This patent application pertains generally to the field of electromagnetic launchers, and specifically to railguns.
Background references include the following references, all of which are hereby incorporated in their entireties into the present patent application:
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'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.
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).
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:
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
In this two-rail 14 system, current is carried along the first rail 14, conducted across a moving armature 30 (see
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'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
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.
In many embodiments, such as illustrated in
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.
As shown in
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'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
1 MPa=1×102 N/cm2
Therefore, the yield strength of steel (800 MPa) is:
8×104 N/cm2
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:
(1.2×107 N/m)/(8×104 N/cm2)=1.5×102 cm2/m
For a 400% engineering safety margin, this becomes:
6.0×102 cm2/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 cm2. 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
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.
What is not shown accurately in
Each separatrix 27 shown in
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
In the embodiment illustrated in
Second Principal Embodiment
Because of the relatively large cross-sectional size of the conducting rails 14 in the embodiment illustrated in
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
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
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
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.
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
Each separatrix 27 shown in
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.
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.
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