Abstract:
An improved railgun, railgun barrel, railgun projectile, and railgun system for accelerating a solid-to-hybrid transitioning armature projectile using a barrel having a bore that does not conform to a cross-sectional profile of the projectile, to contact and guide the projectile only by the rails in a low pressure bore volume so as to minimize damage, failure, and/or underperformance caused by plasma armatures, insulator ablation, and/or restrikes.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/345,920, filed May 18, 2010 and incorporated by reference herein. 
    
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the United States Department of Energy and Lawrence Livermore National Security, LLC for the operation of Lawrence Livermore National Laboratory. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to railguns. More particularly, the invention relates to an improved railgun, railgun barrel, railgun projectile, and railgun system for accelerating a solid-to-hybrid transitioning armature projectile using a barrel having a barrel bore that does not conform to a cross-sectional profile of the projectile, to contact and guide the projectile only by the rails in a low pressure bore volume so as to minimize damage, failure, and/or underperformance caused by plasma armatures, insulator ablation, and/or restrikes. 
     BACKGROUND OF THE INVENTION 
     Railguns are electrical guns known to accelerate projectiles along a pair of electrically conductive rails by permitting a large electric current to pass between the pair of rails by way of a conductive medium of, produced by, or otherwise associated with the projectile. This current interacts with the strong magnetic fields generated by the rails to accelerate the projectile using the same principles as the homopolar motor, and have been known to achieve velocities greater than what is typically achievable by conventional firearms-based technology. For example, railguns have been used in experiments to accelerate gram-sized and larger objects to speeds of 6 km/sec. 
     Various approaches are known for accelerating projectiles using railguns. One approach relies on a column of electrically conductive gas to produce a plasma armature to provide the propulsive force, in the form of gas pressure, to a non-conductive sabot typically made of a high-strength plastic, such as for example Lexan. It is appreciated that a plasma armature involves rail-to-rail plasma arcing. Other operating modes involve (1) using a solid conductor in contact with the rails for producing a solid armature, (2) using a transitioning hybrid armature which operates with both plasma arcing as well as conduction through a solid (plasma-solid-plasma conduction), and (3) tandem operation, which can involve both plasma armature and hybrid armature operation. It is notable, however, that plasma armatures have deleterious effects on railgun operation and projectile velocities, due to for example ablation drag, restrike, etc. Therefore plasma 
     One example railgun is shown in the article “The Gas-Insulated-Railgun” by Tidman, Parker, et. al., which uses a high-pressure gas fill in a steel tube. In this configuration a trailing wake can be produced, and at the wake boundary there is likely to be a region exactly at the “Paschen minimum”, i.e. where the value of pressure for the rail-to-rail gap at which the breakdown voltage is a minimum. This situation is likely to produce restrike arcs in the wake. 
     Another example is shown in the article “Railgun Performance with a Two-Stage Light-Gas Gun Injector” by Hawke, Suseoff, et. al., which involves a projectile-conforming sealed barrel design, along with a solid/hybrid transitioning armature. Because of the conforming barrel bore (i.e. barrel walls substantially conform to the cross-sectional profile of the projectile), it is typical in such designs that a plasma armature forms behind the hybrid armature (i.e. referred to as “tandem” operation). This is likely because the plasma cannot be completely contained within the hybrid and leaks rearward. The conforming barrel bore provides a means to support a non-zero plasma pressure and hence a non-zero plasma conductivity; the hot plasma in the rear of the armature providing a conductive path which inevitably leads to a plasma armature in the rear. In fact the experimental results show that tandem operation commences almost immediately. However, once a plasma armature forms in the back, all the deleterious effects of a plasma armature arise, i.e. ablation drag, restrike, etc. 
     And another example is described in the article “A Transitioning Hybrid Armature Concept” by Trevor James, which involves hybrid armature in a projectile conforming barrel bore, where the “plasma brushes” live in the “legs” of an otherwise “C-shaped” solid armature. The concept attempts to control the plasma armature pressure using radially-inward-directed “exhaust ducts”. This design is likely to operate in the “tandem” mode (hybrid armature plus plasma armature) at ultra-high-velocities, since the hot, conductive exhaust gases are directed rearward in a closed barrel, leading to tandem operation in a similar manner as in the Hawke, Suseoff article. For Ultra-High-Velocity operation tandem operation is not desirable for same reasons as discussed for the Hawke, Suseoff design. 
     As such, there is a need for a railgun designed to operate without and prevent the formation of plasma armatures, to enable high and ultra-high velocities in railgun applications. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention includes a railgun comprising: a barrel having a pair of parallel conductive rails each with a convex rail surface facing the other convex rail surface with only said convex rail surfaces of the rails adapted to contact (“rail-only contact”) a projectile launched through the barrel; a barrel bore defined between and in part by said convex rail surfaces and having a cross-sectional profile (“non-conforming-to-projectile bore profile”) that does not completely conform to a cross-sectional profile of the projectile; and an enclosure surrounding the barrel bore and filled with a low-pressure gas, whereby the rail-only contact, the non-conforming-to-projectile bore profile, and the low-pressure gas in the enclosure contribute to minimizing plasma armature formation and restrikes. 
     Another aspect of the present invention includes a railgun projectile comprising, a nonconductive sabot; and a conductive section connected to the sabot and having a rear conductive section with a pair of contact surfaces each for contacting a corresponding one of the convex rail surfaces and a forward conductive section spanning less than a gap distance between the convex rail surfaces, wherein the rear section and the forward section are adapted so that when the projectile is accelerated along an energized pair of parallel conducting rails, in a first stage of acceleration an electrical current is produced across the rear conductive section as a solid armature to accelerate the projectile until the rear conductive section is ablated prior to reaching the muzzle end of the barrel, and in a second stage of acceleration an electrical current is produced across the forward section as a hybrid armature to accelerate the projectile out through the muzzle end of the barrel. 
     Another aspect of the present invention includes a railgun system comprising: a barrel having a breech end, a muzzle end, a pair of parallel conductive rails extending between the breech and muzzle ends, with each conductive rail having a convex rail surface facing the other convex rail surface with only said convex rail surfaces of the rails adapted to contact (“rail-only contact”) a projectile launched through the barrel; a barrel bore defined between and in part by said convex rail surfaces and having a cross-sectional profile (“non-conforming-to-projectile bore profile”) that does not completely conform to a cross-sectional profile of the projectile; and an enclosure surrounding the barrel bore and filled with a low-pressure gas, whereby the rail-only contact, the non-conforming-to-projectile bore profile, and the low-pressure gas in the enclosure contribute to minimizing plasma armature formation and restrikes; and a projectile having a nonconductive sabot, and a conductive section connected to the sabot and having a rear conductive section with a pair of contact surfaces each for contacting a corresponding one of the convex rail surfaces, and a forward conductive section spanning less than a gap distance between the convex rail surfaces, wherein the rear section and the forward section are adapted so that upon connecting the conductive rails to a voltage source, in a first stage of acceleration an electrical current is produced across the rear conductive section as a solid armature to accelerate the projectile until the rear conductive section is ablated prior to reaching the muzzle end of the barrel, and in a second stage of acceleration an electrical current is produced across the forward section as a hybrid armature to accelerate the projectile out through the muzzle end of the barrel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated into and form a part of the disclosure, are as follows. 
         FIG. 1A  is a cross-sectional view of a first exemplary embodiment of the open barrel (i.e. having non-conforming-to-projectile barrel bore) of the present invention. 
         FIG. 1B  is a cross-sectional view of a first exemplary embodiment of the open barrel (i.e. having non-conforming-to-projectile barrel bore) of the present invention also shown with a layer of energy-absorbing substance on the dielectric surfaces. 
         FIG. 2A  is a cross-sectional view of the exemplary embodiment of  FIG. 2B . 
         FIG. 2B  is a top view of a section of the barrel of the present invention, including an exemplary embodiment of the projectile of the present invention. 
         FIG. 3A-C  show side, top and back end views, respectively of the conductive section of the projectile. 
         FIGS. 4A-B  show time lapsed views of an exemplary projectile embodiment as it is launched in the barrel of the present invention. 
         FIG. 5A  shows another embodiment of the projectile having a front payload. 
         FIG. 5B  shows another embodiment of the projectile having a rear payload. 
         FIG. 6  is a cross-sectional view of another exemplary embodiment of the open barrel shown with ports for introducing a low pressure gas. 
         FIG. 7  is a cross-sectional view of another exemplary embodiment of the open barrel including augementing rails. 
         FIG. 8  is a schematic view of another embodiment of the present invention shown a barrel configuration with segmented primary rails, an injector, and augmenting rails. 
     
    
    
     DETAILED DESCRIPTION 
     Generally, the present invention is a railgun designed to launch a projectile to high velocities and ultra-high velocities (e.g. greater than 6 km/sec, such as 10-15 km/sec) using an “open barrel” architecture (e.g. rail-only contact with projectile and a non-conforming-to-projectile bore profile), a solid-to-hybrid transitioning armature, and low pressure gas fill to increase rail-to-rail breakdown voltage, which together contribute to minimize and avoid the deleterious effects associated with plasma armature formation and tandem operation, which may include restrikes. 
     Turning now to the drawings,  FIG. 1A  shows a first exemplary embodiment of the open barrel structure of the present invention. In particular,  FIG. 1A  shows a barrel  100  having a pair of parallel conductive rails  12  defining in part a barrel bore  16  therebetween (which is shown occupied in part by projectile  10 ). The rails  12  are shown having convective rail surfaces  12 ′ facing each other across the barrel bore  16 . Furthermore, only the convex rail surfaces of the rails are adapted to contact the projectile  10  that is launched through the barrel, and is characterized as “rail-only contact”.  FIG. 1A  also shows a projectile  10  positioned in the barrel bore  16  between the rails  12 . which mate with concave surface of the projectile. The open space above and below the projectile is an important feature for mitigating plasma armature formation and secondard strikes (restrikes). As shown in  FIG. 1A , it is notable that only the convex rail surfaces come in contact with the projectile. No other parts of the barrel, including the dielectric wall surfaces come in contact with the projectile. 
     In this embodiment, dielectric walls  14  are also shown provided and forming an enclosure, which also define in part the barrel bore, together with the convex rail surfaces. The pair of dielectric sidewalls  14  each have a dielectric sidewall surface  14 ′ facing the other dielectric sidewall surface  14 ′, and which are positioned away from the projectile  10  so that the barrel bore  16  is characterized as not completely conforming to the cross-sectional profile of the projectile (“non-conforming-to-projectile bore profile”). In fact, it can be seen that the area covered by the cross-sectional bore profile is larger than the cross-sectional profile of the projectile  10 . While it is appreciated that some conforming to the projectile profile inherently exists dues to the mating surfaces between the projectile and the convex rail surfaces, it is not completely conforming since no other barrel surface is positioned adjacent the projectile. 
     Generally, the open barrel configuration of the present invention has a “non-conforming-to-projectile” barrel bore, which, as used herein in the claims, is a type surrounded and defined by a barrel wall or walls which do not substantially conform to the diametric or cross-sectional profile of the projectile, and thus subjects the projectile to contact on only a limited number (less than all) of its sides by the barrel wall(s) for guiding the projectile as it travels through the barrel. In the present invention, the barrel wall sections which do come in contact with the projectile are a pair of conductive rails (rail-only contact) which also supply the electromotive force which launches the projectile through the barrel. All other projectile surfaces not in contact with or adjacent the conductive rails are adjacent an open space, which may be the exterior environment in the case of a rail-only, partially wall-less design, or the hollow space inside an enclosed barrel chamber. In contrast, a projectile conforming barrel bore is a bore which substantially conforms to the diametric or cross-sectional profile of the projectile. In particular, the barrel wall or walls of the barrel substantially conform to the diametric or cross-sectional profile of the projectile, as exemplified by the gun barrels of most conventional firearms, and thus subjects the projectile to contact on all sides (with the exception of the front and rear) by the barrel walls for guiding the projectile as it travels through the barrel. 
     The present invention also relies on a low-pressure (e.g. all the way down to a perfect vacuum) gas fill, with the ambient pressure below the Paschen minimum, and hence restrike in the wake is averted. In particular, the enclosure  14  in  FIG. 1A  surrounding the barrel bore is filled with a low-pressure gas. The reason for a “low pressure” gas fill is that certain gases such as SF6 have a greater minimum breakdown voltage than air, and hence a prefill of this gas (followed by a pumping down to a low pressure) is desirable for increased performance. “Low-pressure” may include, for example, 2 atm or less pressure. In particular, the low-pressure gas is of a type capable of increasing rail-to-rail breakdown voltage in the presence of high temperature metal vapor. The gas is preferably supplied via ports  36  shown in  FIG. 6  showing embodiment  600 . The open barrel architecture and low-pressure gas help eliminate plasma armature or tandem operation, since without sidewalls closely positioned adjacent the projectile the gas in the tail cannot maintain a non-zero pressure, and hence cannot sustain a conductive path. 
     And  FIG. 1B  shows a second exemplary embodiment of the open-barrel structure of the present invention. In particular,  FIG. 1B  shows the dielectric surfaces coated with an energy-absorbing material  18 , as known in the art, capable of vaporizing to a low-temperature dielectric gas, which also prevents and minimizes restrikes. 
       FIGS. 2A-2B  show an exemplary projectile  10  positioned in the barrel bore of the present invention. In particular, the projectile  10  has a nonconductive sabot  20  (may include payload) at a leading end, and a conductive section connected to the sabot. The conductive section includes a rear conductive section  11 ′ shown as a pair of C-shaped “wings”  24  with a pair of contact surfaces, each contact surface for contacting a corresponding one of the convex rail surfaces of the conductive rails  12 , and a forward conductive section  11  spanning less than a gap distance between the convex rail surfaces. Additional views of the projectile and its two sections are shown in  FIGS. 3A-C . 
     And  FIGS. 4A and 4B  show the transition from solid armatures ( FIG. 4A ) to a hybrid armature ( FIG. 4B ) of the conductive section of the projectile as it is launched in the direction  30 . A sabot is shown ahead of the conductive section, and plasma arcing is shown at  28  in  FIG. 4B . In particular, the rear section and the forward section are adapted so that upon connecting the conductive rails to a voltage source, in a first stage of acceleration (shown in  FIG. 4A ), an electrical current is produced across the rear conductive section as a solid armature to accelerate the projectile. In particular, the rear conductive section is configured to be ablated prior to reaching the muzzle end of the barrel. And in a second stage of acceleration an electrical current is produced across the forward section as a hybrid armature to accelerate the projectile out through the muzzle end of the barrel. The rear section is preferably made of a material with a melting point less than a predetermined operating temperature of the railgun so as to melt away and reduce parasitic mass as the armature accelerates along the rails. 
     And  FIGS. 5A and 5B  show two exemplary embodiments  500  and  501  of the projectiles showing a sabot  32  with payloads in front and to the rear of the conductive section. In particular,  FIG. 5B  shows a tail or shroud  34  which extends to the rear of the conductive section. 
       FIG. 7  show another embodiment  700  showing the use of augmenting rails  38  which augment the primary rails by decreasing the required armature current for a given acceleration, and/or increase the stagnation velocity of free-running precursor or secondary arcs. At least one additional pair of parallel conductive rails (“augmenting rails”) may be positioned adjacent the first pair of parallel conductive rails.  FIG. 8  also shows the use of augmenting railsprimary rails in embodiment  800 .  FIG. 8  also shows the segmentation of the primary rails into multiple sections  40  and  41 , where a current  42   a  is provided for segment  40  while a different current  42   b  is provided for segment  41 . The pair of conductive rails are segmented along the length of the barrel with each rail segment powered independently from other segments. Projectile pre-acceleration is also shown in  FIG. 8  using an injector  48  for injecting the projectile at the breech end of the barrel. 
     While particular embodiments and parameters have been described and/or illustrated, such are not intended to be limiting. Modifications and changes may become apparent to those skilled in the art, and it is intended that the invention be limited only by the scope of the appended claims.