Electromagnetic launcher with improved rail energy recovery or dissipation

An electromagnetic projectile launcher is provided with muzzle circuitry which increases the rate of dissipation of inductively stored rail energy following the launch of a projectile and can be used to recover a portion of that stored energy for use in successive launches. The muzzle circuitry includes a pair of conductors which lie adjacent to the projectile launching rails and conduct current in a direction which is opposite to the current flow in the projectile rails. These conductors may be connected to a pulse transformer to increase the post-launch current of an inductive energy storage device. In an alternative embodiment, a capacitive muzzle circuit is used to store the inductive rail energy and inject current into the projectile launching rails during the launch of a successive projectile.

BACKGROUND OF THE INVENTION 
This invention relates to electromagnetic projectile launchers and more 
particularly to such launchers which include structures for recovering 
inductively stored rail energy or for accelerating current decay following 
the launch of a projectile. 
In a simple parallel rail electromagnetic launcher system, the inductively 
stored rail energy at the time of projectile exit is equal to 1/2 
L'.times.I.sup.2.sub.m where L' is the barrel inductance gradient, .times. 
is the barrel length, and I.sub.m is the muzzle current. The instantaneous 
accelerating force on a projectile is 1/2 L'I.sup.2 and, therefore, the 
total energy E imparted to a projectile, neglecting friction, is given by 
the equation: 
EQU E=.intg.Fdx=.intg.1/2 L'I.sup.2 dx (1) 
If launcher barrel length is to be minimized, the maximum allowable force 
must be sustained throughout the bore length. In that case, the current 
must stay constant at its maximum level and, immediately following a 
projectile launch, the remaining rail inductive energy will be 1/2 
L'.times.I.sup.2 which is then exactly equal to the projectile kinetic 
energy. Thus, for minimum barrel length and constant accelerating current, 
neglecting all rail resistance, 50% of the energy supplied at the breech 
will remain inductively stored by the rails and this energy is normally 
wasted. In practice, and expecially if high efficiency is desired, the 
projectile is allowed to exit only after a significant current drop 
compared to the initial breech current, but this involves the penalty or 
detriment of a considerably longer barrel. 
If the barrel has external augmenting turns, then at least some of the 
post-projectile-exit inductively stored rail energy can be returned to the 
inductive storage in the power supply. Such a launching system is 
disclosed in a copending commonly assigned application by Kemeny et al. 
entitled "Electromagnetic Projectile Launcher With Energy Recovering 
Augmenting Field And Minimal External Field", filed Apr. 23, 1981, Ser. 
No. 256,745 (W.E. Case 49,429). The beneficial energy conservation 
illustrated in that application is available because of the augmenting 
turns and is unavailable for a simple parallel rail launcher. For a simple 
parallel rail launcher configuration, a muzzle shunt resistor can be used 
to dissipate inductively stored rail energy. The rate of current 
commutation into the muzzle resistor is improved by adding higher 
impedance sections to the projectile launching rails at the muzzle ends. 
The shunting resistor can be designed such that it does not produce an 
excessive voltage drop but still dissipates most of the energy and thus 
results in less heating of the projectile launching rails. In general, the 
rate of current decay after firing will be substantially slower than the 
current decay during the actual projectile acceleration, and for very 
rapid burst firing, this relatively slow decay will limit the attainable 
burst firing rate. Current decay can be accelerated by increasing the 
resistance of the muzzle shunt impedance, which decreases the circuit time 
constant, but this results in higher voltage across the shunt and more 
difficulty in commutating current into the shunt. The present invention 
provides launching systems which improve the rate of current decay after 
projectile exit and can recover a portion of the inductively stored rail 
energy for use in accelerating a successive projectile. 
SUMMARY OF THE INVENTION 
An electromagnetic projectile launcher constructed in accordance with the 
present invention comprises: a pair of conductive rails having a breech 
end and a muzzle end; a source of direct current connected to the rails; 
means for conducting current between the rails and for propelling a 
projectile along the rails; and means for increasing the rate of reduction 
of magnetic flux between a portion of the rails following the exit of the 
projectile. The means for increasing the rate of reduction of magnetic 
flux may include a pair of conductors lying adjacent to the conductive 
rails and connected to the muzzle end of the rails such that current flow 
through the conductors produces magnetic flux between the rails which is 
in a direction opposite to the magnetic flux produced by current flowing 
in the rails. A resistor may be connected in series with these conductors 
to dissipate the rail energy or the conductors may themselves have 
sufficient resistance to limit premature and parasitic current flow to 
less than a predetermined magnitude. In addition, switching means may be 
connected in series with these conductors to prevent premature current 
flow and to start current flow at the optimum time. Increased coupling may 
be provided by making the conductive rails and conductors substantially 
coaxial. 
The launchers of this invention can also be configured to recover at least 
a fraction of the inductively stored projectile rail energy following the 
launch of a projectile by including a pulse transformer having first and 
second windings wherein the first winding is connected in series with the 
power source and the second winding is connected in series with the 
conductors used to increase the rate of flux reduction. In an alternative 
embodiment, energy recovery can be provided by connecting a capacitor 
array between the projectile launching rails at the muzzle end. Switching 
means must then be provided for initially transferring the rail inductive 
energy to the capacitor array and storing it there. Further means may be 
provided for discharging the capacitor array into the rails to increase 
current during the launch of a successive projectile. 
The launchers of this invention accelerate projectiles in accordance with a 
method which comprises the steps of: passing a current through a pair of 
projectile launching rails and through a means for conducting current 
between the rails to create magnetic fields which interact to propel the 
means for conducting current and an associated projectile along the rails; 
and increasing the rate of injection of muzzle current into a muzzle shunt 
circuit by folding conductors in the muzzle shunt circuit toward the 
breech end of the projectile launching rails.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, FIG. 1 is a schematic diagram of an 
electromagnetic projectile launching system constructed in accordance with 
one embodiment of the present invention. A pair of generally parallel 
conductive projectile launching rails 10 and 12 are connected to a source 
of high current 14 which includes, for example, the series connection of a 
generator 16, switch 18, and inductive energy storage device 20. Of 
course, other known current sources can also be used in the launching 
systems of this invention. An arc or metallic conductive armature 22 is 
located between the projectile launching rails and serves as means for 
conducting current between the rails and for propelling a projectile along 
the rails. A firing switch 24 is connected to the breech end of the 
projectile launching rails 10 and 12. Insulating or higher resistance rail 
segments 26 and 28 are provided adjacent to the muzzle end of projectile 
launching rails 10 and 12, respectively, to improve the rate of current 
commutation into the means for increasing the rate of reduction of 
magnetic flux in a portion of the projectile launching rails following the 
launch of a projectile. This means for increasing the rate of reduction of 
magnetic flux includes a pair of conductors 30 and 32 which are connected 
to points A and B adjacent to the muzzle ends of projectile launching 
rails 10 and 12, respectively, and are positioned closely adjacent to 
rails 10 and 12 such that current flow in each rail and the adjacent 
conductor is in the opposite direction. Resistor 34 is connected to 
conductors 30 and 32, for example at points C and D. Electromagnetic 
projectile launching systems have been proposed which are similar to the 
launcher of FIG. 1, but wherein the resistor 34 is connected directly 
across points A and B. That configuration permits energy dissipation but 
current decay is rather slow and results in complete wastage of the 
inductively stored energy which remains after a projectile has been 
launched. This invention utilizes conductors 30 and 32 to accelerate 
energy dissipation, which is favorable, especially for a rapid fire 
scenario but can, in other embodiments, additionally return some of this 
energy back to the inductor loop of the power supply thus conserving 
energy, increasing efficiency and resulting in more shots for a given 
magnitude of prestored energy. Furthermore, by not wasting all of the 
post-firing rail inductive energy, higher efficiencies are obtainable at 
higher muzzle-to-breech current ratios and therefore there is less penalty 
due to efficiency loss for near-to-constant current acceleration, which, 
of course, results in reduced barrel length at still high efficiency 
levels. 
The operation of the launcher shown in FIG. 1, which does not include 
energy recovery means, will be discussed with reference to the graph of 
FIG. 2. In order to launch a projectile, switches 18 and 24 are initially 
closed and inductive energy storage device 20 is charged to a 
predetermined firing current level. The current in the system is 
illustrated by the solid curve 36 in FIG. 2. At time T.sub.1, firing 
switch 24 is opened and current is commutated into projectile launching 
rails 10 and 12 and through armature 22 to propel the armature along the 
rails. Current in the system rapidly decreases as shown by curve 36 in 
FIG. 2 between times T.sub.1 and T.sub.2. At time T.sub.2, armature 22 
passes from the muzzle end of rails 10 and 12 into the insulating or 
higher impedance rail segments 26 and 28 downstream of the shunt 
attachment points A and B. The resulting increasing voltage across 
attachment points A and B will hasten commutation of current into the 
shunt loop which comprises the series connection of conductor 30, resistor 
34 and conductor 32. Conductors 30 and 32 essentially parallel the 
projectile launching rails, but have current flow opposite to that in the 
rails. Because of this geometry, the firing rails in series with the shunt 
loop have a lower inductance than the firing rail loop alone. The current 
will therefore now rapidly rise during commutation and then, in turn, more 
rapidly decay. The rapid rise of current is due to substantial 
conservation of magnetically stored energy during the commutation of 
current into a lower inductance loop. A rather similar current increase 
procedure is described in my copending application Ser. No. 443,730, filed 
Nov. 22, 1982 and entitled "Burst Firing Electromagnetic Launcher 
Utilizing Variable Inductance Coils", (W.E. Case 50,706). The broken curve 
38 of FIG. 2 approximates the current decay of a system wherein resistor 
34 is connected directly to points A and B. The more rapid decay after 
time T.sub.2, illustrated by curve 36, is due to a decrease in the circuit 
time constant L/R wherein the inductance L has decreased and the 
resistance R of the system has increased through the addition of shunt 
loop ACDB. 
It should be understood that although a separate muzzle resistor 34 has 
been illustrated, such a separate unit is not required as the loop ACDB 
can be designed to have the required or optimum resistance by simply using 
a cable size and material which yields the desired resistance. Curve 36 in 
the graph of FIG. 2 gives an estimate of the current attainable with the 
embodiment of FIG. 1. As can be seen, the current first rises rapidly 
during the commutation and then more rapidly decays as compared with 
broken curve 38. With everything else unchanged, the rapidity of 
commutation will be improved by increasing the voltage across the muzzle 
loop terminations A and B. Thus, increased muzzle velocity yields a higher 
commutation voltage and the desired faster commutation. The ratio of the 
current after commutation to the initial muzzle current is, under ideal 
conditions, equal to the square root of the ratio of the initial rail loop 
inductance to the lower inductance of the final loop which includes the 
muzzle shunt circuitry. The ratio of these inductances depends on how 
large a fraction of the total rail length is paralleled by the conductors 
30 and 32 and on the extent by which current in these conductors opposes 
or reduces the initial rail bore flux. 
To yield the desired low inductance after current flows in the rails and 
muzzle shunt loop, the shunt leads should closely parallel the rails. FIG. 
3 shows a cross section of projectile launching rails which may be used in 
the launcher of FIG. 1. Projectile launching rails 10 and 12 and muzzle 
shunt circuit conductors 30 and 32 are held in position by an insulating 
support structure 40. In this embodiment, the muzzle shunt leads are 
essentially coaxial with the individual projectile launching rails, thus 
yielding an even lower inductance assembly and hence a higher current peak 
and faster decay. Additionally, the scheme of paralleling the projectile 
rails with the shunt leads carrying current in an opposite direction 
causes very rapid decay of the barrel stray or fringe field in the bore 
length paralleled by the shunt leads, thereby hindering detection by 
hostile magnetic field sensors. 
FIG. 4 is a schematic diagram of an electromagnetic projectile launching 
system which allows the recovery of at least some of the stored inductive 
rail energy which is initially trapped in the projectile launching rail 
loop before the projectile bridges the rails at terminations A and B. In 
this embodiment, the muzzle shunt conductors 30 and 32 have been extended 
and are connected in series with an air core pulse transformer 42 which 
includes a first winding 44 and a second winding 46. The first winding 44 
serves as an auxiliary inductor in series with power supply 14 and the 
turns of the first and second windings cooperatively link the same flux. 
In this configuration, the current pulse produced during commutation and 
current decay in the loop AEFB induces a current in the first winding 44 
and, by using proper winding directions, the current in the first winding 
will be in a direction which increases current in the main storage 
inductor 20 thus recovering energy. The existence of the first winding 44 
as an auxiliary inductor, is not detrimental as, for example without loop 
AEFB, the inductive energy storage in the first winding simply adds to the 
total inductively stored energy. During charging of the main and auxiliary 
inductors, a current will be parasitically induced in the closed loop 
EFBHGAE, where segment HG represents the closed firing switch 24. Whether 
this current flow wastes a significant quantity of energy depends on 
circuit parameters, geometry, and on how fast current is built up in the 
inductors. During firing, very rapid current drop in the first winding 44 
will certainly produce a significant current in the loop which includes 
circuit AEFB and which loop is now closed by the projectile armature or 
plasma 22. Such parasitic current flow can be readily prevented and this 
is accomplished, for example, by a series circuit element such as 
thyristor array 48 which can be triggered by the voltage across 
terminations A and B produced as the projectile passes these points. 
The current magnitude during the commutation pulse into the loop AEFB will 
depend on the circuit parameters. For example, if the reduction in 
inductance due to the paralleling of conductors 30 and 32 to the driving 
rails about equals the addition of inductance due to the turns of the 
secondary winding 46, then the pulse current magnitude will not exceed the 
muzzle current level. If the reduction in inductance due to the 
paralleling well exceeds the addition of inductance by the secondary 
winding 46, then current can increase and the current pulse will resemble 
that of curve 36 following T.sub.2 in FIG. 2. Although the launcher of 
FIG. 4 illustrates an embodiment with shunt conductors 30 and 32 parallel 
to the projectile launching rails 10 and 12, the muzzle rail terminations 
A and B could also be connected to the secondary winding terminations E 
and F by suitable external and preferably very low inductance cabling. In 
such a case, the muzzle plus rail loop EFBHGAE will definitely have a 
higher inductance than the rail loop BHGAB alone and the current after 
commutation will be lower than the muzzle current, current injection into 
the increased inductance will take longer, and pulse transformer operation 
should be less favorable. In general, the launcher configuration of FIG. 4 
will result in both energy recovery to allow more shots for a given 
prestorage energy level, and faster current decay to allow shots in more 
rapid succession. 
FIG. 5 is a schematic diagram of an alternative embodiment of the present 
invention utilizing a muzzle energy recovery system wherein the 
post-firing rail inductive energy is first transferred to a capacitor 
array. In this embodiment, the muzzle shunt circuit includes a series 
connection of diode arrays 52 and 54 and a capacitor bank which includes 
the parallel connection of capacitors 50. When armature 22 passes 
termination points A and B, commutating the current into the capacitor 
array is electrically very favorable since the uncharged capacitors not 
only act initially as a short circuit, but after projectile exit and 
completion of commutation, and hence cessation of current flow, for 
example by arching directly across the rails, the rail current flow will 
go to zero in a normal oscillatory inductor-capacitor circuit manner and 
at this current zero, the circuit can be opened to trap the remaining 
post-firing energy in the capacitor array. That energy can then be used 
for even a long-delayed successive shot. 
FIG. 5 shows circuitry wherein the muzzle capacitors 50 are connected 
through switching devices 56 and 58 to projectile launching rails 10 and 
12 at points J and K. This configuration allows the muzzle capacitors to 
reinject current, through synchronized switching, into the projectile 
rails to raise the attenuated driving current level of a successive 
projectile back to nearer breech current magnitude for successive shots. 
Another alternative is to discharge the capacitors into a pulse-type 
transformer for successive shots in a manner similar to that shown in FIG. 
4. With that alternative, a pulse transformer such as 42 in FIG. 4 would 
be inserted in series with the power supply and the capacitor array would 
be connected to terminals E and F of winding 46 of the transformer through 
switching devices 56 and 58. With the FIG. 5 type of system, useful energy 
recovery, that is, the fraction of post-firing rail inductive energy which 
can be supplied back into the rails for a subsequent shot, can be expected 
to be considerably better than 50%. The disadvantage of the FIG. 5 
capacitor system is that the size and cost of conventional and presently 
available capacitors will make such a system unattractive for mobile 
applications. Nevertheless, such capacitive rail energy storage systems 
may become more versatile as more compact high energy storage capacitors 
become available. 
It should be observed that a commutating voltage which aids current 
injection into the shunt loop will be generated as soon as armature motion 
causes the flux in the shunt loop to increase. For example, in FIG. 1, 
such a voltage is generated when the projectile armature passes connection 
points C and D. This induced voltage is generated until the armature exits 
the launcher. Thus the folding of shunt leads toward the breech not only 
creates a voltage in a direction which aids in commutating current into 
the shunt, but also maintains this voltage for a sufficiently long time to 
allow the commutation of the desired fraction of the current. The 
magnitude of this induced voltage will be comparable to the back EMF and 
can thus reach the value of IL'v where v is the projectile velocity. For 
example, at I=1 MA, L'=0.5 .mu.H/m and v=2000 m/sec, the injecting voltage 
can reach 1000 volts which will certainly hasten current injection into 
the muzzle shunt loop. 
Thus folding of the muzzle shunt circuit leads from their attachment points 
backward toward the breech, causes an earlier application of current 
injection voltage and hastens commutation of current into the shunt loop. 
In FIG. 5, the leads from the projectile rails to circuit elements 52 and 
54 are therefore shown to be folded toward the breech, thereby causing 
earlier energy injection into the muzzle shunting capacitors. It should be 
understood that additional conductors which are electrically connected in 
parallel with the backward folded muzzle shunt circuit conductors can be 
added to the launchers of FIGS. 1, 4 and 5. These additional conductors 
would be symmetrically disposed on opposite sides of the projectile 
launching rails in a manner similar to that shown for the backward folded 
portions of the muzzle shunt circuit conductors in FIGS. 1, 4 and 5. 
Because a voltage which will tend to inject current into the shunt loop is 
generated as soon as flux is injected into this loop by the motion of the 
projectile, a thyristor assembly such as 48 in FIG. 4 will be required if 
the muzzle shunt loop extends over a substantial portion of the projectile 
rails as it does in FIG. 4. This thyristor assembly or other switching 
means is then triggered to conduct as soon as it is desired to start 
injecting current into the shunt loop. 
While the present invention has been described in terms of what are at 
present believed to be the preferred embodiments, it will be apparent to 
those skilled in the art that various changes may be made to these 
embodiments without departing from the scope of the invention. It is, 
therefore, intended that the appended claims cover all such changes.