Multiple rail electromagnetic launchers with acceleration enhancing rail configurations

An internally augmented, parallel rail electromagnetic projectile launching system includes a plurality of projectile launching rail pairs wherein the stacked height of the rails adjacent to the launcher bore decreases from the breech end to the muzzle end of the launcher. The launcher rail pairs may be electrically connected in series with each other and in series with an external current source. Alternatively, a separate though not necessarily entirely independent current source may be provided for each pair of rails and switching means may be included to substantially simultaneously connect each current source to its associated rail pair.

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. 
Parallel rail electromagnetic launchers which utilize a single pair of 
projectile rails require very high currents to achieve projectile 
velocities in excess of those obtained with conventional accelerating 
means such as explosives. In order to achieve a given accelerating force 
with a lower current, various augmentation schemes have been proposed. 
External augmentation is accomplished by placing additional conductors 
outside of the bore to increase bore flux and thereby increase the force 
exerted for a given current level on the armature, or on a sabot by a 
plasma. Internally augmented launchers have additional conductors disposed 
along the interior of the bore. For a given number of conductor pairs, 
internal series augmentation results in the highest force increase for a 
given current or yields the greatest current reduction for a given 
propelling force compared to a simple parallel rail launcher. Thus 
internal series augmentation is highly desirable from high propellant 
force and current reduction considerations. However, when the rail pairs 
in an internally augmented launcher are electrically connected in series, 
the integrity of the individual rail and armature current loops must be 
maintained since failure to do so will result in shorting out loops which 
will cause a drastic reduction in the projectile accelerating force. 
Because of high velocities and high currents, relatively high voltage 
differences exist between adjacent rails and adjacent but distinct current 
paths across the armature. Therefore, successful operation of internally 
series augmented launcher configurations is most likely to be attained for 
lower velocity, large bore massive projectile applications involving 
current conducting armatures or specially constructed sabots for plasma 
separation. A commonly assigned, copending application Ser. No. 381,603, 
filed May 24, 1982 and entitled "Parallel Rail Electromagnetic Launcher 
With Multiple Current Path Armature", U.S. Pat. No. 4,485,720 discloses 
series connected internally augmented launchers with plasma separating 
sabots and is hereby incorporated by reference. 
The potential difference between adjacent conductors in an internally 
augmented launcher can be minimized through the use of multiple sources of 
current wherein each pair of conductors and the associated conductive 
armature path or plasma are supplied by an independent current source. If 
current is supplied by individual and presumably identical current sources 
to each of a number of physically in parallel projectile rail pairs, then 
the total current for this rail configuration is roughly, for the same 
overall configuration, current density and acceleration, identical to the 
current for a simple parallel rail launcher. Internally augmented 
launchers having multiple current sources are also disclosed in the above 
U.S. Pat. No. 4,485,720. 
In a parallel rail electromagnetic launcher, a force accelerates a current 
carrying conductor located 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. 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 magnetic field is produced by the time dependent current 
distribution which exists in the rails not more than roughly 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 or in the close 
vicinity of 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. Additional rail configurations 
improvements have been shown in commonly assigned copending application 
Ser. No. 571,609, filed Jan. 17, 1984 and entitled " Electromagnetic 
Launchers With Improved Rail Configurations". Application Ser. No. 571,609 
provides additional background information and is hereby incorporated by 
reference. 
SUMMARY OF THE INVENTION 
The present invention includes internally augmented, multiple projectile 
rail pair launchers wherein significant acceleration improvements are 
attained by reducing the stacked rail height toward the muzzle end. These 
launchers include at least two pairs of generally parallel conductive 
rails lining a bore and means for conducting current between each rail 
pair and for propelling a projectile along the rails. The total stacked 
height of the surfaces of each of the rails which lie adjacent to the bore 
decreases from the breech end to the muzzle end of the rails, thereby 
increasing the acceleration force on the projectile, caused by current 
flowing through each of the rails and through the means for conducting 
current between the rails, as the means for conducting current travels 
from the breech end to the muzzle end of the rails. 
In one configuration, a separate current source is provided for each pair 
of rails and switching means is included for substantially simultaneously 
connecting each current source to its respective pair of rails. In an 
alternative configuration, the rails are electrically connected in series 
and a single current source is used to provide the propelling current.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, FIG. 1 is a schematic diagram of an internally 
augmented, multiple rail electromagnetic projectile launcher constructed 
in accordance with one embodiment of the present invention. This launcher 
includes a first pair of rails 10 and 12, a second pair of rails 14 and 16 
and a third pair of rails 18 and 20 all lining a bore 22. A plurality of 
sliding metallic conductive armatures 24, 26 and 28 are positioned to make 
sliding electrical contact between the rails of the first, second and 
third pairs respectively, and serve as means for conducting current 
between the rails and for propelling projectile 30 along the rails. These 
conductive armatures are attached to an insulating sabot structure 32 
which provides mechanical support for the armatures and the projectile. 
A separate current source is provided for each pair of rails. This current 
source includes the series connection of a generator 34, which may be for 
example a homopolar pulse generator, a switch 36 and one of several 
inductive energy storage devices 38, 40 and 42. Switches 44, 46 and 48 
serve as means for switching current from the current sources to the 
projectile launching rail pairs. These switches are shown to be ganged for 
substantially simultaneous operation. 
FIG. 2 is a longitudinal cross section of a launcher of this invention 
taken in a plane adjacent to the launcher bore. This figure illustrates 
how the height of each launcher rail, and the total stacked height of the 
rails, decreases from the breech end 50 to the muzzle end 52 of the 
launcher bore. An insulating support structure 54 is provided to support 
the projectile launching rails and guide the projectile assembly. FIGS. 3 
and 4 are transverse cross sections of the launcher of FIG. 2 taken 
adjacent to the breech and muzzle ends of the launcher respectively. These 
figures illustrate how the rectangular cross-sectional area of the rails 
may decrease toward the muzzle end and also how the total stacked height, 
h, of the rail surfaces which are adjacent to the bore decreases from the 
breech end to the muzzle end. Sliding conductive armatures 24, 26 and 28 
are shown in FIG. 3 to extend between the rails of the first, second and 
third pairs respectively. If separate current sources are utilized for 
each conductive rail pair, electrical shorting of the rail pairs at the 
location of the armatures does not reduce the accelerating force on the 
projectile. Therefore, as illustrated in the transverse cross section of 
FIG. 5, a single plasma 56 or a single conductive armature spanning across 
all the rail pairs, can serve as a means for conducting current between 
the rails and for propelling the projectile along the rails. It should be 
noted that the multiple current sources of FIG. 1 are not entirely 
independent. In the design of ultra high current inductors, it is standard 
practice to obtain the high current capability by using a multiplicity of 
parallel inductor windings, each of which conducts the same current 
magnitude and all of which are generally interconnected electrically in 
parallel at the inductor terminals. The present invention takes advantage 
of this standard design by providing a firing switch which is electrically 
connected in series to each of the individual inductor winding circuits 
and across the breech end of each individual rail pair. These switches are 
ganged together so that the breech current is injected simultaneously from 
each inductor winding into each parallel projectile rail pair. It should 
be understood that the inductive energy store need not have a number of 
parallel windings that is precisely the same as the number of projectile 
rail pairs. For example, if an inductive energy store having eight 
windings were used in the launcher of FIG. 1, two windings would be left 
open-circuited and three sets of two windings each would be individually 
connected in parallel or series to feed current into each switch and 
projectile rail pair. For the series connection of two inductor windings 
feeding each rail pair and switch, a favorable configuration would have 
the two inductors connected on opposite sides of each firing switch. 
The placement of individual firing switches into individual inductor 
circuits is not as complicated as it may at first glance appear since at 
ultra high currents, had there only been a single massive switch for a 
simple parallel rail launcher, a multiplicity of parallel conductors or 
bus bars would still most likely be required to conduct the firing current 
which may be for example 1.5 megamperes. The launcher of FIG. 1 simply 
keeps these parallel conductors electrically insulated from each other 
with each, or each set, now feeding into an individual firing switch. 
Because all current sources, rails and switches are in essence identical 
and in a geometrically and electrically parallel array, voltage 
differences between adjacent bus bars, rails or switches, will be minor 
and due to these low voltage differences there will be no insulation 
problems. Furthermore, with each firing switch subjected to only a 
fraction of the total current, for example one third in FIG. 1, the 
likelihood of switch damage due to arcing and current concentration 
effects during switching transients will be far reduced. 
The electrical connection of FIG. 1 which involves separate and parallel 
inductor windings for providing current to a parallel array of projectile 
rail pairs allows shorting at, for example, the armature without a 
reduction in armature acceleration force. Thus the armature need not have 
a multiplicity of separate current paths. This allows for a simpler 
armature construction and of course readily permits plasma drive. During a 
projectile launch, the magnitude of the driving current will decrease as 
the projectile travels toward the muzzle. Since the integrity of the 
individual current paths across the armature need not be preserved, the 
total stacked height of the parallel rail pairs can now be reduced as 
allowed by the driving current reduction and reduced current residence 
time toward the muzzle. Thus, no matter how many parallel projectile rail 
pairs are used in a given launcher, their total stacked height can now be 
reduced commensurate with the driving current reduction experienced as the 
armature approaches the muzzle, and the full benefits of projectile rail 
height reduction will be attained for any number of projectile rail pairs. 
Another and more subtle projectile acceleration improvement will result 
from the physically and electrically in parallel connection of multiple 
projectile rail pairs, compared to using a single massive parallel rail 
pair launcher configuration. During a projectile launch, current tends to 
excessively concentrate toward the extremities of the projectile rails as 
illustrated by the computer generated plot in FIG. 6. That figure 
approximates the magnetic field distribution 58 just behind the projectile 
for copper launch rails 13 millimeters wide by 50 millimeters tall with a 
sliding armature having a velocity in the order of one thousand meters per 
second. FIG. 6 was created by an eddy current solution of a finite element 
program to show this current concentration effect graphically. The effect 
would be more pronounced for even higher armature velocities and thus 
results in greater current concentration and heating at the corners of the 
sliding armature contact. 
Excessive current concentration toward the rail extremities adversely 
affects the inductance gradient L', which is always increased by having 
more current concentration toward the horizontal bore center line. The 
inductance gradient for a simple parallel rail launcher having a single 
parallel rail pair, with rail 60 being one of the rails, in the high 
velocity regime, will only be an estimated 0.389 microhenries per meter 
which is 19% below the inductance gradient of 0.478 microhenries per meter 
for an evenly distributed current. Thus another acceleration improvement 
inherent in the parallel connections of FIG. 1 is the decrease in 
acceleration reducing current concentrations in the projectile rails. 
Additionally, by forcing a more uniform current distribution in the 
stacked projectile rails and the armature, damage and wear due to 
excessive current concentration in both the rails and armature conductors 
and in the armature contact areas will be beneficially reduced. 
Furthermore, by having more uniform current flow across the armature, the 
armature acceleration force will be more uniform over the bore area 
thereby simplifying the armature and sabot structural design. 
From the discussion up to this point, it should be understood that using 
the individual windings of a single inductive energy store, in such a 
manner that each winding or set of windings feeds a firing switch which 
injects current into an individual rail pair of a multiple rail pair 
launcher, presents no significant increase in complexity over using a 
single set of massive rails subjected to the total accelerating current. 
The beneficial effects of the parallel rail pair configuration are that a 
more uniform current density in the rails and armatures will yield a 
higher acceleration force, and less wear and less heating, than would be 
induced by excessive local current densities if the rails were undivided. 
Another advantage of the FIG. 1 configuration is that the inductors will 
force substantially equal current flow through each firing switch, which 
will improve switching performance. Additionally, the advantage of 
modularity is attained. For example, if one has a firing switch suitable 
for 0.5 MA, then a 2.0 MA launcher can use four parallel rail pairs, 
making it unnecessary to go to the expense of having to develop a 2.0 MA 
switch. 
FIG. 7 is a schematic diagram of an alternative embodiment of the present 
invention. In this embodiment, a first pair of rails 62 and 64, a second 
pair of rails 66 and 68 and a third pair of rails 70 and 72 are all 
positioned generally parallel to each other along the bore 74. Sliding 
conductive armatures 76, 78 and 80 serve as means for conducting current 
between the first, second and third rail pairs respectively and for 
propelling a projectile 82 along the rails. An insulating sabot structure 
84 serves to support the armatures and projectile and includes insulating 
tabs 86 and 88 which prevent shorting between the armatures. Similar sabot 
structures are disclosed in the previously discussed commonly assigned 
U.S. Pat. No. 4,485,720. The rail pairs of FIG. 7 are electrically 
connected in series with each other and in series with a current source 90 
which may comprise the series connection of a generator 92, switch 94 and 
inductor 96. A switch 98 serves as means for switching current from the 
current source 90 to the breech of the series connected projectile 
launcher rails. If a simple parallel rail launcher, that is one containing 
a single pair of launcher rails, is assumed to have the same bore area and 
rail cross section as the launcher of FIG. 7, then the simple parallel 
rail launcher design will have an accelerating force equal to 
EQU F.sub.1 =1/2L'.sub.1 I.sup.2.sub.1 (1) 
where I.sub.1 is the total current which is assumed to be uniformly 
distributed over the rail cross-sectional areas, and L'.sub.1 is the 
inductance gradient for such a simple parallel rail launcher design, which 
will be about 0.5 microhenries per meter. To develop the same acceleration 
force F.sub.2 in the series augmented launcher of FIG. 7, neglecting the 
minor inter-rail insulation area, the current density in the FIG. 7 rails 
will have to be precisely that of the simple parallel rail launcher, since 
the force on the armature is only a function of the vector cross product 
of the current density and the magnetic field density. To therefore get 
the same force, the current in the launcher rails of FIG. 7, I.sub.2 will 
have to be equal to 1/3I.sub.1 and: 
##EQU1## 
and hence: 
EQU L'.sub.2 =9L'.sub.1 =N.sup.2 L'.sub.1 (3) 
where N equals the number of series connected rail pairs for equivalent 
rail cross-sectional area designs. This simply demonstrates that an 
internally augmented series connected parallel rail launcher exhibits 
force augmentation by about a factor of N.sup.2 for a fixed current. It 
should also be understood, that for the same force, rail current densities 
will be identical for a simple parallel rail launcher and internally 
series augmented versions. 
When the launcher of FIG. 7 is operated at high velocities and high 
currents, high voltage differences, which can reach the order of a few 
kilovolts, may occur between adjacent rails and between the individual and 
distinct current paths or loops of the armature. This is why insulating 
tabs 86 and 88 are shown to be included on the sabot structure 84. 
The computer generated magnetic flux distributions of FIGS. 8 and 9 can now 
be used to illustrate the force augmentation provided for internally 
augmented launcher configurations. If a simple parallel rail launcher, 
having a single pair of launcher rails, has a 50.times.50 millimeter bore 
and a 50.times.13 millimeter rail cross section, it will have a calculated 
inductance gradient L'.sub.S of 0.478 microhenries per meter for a uniform 
current distribution. By using two pairs of series connected rails as 
shown in FIG. 8 wherein rails 100 and 102 represent one rail of each of 
the series connected pairs, and neglecting the very minor effect of the 
inter-rail pair insulation, one obtains a four fold increase in the 
inductance gradient L' since the inductance gradient is proportional to 
the number of turns squared. In that case, the configuration of FIG. 8 
results in an inductance gradient L'.sub.A equal to 1.912 microhenries per 
meter for a uniform current distribution in the rails. 
FIG. 9 illustrates the magnetic flux distribution toward the muzzle end of 
the launcher. Assuming a muzzle current reduction of 30%, the conducting 
rail height or perimeter is therefore allowed to be reduced by at least 
30% and in the FIG. 9 flux plot it has been additionally assumed that the 
current penetration in the wake of the now more rapidly moving projectile 
averages only 5 millimeters. For a simple parallel rail launcher having 
the same dimensions as the bore of the launcher of FIG. 9, the inductance 
gradient would be 0.592 microhenries per meter. By using the internally 
augmented configuration of FIG. 9, the inductance gradient is increased 
four-fold to 2.368 microhenries per meter. Thus by internal series 
augmentation and rail height reduction toward the muzzle, a four fold 
force augmentation at the breech has been achieved compared to a simple 
parallel rail launcher at the same current level. Toward the muzzle, in 
comparison to the breech of a simple launcher, the force augmentation is 
now a factor of 4.95, or an additional 24% improvement. More precisely, 
18% is due strictly to rail height reduction and the rest is due to 
reduced current penetration into the rail thickness. 
The two rail pair internally series augmented configuration is particularly 
attractive since there are only two conducting paths across the armature 
which must be insulated from each other. However, if an additional rail 
pair is added while maintaining the same rail and bore area, an additional 
force factor of only 2.25 (9 divided by 4) is achieved. Furthermore, since 
the central rail pair height must remain substantially unchanged as the 
location of the inter-rail insulation must remain fixed in order to 
maintain the separate current paths in the armature, a 30% rail height 
reduction justified by current reduction, can now effectively only be 
applied to the other rail pairs and hence the overall rail height 
reduction will not be 30%, but instead only around 20%. Therefore there is 
less enhancement of acceleration force available toward the muzzle end. 
If the rail configuration illustrated in FIGS. 8 and 9 is supplied by a 
pair of parallel current sources and because the inductances in the two 
rail pairs are substantially identical, the voltage differences across the 
insulation between the adjacent rail pairs will be zero. For a three 
projectile rail pair launcher, this is not quite the case since the middle 
projectile rail pair will have a somewhat higher inductance and hence due 
to the inductive voltage component and differences in energy consumption, 
there will be voltage differences in the order of tens of volts across the 
insulation between adjacent rails. For a larger number of rail pairs in a 
parallel array, transposition can be used advantageously to assure that 
energy flow to each rail pair is closely matched. For example, with four 
stacked projectile rail pairs, one inductor circuit can provide the 
current to the upper left rail and the second from the top on the right. 
By similar criss-crossing rail pairs, all four will have the same 
inductance, energy flow to them will be better matched, and voltage 
differences across insulators will be reduced. Obviously, this 
transposition is only feasible when the now transposed projectile rail 
pairs are all shorted at the armature or plasma, or if the armature has 
distinct criss-crossed current paths to interconnect across the transposed 
projectile rails. 
FIGS. 10 and 11 are transverse cross sections of a circular bore launcher 
constructed in accordance with the invention. Bore 104 in support 
structure 106 is lined by three pairs of projectile launching rails 108 
and 110, 112 and 114, and 116 and 118. FIG. 10 is taken near the breech 
end and FIG. 11 is taken near the muzzle end. Note that the portion of the 
bore circumference spanned by the rails and, if desired, the width of the 
rails decreases from the breech end to the muzzle end in a manner similar 
to that illustrated in FIG. 2. 
Although the present invention has been described in terms of what are at 
present believed to be its preferred embodiments, it will be apparent to 
those skilled in the art that various changed may be made without 
departing from the scope of the invention. It is therefore intended that 
the appended claims cover all such changes.