Patent Grant
10041757
This disclosure generally relates to systems and methods for improving the accuracy of large guns.
Conventional guns (such as the M198 or M777 155-mm howitzer or large naval guns) rely on chemical propellants which limit their muzzle energy, range, and down-range accuracy. Multiple factors (such as powder temperature) may cause the muzzle velocity of a conventional projectile to vary a few percent from nominal. In some guns a change of just 1° C. in the chemical propellant can cause a 1.5 m/sec change in muzzle velocity, where every 1 m/sec variation from nominal muzzle velocity in a conventional projectile means the ordinance will be off target 30-40 m down range. For example, a 3% deviation from a nominal muzzle velocity of 800 m/sec is 24 m/sec, which could cause the projectile to be delivered almost 1 km away from its desired target. Conventional guns also suffer from barrel wear as they fire more and more rounds. Barrel wear may cause the projectile to leave the gun slightly off center, resulting in a potentially unpredictable trajectory, thereby further reducing down-range accuracy. Additionally, current weapons systems have reached a limit for muzzle velocity with existing explosives.
Coil guns are electromagnetic guns that use the Lorentz force to accelerate a projectile with a conducting armature. For high-speed applications, induction coil guns use magnetic coupling to drive current in the armature without direct electrical contact between the barrel and projectile. Some induction coil guns consist of short-length, solenoidal electromagnets that are stacked end to end. The coils are energized sequentially to create a wave of electromagnetic energy moving from breech to muzzle in order to accelerate the armature. Active tracking of the projectile location during launch provides precise feedback to control when the coils will be triggered to create the electromagnetic wave that propels the projectile.
Existing solutions of bringing electromagnetically propelled weaponry to the battlefield require complete re-design and re-build of existing systems. There is presently no electromagnetic solution known to the authors that can be installed or mounted on existing weapons platforms without major modifications. There is also no known solution to controlling muzzle velocity of conventional guns that use chemical propellant. Guided munitions can be used to control accuracy, but they are very expensive compared to unguided munitions.
It would be desirable to provide a system that can actively control the muzzle velocity of a projectile as it leaves a gun by detecting the velocity of the projectile as it leaves the gun and then adjust its velocity to a target velocity. Preferably this system would be easily retrofit onto existing guns so that minimal or no re-design of the gun or projectile would be necessary.
The subject matter disclosed in detail below is directed to 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.
In accordance with the embodiments disclosed herein, a section of coil gun can be attached to the end of a conventional gun barrel (similar to installation of a suppressor on small arms) and used to electromagnetically control the muzzle velocity of a conventional projectile fired from that gun barrel. A short section (e.g., ˜1 m) of coil gun, with active feedback fire control, attached to the end of a conventional gun can be used to control, and even enhance, the muzzle velocity of a conventional gun. A longer section of coil gun could be used to significantly enhance the muzzle energy of a conventional projectile. These coil guns can be designed to retrofit onto an existing platform and require minimal if any changes to the projectile.
The systems In accordance with the embodiments disclosed herein further comprise detection electronics for detecting the muzzle velocity of the projectile as it exits the gun barrel and high-current, high-voltage switching circuits which connect the coils of the coil gun to a compact self-contained source of electrical power. The power supply may comprise a multiplicity of moderately high-energy-density capacitors and a generator (for charging the capacitors) that can be mounted on a tank or, in the case of artillery, in a small truck or trailer.
One aspect of the subject matter disclosed in detail below is a system that is capable of firing a projectile using chemical propellant, which system comprises: a gun barrel having a muzzle; a barrel extension attached to the muzzle of the gun barrel, the barrel extension being coaxial with the gun barrel; a multiplicity of electrically conductive coils arranged in sequence along the axis of the barrel extension and surrounding respective axial portions of the barrel extension; a multiplicity of sources of electrical current; a multiplicity of switches, each of the switches being connected to a respective coil and to a respective source of electrical current; a sensor system capable of detecting positions of a projectile as it exits the muzzle; and control electronics programmed or configured to alter the state of one or more of the multiplicity of switches based on signals output by the sensor system. The gun barrel may be part of a tank, a howitzer, a naval gun, a rifle, or other similar large gun.
In accordance with some embodiments of the system described in the preceding paragraph, the control electronics are programmed or configured to perform the following operations: (a) generate data representing a present velocity of the projectile based on the signals output by the sensor system; (b) compare the data representing a present velocity of the projectile with data representing a target velocity of the projectile; and (c) generate switching control signals for controlling the state of the switches in a manner that causes the coils to generate electromagnetic forces that reduce a difference between the present and target velocities.
In accordance with some embodiments, the sensor system comprises: 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 and located to send a second signal when said portion of the projectile arrives at a second axial position at a second time subsequent to said first time. 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 can be controlled to cause at least one of the coils 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.
In accordance with one implementation, the sources of electrical current comprise respective capacitor banks; each capacitor bank is connected to a respective switch; and each sensor may comprise a respective light emitter and a respective photodetector arranged to receive light from the respective light emitter.
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.
Another aspect of the subject matter disclosed herein is a method for retrofitting a gun that is capable of firing a projectile using chemical propellant. The retrofitting method comprises: mounting a multiplicity of electrically conductive coils at spaced intervals outside and along a length of barrel extension having a smooth bore; and coupling the barrel extension to the barrel of a gun such that the smooth bore of the barrel extension is aligned with a smooth bore of the gun barrel. In some embodiments, the gun further comprises a muzzle brake attached to a muzzle of the gun barrel, the barrel extension being attached to the muzzle brake. Again the gun may be a tank, a howitzer, a naval gun, a rifle, or other similar large gun.
A further aspect is a method for adjusting a velocity of a projectile propelled by a gun using chemical propellant, the method 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 determined in step (b) 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 in dependence on the results of step (c). In the disclosed embodiments, step (d) comprises energizing one or more electrically conductive coils disposed forward of the muzzle to increase or decrease the projectile velocity depending on whether the present velocity is less or greater than the target velocity.
In accordance with some embodiments, steps (b) through (d) are iteratively performed until the present velocity differs from the target velocity by less than a specified threshold. In accordance with other embodiments, step (d) comprises energizing multiple coils in accordance with a specified firing sequence which is selected in dependence on the magnitude of the difference between the present and target velocities.
Yet another aspect of the subject matter disclosed herein is an apparatus for launching a projectile comprising: a first gun barrel section having a breech and a muzzle; a second gun barrel section coupled to and aligned with the first gun barrel section; a multiplicity of electrically conductive coils arranged in sequence along the second gun barrel section and surrounding respective axial portions of the second gun barrel section; a multiplicity of switches connected to respective coils of the multiplicity of coils; and a multiplicity of capacitor banks connected to respective switches of the multiplicity of switches. This apparatus may further comprise a muzzle brake attached to and disposed between the first and second gun barrel sections. In accordance with various embodiments, the first gun barrel section is a barrel of a tank, a howitzer, a naval gun, a rifle, or other similar large gun.
Other aspects of improved systems and methods for electromagnetically controlling or boosting the muzzle velocity of a large gun are disclosed below.
Reference will hereinafter be made to the drawings in which similar elements in different drawings bear the same reference numerals.
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.
A power supply (not shown in
Referring again to
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.
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 1011 A/sec.
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 3×1012 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).
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
{right arrow over (F)}=∫surface·{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 Bθ=0 as well. The stress tensor can now be written
Now it will be assumed for simplicity that the magnetic field at Maxwell surfaces 1 and 2 (indicated by respective vertical dotted lines in
where the integral is over the length L of Maxwell surface 3 (indicated by horizontal dotted lines in
where xc=rp/rc is a geometric coupling factor between the radius of the projectile rp and the radius of the coils re. This result satisfies a few key features. First, if no projectile is present, rp=0, the system is force free as it must be. Second, it shows that there is no acceleration if Bu=Bd, again as it must be. Finally, it shows that if Bu>Bd, the projectile 18 speeds up; and if Bu<Bd the projectile 18 slows down.
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 Bd=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 Bu=0 and increasing Bd.
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:
where ip and ic are the currents in the projectile and coils respectively.
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 vp=800 m/sec; projectile mass mp=45 kg; armature (the conducting part of the projectile) length lp=10 cm; radius rp=77.5 mm; and the desired velocity correction Δv=1 m/sec. For this example, a single coil with length lc=3 cm and radius rc=81.5 mm will be used.
The armature will pass completely through the coil in tcl=(lp+lc)/vp=162.5 μsec. The time for half of the armature to pass into the coil is tc2=lp/2vp=62.5 μsec. The rise time of the coil necessary to accelerate the projectile will be some time in between these and can be approximated by tc=(lp+2lc)/2vp=100 μsec. This will also be approximately the time over which the acceleration acts.
To effect Δv=1 m/sec over 100 μsec, an acceleration a=10 km/sec2 is needed, which is modest. If the above parameters are put into Eq. (4), the result of the calculation is a≈1750B2. This means that a magnetic field B≈2.4 T is needed, which is again modest. A 100-kA current in a single loop will give B˜0.126 T. So to accomplish a Δv=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 Δv (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.
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 Δv=±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/sec2 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 Δv≈±1 m/sec to the projectile.
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 1010 A/sec. The current transfer rate and coil inductance sets the voltage required for this system.
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.
For velocity corrections Δv=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 m3. 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.
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.
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.
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.
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.
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.
The coil gun partly depicted in
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.
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
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.
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.
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.
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.
This application is a divisional of and claims priority from U.S. patent application Ser. No. 14/282,376 filed on May 20, 2014, which issued as U.S. Pat. No. 9,562,736 on Feb. 7, 2017.
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| Number | Date | Country | |
|---|---|---|---|
| 20170307325 A1 | Oct 2017 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14282376 | May 2014 | US |
| Child | 15403592 | US |
