Electromagnetic muzzle velocity controller and booster for guns

Abstract

Systems and methods for electromagnetically controlling the muzzle velocity of a conventional gun using a coil gun on a barrel extension. This method can also provide an electromagnetically induced increase to muzzle velocity beyond that capable by conventional explosives. With higher muzzle velocity, the weapons will have longer range, higher penetrating power and stand-off distances. A section of coil gun can also be used to center the projectile in the barrel to control the exit trajectory. Using a coil gun to control muzzle velocity and center the projectile can be a retrofit to existing weapons that would greatly increase their down-range accuracy.

Claims

1. A method for adjusting a velocity of a projectile that has been propelled through a barrel of a gun, comprising: mounting a multiplicity of electrically conductive coils at spaced intervals outside and along a length of barrel extension having a smooth bore; coupling the barrel extension to the barrel of the gun such that the smooth bore of the barrel extension is aligned with a smooth bore of the barrel of the gun; generating electrical energy; using the generated electrical energy to charge capacitors; after the capacitors have been charged, igniting chemical propellant to cause a projectile to be propelled from a breech to a muzzle of the barrel of the gun; and adjusting the velocity of the projectile by generating an electromagnetic force in a space that is forward of the muzzle using the multiplicity of electrically conductive coils, wherein the electromagnetic force is generated by supplying electrical energy from the capacitors to the multiplicity of electrically conductive coils.

2. The method as recited in claim 1, wherein the gun further comprises a muzzle brake attached to a muzzle of the gun barrel, the barrel extension being attached to the muzzle brake.

3. The method as recited in claim 1, wherein the gun is a tank, a howitzer, a rifle, naval gun, or other large gun.

4. The method as recited in claim 1, further comprising: connecting the electrically conductive coils to the capacitors by way of respective switches; and mounting a multiplicity of sensors to the barrel section at respective axial positions along the length of the barrel section.

5. The method as recited in claim 4, further comprising configuring a computer to control the states of the switches based on the states of the sensors.

6. A method for adjusting a velocity of a projectile that has been propelled through a barrel of a gun, said method comprising: (a) generating electrical energy; (b) using the generated electrical energy to charge capacitors; (c) after the capacitors have been charged, igniting chemical propellant to cause a projectile to be propelled from a breech to a muzzle of a gun barrel; (d) determining a present velocity of the projectile after at least a portion of the projectile has exited the muzzle; (e) comparing the present velocity to a target velocity; and (f) adjusting the velocity of the projectile by generating an electromagnetic force in a space that is forward of the muzzle, wherein the electromagnetic force is generated by supplying electrical energy from the capacitors to a multiplicity of electrically conductive coils.

7. The method as recited in claim 6, wherein step (f) comprises increasing the velocity of a projectile when the present velocity is less than the target velocity.

8. The method as recited in claim 6, wherein step (f) comprises decreasing the velocity of a projectile when the present velocity is greater than the target velocity.

9. The method as recited in claim 6, wherein steps (d) through (f) are iteratively performed until the present velocity differs from the target velocity by less than a specified threshold.

10. The method as recited in claim 6, wherein step (f) comprises energizing one or more electrically conductive coils disposed forward of the muzzle.

11. A method for adjusting a velocity of a projectile that has been propelled through a barrel of a gun, comprising: (a) igniting chemical propellant to cause a projectile to be propelled from a breech to a muzzle of a gun barrel; (b) determining a present velocity of the projectile after at least a portion of the projectile has exited the muzzle; (c) comparing the present velocity to a target velocity; and (d) adjusting the velocity of the projectile by generating an electromagnetic force in a space that is forward of the muzzle, wherein step (d) comprises energizing multiple coils in accordance with a specified firing sequence which is selected from a plurality of different specified firing sequences in dependence on the magnitude of the difference between the present and target velocities.

12. The method as recited in claim 6, further comprising sensing a presence of the projectile at a multiplicity of axial positions along a length of a barrel section attached to the muzzle.

13. The method as recited in claim 1, wherein the barrel extension is attached to a muzzle of the barrel.

14. A method for adjusting a velocity of a projectile that has been propelled through a barrel of a gun, said method comprising: generating electrical energy; using the generated electrical energy to charge capacitors; after the capacitors have been charged, igniting chemical propellant to cause a projectile to be propelled from a breech to a muzzle of the barrel of the gun; and adjusting the velocity of the projectile by generating an electromagnetic force in a space that is forward of the muzzle, wherein the electromagnetic force is generated by supplying electrical energy from the capacitors to a multiplicity of electrically conductive coils disposed forward of the muzzle.

15. The method as recited in claim 14, further comprising: attaching a barrel section having a multiplicity of electrically conductive coils to the muzzle; and sensing a presence of the projectile at a multiplicity of axial positions along a length of the barrel section.

16. The method as recited in claim 14, further comprising: attaching a barrel section having a multiplicity of electrically conductive coils to a muzzle brake that is attached to the barrel; and sensing a presence of the projectile at a multiplicity of axial positions along a length of the barrel section.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A through 1C are diagrams illustrating respective positions at respective instances of time of a projectile being accelerated by a coil gun.

(2) FIGS. 2A and 2B are diagrams representing sectional and end views respectively of a section of coil gun.

(3) FIG. 3 is a diagram representing a projectile-coil system for solving a Maxwell stress tensor.

(4) FIG. 4 is a diagram representing a side view of a howitzer equipped with a section of a coil gun at the end of a barrel.

(5) FIG. 5 is a diagram representing a side view of an M1 Abrams tank equipped with a section of a coil gun at the end of a barrel.

(6) FIG. 6 is a diagram representing a side view of a sniper rifle equipped with a section of a coil gun at the end of a barrel.

(7) FIG. 7 is a block diagram showing electrical components of a system for providing electromagnetic assistance to a conventional gun.

(8) FIG. 8 is a diagram representing a projectile-coil system that uses optical detection of the axial position of a launched projectile.

(9) FIG. 9 is a flowchart indicating steps of a process for electromagnetically achieving a target muzzle velocity of a projectile using sensor feedback.

(10) FIG. 10 is a flowchart indicating steps of a process for electromagnetically achieving a target muzzle velocity of a projectile using a look-up table.

(11) Reference will hereinafter be made to the drawings in which similar elements in different drawings bear the same reference numerals.

DETAILED DESCRIPTION

(12) A coil gun is an electromagnetic launch device that uses a series of coaxial magnetic field-producing coils, stacked end to end to form a barrel, which are energized in sequence to accelerate or decelerate an electrically conductive projectile. FIGS. 1A through 1C illustrate a 10-stage coil gun 10 accelerating a projectile 18. The projectile 18 comprises an element (e.g., a coil, shell, ring or jacket) made of electrically conductive material (e.g., aluminum or copper), referred to herein as the armature. In other embodiments, the armature could be a sabot. The electrically conductive projectile 18 depicted in FIGS. 1A-1C is fired conventionally using chemical propellant. To create a traveling electromagnetic wave in the barrel that is nearly synchronous with the location of the armature, a real-time detector (not shown in FIGS. 1A-1C) locates the projectile and the coil gun's firing system generates the trigger to switches that connect the individual coils to a power supply. The resulting transient electromagnetic wave induces a current in the armature. FIGS. 1A-1C show respective positions of the moving projectile 18 and lines of respective magnetic fields 20 produced by the energized coils 12 at respective instances of time.

(13) FIGS. 2A and 2B represent sectional and end views respectively of a short section of a coil gun 10. This coil gun section comprises multiple (in this example, ten) magnetic field-producing coils 12 surrounding a smooth bore barrel 14. The coils 12 are enclosed in an outer casing 16. This short section of a coil gun can be added to the end of an existing gun barrel (not shown).

(14) A power supply (not shown in FIG. 2A) can be sequentially connected to the coils 12 by switches (also not shown) to provide short bursts of electrical energy during firing of the gun. Control electronics (not shown in FIG. 2A) in the coil gun section measure the velocity of the projectile entering the coil gun based on feedback from sensors (also not shown) and synchronize the energization of coils 12 to increase and/or regulate the muzzle velocity of the projectile.

(15) Referring again to FIGS. 1A-1C, a coil gun is essentially a linear motor wherein the coils 12 function as the stator and the projectile 18 functions as the armature. Acceleration is accomplished by means of the Lorentz force (JB) between the radial magnetic field from the coils 12 and the azimuthal current induced in the projectile 18. Typically coil guns are meant to be stand-alone devices that can launch projectiles to velocities in excess of 2 km/sec purely inductively using no chemical propellant. However, this does not need to be the case. A small section of coil gun of the type partly depicted in FIGS. 2A and 2B can be used to augment or precisely control the muzzle velocity of a conventional gun.

(16) The electromagnetic assist concept presented herein can be implemented to precisely regulate the muzzle velocity of the projectile. If enough energy is available, the concept could also be implemented to significantly increase the velocity. For regulating muzzle velocity, the firing time of the coils cannot be preprogrammed (as might be done in a low-velocity coil gun) because prior to firing, it will not be known whether the projectile needs to be sped up or slowed down until it reaches the end of the barrel. The same is true if one were to use a coil gun solely to enhance the muzzle velocity. Accordingly, some way of sensing the projectile position, calculating its velocity, and then firing the coils at the appropriate time should be provided.

(17) The primary issues with coil guns revolve around power delivery to the coils. All of the kinetic energy which a coil gun imparts to a projectile must be supplied to the coils in the form of electrical energy. This is typically done using a multiplicity of capacitor banks, each capacitor bank in turn comprising a respective multiplicity of capacitors. Each coil is energized by its own capacitor bank. These capacitor banks can be large, and as the projectile velocity increases, larger voltages and energies are required to accelerate the projectile. Switching the current can also be an issue. At low velocity and low voltage, the currents required and switching times are low enough that an ignitron or even a silicon-controlled rectifier can be used. However, for high-acceleration, high-velocity applications, the switches may need to be able to hold off more than 50 kV and switch more than 10.sup.11 A/sec.

(18) The energy density of modern capacitors enables the production of high-voltage, high-capacity devices available in small packages. This technology enables bank energies in the 100 kJ range (suitable for muzzle velocity regulation) which can fit on a desktop. In addition, advances in switching technology have produced improved solid-state switches, such as insulated-gate bipolar transistors (IGBT) capable of actively switching (turning on and off) large currents at tens of kilovolts. In the alternative, thyratron switches can now deliver 310.sup.12 A/sec at 75 kV. This is adequate to meet the needs of coil guns capable of accelerating a large mass (>3 kg) to hypervelocity (i.e., >2 km/sec).

(19) The following is a simple analytic model of acceleration from a coil gun using the Maxwell stress tensor to calculate the magnetic force exerted on a projectile by a series of axially spaced coils. The force on the projectile can be found by simply solving the stress tensor for the projectile-coil system schematically depicted in FIG. 3. Although FIG. 3 shows a relatively short projectile 18 surrounded by respective portions of relatively long coils 12a and 12b, the concept holds for a longer projectile inside a set of shorter coils. If there are an upstream magnetic field B.sub.u and a downstream magnetic field B.sub.d, the force on the projectile is given by
{right arrow over (F)}=.sub.surface.Math.{right arrow over (n)}dArea(1)
where is the Maxwell stress tensor. The projectile is conducting so there is no electric field, E=0, inside the projectile 18 and the azimuthal field B.sub.=0 as well. The stress tensor can now be written

(20) $\begin{array}{cc}\stackrel{.Math.}{T}=\frac{\stackrel{.fwdarw.}{B}\stackrel{.fwdarw.}{B}}{{}_0}-\frac{1}{2{}_0}\stackrel{.Math.}{I}\left(B^2\right)=\frac{1}{{}_0}\left(\begin{array}{ccc}{B}_r^2& 0& B_r{B}_z\\ 0& -\frac{B^2}{2}& 0\\ B_z{B}_r& 0& B_z^2-\frac{B^2}{2}\end{array}\right)& \left(2\right)\end{array}$
Now it will be assumed for simplicity that the magnetic field at Maxwell surfaces 1 and 2 (indicated by respective vertical dotted lines in FIG. 3) is axial only, i.e., the radial magnetic field B.sub.r=0 and B=B.sub.z. This is a valid assumption for the case of long coils, but not necessarily valid for short coils. The force on the projectile due to the upstream and downstream magnetic fields is

(21) $\begin{array}{cc}\stackrel{.fwdarw.}{F}=F_z=-\frac{r_c^2}{2{}_0}\left(B_u^2-B_d^2\right)+\frac{2r_c^2}{{}_0}\underset{0}{\overset{L}{}}{B}_r{B}_z\mathrm{dz}& \left(3\right)\end{array}$
where the integral is over the length L of Maxwell surface 3 (indicated by horizontal dotted lines in FIG. 3). This result shows that the magnetic field tension (i.e., the first term on the right in Eq. (3)) acts to pull the projectile 18 toward the higher field. The second term on the right in Eq. (3) is the shear term due to the radial field. This is the field that accelerates the projectile 18. Since all the coil currents are azimuthal, J=J.sub., the induced current in the conducting projectile, must also be azimuthal. The Lorentz force on the projectile 18 due to the axial magnetic field B.sub.z is then radial, or attempting to compress the projectile 18, while the radial magnetic field B.sub.r is axial, accelerating the projectile 18. Remembering that F=ma and solving for the acceleration on the projectile 18, we get

(22) $\begin{array}{cc}\stackrel{.fwdarw.}{a}=a_z=\frac{r_c^2}{2{}_0{m}_p}\left(\frac{x_c^2}{1-x_c^2}\right)\left(B_u^2-B_d^2\right)& \left(4\right)\end{array}$
where x.sub.c=r.sub.p/r.sub.c is a geometric coupling factor between the radius of the projectile r.sub.p and the radius of the coils r.sub.e. This result satisfies a few key features. First, if no projectile is present, r.sub.p=0, the system is force free as it must be. Second, it shows that there is no acceleration if B.sub.u=B.sub.d, again as it must be. Finally, it shows that if B.sub.u>B.sub.d, the projectile 18 speeds up; and if B.sub.u<B.sub.d the projectile 18 slows down.

(23) The result in Eq. (4) is important because it shows that a coil gun can be used to both speed up and slow down a projectile. Typically the downstream magnetic field is kept B.sub.d=0 and the upstream field is increased sequentially in the coils so as to positively accelerate the projectile to a high velocity. In the context of this work, however, the desire is primarily to control the muzzle velocity of the projectile (possibly to enhance it), which may require slowing the projectile by making B.sub.u=0 and increasing B.sub.d.

(24) It should be noted that Eq. (4) is only approximate for a real system. In practice, the projectile will have finite conductivity and the flux from the coils will bleed into the projectile, thereby reducing the acceleration. Also, Eq. (4) was derived using long coils, whereas in practice, coils may be short relative to the length of the projectile in order to keep the magnetic gradient and therefore the acceleration on the projectile as constant as possible. Finally, Eq. (4) provides a handy formula that can give the acceleration based on known coil and projectile geometries and magnetic fields. Other methods of calculating acceleration require more complex methods of calculating the change of mutual inductance M between the coils and the projectile:

(25) $\begin{array}{cc}{F}_z=i_p{i}_c\frac{\mathrm{dM}}{\mathrm{dz}}& \left(5\right)\end{array}$
where i.sub.p and i.sub.c are the currents in the projectile and coils respectively.

(26) To get an idea of what kind of velocity change a coil gun may be able to achieve, it is useful to put some basic design parameters into Eq. (4). In this example, the following conditions will be assumed: a nominal muzzle velocity v.sub.p=800 m/sec; projectile mass m.sub.p=45 kg; armature (the conducting part of the projectile) length l.sub.p=10 cm; radius r.sub.p=77.5 mm; and the desired velocity correction v=1 m/sec. For this example, a single coil with length l.sub.c=3 cm and radius r.sub.c=81.5 mm will be used.

(27) The armature will pass completely through the coil in t.sub.cl=(l.sub.p+l.sub.c)/v.sub.p=162.5 sec. The time for half of the armature to pass into the coil is t.sub.c2=l.sub.p/2v.sub.p=62.5 sec. The rise time of the coil necessary to accelerate the projectile will be some time in between these and can be approximated by t.sub.c=(l.sub.p+2l.sub.c)/2v.sub.p=100 sec. This will also be approximately the time over which the acceleration acts.

(28) To effect v=1 m/sec over 100 sec, an acceleration a=10 km/sec.sup.2 is needed, which is modest. If the above parameters are put into Eq. (4), the result of the calculation is a1750B.sup.2. This means that a magnetic field B2.4 T is needed, which is again modest. A 100-kA current in a single loop will give B0.126 T. So to accomplish a v=1 m/sec in a single coil, 20 turns and about 10 kV would be needed. This is all idealized, but still very reasonable and even when one considers practical considerations of a real system, the voltages and currents required do not vary much from here. Also one should bear in mind that this is for a single coil. In actuality it would not be unreasonable to have 10 or more coils (particularly if they are only 3 cm long) in the system and the voltage, current, and turns per coil can be scaled up to allow larger v (larger acceleration), or lower fields (i.e., voltage and current). It should be noted that for this example, the change in muzzle energy is about 36 kJ.

(29) The results of the above-presented analytic model provide an idea of what may be necessary for an electromagnetic system to assist a gun to achieve more predictable muzzle velocities. The system should be capable of applying velocity corrections v=25 m/sec to a projectile having a nominal muzzle velocity of 800 m/sec. In the ideal case this requires an acceleration of 20 km/sec.sup.2 for 1.25 msec for a system that is 1 m long. For this case one can envision a system with twenty-five coils, each 4 cm long (including the gap between coils), with each coil capable of imparting a velocity correction v1 m/sec to the projectile.

(30) In view of the foregoing, the magnetic field is preferably about 3.4 T in the coils. There are also coil design considerations. While more turns in a coil will increase the magnetic field for the same current, more turns will also increase the inductance, requiring a higher voltage. These conditions should be balanced given that the time the armature spends in the coil sets its rise time. This will require a few hundred kiloamperes and multi-turn coils with di/dt on the order of 10.sup.10 A/sec. The current transfer rate and coil inductance sets the voltage required for this system.

(31) Unlike a typical coil gun that only positively accelerates a projectile, the system disclosed herein is capable of both speeding up and slowing down a projectile. In a typical coil gun, coil voltages and risetimes are tailored to the increasing velocity of the projectile. In this case all of the coils should be designed with the same risetime, voltage, and current. This should be acceptable given that one purpose is to regulate the velocity of the projectile around a nominal value and it can be assumed that under normal conditions, the projectile velocity will not be more than a few percent from that value. The amount of acceleration will be set by hardware or software that determines the initial muzzle velocity and fires or does not fire coils in such a manner as to achieve the desired acceleration.

(32) For velocity corrections v=25 m/sec at a projectile velocity of 800 m/sec, the kinetic energy of the projectile would need to be changed by less than 1 MJ. This would require approximately a 2-MJ capacitor bank. Typical high-energy-density capacitors, as of the filing date, range from 1.0 to 1.8 J/cc, which would take a volume between 1 and 2 m.sup.3. This bank size would easily fit in a small truck or trailer, which is not an unreasonable amount of extra support for a piece of artillery. There are a wide range of capacitors available in the voltage, current, and capacitance range required for this application that also fit this energy density. Although there are also much higher-energy-density capacitors available, their shot lifetime is, as of the filing date, too short (thousands of shots versus tens or hundreds of thousands of shots). There would be a need for generators to charge the banks between shots and rapid charging technology would be required to meet the current firing rate of common guns.

(33) A small section of coil gun can be used to control the muzzle velocity of a conventional projectile fired from a conventional gun, such as a howitzer M777. This can be used, for example, to correct for muzzle velocity differences due to changes in powder temperature, and control the muzzle velocity to less than 1 m/sec from the nominal velocity. This results in much greater down-range accuracy of the gun. A conventional gun can be retrofitted with a section of coil gun by forming threads on the exterior of the muzzle end of the barrel of the conventional gun and providing a coil gun section comprising a barrel extension having internal threads on the end to be attached to the gun barrel. The coil gun could then be screwed onto the end of the gun barrel and locked in place by any conventional means. Other means could be used to attach the coil gun to the gun barrel.

(34) FIG. 4 is a side view of a howitzer 30 equipped with a section of a coil gun 10 attached to a muzzle brake 34, which is in turn attached to the muzzle of a gun barrel 32. (A muzzle brake generally is the area at the end of a gun where the propellant gasses are vented as the projectile leaves the muzzle.) The coil gun 10 may comprise a multiplicity of coils 12 arranged as shown in FIG. 2A. The coil gun 10 may further comprise two or more position sensors for detecting when a portion of launched projectile arrives at respective axial positions relative to the muzzle. For example, a pair of sensors may be mounted to the muzzle brake 34 to provide feedback data from which the muzzle velocity of a projectile can be determined. Additional sensors can be provided inside the section of coil gun 10 for detecting the present velocity of the projectile at various axial positions along the coil gun axis. The power supply (e.g., capacitor banks charged by a generator) and control electronics for energizing the coils 12 are not shown in FIG. 4, but would be arranged as generally depicted in FIG. 7). The coils 12 can be energized in various ways to achieve a desired adjustment of the projectile velocity in dependence on the present velocity of the projectile, as will be described in more detail below with reference to FIGS. 9 and 10.

(35) FIG. 5 is a side view of an M1 Abrams tank 31 equipped with a section of a coil gun 10 attached to the muzzle of a gun barrel 32. The coil gun 10 may comprise a multiplicity of coils 12 arranged as shown in FIG. 2A. The coil gun 10 may further comprise two or more position sensors for detecting when a portion of launched projectile arrives at respective axial positions relative to the muzzle. For example, a pair of sensors may be disposed between the muzzle of gun barrel 32 and the start of the first coil of coil gun 10 to provide feedback data from which the muzzle velocity of a projectile can be determined. Additional sensors can be provided inside the section of coil gun 10 for detecting the present velocity of the projectile at various axial positions along the coil gun axis. The power supply 22 may be mounted on the exterior of a rear portion of the tank 31 and connected to the coil gun 10 by means of an electrical cable 36, as shown in FIG. 5. The control electronics for selectively energizing the coils to produce a desired electromagnetic force are not shown in FIG. 5, but will be described later with reference to FIGS. 7 and 8. The coils 12 can be energized in various ways to achieve a desired adjustment of the projectile velocity in dependence on the present velocity of the projectile, as will be described in more detail below with reference to FIGS. 9 and 10.

(36) FIG. 6 is a side view of a sniper rifle 33 equipped with a section of a coil gun 10 at the end of a gun barrel 32. Again the coil gun 10 may comprise a multiplicity of coils 12 arranged as shown in FIG. 2A. The coil gun 10 may further comprise two or more position sensors for detecting when a portion of a bullet arrives at respective axial positions relative to the muzzle. For example, a pair of sensors may be disposed between the muzzle of gun barrel 32 and the start of the first coil of coil gun 10 to provide feedback data from which the muzzle velocity of a bullet can be determined. Additional sensors can be provided inside the section of coil gun 10 for detecting the present velocity of the bullet at various axial positions along the coil gun axis. A power supply 22 may be connected to the coil gun 10 by means of an electrical cable 36, as shown in FIG. 6. The control electronics for selectively energized the coils to produce a desired electromagnetic force are not shown in FIG. 6, but will be described later with reference to FIGS. 7 and 8. The coils of coil gun 10 can be energized in various ways to achieve a desired adjustment of the projectile velocity in dependence on the present velocity of the projectile, as will be described in more detail below with reference to FIGS. 9 and 10.

(37) Basic calculations would show that the electromagnetic assistance concept disclosed herein is practical in terms of size of coils, size of capacitor banks, bank energy, current, and voltage. In the case of tanks and howitzers, the coils themselves can total about a meter in length and the banks themselves, with moderately high-energy-density capacitors, can fit on a tank or in a small truck or trailer that would accompany a howitzer.

(38) FIG. 7 shows some electrical components of a system for providing electromagnetic assistance to a conventional gun that utilizes chemical propellant. A power supply can be sequentially connected to the coils 12 by a multiplicity of switches 24 to provide short bursts of electrical energy (i.e., current pulses) during firing of the gun. The switches 24 may comprise insulated-gate bipolar transistors, thyratron switches, or other suitable switches. In accordance with the embodiment shown in FIG. 7, the power supply comprises a multiplicity of capacitor banks 25 charged by a generator 38. Each capacitor bank 25 comprises a respective multiplicity of capacitors. Each of the switches 24 is connected to a respective coil 12 and to a respective capacitor bank 25. Control electronics 28 in the coil gun section measure the velocity of the projectile entering the coil gun based on feedback from sensors 26 and synchronize the closing of switches 24 and the firing of coils 12 to increase and/or regulate the muzzle velocity of the projectile. More specifically, the sensors 26 may include a first sensor configured and located to send a first signal when a portion of a projectile arrives at a first axial position at a first time and a second sensor configured to send a second signal when the same portion of the projectile arrives at a second axial position at a second time subsequent to the first time. Based on the information represented by the characteristics of the first and second signals, the control electronics 28 can alter the states of one or more of the multiplicity of switches 24 to produce electromagnetic forces for adjusting the velocity of an electrically conductive projectile.

(39) In accordance with some embodiments, the control electronics 28 are programmed or configured to perform the following operations: (a) generate a signal representing a present velocity of the projectile based on first and second signals; (b) compare the signal representing a present velocity of the projectile with a signal representing a target velocity of the projectile; and (c) generate switching control signals for controlling the states of the switches 24 in a manner that causes the coils 12 to generate electromagnetic forces that reduce a difference between the present and target velocities. Operation (a) may comprise calculating the present velocity based on a distance between the first and second sensors and a time interval separating the first and second times. The states of the switches 24 can be controlled to cause at least one of the coils 12 to generate an electromagnetic force which will increase or decrease the velocity of a projectile depending on whether the present velocity is less or greater than the target velocity.

(40) It should be appreciated that the control electronics 28 may be implemented in hardware, software or firmware. For example, the controller may comprise a computer or a processor programmed to perform calculations and execute operations. In the alternative, the controller may take the form of hard-wired control units implemented through use of sequential logic units, featuring a finite number of gates that can generate specific results based on the instructions that were used to invoke those responses. Hard-wired control units have a fixed architecture, i.e., they require changes in the wiring if the instruction set is modified or changed.

(41) FIG. 8 is a diagram representing a projectile-coil system that uses optical detection of the axial position of a launched projectile 18. FIG. 8 shows a projectile 18 inside a smooth bore of a barrel extension 14 of a coil gun. The projectile 18 has a velocity vector which is indicated by the horizontal solid arrow in FIG. 8. Respective portions of the barrel extension 14 are surrounded by a multiplicity of coils. FIG. 8 only shows three coils 12a, 12b and 12c; other coils and the rest of the barrel extension are not shown. The barrel extension 14 is coupled to a gun barrel (not shown in FIG. 8) such that the smooth bore of the barrel extension 14 is aligned with a smooth bore of the gun barrel.

(42) In an embodiment that regulates the projectile velocity, the coils may be configured to have the same risetime, voltage, and current. For an embodiment that increases the projectile velocity, those parameters would need to change for coils downstream of the projectile for increased velocity.

(43) The coil gun partly depicted in FIG. 8 is equipped with a multiplicity of sensors, which may be placed in a multiplicity of pairs of diametrally opposed openings 8 formed in the barrel extension 14. The opening 8 may be formed in the gaps between neighboring coils. FIG. 8 depicts two pairs of openings respectively centered at axial positions A and B (indicated by downward solid arrows in FIG. 8). In the implementation shown in FIG. 8, each sensor comprises a respective light emitter 6 and a respective photodetector 4 arranged to receive light (indicated by upward dashed arrows in FIG. 8) from the respective light emitter 6 in the absence of an intervening obstruction. (There are also proximity sensors that can be used, that also use a light emitter and detector, but they detect light reflected off the projectile instead of when the light is blocked.) When the nose of the projectile 18 intersects the path of the light directed toward a photodetector 4, the light will become obstructed and the electrical signal being output by the photodetector 4 will cease at the instant in time when the nose of the projectile crosses that path. Thus the photodetectors 4 seen in FIG. 8 can produce first and second signals which indicate the respective instances in time when the nose of the projectile arrived at axial positions A and B. The first signal can be used to generate a starting pulse for a digital counter and the second signal can be used to generate a stop pulse for the digital counter. The resulting count represents the duration of time for the projectile to travel a distance equal to the distance separating axial positions A and B. Thus the present projectile velocity can be calculated by dividing the separation distance by the duration of time.

(44) In the alternative, external laser-based diagnostics could be used to monitor the position and velocity of the projectile in a coil gun during launch. The energizing of each coil is then based on the true position of the projectile with respect to the coils to provide optimum thrust. The coils are only energized if the projectile's present velocity falls outside an accepted tolerance band around a target velocity. The coils can be energized to adjust the project velocity to achieve a desired precision relative to a target velocity.

(45) The switching configurations could be pre-stored or switch closure times could be computed on the fly. Respective examples of such switching configurations will now be described with reference to FIGS. 9 and 10.

(46) FIG. 9 is a flowchart indicating steps of a process for electromagnetically achieving a target muzzle velocity of a projectile using sensor feedback. More specifically, the method entails adjusting a velocity of a projectile propelled by a gun using chemical propellant. First, chemical propellant is ignited to cause a projectile to be propelled from a breech to a muzzle of a gun barrel (step 40 in FIG. 9). As the projectile exits the muzzle (step 42), sensors detect respective first and second times of arrival of the projectile at first and second axial positions respectively. Based on this time information and the distance separating the first and second axial positions, the muzzle velocity is calculated (step 44). The calculated muzzle velocity is then compared to a target muzzle velocity (step 46). Next, a determination is made whether the calculated muzzle velocity is greater than the target muzzle velocity (step 48). If the calculated muzzle velocity is greater than the target muzzle velocity, then the control electronics will cause the next coil (which may be the first coil) to be fired (i.e., energized) to decrease the velocity of the projectile by generating an electromagnetic force in a space that is forward of the muzzle (step 52). If a determination is made in step 48 that the calculated muzzle velocity is not greater than the target muzzle velocity, then a determination is made whether the calculated muzzle velocity is less than the target muzzle velocity (step 50). If the calculated muzzle velocity is less than the target muzzle velocity, then the control electronics will cause the next coil (which may be the first coil) to be fired (i.e., energized) to increase the velocity of the projectile by generating an electromagnetic force in a space that is forward of the muzzle (step 54). If a determination is made in step 50 that the calculated muzzle velocity is not less than the target muzzle velocity, then the next coil is not fired (step 58).

(47) After the next coil has been fired, the present velocity of the projectile can be calculated based, for example, on old information from the sensor at the second axial position and new information from a sensor situated at a third axial position (step 56). The newly calculated present projectile velocity is then again compared to the target muzzle velocity (step 46). Steps 46, 48, 50, 52, 54 and 56 are iteratively performed until the present projectile velocity is within a specified tolerance of the target muzzle velocity, i.e., until the present velocity differs from the target velocity by less than a specified threshold.

(48) FIG. 10 is a flowchart indicating steps of a process for electromagnetically achieving a target muzzle velocity of a projectile using a look-up table. First, chemical propellant is ignited to cause a projectile to be propelled from a breech to a muzzle of a gun barrel (step 40 in FIG. 10). As the projectile exits the muzzle (step 42), sensors detect respective first and second times of arrival of the projectile at first and second axial positions respectively. Based on this time information and the distance separating the first and second axial positions, the muzzle velocity is calculated (step 44). The calculated muzzle velocity is then compared to a target muzzle velocity and the difference between those velocities is computed (step 46). Next, the velocity difference is input as an address to a look-up table that stores a multiplicity of specified timing sequences for firing of the coils of the coil gun (step 60). Based on the input address, a predetermined timing sequence is read out from the look-up table. The coils are then fired in accordance with that timing sequence (step 62) to achieve the desired change (i.e., delta) in velocity of the projectile.

(49) While systems and methods for electromagnetically assisting the launching of chemically propelled projectiles have been described with reference to various embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the claims set forth hereinafter. In addition, many modifications may be made to adapt the teachings herein to a particular situation without departing from the scope of the claims.

(50) The method claims set forth hereinafter should not be construed to require that the steps recited therein be performed in alphabetical order (alphabetical ordering in the claims is used solely for the purpose of referencing previously recited steps) or in the order in which they are recited. Nor should they be construed to exclude two or more steps or portions thereof being performed concurrently or to exclude any portions of two or more steps being performed alternatingly.

(51) As used in the claims, the term velocity means the magnitude of the velocity vector, i.e., speed, and is not intended to require direction information, which is assumed to be constant during firing of the projectile. As used in the claims, the term muzzle means the end of a gun barrel from which the projectile will exit.