Electromagnetic launchers with improved rail configurations

An electromagnetic projectile launcher is provided with a pair of parallel conductive projectile launching rails which are generally T-shaped with a significant current carrying cross section located symmetrically around a horizontal bore center line. Augmenting conductors which may be either series or parallel connected are located adjacent to the projectile launching rails. In favorable embodiments, force augmentation is also achieved by a reduction in launcher rail height from the breech to the muzzle end.

BACKGROUND OF THE INVENTION 
This invention relates to electromagnetic projectile launchers and more 
particularly to such launchers which utilize projectile launching rails 
having controlled cross-sectional shapes to improve projectile 
accelerating forces. 
In a parallel rail electromagnetic launcher, a force accelerates a current 
carrying conductor in a magnetic field and this force is equal to the 
vector cross product of the current density and the magnetic flux density. 
It can be shown that this force is equal to 1/2 L'I.sup.2 where L' is the 
inductance gradient of the parallel rail configuration and I is the 
current. Since the magnetic field which interacts with and therefore 
accelerates a current carrying conducting armature or plasma is primarily 
produced by the conducting rails just in the wake of the armature, for 
example, the field produced by the time dependent current distribution 
which exists in the rails not more than about three bore widths behind the 
armature, the accelerating force is similarly a function of that 
instantaneous current distribution in the conducting rails right in the 
vicinity of the armature or driving plasma. Therefore it should be 
understood that the significant value of L' is the inductance gradient 
existing in the current conducting rails right behind the armature. Simple 
parallel rail launchers of the prior art have used rectangular cross 
section projectile launching rails and designs have been proposed wherein 
the bore is circular and the rails are then essentially formed from 
annular sectors. Although such rails have been shown to be practical, 
substantial projectile acceleration improvements can be obtained by 
utilizing rail cross sections which result in higher acceleration forces 
in, for example, the breech area, and other configurations which can be 
utilized to increase accelerating force where the projectile is already 
traveling at a high speed and when the accelerating current has been 
reduced. The present invention utilizes particular rail configurations to 
achieve acceleration improvements. 
SUMMARY OF THE INVENTION 
An electromagnetic projectile launcher constructed in accordance with the 
present invention comprises: a pair of generally parallel conductive 
projectile launching rails, each having an internal surface, an external 
surface and a longitudinal protuberance extending from the external 
surface; a source of current connected to the rails; and means for 
conducting current between the rails and for propelling a projectile along 
the rails. In the preferred embodiment, the longitudinal protuberance is 
generally disposed along the mid-plane of the rails. The internal and 
external rail surfaces may be planar or arcuate. To increase accelerating 
forces, augmenting conductors can be inserted adjacent to the external 
surface and on opposite sides of the longitudinal protuberance. In an 
alternative embodiment, projectile launching rails of rectangular cross 
section, with decreasing cross-sectional area toward the muzzle, can be 
utilized alone or in conjunction with a pair of augmenting conductors 
disposed adjacent to the external surface of each of the projectile 
launching rails. These conductors may be connected either in series or in 
parallel and the force on a projectile is further increased by decreasing 
the height of the projectile launching rails as the rails extend from the 
breech end to the muzzle end of the launcher.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, FIG. 1 is a schematic diagram of a prior art 
parallel rail electromagnetic projectile launcher which may be modified to 
include a launching rail system in accordance with the present invention. 
This launcher comprises a pair of generally parallel conductive projectile 
launching rails 10 and 12 which are connected in series with augmenting 
conductors 14, 16, 18 and 20 such that, during projectile acceleration, 
augmenting conductors 14 and 16 conduct current in the same direction as 
projectile launching rail 10, while augmenting conductors 18 and 20 carry 
current in the same direction as projectile launching rail 12. A sliding 
conductive armature or plasma 22 serves as means for conducting current 
between the projectile launching rails and for propelling a projectile 
along those rails. A source of current 24 comprising the series connection 
of generator 26, switch 28 and optional inductor 30 is connected to 
augmenting conductor 16 and projectile launching rail 12. Firing switch 32 
is connected across projectile launching rails 10 and 12 at the breech and 
serves to conduct current during the initial charging of inductor 30 and 
the augmenting conductors. 
FIG. 2 is a cross-sectional view of a prior art augmented electromagnetic 
projectile launcher rail assembly which can be used in the launcher 
circuit of FIG. 1. This represents a twice augmented system where the 
projectile launching rails 10 and 12 line a bore 34 and have rectangular 
cross sections. Augmenting conductors 14, 16, 18, and 20 are positioned 
adjacent to the projectile launching rails and also have a rectangular 
cross section. All of the rails and conductors are held in place by a 
rigid insulating restraining structure 36. If augmenting conductors 16 and 
20 were removed, a singly augmented system would remain. Similarly, if 
augmenting conductors 14 and 18 were also removed, a simple parallel rail 
launcher would result. A force augmentation factor can be defined as the 
ratio of the projectile propelling force in an augmented launcher to that 
force which would result from a simple parallel rail launcher which 
contains only projectile launching rails such as 10 and 12 of FIG. 2. 
Prior art externally augmented parallel rail launcher configurations, such 
as those of FIG. 2, ideally have a force augmentation factor of three for 
once augmented and five for twice augmented systems. Actual experiment and 
detailed computer calculations have shown that the force augmentation 
factors are well below these ideal values and are actually, for a 
practical configuration, roughly 2.3 and 3.6 respectively. The launchers 
of the present invention utilize projectile launching rails which have 
particular cross-sectional shapes to achieve projectile accelerating 
forces in excess of those available in a simple parallel rail launcher 
with rails having a rectangular cross section. In augmented embodiments, 
the present invention configurations yield force augmentation factors 
which can be above the previously considered maximum values of three for 
once augmented and five for twice augmented systems. 
Since the accelerating force is equal to 1/2 L'I.sup.2, increasing the 
inductance gradient clearly increases the force if the propelling current 
is held constant. For the sake of comparison, a simple square bore 
parallel rail launcher such as one which contains only projectile 
launching rails 10 and 12 of FIG. 2, has a computed inductance gradient 
L'.sub.PA =0.478 .mu.H/m. Near the breech region, when the projectile is 
still moving slowly, the rail current distribution should be uniform and 
the accelerating force will then be 1/2 L'.sub.PA I.sup.2. FIG. 3 is a 
cross-sectional view of a pair of conductive projectile launching rails 
10a and 12a in accordance with one embodiment of the present invention. 
Each of these rails includes an internal surface 38 adjacent to bore 34 
and an external surface 40. A longitudinal protuberance 42 extends from 
each external surface 40 and is generally disposed along the mid-plane 43 
of the rails. The rails are held by an insulating restraining structure 
44. By using rails having this generally T-shaped cross section, there 
would still be substantially uniform current density in the rails in the 
wake of a just starting or slowly moving projectile, but now, the computed 
inductance gradient L' .sub.PI =0.606 .mu.H/m and a 27% increase in 
initial accelerating force, over that of a simple parallel launcher, has 
thus been achieved. This was accomplished by using the same conductor 
total cross sectional area but concentrating more conductor area near the 
horizontal centerline or mid-plane 43 of the rail configuration. This rail 
configuration can be beneficially utilized for two or three bore widths in 
the rear of the starting position of the armature to give a higher initial 
acceleration and for that length behond the armature starting location 
wherein near uniform current density is still attained closely enough 
behind the armature so that the FIG. 3 type of rail geometry increases the 
inductance gradient. In general, the lower the acceleration and the larger 
the bore, the longer the distance for which it will be advantageous to use 
the FIG. 3 type of rail geometry. Such configurations are particularly 
attractive for heavy projectile acceleration applications, such as missile 
or torpedo launching. It should be pointed out that there is no 
operational disadvantage to using the FIG. 3 rail cross section throughout 
the bore length. The areas adjacent to external surface 40 on either side 
of longitudinal protuberance 42 can, for example, be utilized to locate 
coolant passages. 
Once a projectile travels faster than a few hundred meters per second, 
current penetration is so slow relative to armature speed that the field 
which produces the accelerating force is generated by current concentrated 
in a relatively thin skin depth, and the remainder of the rail cross 
section serves primarily for lowering both the time averaged rail ohmic 
resistance and the average adiabatic rail temperature after firing and 
temperature equalization. A rough approximation of the current diffusion 
into the rail metal can be obtained from the following formula: 
##EQU1## 
where t=time in seconds for the current to penetrate to the depth .delta. 
in meters 
.mu.=4.pi..times.10.sup.-7 in non-ferromagnetic materials 
.rho.=resistivity in .OMEGA.-m 
Using this formula, if an armature is traveling at a steady speed of 1,000 
meters/sec, then penetration to only 0.5 cm of the copper rails will only 
occur at about 60 cm behind the armature and if the bore width were 5 cm, 
penetration to 0.5 cm would only occur in 12 bore widths behind the 
armature. Therefore, the accelerating force for this case at this instant 
would have to be calculated based on a rather thin current layer at the 
inner faces of the parallel rails. 
Unfortunately, having a rather low rail thickness for the rails does not 
significantly change the inductance gradient. For example, using a uniform 
current density, a reduction in rail thickness of about 60%, for example 
from 1.3 cm to 0.5 cm, only results in a 5% increase in the inductance 
gradient over the reference value of 0.478 .mu.H/m given previously for a 
simple parallel rail launcher. 
Pulse power systems utilized for accelerating a projectile in a parallel 
rail laucher have the characteristic of high breech currents and at least 
somewhat reduced currents at higher projectile velocities and towards the 
muzzle. For example, assume that for a particular acceleration scenario, 
the height of the bore rails 10b and 12b, in the breech region, as 
illustrated in FIG. 4 is adequate for conducting, to and from the 
armature, the maximum accelerating current with acceptable rail and 
armature wear. Calculations have been made for copper rails, which 
indicate that starting at room temperature, up to about 43 kiloamp per mm 
of rail height, or conducting rail perimeter, is allowable without rail 
surface melting. Experiments have actually somewhat exceeded this limit 
without rail damage. If then, towards the muzzle, the armature current 
level is reduced by for example 30%, the projectile rail height or 
conducting rail internal bore surface may also be reduced by 30% without 
exceeding the acceptable kiloamp/mm of rail height level. Furthermore, at 
higher velocities, the rail surface temperature rise resulting from the 
rail to armature heat flux is decreased because of the shorter residence 
time of the armature or plasma at a given rail location, and therefore, 
even more than a 30% rail height reduction is quite likely to be 
acceptable. Additionally, because the current level toward the muzzle end 
of the projectile rails is lower, the projectile rails toward the muzzle 
can have a lower cross-sectional area without exceeding an acceptable 
temperature limit after the temperature across the rail area has roughly 
equalized. Reducing the projectile rail cross-sectional area toward the 
muzzle is highly desirable. This can reduce barrel weight since rail mass 
per unit length adjacent to the breech end is greater than rail mass per 
unit length adjacent to the muzzle end. Barrel weight near the muzzle end 
can be further reduced since the rail spreading forces near the muzzle end 
are lower and lighter or less material can be used as rail spreading force 
restraining structural members toward the muzzle. However, just reducing 
the rail height by only 30%, for example, from 5 cm to 3.5 cm is 
calculated to result in a significant 18% acceleration force improvement 
over the full height, 0.5 cm boundary layer configuration. 
FIG. 5 is a cross section of the same parallel rail launcher as FIG. 4, but 
taken near the muzzle end. FIG. 5 shows that the reduction in rail height 
can be obtained by physically reducing rail height and replacing the lost 
volume with a portion of the insulating restraining structure 46. In this 
case, if one uses a metal armature, the extreme upper and lower armature 
portions will ride on the insulation which would be quite harmless and 
contact voltage drops can be expected to be too low to cause arcing. 
Alternatively, the rail area where no contact is desired can be simply 
recessed by machining away some of the rail contact surface metal so that 
the armature cannot contact this area. This latter scheme is of course not 
feasible for plasma drive systems. 
FIG. 7 shows the addition of augmenting conductors to the rail system of 
FIG. 3. In accordance with this invention, the two augmenting turns 
associated with each projectile launching rail may be connected in series 
or parallel. For the purposes of the discussion below, the terms once 
externally augmented and one external augmenting turn are considered to be 
synonymous. They both indicate that augmentation is by one turn conducting 
the full projectile current, but that one turn may physically consist of a 
number of augmenting conductors which are connected electrically in 
parallel. The augmenting conductors are geometrically in parallel with and 
adjacent to the projectile launching rails. As stated previously, a 
computer calculated inductance gradient for the simple parallel rail 
launcher having a uniform current distribution and the structure 
represented by only projectile launching rails 10 and 12 of FIG. 2, is 
L'.sub.PA =0.478 .mu.H/m. This calculation was based on a 50 mm.times.50 
mm bore and a rail cross section of 50 mm.times.13 mm. If augmenting 
conductors 14a, 16a, 18a and 20a of FIG. 7 are assumed to have a 20 
mm.times.20 mm square cross section and are electrically connected in 
parallal as illustrated in FIG. 6, and if the interrail insulation 
thickness is neglected, then the resultant inductance gradient L'.sub.2 
computes to be equal to 1.663 .mu.H/m or 3.48 times L'.sub.PA. This 
equates to a force augmentation factor of 3.48 which is 16% above the 
previously considered theoretical maximum factor of three for a once 
externally augmented system. It should be understood that L'.sub.2 =1.663 
.mu.H/m is the value of L' in the formula for force, F=1/2 L'I.sup.2, that 
is, the value of L' which produces the projectile accelerating force. 
Computationally, the L' which produces the acceleration is the value of L' 
for only the augmenting conductors which carry current prior to the launch 
subtracted from the L' of the configuration which exists in the wake of 
the projectile during acceleration. The value of L'.sub.2 as computed 
above assumes uniform current distribution in both the augmenting 
conductor cross sections and also in the T-shaped projectile launching 
rails. Such a uniform current distribution in the wake of the projectile 
will be approached throughout the bore length for large bore, low 
velocity, low acceleration applications involving heavy projectiles. For 
high velocity, high acceleration applications, the computed value of 
L'.sub.2 will be appropriate only at and near the breech where the 
velocity is still moderate. 
If the FIG. 7 configuration is used for a smaller bore, high acceleration 
and high velocity application, then the projectile rail current in the 
wake of the armature will be confined primarily to a thin layer facing the 
bore. This thin layer may be assumed to be a 5 mm layer, with no current 
flow in the longitudinal protuberance 42 between the augmenting 
conductors. This results in an effective inductance gradient value of 
1.361 or 2.85 times the inductance gradient of the reference simple 
parallel rail launcher. Thus, in the high velocity rail bore portion, 
there will be a measurable decrease in the accelerating force but near to 
an augmentation factor of 3 will still be obtained. 
As already explained, in a pulse power system current decreases as the 
projectile passes down the bore. Therefore, toward the muzzle, the 
projectile rail height, or conducting projectile rail perimeter, can also 
be reduced. FIGS. 8 and 9 show-cross sectional views of a launcher rail 
system constructed in accordance with this invention, taken near the 
breech and near the muzzle ends respectively. The rail configuration of 
FIG. 8 is similar to that of FIG. 7. However, when FIGS. 8 and 9 are 
considered together, projectile launching rails 10b and 12b are seen to 
have a height reduction of approximately 30%. This rail height reduction 
is assumed to be justified by a roughly corresponding current reduction. 
The inductance gradient for FIG. 9 now computes to be 1.575 or 3.29 times 
the reference system inductance gradient. It should be noted that when the 
inductance gradient of FIG. 9 was calculated, it was assumed that no 
current flows in longitudinal protuberance 42. 
Up to this point, we have shown that favorable once augmented 
electromagnetic launcher bore configurations as represented by FIGS. 7, 8 
and 9 will result in force augmentation factors above 3 at the same 
projectile current as an unaugmented barrel. Thus these preferred 
embodiments attain far more improvement compared to the augmentation 
factors of about 2.2 and 2.4 which have been experimentally measured and 
detail computed for the prior art once augmented designs similar to that 
shown in FIG. 2. 
It should be observed that the electrically in-parallel connection of the 
two augmenting conductors as illustrated in FIG. 6, is particularly 
attractive as it reduces the current density in the augmentation 
conductors. If an electromagnetic launcher is powered by a homopolar 
generator-inductor pulse compression system as shown in FIG. 6, then the 
augmenting turn conducts the charging current prior to the launch and it 
is necessary from an efficiency view point to have low resistance 
augmenting conductors as the charging period to full launch current tends 
to be relatively long, compared to the projectile acceleration duration. 
Furthermore, the lower current density in the augmenting conductors is 
additionally required to prevent their overheating. For the FIG. 7 
configuration and uniform current distribution, and if interrail 
insulation is neglected, the current density in the augmenting conductors 
is calculated to be only about 56% of the projectile rail current density 
for the parallel augmenting conductor configuration and therefore for the 
same current flow duration, the augmenting turn would only experience 32% 
of the temperature rise of the projectile rails. 
If the augmenting conductors of FIG. 7 were connected in series, each of 
the augmenting conductors then conducts the full projectile current thus 
yielding a twice externally augmented launcher configuration. For a 
uniform current distribution, the inductance gradient of this 
configuration is 2.509 .mu.H/m, which is equal to 5.25 times the 
inductance gradient of the reference system and thus again above the 
previously considered unattainable limit of 5 for twice augmented designs. 
This yields a 44% improvement over the experimental and calculated 
augmentation factor which averages about 3.65 for the FIG. 2 prior art 
type of configuration for low velocity, low acceleration, larger bore 
launchers. For higher velocity smaller bore launchers, that improvement 
would be experienced during the initial acceleration with a somewhat 
reduced improvement in the higher velocity bore portion. In the high 
velocity bore portion, projectile rail height could then again be reduced 
to increase the inductance gradient and thus improve acceleration. 
If the augmenting conductors of the FIG. 7 configuration are connected in 
series, this configuration could only be effectively used with a 
capacitively powered launcher, since the augmenting turns would have an 
excessive current density and hence excessive resistance and temperature 
rise if a homopolar generator-inductor pulse power source were utilized. 
For effective and efficient use with a homopolar generator-inductor pulse 
compression system, the horizontal dimensions of the augmenting turns can 
be roughly doubled which will still yield a force augmentation factor 
above 5 compared to the reference system. 
It should be noted that the preferred embodiments which involve a 
projectile rail configuration which is generally T-shaped, can be adapted 
to a round bore as shown in FIG. 10. In that Figure, a pair of generally 
parallel projectile launching rails 54 and 60 are arranged adjacent to 
bore 52. Each of these rails includes an arcuate portion 58 and 60 with an 
arcuate internal surface 62 and an external surface 64 which may also be 
arcuate. Longitudinal protuberances 66 and 68 are shown to extend from the 
external surface 64 of rails 54 and 56 respectively. 
The electromagnetic launchers of this invention utilize particular rail 
shapes to increase projectile acceleration by increasing the inductance 
gradient of the rail assembly. Projectiles are accelerated in these 
launchers by a method which comprises the steps of: causing current flow 
through a pair of generally parallel projectile launching rails and 
through an armature, or means for conducting current between the rails and 
for propelling a projectile along the rails; and increasing the inductance 
gradient toward the muzzle end of the rails by decreasing rail height. 
Alternatively or additionally, the inductance gradient can be increased by 
using a rail configuration which concentrates the current carrying area of 
the rails near the mid-plane of the launcher bore. 
Although the present invention has been described in terms of what are at 
present believed to be its preferred embodiments, it will be apparent that 
various changes may be made without departing from the scope of the 
invention. It is therefore intended that the appended claims cover all 
such changes.