 . The bore  11  is directly exposed to the atmosphere (ambient gases) along its entire length. 
     First Principal Embodiment 
       FIG. 1  shows an isometric view of a first principal embodiment of the present invention. The spacing between the rails  14  is typically 30 cm for the particular design used to calculate results discussed here, including the inductance (L′=0.40 uH/m) and the magnetic computational outputs presented below. The results presented are not optimized at the system level. For example, with all else being held constant, if the gun bore  11  is increased from 30 cm to 40 cm, L′ increases from 0.40 uH/m to 0.47 uH/m. 
     In this two-rail  14  system, current is carried along the first rail  14 , conducted across a moving armature  30  (see  FIG. 2 ), and then returned in the opposite direction along the second, parallel, and opposing rail  14 . 
     Image currents create the opportunity for a two-part rail  14 , 6 . The first part  6 , where sliding contact between the rail  6  and the armature  30  is made, is made of steel or a similar highly wear-resistant material. The second part  14 , which makes up the bulk of the rail  14 , 6  and which carries the bulk of the current to and from the generator, is made of copper or copper alloy. This design allows for a large enough rail  14 , 6  size to both accommodate rail  14 , 6  cooling and a reduced resistance per unit length, to more than counter the increased power loading due to the introduction of image currents on the outer surfaces of barrel sections  13 . 
     Circular rail part  14  is preferably recessed within its corresponding barrel section  13 . While other materials could be used for plate  6 , steel is usually the material of choice, even though the thermal expansion coefficients of steel and copper alloys are quite different. Other materials, such as tungsten alloy, tungsten copper eutectic, or a tungsten copper alloy, could be used for plate  6 . This would better match the CTE&#39;s of the two parts  14 , 6 . 
     Plate  6  is explosion bonded, or otherwise firmly attached, to the copper alloy rail part  14 . Each plate  6  preferably has small periodically spaced slots  7 . The slots  7  are perpendicular to the long (z) axis of the rail  14 , 6 . Slots  7  extend all the way through plate  6 . In this way, as the copper  14  expands and contracts at a greater rate than the steel  6 , the small sections of steel  6  can absorb the small differential stresses and strains that occur as the temperature cycles between each railgun  1  shot. 
     In an alternate embodiment, the two parts  14 , 6  of the rail can be replaced with a single part  14  made entirely out of a single material, such as tungsten copper alloy. 
     The copper or copper alloy part  14  contains a large continuous or sectional channel  5  interior to the rail part  14 . Channel  5  allows for the flow of coolant, either along the entire rail  14 , 6  length, or, preferably, along a plurality of rail  14 , 6  sections. In this embodiment, holes  17  can be machined into the barrel sections  13 , and coolant can be exchanged at varying gun  1  locations. The total heat deposited varies along the rail  14 , 6 . Therefore, rail  14 , 6  cooling can be better managed in sections, as provided by this embodiment, 
     While not shown, the gun barrel  13  can be cooled independently of the rails  14 , 6 . 
     The barrel sections  13  provide structural integrity to the railgun  1 . In conjunction with the retention frames  32 , sections  13  contain the strong outward lateral forces that are produced by the magnetic pressure from the (typically very high) currents flowing through the rails  14 , 6 . 
     Typically, the two barrel sections  13  are electrically connected to each other at just one location along the length of the gun  1 , namely, at the region of the base  23 , as shown in  FIGS. 7 ,  8 , and  9 . However, in some embodiments, the two barrel sections  13  are not electrically connected to each other or to any other electrically conductive mass (e.g., base  23 , turret  20 , or deck  4 ) at all, i.e., they are made to “float” electrically. This “electrically floating” design can be used for all the embodiments illustrated herein, i.e., those depicted in  FIGS. 1 through 6 , as well as  FIGS. 10 through 16 . 
     Barrel sections  13  are typically made of a high-strength metal, such as steel. The outer surfaces of the barrel sections  13  may be lined with electrically conductive linings  16 , so that these outer surfaces are electrically conductive to a higher degree. This facilitates the return of image currents from the muzzle end  22  to the breech end  21  with lower losses. When used, linings  16  are fabricated of a very highly electrically conductive material, such as copper or copper alloy. 
     The two barrel sections  13  are spaced apart from each other, at a uniform distance, throughout the length of the railgun, and are normally identical to each other. 
       FIG. 2  is a cross-sectional view of a barrel section  13  and its embedded current carrying rail part  14 . In this particular embodiment, barrel section  13  has a square or non-square but rectangular cross-section. Typical dimensions are 55 cm×52 cm for the cross-section of barrel section  13 , 30 cm diameter for rail part  14 , and 2.5 cm thickness for insulator  18 . The gun bore  11  is represented by the volume between the two facing steel plates  6 .  FIG. 2  illustrates the left-hand plate  6 . The bore  11  volume is entirely open to the atmosphere (ambient gases) from the breech  21  to the muzzle  22 , except when armature  30  and any accompanying projectile  31  pass through the bore  11 . Armature  30  can itself be the payload, or it can propel a separate projectile  31  which constitutes the payload. 
     In many embodiments, such as illustrated in  FIG. 2 , the barrel  13  surfaces are lined with copper or copper alloy lining  16 . In some circumstances, such as to save weight, aluminum or aluminum alloy might be used for linings  16 . The region between the current carrying rail parts  14  and the gun barrel  13  is mostly filled with a dielectric  18 . Kevlar is the dielectric  18  of choice, although in other applications, Phenolic, ceramic, or a ceramic composite can be used. The geometry is designed to insure that there is no direct line of sight between the dielectric  18 /air interface and the sliding contact region between the rail  14 , 6  and the armature  30 . The purpose of this geometrical constraint is to prevent direct UV illumination of these surfaces. It also prevents direct liquid metal (emanating from the armature  30 /rail  14 , 6  interface) or other direct sputtering or evaporative induced coating of the insulator  18  surface. Additional baffles can be added to further protect this dielectric  18 /air interface, if required. 
     Plate  6  is concave (from the point of view of bore  11 ). This allows for an effective mechanical capture and guidance of the armature  30  and payload  31  along the gun bore  11 . It also helps to insure that any liquid metal jetted from the rail  14 , 6 /armature  30  interface will be ejected directly into the outside region of the gun  1  and well away from the insulators  18 . 
       FIG. 3  shows two of the retention frames  32  that hold the barrel sections  13  in place during a shot. In practice, several retention frames  32  are used, spaced apart along the length of the gun  1 .  FIG. 3  shows a partial assembly of the barrel sections  13  at a region other than the region near the base  23  and turret  20 . All inward facing surfaces of the retention frames  32  that would otherwise come in contact with the electrically conductive barrel  13  surfaces are lined with Kevlar or other suitable dielectric  35 . This is to prevent the electrical interconnection of the two barrel sections  13  with each other. These dielectric sections  35 , like dielectrics  18 , are always under compression. 
     As shown in  FIGS. 3 and 4 , each frame  32  has two wedge sections  33  with sharp edges, positioned on the top and bottom of the frame  32 . Wedges  33  prevent the hot, high velocity liquid metal that is jetted from the armature  30 /rail  14 , 6  interface from coming into contact with this portion of the retention frames  32 . The sharp edges are used to prevent any appreciable backsplash of the hot liquid metal. The liquid metal is released into the atmosphere, where it is burned off. 
     During the short periods that the jetted liquid metal comes into contact with any of the retention frames  32 , an alternate conducting path is produced for current flow between the rails  14 , 6  other than through the armature  30 . This current path is highly resistive and highly inductive compared to the normal path through the armature  30 . Therefore, relatively little current flows along this path. The retention frames  32  can be coated with a non-conductor along their beveled surfaces  33  to prevent current flow along this path. 
     The pitch of the retention frames  32  (distance between adjacent frames  32  along the z axis) is large compared to the thickness of each frame  32 . This is important so as not to reduce the rail  14 , 6  inductance appreciably. This also helps keep the added weight in check and is possible for two reasons. The frame  32  height can be increased as necessary to insure that the induced stress in the frame  32  due to the rail  14 , 6  current-induced magnetic pressure is well within the stress limit of the (typically steel) material from which the frames  32  are fabricated. Secondly, the barrel sections  13  are substantial in physical size and prevent outward deflection of the rails  14 , 6  in the regions between retention frames  32 . 
     Each retention frame  32  contains at least one (typically horizontal) separation slot  34 , cut completely through the frame  32 , located in the vicinity of a gun barrel  13  back surface  12 . This prevents the complete encirclement of the rails  14 , 6  by a conductor which would otherwise reduce the gun  1  inductance per unit length [References 2, 3, 4]. Because of the size and design of the retention frames  32 , these slots  34  do not compromise the frame&#39;s  32  structural integrity. The large frame size  32  on the barrel backside  12  reduces the inductance only marginally, as the flux density in this region is low. The conservative computer modeling estimate for this flux density is approximately between 0.2% and 0.3%. 
     The following is a calculation of the cross-sectional area of the retention frames  32  required to prevent separation of the rails  14 , 6  so that the retention frames  32  do not fail. Superimposed on  FIG. 5  is a contour  8  used for the computer-based net outward force calculation (1.2×10 7  N/m) for a current of 6 MA flowing in each current carrying rail part  14 . A tensile strength of 800 MPa for heat treated steel is assumed.
 
1 MPa=1×10 2  N/cm 2  
 
     Therefore, the yield strength of steel (800 MPa) is:
 
8×10 4  N/cm 2  
 
     The cross-sectional area per unit length along the gun bore  11  axis required to prevent lateral rail  14 , 6  expansion at the yield strength of the material is then:
 
(1.2×10 7  N/m)/(8×10 4  N/cm 2 )=1.5×10 2  cm 2 /m
 
     For a 400% engineering safety margin, this becomes:
 
6.0×10 2  cm 2 /m
 
     For every meter along the gun bore  11  length, the steel cross-sectional area that spans the bore  11  region required with a safety factor of 400% is given by the above. Because there are two segments (one on the top and one on the bottom) to each retention frame  32  that spans the gun bore  11 , each such steel cross-section is 300 cm 2 . If each frame  32  element were 30 cm in height, the retention frame  32  width would be 10 cm. This represents a mechanical transparency factor of 90%. However, the magnetic transparency factor will be higher, as the flux lines are ducted around the frames  32 , preserving a high degree of the energy density on both sides of the frame  32 . However, and for example, the optimized mechanical design might call for two retention frames  32  per meter, each 5 cm in width. 
     The 400% engineering safety margin accounts for a number of factors, including mechanical safety to failure. Of equal importance are such factors as magnetically induced lateral rail  14 , 6  displacement at the point of armature  30  contact. The overall mechanical system must be sufficiently rigid to insure that rail  14 , 6  spacing and planarity specifications are met. 
     The compressive strength of Kevlar is variously given in the literature as being between 200 Mpa and 300 Mpa, and with heat treatment can be as high as 500 MPa [Reference 5]. The insulation  35  surface area used on the backsides  12  of the barrel sections  13  can be designed to accommodate similar engineering safety margins as that used above. 
     Magnetic Analysis 
       FIG. 5  shows a two-dimensional magnetic field line plot for the embodiment where the two barrel sections  13  are electrically connected to each other at the gun base  23  only. The two barrel sections  13  remain electrically isolated from each other everywhere else along the gun barrel  13  length, from the breech  21  to the muzzle  22 . Physically, this means that the two barrel sections  13  are mechanically secured to the gun base  23  with direct metal-to-metal connections. The retention frames  32  were not included in the computer run that was used for  FIG. 5 . Inclusion of frames  32  would alter the results by approximately 2% to 3%. The field line plot of  FIG. 5  was taken 100 micro-seconds into the pulse. Copper was used to simulate the barrel sections  13 . This simulated the copper linings  16  typically used around steel barrel  13  walls. Due to computer program limitations, each plate  6  was simulated using copper fused with its corresponding copper rail part  14 . There was no accounting for transient thermal heating of the electrodes  14  that would otherwise drive the surface currents deeper into the electrodes  14  with time. Version 5.7 of Quickfield [Reference 6] was used for all computational work presented herein. 
     A substantial amount of additional steel can be added to the gun barrel sections  13  without compromising the railgun  1  inductance. For example, in one computer simulation in which a substantial amount of steel was added laterally to the backside  12  of each barrel section  13 , the inductance per unit length decreased from 0.40 uH/m to 0.39 uH/m. This is approximately a 2.5% reduction in the inductance. 
       FIG. 6  shows an enlargement of the space around one of the current carrying copper rails  14 , 6 . The field line density has been increased to show more precisely where the surface currents will flow after the armature  30  has passed and before significant current diffusion into the conductors  14 , 6  has occurred. Drive currents flow on the surfaces of rail  14 , 6 , and image currents flow on surfaces of the steel barrel  13 . Once the armature  30  has passed, a high percentage of the rail  14 , 6  current is drawn into the small gap  26  between the copper rail part  14  and the barrel  13 . By proper design, the rail  14 , 6  current is initially distributed uniformly over the significantly enhanced surface area of the enlarged rail  14 , 6 , 26 . Later, as the current diffuses into the bulk of the rail  14 , it does so more uniformly. This minimizes energy dissipation in the rail  14 , 6 . 
     What is not shown accurately in  FIG. 6 , as Version 5.7 of Quickfield is unable to simulate such, is that due to the higher resistivity of the steel plate  6 , compared with the copper rail part  14 , a higher percentage of the rail  14 , 6  current on the front surface quickly migrates to the copper part  14  than is shown in  FIG. 6 . Some of the surface current immediately migrates to the copper  14  surface because of the relatively higher resistivity of the steel  6 . Second, the higher steel  6  resistivity and ensuing heating of this material then drives additional surface current to the copper  14  surface, and leads to faster current diffusion into the bulk steel  6  and subsequent flow in the underlying copper part  14 . Version 5.7 of Quickfield does not dynamically simulate material temperature increases and self-correct for these related resistivity changes. 
     Each separatrix  27  shown in  FIG. 6  denotes a location on the barrel  13  surface where the surface current changes direction. These currents reconnect at the base  23  region, where the rail  14 , 6  currents originate, and in the vicinity of the armature  30 . Energy flow into the combined rail  14 , 6  and copper lining  16  can be relatively low, as compared to existing railgun designs. In addition, the design described in this patent application allows for multiple firings, as the rails  14 , 6  are actively cooled. 
     The barrel  13  surface current density is highest on the surface that faces the copper rail part  14 . It is of value that the lining  16  be particularly thick in this region, as shown in  FIG. 2 . The area over which the image currents flow in the opposite direction is substantially larger, the surface current densities are lower there, and therefore the lining  16  thickness can be thinner there. See  FIGS. 2 and 6 . Adjustment of the lining  16  thickness in this region can be further adjusted, based on the specific surface current density distribution around the barrel  13  circumference. 
       FIGS. 7 ,  8 , and  9  illustrate different techniques for electrically coupling the two barrel sections  13  together at the base  23 .  FIG. 7  illustrates an electrically conductive clamping member  24 , generally in the shape of the letter “C” lying on its back, positioned at the base  23  region. Clamping member  24  provides mechanical support as well as electrical connectivity between the two barrel sections  13 . In each of  FIGS. 7 ,  8 , and  9 , member  24  can be as long (in the z direction) as needed for its mechanical purposes, although lengthening member  24  causes a reduction in L′ in the regions of the base  23 . Clamping member  24  is supported by and electrically connected to an electrically conductive support base  23  that rests on and is electrically connected to an electrically conductive turret section  20 . In the illustrated embodiment, turret section  20  rotates with respect to, is mounted on, and is electrically connected to, a flat electrically conductive surface  4 , such as the deck of a ship. The  FIG. 7  embodiment illustrates the rails  14 , 6  passing through an “open” contact area in the region of the base  23 . 
     In the embodiment illustrated in  FIG. 8 , on the other hand, electrically conductive clamping member  24  surrounds the barrel sections  13  and rails  14 , 6  in the region of the base  23 , forming a full “closed” contact area (aperture). In other respects,  FIG. 8  is identical to  FIG. 7 . The magnetic field geometry in the gun bore  11  is the same for the  FIGS. 7 and 8  embodiments. This has been confirmed experimentally. 
       FIG. 9  is identical to  FIG. 8 , except that a dielectric shell  10  has been added. Shell  10  has the same outer dimensions as clamping member  24 , is hollow, extends along the entire length of the railgun  1 , and surrounds all the other components in the system, including retention frames  32 . Shell  10  allows the magnetic field to escape into the region outside of the barrel  13  region and therefore maintain a high L′. This protects the rails  14 , 6  from the surrounding environment. It also allows the rail  14 , 6  region to be filled with an inert gas, such as helium, nitrogen, or argon. This prevents the hot liquid metal, mostly aluminum, that is jetted from the armature  30 /rail  14 , 6  interface from immediately bursting into flames all along the gun bore  11  as the armature  30  accelerates through the bore  11 . The jetted metal can collect and solidify on the dielectric  10  cover, which can be easily replaced as required. 
       FIG. 10  illustrates an alternative embodiment that differs from the  FIG. 1  embodiment in two respects: first, the cross-section of each barrel section  13  is not a square, but rather a non-square rectangle. This technique can be fruitfully used to add additional mass to the structure, e.g., for reasons of increased mechanical support. The second difference in this  FIG. 10  embodiment is that the barrel sections  13 , when viewed from the top or the bottom, are tapered, with these sections  13  being wider at the breech end  21  than at the muzzle end  22 . The purpose of the tapering is to extend the length of the bore  11  compared with a non-tapered design. Lengthening the bore  11  can advantageously cause either a higher exit velocity for the projectile  31  for a given set of design parameters, or, alternatively, a relaxation in these design parameters for a given exit velocity. This tapering technique can be used with all of the embodiments of the present invention that are described herein. 
       FIG. 11  is a top view of this tapered alternative embodiment, with the retention frames  32  being oriented orthogonal to the gun bore  11  axis. Note that the tapering is stepwise rather than continuous, i.e., there is no tapering where the retention frames  32  are located. This is done for ease of mechanical assembly and for increased mechanical strength. 
     Second Principal Embodiment 
     Because of the relatively large cross-sectional size of the conducting rails  14  in the embodiment illustrated in  FIGS. 1 through 6 , the inductance per unit length (L′) was limited from 0.4 uH/m to 0.47 uH/m for a bore  11  spacing of between 30 cm and 40 cm. The current carrying rail  14  size was made large, in part to accommodate the substantial open channel  5  interior to the rail part  14  used to flow cooling fluid through the rails  14 , 6  (or sections thereof). It is primarily through the reduction in the cross-sectional size of the rail part  14  that further reductions in L′ can be attained. It is the objective of the embodiment illustrated in  FIGS. 12 through 16  to increase the inductance per unit length (L′ in units of uH/m). Inductance per unit length in the range of 0.6 uH/m is desirable. This second principal embodiment can achieve this goal. 
     In this second principal embodiment, a reduction in the size of the rail  14 , 6  is achieved by eliminating the interior cooling channel  5 , and moving the rail  14 , 6  to the outer surface of the steel barrel section  13 . Cooling of the rails  14 , 6  can be achieved by use of a water (or other evaporative fluid) spray directed to the outer surfaces of the rails  14 , 6  after each shot. As shown in  FIG. 12 , this fluid spray can flow through a plurality of nozzles  9  fabricated on at least one inside surface of a barrel section  13 , above and/or below the rail  14 , 6 . The nozzles  9 , part of the thermal management system, carry water or other coolant for the rails  14 , 6  and are usually pointed in the direction of the rails  14 , 6 . 
       FIG. 12  shows an isometric view of the second principal embodiment. The spacing between the rails  14 , 6  is 30 cm for the particular design used to calculate results discussed here, including the inductance (L′=0.55 uH/m), and the magnetic computational outputs presented below. The results presented are not optimized at the system level. For example, with all else being held constant, if the gun bore  11  is increased from 30 cm to 40 cm, L′ increases from 0.55 uH/m to 0.63 uH/m. 
     This second principal embodiment is also a two-rail  14 , 6  system. Current is carried along the first rail  14 , 6  conducted across the moving armature  30 , and then returned in the opposite direction along the second, parallel, and opposing rail  14 , 6 . Each rail  14  typically consists of a single part made of copper, copper alloy, tungsten copper alloy, tungsten copper eutectic, or a similar wear-resistant but highly electrically conductive material. Alternatively, a wear-resistant cap  6  can be fabricated onto primary rail part  14 , as shown in  FIG. 12 . Cap  6  is typically fabricated of steel, tungsten alloy, tungsten copper eutectic, or tungsten copper alloy. 
     Because of the design simplicity in this embodiment, the rail  14 , 6  can be made to be removable from the barrel  13  to facilitate easy replacement of the rail  14 , 6 . 
     Shown in  FIG. 13  is a detailed view of a gun barrel section  13  and its attached current carrying rail  14 , 6 . Typical dimensions are 52 cm by 50 cm for the cross-section of barrel section  13 , and 8 cm for the thickness of insulator  18 . These dimensions are self-consistent with the inductance calculation results noted above. 
     The region between each current carrying rail  14 , 6  and its associated barrel section  13  is filled with a dielectric  18 . Kevlar is the dielectric  18  of choice, though Phenolic, ceramic, or a ceramic composite can be used. The geometry is designed such that there is no direct line of sight between the insulator  18 /air interface and the sliding contact region between the rail  14 , 6  and the armature  30 . This geometry advantageously prevents direct UV illumination of these surfaces. It also prevents direct liquid metal (emanating from the sliding contact  14 , 6 , 30 ) or other direct sputtering or evaporative induced coating onto the insulator  18  surface. Additional baffles can be added to further protect the insulator  18  if required. 
     Each rail  14 , 6  is convex in shape from the point of view of the bore  11 . This allows for mechanically secure capture and guidance of the armature  30  and any payload  31  along the gun bore  11 . The top and bottom portions of each rail part  14  are made to be vertical, to redirect the jetted liquid metal from the sliding rail  14 , 6 /armature  30  interface away from the insulator  18  region and directly out of the gun bore  11 . 
       FIG. 14  shows a partial assembly of the barrel  13  at a region other than the base  23  and turret  20 . This includes the current carrying rails  14 , 6 , barrel sections  13 , and now the retention frames  32  that mostly encircle the rail  14 , 6  and barrel  13  assemblies. FIG.  14 , 6  shows two of the retention frames  32  that hold the two barrel sections  13  in place during a shot. All inward facing surfaces of the retention frames  32  that would otherwise come in contact with the barrel sections  13  are lined with Kevlar or other suitable dielectric  35 . This is to prevent the electrical interconnection of the two barrel sections  13  with each other at all but one region along the length of the railgun  1 . (For the embodiment where the barrel sections  13  are floating, there is no region where the barrel sections  13  are electrically interconnected.) These dielectric sections  35 , like dielectrics  18 , are always under compression. 
     The pitch of the retention frames  32  (distance between adjacent frames  32 ) is large compared to the thickness of each frame  32 . This is done so as not to reduce the inductance appreciably. This is also a weight-saving feature and is made possible for two reasons. First, the frame  32  height can be increased as necessary to insure that the induced stress in the frame  32  due to the rail  14 , 6  current-induced magnetic pressure is well within the stress limit of the steel or other strong material that frame  32  is made of. Second, the barrel sections  13  are substantial in physical size, and themselves help to prevent outward deflection of the rails  14 , 6  in zones between each pair of retention frames  32 . 
     Magnetic Analysis 
       FIG. 15  shows a two-dimensional magnetic field line plot in which the two barrel sections  13  are electrically connected to each other at the gun base  23  only. The two barrel sections  13  remain electrically isolated from each other everywhere else along the gun barrel  13  length, from the breech  21  to the muzzle  22 . Physically, this means that the two barrel sections  13  are mechanically secured to the base  23  with direct metal-to-metal connections. The retention frames  32  were not included in the computer run upon which  FIG. 15  is based. Their inclusion would alter the results by approximately 2% to 3%. This field line plot was taken 100 micro-seconds into the pulse. Copper was used to simulate steel as the material for the barrel sections  13 . This simulated the copper linings  16  that are typically used around the barrel  13  walls. 
       FIG. 16  shows an enlargement of the space around one of the current carrying copper rails  14 . The field line density has been increased to show more precisely where the surface currents flow. Drive currents flow on surfaces of the copper rail  14 , and image currents flow on surfaces of the barrel  13 . Once the armature  30  has passed, a percentage of the rail  14  current is drawn onto the back rail surface  15  and into the gap  28  between the rail  14  and the barrel section  13 . By proper design, the rail  14  current is distributed uniformly over the entire surface area of the rail  14 . This advantageously minimizes energy dissipation in the rail  14 . 
     Each separatrix  27  shown in  FIG. 16  denotes the location on the barrel  13  surface where the surface current changes direction. These currents reconnect at the base  23  region, where the rail  14  currents originate, and in the vicinity of the armature  30 . Energy flow into the combined copper rail  14  and copper lining  16  can be relatively low, as compared with existing railgun designs. In addition, this design allows for multiple firings with minimal degradation of the rails  14 , 6 , as the rails  14 , 6  can be actively cooled using fluid jet techniques. 
     The above description is included to illustrate the operation of the preferred embodiments, and is not meant to limit the scope of the invention. The scope of the invention is to be limited only by the following claims. From the above discussion, many variations will be apparent to one skilled in the art that would yet be encompassed by the spirit and scope of the present invention. For example, in the two principal embodiments included in the above description, the barrel sections  13  had a square or non-square rectangular cross-section. However, the barrel sections can have any number of cross-sections  13 , including but not limited to triangular, circular, elliptical, or trapezoidal. Similarly, the cross-sections of the current carrying rails  14  are not limited to any specific shapes or sizes.