Lightweight high L' electromagnetic launcher

Methods and apparatus related to design and construction of lightweight and mobile railgun barrels having high L' (inductance per unit length). The barrels comprise stacked metallic annular laminations in combination with prestressed tension elements to add radial and longitudinal stiffness. Laminations have elongated annular shapes to accommodate a plurality of longitudinal rails within the barrel, and the lamination shape is optimized to contain radial forces (tending to spread the rails) associated with the launching of railgun projectiles. Longitudinal stiffness is obtained from tension elements spirally wound around the barrel at a preferred angle with the barrel longitudinal axis. Radial stiffness is obtained by injecting a settable plastic fluid between barrel laminations and internal (bore) components, the latter being held as a subassembly with a longitudinal shrink tube.

BACKGROUND 
1. Field of the Invention 
The invention relates to methods and apparatus for maintaining rail spacing 
and minimizing rail curvature in electromagnetic launchers (railguns). 
2. Design Considerations for Mobile Railguns 
Railguns use electromagnetic force to accelerate a projectile which 
completes an electrical path incorporating two or more substantially 
parallel conducting rails. Peak electrical currents exceed 3,000,000 
amperes (3.0 MA) in certain guns, and the resulting magnetic pressure may 
result in a total (radial) force acting to separate the rails in excess of 
90 kips/in. Actual movement of the rails, however, should not be so great 
that electrical contact is lost or substantially reduced between either 
rail and the projectile. 
Should rail-projectile contact be lost, the resulting plasma contact would 
generally have a higher associated voltage drop than a metal-to-metal 
contact, thus reducing current flow and the amount of magnetic force 
applied to the projectile. Thus in stationary railguns, high-strength (and 
relatively heavy) structural members are employed to provide the 
substantial restraining forces needed to prevent rail separation. The 
required structural mass may be reduced somewhat through the use of 
preloaded elements (e.g., ceramic rail supports heavily preloaded by an 
outer steel shell), but the resulting railgun barrel is still too heavy 
for mobile applications. Further weight reductions require replacement of 
ceramic rail supports with members having lower preload requirements, and 
replacement of a steel outer shell with a lighter preloading member. 
Weight reductions in structural elements must not, however, compromise the 
stiffness required to keep the railgun barrel straight within narrow 
tolerances. This is because the muzzle velocity of railgun projectiles may 
range from two to ten kilometers per second. Thus, the rails and their 
supporting structures comprising the railgun barrel must have the 
requisite stiffness to avoid droop or sag which would impair performance 
by deviating the projectile path from a straight line. If the railgun is 
to be mobile, the barrel must also be sufficiently light and stiff to 
allow the use of conventional recoil and aiming mechanisms (i.e., the 
barrel should be stiff enough to withstand abrupt changes in longitudinal 
acceleration and high slew rates). 
Another aspect of railgun design is maximization of muzzle energy, which 
for a given rail current is approximately linearly related to the 
inductance per unit length (L') of the rails within a railgun barrel. 
L'tends to be reduced by the presence of metal rail supports or any other 
electrical conductor near the rails if substantial eddy currents can be 
induced parallel to the longitudinal axis of the barrel. Ceramic rail 
supports are thus theoretically superior to metal supports because they 
are electrical insulators, but the large preload requirement needed to 
prevent cracking of the insulators is not compatible with weight 
constraints in a mobile railgun. 
As a substitute for ceramic rail supports, stacked iron or steel 
laminations have been used to fore railgun barrels and to provide needed 
radial support for the rails. Laminated metal rail supports have less 
longitudinal stiffness than solid metal, but eddy current losses in the 
laminations are substantially smaller than such losses would be in 
comparable solid metal supports if the laminations are sufficiently thin 
and electrically insulated from one another. 
To increase longitudinal stiffness, stacked laminations are bonded together 
(preferably by the insulating material separating them from each other and 
from the rails). Without additional support, however, bonded laminations 
have limited resistance to transverse forces (e.g., due to gravity or 
barrel inertial forces) which tend to distort the railgun barrel along its 
longitudinal axis (which substantially parallels or is coincident with the 
direction of projectile travel). Hence, laminations have heretofore been 
used in railguns having fixed mounts where longitudinal railgun barrel 
stiffness could be provided by additional (fixed) structural systems 
(e.g., longitudinal bolts). No design for railgun barrels comprising 
stacked laminations and having the strength, stiffness and lightness 
required for mobile applications has been demonstrated or proposed. 
SUMMARY OF THE INVENTION 
The present invention substantially overcomes shortcomings in prior railgun 
barrels and methods for making them, relating particularly to railguns 
which are sufficiently strong, lightweight and stiff for mobile 
applications. Each railgun barrel of the present invention comprises a 
plurality of substantially parallel longitudinal rails disposed within and 
substantially parallel to the longitudinal axis of a hollow cylindrical 
stack of annular steel laminations. At least two longitudinal rails are 
disposed to form a portion of the railgun barrel bore, such rails being 
separated throughout their length by longitudinal insulating spacers. In 
preferred embodiments, the longitudinal rails and longitudinal spacers are 
disposed alternately and symmetrically around a longitudinal void, the 
void comprising the barrel bore, and the rails and spacers collectively 
forming the bore wall. 
Each lamination (and each lamination stack) has inner and outer contours, 
and each lamination is thin enough to substantially inhibit eddy current 
losses and the associated undesirable reduction of L'for rails which are 
longitudinally oriented adjacent to the inner contour of the lamination 
stack. 
Laminations are further described as planar structures having substantially 
parallel first and second sides, and an edge which comprises the outer 
contour of each lamination. Entirely enclosed by the outer contour is an 
inner contour which defines a center void portion lying within the plane 
of each lamination. Inner and outer contours are continuous, and when a 
plurality of laminations is stacked, the inner contours combine to form a 
cylinder having an outer surface, an inner surface, and a longitudinal 
axis substantially parallel to or colinear with the railgun bore. 
Longitudinal rails and insulating spacers are preferably disposed adjacent 
to said inner surface. The cylinder wall thickness at any point 
corresponds to the distance separating the inner and outer contours of the 
lamination at that point. 
The outer surface of a lamination stack generally lies within the 
(preferably) cylindrical outer surface of the railgun barrel, the 
intervening space being substantially occupied by longitudinal preload 
elements laid adjacent to the lamination stack. Radial preload elements, 
as explained below, are preferably applied adjacent to the lamination 
stack inner surface in railgun barrels of the present invention. Hence, 
the presence of radial preload elements does not substantially alter the 
barrel outer surface. 
Examples of preload elements include prestressed longitudinal tension 
preload elements (e.g., S-glass fibers) coupled (bonded) to the outer 
surface of lamination stacks with a predetermined tension level. The 
tension elements are spirally wound around the lamination stack at an 
angle between about 0.degree. and 11.degree. to the longitudinal axis of 
the lamination stack. Requirements for such longitudinal tension preload 
elements in rail gun barrels constructed according to the present 
invention include relatively light weight and sufficiently high electrical 
resistivity (i.e., the tension elements should effectively be electrical 
insulators). 
Longitudinal preload elements are ideally applied substantially parallel to 
the longitudinal axis of the barrel, but in practice they must either be 
bound to the barrel by hoop windings (the planes of which are 
substantially perpendicular to the longitudinal axis), or be wound at a 
small angle .alpha.to the longitudinal axis to prevent sagging away from 
the lamination stack. Because hoop windings impose a weight penalty while 
contributing nothing to longitudinal stiffness, winding at an angle 
.alpha.to the longitudinal axis is preferred. In railgun barrels made 
according to the present invention, the preferred angle .alpha.is about 
6.degree.. 
Longitudinal preload elements in preferred embodiments of railgun barrels 
made according the present invention comprise S-glass fibers applied under 
tension to the lamination stack at the angles noted above. The 
longitudinal components of tension forces in each element combine to yield 
the total preload force applied to the lamination stack. If, during 
application of the longitudinal tension preload elements, the lamination 
stack is simultaneously compressed, the subsequent release of such 
compression adds to the tension force applied to the longitudinal tension 
elements during winding, thus resulting in a larger total preload. 
Laminations are insulated from each other and from the rails by, e.g., 
nonconducting epoxy or composite material layers. Individual laminations 
are substantially flat and may be stamped or cut from, for example, half 
hard type 301 stainless steel stock. When stacked according to the present 
invention, perpendiculars to flat portions of each lamination are 
substantially parallel to the longitudinal axis of the railgun barrel. 
Each lamination completely encloses a thin section of the barrel bore 
transverse to the longitudinal axis and containing a thin section of each 
longitudinal rail and each insulating longitudinal rail spacer. Rail 
spacers, together with the rails, comprise the bore components of the 
railgun barrel, defining the bore cross-sectional shape and acting to 
prevent electrical contact with the projectile except through the intended 
rails. 
Laminations must have sufficient radial stiffness to counter the magnetic 
pressure acting to increase separation of the rails when a projectile is 
fired. Radial lamination stiffness in relation to weight may be optimized 
by making portions of each lamination which extend between the rails as 
straight as possible, consistent with the requirement that such portions 
enclose the bore components (specifically, the insulating rail spacers). 
Such a configuration results in an elongated substantially hexagonal 
lamination shape. 
To prevent forces tending to separate the rails from opening gaps between 
the rails and longitudinal insulating spacers forming the bore 
cross-section (i.e., to prevent further elongation of the above elongated 
hexagonal shape), the bore components (i.e., rails and longitudinal 
spacers) are preloaded in compression by injection of a settable plastic 
fluid material (e.g., an epoxy resin) under a predetermined pressure of, 
preferably, 2,000 to 4,000 psi between the bore components and the 
lamination stack. Pressure is then maintained on the plastic material 
until it cures (sets) to a solid form and the compressive preload on bore 
components becomes permanent. Properly applied, such a radial preload 
substantially prevents leakage of contaminates to the region between the 
bore and the interior wall of the lamination stack during firing of the 
railgun. If not prevented, such leakage can eventually result in 
electrical short circuit paths between the rails. 
Radial preloading is facilitated in construction of railgun barrels 
according to the present invention if the bore components are assembled 
outside of the lamination stack and temporarily held in fixed positional 
relationship with a longitudinally applied shrink-fit plastic tube. A 
sleeve comprising S-glass in a 45.degree. weave is then slipped over the 
plastic tube, after which the sub-assembly comprising the rails, 
longitudinal insulating spacers, longitudinal shrink-fit tube and S-glass 
sleeve is then inserted into the lamination stack cylinder and the ends 
sealed to allow injection of the settable plastic material between the 
S-glass sleeve and the inner wall of the lamination stack. During this 
injection, the plastic tube prevents extrusion of the injected material 
between the rails and longitudinal insulating spacers, thus preserving the 
designed bore cross-section and allowing application of sufficient 
injection pressure to obtain satisfactory radial preload of the bore 
components. The S-glass sleeve acts to stabilize the settable plastic 
material and tends to prevent extrusion of the material under high 
pressure.