Electromagnetic guns - Revision history

Lwcamp at 19:00, 7 March 2026

2026-03-07T19:00:12Z

← Older revision		Revision as of 12:00, 7 March 2026
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	== References ==		== References ==

	[[Category:Warfare]][[Category:Physics & Engineering]][[Category:Engineering]][[Category:Military Technology]]		[[Category:Warfare]][[Category:Physics & Engineering]][[Category:Physics & Math & Engineering]][[Category:Engineering]][[Category:Military Technology]]

Lwcamp: /* Charging equipment */

2026-02-17T16:15:12Z

Charging equipment

← Older revision		Revision as of 09:15, 17 February 2026
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	=== Charging equipment ===		=== Charging equipment ===

	An electromagnetic gun requires high power to be delivered in a very short pulse, on the order of a millisecond long. For artillery or naval canons, this can result in an instantaneous power of tens to hundreds of gigawatts! Unless the electromagnetic gun is directly plugged in to the full electrical output of a major regional power generating station (and you are willing to cause blackouts when it fires), you won't be able to directly deliver that kind of power from your main power supply. Instead, you will need to gradually build up energy over time, [[Energy_Storage\|storing it in some kind of equipment]] that can deliver the stored energy in a very fast pulse. Common ways to do this include charging up capacitor banks or spinning up a compulsator (basically a flywheel attached to a generator). At least one research program used a massive inductor to store the energy<ref name="RashleighMarshall1978">S. C. Rashleigh and R. A. Marshall, "Electromagnetic acceleration of macroparticles to high velocities", Journal of Applied Physics 49, 2540-2542 (1978)</ref>~~. Other potential energy accumulation systems include other varieties of [[Energy_Storage#Flywheels\|flywheel energy storage]] as well as~~ [[Superconductive_Magnetic_Energy_Storage\|~~superconducting magnetic energy storage~~]] ~~(which is still an inductor, but a particularly~~ useful ~~form of inductor~~ if you have the tech ~~to make it convenient)~~.		An electromagnetic gun requires high power to be delivered in a very short pulse, on the order of a millisecond long. For artillery or naval canons, this can result in an instantaneous power of tens to hundreds of gigawatts! Unless the electromagnetic gun is directly plugged in to the full electrical output of a major regional power generating station (and you are willing to cause blackouts when it fires), you won't be able to directly deliver that kind of power from your main power supply. Instead, you will need to gradually build up energy over time, [[Energy_Storage\|storing it in some kind of equipment]] that can deliver the stored energy in a very fast pulse. Common ways to do this include charging up [[Energy_Storage#Capacitors\|capacitor banks]] or spinning up a [[Energy_Storage#Flywheels\|compulsator]] (basically a flywheel attached to a generator). At least one research program used a massive inductor to store the energy<ref name="RashleighMarshall1978">S. C. Rashleigh and R. A. Marshall, "Electromagnetic acceleration of macroparticles to high velocities", Journal of Applied Physics 49, 2540-2542 (1978)</ref>, along the same lines [[Superconductive_Magnetic_Energy_Storage\|superconductive inductors]] might be useful in this regards if you have the tech for them.

	In most cases, the energy accumulator system will store enough energy for one shot. Your rate of fire will depend on the time it takes for your primary power supply to build up enough energy for one shot.		In most cases, the energy accumulator system will store enough energy for one shot. Your rate of fire will depend on the time it takes for your primary power supply to build up enough energy for one shot.

Lwcamp: /* Induction coilguns */

2025-12-03T18:29:56Z

Induction coilguns

← Older revision		Revision as of 11:29, 3 December 2025
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	For a coilgun acting like a conventional gun, the field will ramp up and then down over a time scale of milliseconds or less. For these conditions, the skin depth of good conductors like copper or aluminum will be a few millimeters. Meanwhile, the London depth of superconductors is generally less than a micrometer.		For a coilgun acting like a conventional gun, the field will ramp up and then down over a time scale of milliseconds or less. For these conditions, the skin depth of good conductors like copper or aluminum will be a few millimeters. Meanwhile, the London depth of superconductors is generally less than a micrometer.

	For an armature made of normal conductors, as the flux migrates into the conductor the driving coils become less effective. A solution is to let the position of the driving coil "slip" further down the length of the armature to fresher conductor where the magnetic flux has not yet penetrated. In this sense the armature moves ahead of the magnetic wave propagating up the barrel. This is entirely analogous to the slip of a rotary brushless asynchronous induction motor between the angle of the rotating magnetic field and the angle of the armature. Another solution is to keep the driving coil in the same position relative to the armature but to switch the direction of the magnet, called <i>current reversal</i>. Now the flux frozen in to the conductor of the armature helps to attract it even harder to the switched direction magnet. Again, this is analogous to the rotary induction motor with the change in phase of the field at any given angle relative to the armature. These two mechanisms can be used together~~, as well~~.		For an armature made of normal conductors, as the flux migrates into the conductor the driving coils become less effective. A solution is to let the position of the driving coil "slip" further down the length of the armature to fresher conductor where the magnetic flux has not yet penetrated. In this sense the armature moves ahead of the magnetic wave propagating up the barrel. This is entirely analogous to the slip of a rotary brushless asynchronous induction motor between the angle of the rotating magnetic field and the angle of the armature. Another solution is to keep the driving coil in the same position relative to the armature but to switch the direction of the magnet, called <i>current reversal</i>. Now the flux frozen in to the conductor of the armature helps to attract it even harder to the switched direction magnet. Again, this is analogous to the rotary induction motor with the change in phase of the field at any given angle relative to the armature. These two mechanisms can be used together.

	An induction coilgun can launch a solid continuous slug of metal or other conductor. This has the disadvantage that the driving fields and currents are concentrated within about a skin depth into the driving jacket. For projectiles thicker than a few millimeters, this concentrates the induced current in a thin section at the surface, leading to increased resistive heating and possible melting of the driver jacket. This can be avoided by using a wound coil for the driver jacket, which more uniformly distributes the current through the depth of the projectile<ref>I. R. Shokair, M. Cowan, R. J. Kaye, and B. M. Marder, "performance of an Induction Coil Gun", report SAND93-1358 (1993) https://digital.library.unt.edu/ark:/67531/metadc1280251/</ref>. For smallarms with bullet diameters of only a few millimeters, the skin depth may be large enough to allow acceleration over much of the volume of the projectile without using windings in the projectile.		An induction coilgun can launch a solid continuous slug of metal or other conductor. This has the disadvantage that the driving fields and currents are concentrated within about a skin depth into the driving jacket. For projectiles thicker than a few millimeters, this concentrates the induced current in a thin section at the surface, leading to increased resistive heating and possible melting of the driver jacket. This can be avoided by using a wound coil for the driver jacket, which more uniformly distributes the current through the depth of the projectile<ref>I. R. Shokair, M. Cowan, R. J. Kaye, and B. M. Marder, "performance of an Induction Coil Gun", report SAND93-1358 (1993) https://digital.library.unt.edu/ark:/67531/metadc1280251/</ref>. For smallarms with bullet diameters of only a few millimeters, the skin depth may be large enough to allow acceleration over much of the volume of the projectile without using windings in the projectile.

Lwcamp: /* Induction coilguns */

2025-12-03T18:28:27Z

Induction coilguns

← Older revision		Revision as of 11:28, 3 December 2025
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	A conductor that is not a perfect conductor will initially screen the magnetic field from its interior with the field inside the conductor falling off with a characteristic length scale called the skin depth. At a small fraction of the skin depth from the surface, the magnetic field will be at nearly its full strength – there is not enough circulating current outside that depth to create enough of an electromagnet to cancel the field. But at several skin depths deep, the magnetic field will be almost entirely screened by the circulating current and will fall to near zero and the deeper you go the closer to zero the field gets. (For the mathematically inclined, the function is B(x) = B<sub>0</sub> exp[-x/λ] where lambda is the skin depth, B<sub>0</sub> is the external field strength, and B(x) is the field strength at distance x.) However, because the conductor is not perfect the current will be reduced over time by electrical resistance. As the current turns into heat, there is not enough current to continue cancelling the field and the magnetic flux begins to migrate into the conductor.		A conductor that is not a perfect conductor will initially screen the magnetic field from its interior with the field inside the conductor falling off with a characteristic length scale called the skin depth. At a small fraction of the skin depth from the surface, the magnetic field will be at nearly its full strength – there is not enough circulating current outside that depth to create enough of an electromagnet to cancel the field. But at several skin depths deep, the magnetic field will be almost entirely screened by the circulating current and will fall to near zero and the deeper you go the closer to zero the field gets. (For the mathematically inclined, the function is B(x) = B<sub>0</sub> exp[-x/λ] where lambda is the skin depth, B<sub>0</sub> is the external field strength, and B(x) is the field strength at distance x.) However, because the conductor is not perfect the current will be reduced over time by electrical resistance. As the current turns into heat, there is not enough current to continue cancelling the field and the magnetic flux begins to migrate into the conductor.

	For many practical purposes, superconductors are perfect conductors. Like regular conductors, a magnetic field will penetrate into them with a characteristic skin depth called the London penetration depth. Unlike regular conductors, the superconductor suffers no electrical resistance so the flux never penetrates deeper into the superconductor over time. Unlike other conductors, superconductors expel all magnetic fields from their interior at the moment they form (the <i>Meissner effect</i>) so any superconductive region will always have no magnetic field in it. There are some kinds of superconductor that allow narrow threads of non-superconductive regions to pass through them that contain magnetic flux if the outside field gets high enough (so-called Type-II superconductors), while others continue to expel external fields until the fields get too high and turn the superconductivity off (at the <i>critical field</i>).		For many practical purposes, superconductors are perfect conductors. Like regular conductors, a magnetic field will penetrate into them with a characteristic skin depth called the London penetration depth. Unlike regular conductors, the superconductor suffers no electrical resistance so the flux never penetrates deeper into the superconductor over time. Unlike other conductors, superconductors expel all magnetic fields from their interior at the moment they form (the <i>Meissner effect</i>) so any superconductive region deeper than several London depths will always have no magnetic field in it. There are some kinds of superconductor that allow narrow threads of non-superconductive regions to pass through them that contain magnetic flux if the outside field gets high enough (so-called Type-II superconductors), while others continue to expel external fields until the fields get too high and turn the superconductivity off (at the <i>critical field</i>).

	For a coilgun acting like a conventional gun, the field will ramp up and then down over a time scale of milliseconds or less. For these conditions, the skin depth of good conductors like copper or aluminum will be a few millimeters. Meanwhile, the London depth of superconductors is generally less than a micrometer.		For a coilgun acting like a conventional gun, the field will ramp up and then down over a time scale of milliseconds or less. For these conditions, the skin depth of good conductors like copper or aluminum will be a few millimeters. Meanwhile, the London depth of superconductors is generally less than a micrometer.

Lwcamp: /* Recoil */

2025-11-27T03:07:06Z

Recoil

← Older revision		Revision as of 20:07, 26 November 2025
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	=== Recoil ===		=== Recoil ===

	Newton's third law of motion stipulates that whenever you push on something, it pushes back on you just as much. In a closed system, you would get no net movement. If something gets pushed away, the rest of the system recoils back in the opposite direction.		[[File:Comparison_of_the_recoil_of_conventional_and_electromagnetic_cannon.png\|thumb\|A comparison of the recoil impulse from a powder gun (green), electromagnetic gun (red), and a powder gun with a muzzle brake (blue) for a constant-energy launch (varying the mass of the projectile with speed to keep the total kinetic energy constant). Data from <ref>Edward M. Schmidt, "Comparison of the recoil of conventional and electromagnetic cannon", Shock and Vibration 8 (2001) 141–145 https://doi.org/10.1155/2001/590948</ref>]]Newton's third law of motion stipulates that whenever you push on something, it pushes back on you just as much. In a closed system, you would get no net movement. If something gets pushed away, the rest of the system recoils back in the opposite direction.

	In a conventional firearm this recoil comes from the hot high pressure gas pushing the bullet down the barrel. The pressure of the gas pushing back on the breach face gives the gun its kick. In electromagnetic guns, this recoil force comes from the same interactions of currents and fields in the gun that push back on the gun as pushes out the projectile. In a railgun, the current in a high field might push the projectile one way, but that current loop must close somewhere and the field will push back on the rest of the current loop in the gun. In an induction coilgun, the electromagnet in the barrel that pushes on the induced electromagnet in the projectile is in turn pushed back by that same induced electromagnet. A ferromagnetic coilgun, the same interaction occurs in the electromagnet in the stator and the permanent magnets in the armature.		In a conventional firearm this recoil comes from the hot high pressure gas pushing the bullet down the barrel. The pressure of the gas pushing back on the breach face gives the gun its kick. In electromagnetic guns, this recoil force comes from the same interactions of currents and fields in the gun that push back on the gun as pushes out the projectile. In a railgun, the current in a high field might push the projectile one way, but that current loop must close somewhere and the field will push back on the rest of the current loop in the gun. In an induction coilgun, the electromagnet in the barrel that pushes on the induced electromagnet in the projectile is in turn pushed back by that same induced electromagnet. A ferromagnetic coilgun, the same interaction occurs in the electromagnet in the stator and the permanent magnets in the armature.

Lwcamp: /* Induction coilguns */

2025-11-04T20:13:08Z

Induction coilguns

← Older revision		Revision as of 13:13, 4 November 2025
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	<li>The magnetic flux going through the loop created by the driven currents opposes the initial change in magnetic flux from the original magnetic field.		<li>The magnetic flux going through the loop created by the driven currents opposes the initial change in magnetic flux from the original magnetic field.
	</ul>		</ul>
	A consequence of this is that for a perfect conductor, the flux through any closed loop surrounded by the conductor will never change. Because the interior of any object has a closed loop of the material of the object around it, the flux through a perfectly conductive material will always stay the same. If the conductor starts out with zero flux in its interior, any changing magnetic field will induce currents in the conductor that act to cancel the change in flux.		A consequence of this is that for a perfect conductor, the flux through any closed loop surrounded by the conductor will never change. Because the interior of any object has a closed loop of the material of the object around it, the flux through a perfectly conductive material will always stay the same. If the conductor starts out with zero flux in its interior, any changing magnetic field will induce currents in the conductor that act to cancel the change in flux. (We'll get to imperfect conductors in just a moment! This is to get the basic idea across.)
	<ul>		<ul>
	<li>Because the conductor has current circulating around its perimeter, it creates an electromagnet. As with any magnet, it can be attracted toward or repelled from another magnet with a non-zero magnetic field gradient.		<li>Because the conductor has current circulating around its perimeter, it creates an electromagnet. As with any magnet, it can be attracted toward or repelled from another magnet with a non-zero magnetic field gradient.

Lwcamp: /* Induction coilguns */

2025-11-04T03:41:54Z

Induction coilguns

← Older revision		Revision as of 20:41, 3 November 2025
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	For an armature made of normal conductors, as the flux migrates into the conductor the driving coils become less effective. A solution is to let the position of the driving coil "slip" further down the length of the armature to fresher conductor where the magnetic flux has not yet penetrated. In this sense the armature moves ahead of the magnetic wave propagating up the barrel. This is entirely analogous to the slip of a rotary brushless asynchronous induction motor between the angle of the rotating magnetic field and the angle of the armature. Another solution is to keep the driving coil in the same position relative to the armature but to switch the direction of the magnet, called <i>current reversal</i>. Now the flux frozen in to the conductor of the armature helps to attract it even harder to the switched direction magnet. Again, this is analogous to the rotary induction motor with the change in phase of the field at any given angle relative to the armature. These two mechanisms can be used together, as well.		For an armature made of normal conductors, as the flux migrates into the conductor the driving coils become less effective. A solution is to let the position of the driving coil "slip" further down the length of the armature to fresher conductor where the magnetic flux has not yet penetrated. In this sense the armature moves ahead of the magnetic wave propagating up the barrel. This is entirely analogous to the slip of a rotary brushless asynchronous induction motor between the angle of the rotating magnetic field and the angle of the armature. Another solution is to keep the driving coil in the same position relative to the armature but to switch the direction of the magnet, called <i>current reversal</i>. Now the flux frozen in to the conductor of the armature helps to attract it even harder to the switched direction magnet. Again, this is analogous to the rotary induction motor with the change in phase of the field at any given angle relative to the armature. These two mechanisms can be used together, as well.

	An induction coilgun can launch a solid continuous slug of metal or other conductor. This has the disadvantage that the ~~field can only penetrate~~ into the driving jacket ~~up to a skin depth~~. For projectiles thicker than a few millimeters, this concentrates the induced current in a thin section at the surface, leading to increased resistive heating and possible melting of the driver jacket. This can be avoided by using a wound coil for the driver jacket, which more uniformly distributes the current through the depth of the projectile<ref>I. R. Shokair, M. Cowan, R. J. Kaye, and B. M. Marder, "performance of an Induction Coil Gun", report SAND93-1358 (1993) https://digital.library.unt.edu/ark:/67531/metadc1280251/</ref>. For smallarms with bullet diameters of only a few millimeters, the skin depth may be large enough to allow acceleration over much of the volume of the projectile without using windings in the projectile.		An induction coilgun can launch a solid continuous slug of metal or other conductor. This has the disadvantage that the driving fields and currents are concentrated within about a skin depth into the driving jacket. For projectiles thicker than a few millimeters, this concentrates the induced current in a thin section at the surface, leading to increased resistive heating and possible melting of the driver jacket. This can be avoided by using a wound coil for the driver jacket, which more uniformly distributes the current through the depth of the projectile<ref>I. R. Shokair, M. Cowan, R. J. Kaye, and B. M. Marder, "performance of an Induction Coil Gun", report SAND93-1358 (1993) https://digital.library.unt.edu/ark:/67531/metadc1280251/</ref>. For smallarms with bullet diameters of only a few millimeters, the skin depth may be large enough to allow acceleration over much of the volume of the projectile without using windings in the projectile.

			Coilguns can in principle be quite efficient. Estimates of room temperature aluminum armatures with low launch pressures and large sizes suggest efficiencies in excess of 80%, while higher pressure (~0.2 GPa) and more moderate sized (10-20 cm bore diameters) coilguns might reach 40% to 50% efficiency at turning electrical energy into the kinetic energy of the armature and projectile<ref name="Cowan_1993">M. Cowan, E. C. Cnare, B. W. Duggin, R. J. Kaye, B. M. Marder, I. IL Shokair, "The Continuing Challenge of Electromagnetic Launch", https://www.osti.gov/servlets/purl/10177176</ref>. These efficiencies will improve with improved electrical conductivity of the armature, which could be achieved either by cooling the aluminum to cryogenic temperatures before launch or by using materials with higher room temperature conductivity. This could include copper (with roughly a factor of two higher conductivity), exotic future materials, or superconductors.

	The upper limit to the speed of an induction coilgun using ordinary conductors on the projectile is where the resistive heating from the induced currents melts the conductor in the projectile or sabot. One study<ref name="Winterberg EMRG">F. Winterberg, "The electromagnetic rocket gun", Acta Astronautica Vol. 12, No. 3, pp. 155-161, 1985</ref> suggests this will happen for speeds of more than approximately 20 km/s. This limit does not apply to superconductors, as long as the driving field is kept below the superconductor's critical field.		The upper limit to the speed of an induction coilgun using ordinary conductors on the projectile is where the resistive heating from the induced currents melts the conductor in the projectile or sabot. One study<ref name="Winterberg EMRG">F. Winterberg, "The electromagnetic rocket gun", Acta Astronautica Vol. 12, No. 3, pp. 155-161, 1985</ref> suggests this will happen for speeds of more than approximately 20 km/s. This limit does not apply to superconductors, as long as the driving field is kept below the superconductor's critical field.

Lwcamp: /* Induction coilguns */

2025-11-04T03:18:41Z

Induction coilguns

← Older revision		Revision as of 20:18, 3 November 2025
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	<ul>		<ul>
	<li>A change in the net amount of magnetic field going through the loop (the magnetic <i>flux</i>) creates an electric field around the loop proportional to the change in magnetic flux.		<li>A change in the net amount of magnetic field going through the loop (the magnetic <i>flux</i>) creates an electric field around the loop proportional to the change in magnetic flux.
	<li>If the loop is conductive, the electric field drives a current flow in the loop.		<li>If the loop is conductive, the electric field drives a current flow in the loop, called an <i>eddy current</i>.
	<li>Currents create a magnetic field circulating around them.		<li>Currents create a magnetic field circulating around them.
	<li>The magnetic flux going through the loop created by the driven currents opposes the initial change in magnetic flux from the original magnetic field.		<li>The magnetic flux going through the loop created by the driven currents opposes the initial change in magnetic flux from the original magnetic field.
	</ul>		</ul>
			A consequence of this is that for a perfect conductor, the flux through any closed loop surrounded by the conductor will never change. Because the interior of any object has a closed loop of the material of the object around it, the flux through a perfectly conductive material will always stay the same. If the conductor starts out with zero flux in its interior, any changing magnetic field will induce currents in the conductor that act to cancel the change in flux.
			<ul>
			<li>Because the conductor has current circulating around its perimeter, it creates an electromagnet. As with any magnet, it can be attracted toward or repelled from another magnet with a non-zero magnetic field gradient.
			<li>So to make a coilgun, use a stator electromagnet to ramp up a magnetic field in the vicinity of a conductive armature. This induces another electromagnet in the armature, which will be attracted to the stator electromagnet from their mutual magnetic fields.
			</ul>
			As with the ferromagnetic coilgun, a series of electromagnets are energized along the bore to create a traveling magnetic wave moving up the barrel just in front of the armature. The electromagnets pull the armature towards them via this induction process.

	~~As with the~~ ferromagnetic ~~coilgun, a series~~ of ~~electromagnets are energized along the bore~~ to create a traveling magnetic wave moving up the barrel. A changing magnetic field induces eddy currents in any conductors exposed to it that create an electromagnet that opposes the applied field. The opposing fields then push the projectile out of the barrel.		Induction does not saturate, such that induction coilguns can potentially be used at very high speeds where ferromagnetic coilguns would have too low of efficiency to be competitive.

	~~Because induction requires~~ a ~~changing~~ magnetic field, the ~~projectile continually slips behind~~ the ~~portion~~ of the ~~wave~~ at ~~constant~~ strength, ~~requiring a more complicated~~ field ~~behavior~~ to ~~maintain inductive propulsion~~. ~~In principle~~, ~~a superconducting projectile~~ (~~or sabot~~) ~~with a critical~~ field ~~above~~ the ~~magnetic~~ field strength in the ~~bore could~~ be ~~accelerated without slipping~~, ~~providing~~ the ~~benefits of inductive acceleration without~~ the ~~need for complicated field behavior~~.		A conductor that is not a perfect conductor will initially screen the magnetic field from its interior with the field inside the conductor falling off with a characteristic length scale called the skin depth. At a small fraction of the skin depth from the surface, the magnetic field will be at nearly its full strength – there is not enough circulating current outside that depth to create enough of an electromagnet to cancel the field. But at several skin depths deep, the magnetic field will be almost entirely screened by the circulating current and will fall to near zero and the deeper you go the closer to zero the field gets. (For the mathematically inclined, the function is B(x) = B<sub>0</sub> exp[-x/λ] where lambda is the skin depth, B<sub>0</sub> is the external field strength, and B(x) is the field strength at distance x.) However, because the conductor is not perfect the current will be reduced over time by electrical resistance. As the current turns into heat, there is not enough current to continue cancelling the field and the magnetic flux begins to migrate into the conductor.

	~~Induction does not saturate~~, ~~such that induction coilguns can potentially be used~~ at ~~very high speeds where ferromagnetic coilguns would~~ have ~~too low~~ of ~~efficiency~~ to ~~be competitive~~.		For many practical purposes, superconductors are perfect conductors. Like regular conductors, a magnetic field will penetrate into them with a characteristic skin depth called the London penetration depth. Unlike regular conductors, the superconductor suffers no electrical resistance so the flux never penetrates deeper into the superconductor over time. Unlike other conductors, superconductors expel all magnetic fields from their interior at the moment they form (the <i>Meissner effect</i>) so any superconductive region will always have no magnetic field in it. There are some kinds of superconductor that allow narrow threads of non-superconductive regions to pass through them that contain magnetic flux if the outside field gets high enough (so-called Type-II superconductors), while others continue to expel external fields until the fields get too high and turn the superconductivity off (at the <i>critical field</i>).

			For a coilgun acting like a conventional gun, the field will ramp up and then down over a time scale of milliseconds or less. For these conditions, the skin depth of good conductors like copper or aluminum will be a few millimeters. Meanwhile, the London depth of superconductors is generally less than a micrometer.

			For an armature made of normal conductors, as the flux migrates into the conductor the driving coils become less effective. A solution is to let the position of the driving coil "slip" further down the length of the armature to fresher conductor where the magnetic flux has not yet penetrated. In this sense the armature moves ahead of the magnetic wave propagating up the barrel. This is entirely analogous to the slip of a rotary brushless asynchronous induction motor between the angle of the rotating magnetic field and the angle of the armature. Another solution is to keep the driving coil in the same position relative to the armature but to switch the direction of the magnet, called <i>current reversal</i>. Now the flux frozen in to the conductor of the armature helps to attract it even harder to the switched direction magnet. Again, this is analogous to the rotary induction motor with the change in phase of the field at any given angle relative to the armature. These two mechanisms can be used together, as well.

	An induction coilgun can launch a solid continuous slug of metal or other conductor. This has the disadvantage that the field can only penetrate into the driving jacket up to a skin depth. For projectiles thicker than a few millimeters, this concentrates the induced current in a thin section at the surface, leading to increased resistive heating and possible melting of the driver jacket. This can be avoided by using a wound coil for the driver jacket, which more uniformly distributes the current through the depth of the projectile<ref>I. R. Shokair, M. Cowan, R. J. Kaye, and B. M. Marder, "performance of an Induction Coil Gun", report SAND93-1358 (1993) https://digital.library.unt.edu/ark:/67531/metadc1280251/</ref>. For smallarms with bullet diameters of only a few millimeters, the skin depth may be large enough to allow acceleration over much of the volume of the projectile without using windings in the projectile.		An induction coilgun can launch a solid continuous slug of metal or other conductor. This has the disadvantage that the field can only penetrate into the driving jacket up to a skin depth. For projectiles thicker than a few millimeters, this concentrates the induced current in a thin section at the surface, leading to increased resistive heating and possible melting of the driver jacket. This can be avoided by using a wound coil for the driver jacket, which more uniformly distributes the current through the depth of the projectile<ref>I. R. Shokair, M. Cowan, R. J. Kaye, and B. M. Marder, "performance of an Induction Coil Gun", report SAND93-1358 (1993) https://digital.library.unt.edu/ark:/67531/metadc1280251/</ref>. For smallarms with bullet diameters of only a few millimeters, the skin depth may be large enough to allow acceleration over much of the volume of the projectile without using windings in the projectile.

Lwcamp: /* Induction coilguns */

2025-11-04T02:39:38Z

Induction coilguns

← Older revision		Revision as of 19:39, 3 November 2025
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	==== Induction coilguns ====		==== Induction coilguns ====

	An induction coilgun is what you get when you unroll a brushless asynchronous electric motor into a line. The projectile must be made of a conductive material or enclosed in a conductive sabot. The projectile does not have to be magnetic – in fact, it should not be; materials such as iron should be avoided in the driver shell and if present should only be in a core region shielded from the accelerating field by an outer layer of suitable skin depth to attenuate the field. As with the ferromagnetic coilgun, a series of electromagnets are energized along the bore to create a traveling magnetic wave moving up the barrel. A changing magnetic field induces eddy currents in any conductors exposed to it that create an electromagnet that opposes the applied field. The opposing fields then push the projectile out of the barrel. Because induction requires a changing magnetic field, the projectile continually slips behind the portion of the wave at constant strength, requiring a more complicated field behavior to maintain inductive propulsion. In principle, a superconducting projectile (or sabot) with a critical field above the magnetic field strength in the bore could be accelerated without slipping, providing the benefits of inductive acceleration without the need for complicated field behavior.		An induction coilgun is what you get when you unroll a brushless asynchronous electric motor into a line. The projectile must be made of a conductive material or enclosed in a conductive sabot. The projectile does not have to be magnetic – in fact, it should not be; materials such as iron should be avoided in the driver shell and if present should only be in a core region shielded from the accelerating field by an outer layer of suitable skin depth to attenuate the field.

			To understand how an induction coilgun works, first we discuss one of the fundamental properties of electromagnetism. When a magnetic field changes, it <i>induces</i> an electric field circulating around it. A convenient way to think of this is that for any closed loop:
			<ul>
			<li>A change in the net amount of magnetic field going through the loop (the magnetic <i>flux</i>) creates an electric field around the loop proportional to the change in magnetic flux.
			<li>If the loop is conductive, the electric field drives a current flow in the loop.
			<li>Currents create a magnetic field circulating around them.
			<li>The magnetic flux going through the loop created by the driven currents opposes the initial change in magnetic flux from the original magnetic field.
			</ul>

			As with the ferromagnetic coilgun, a series of electromagnets are energized along the bore to create a traveling magnetic wave moving up the barrel. A changing magnetic field induces eddy currents in any conductors exposed to it that create an electromagnet that opposes the applied field. The opposing fields then push the projectile out of the barrel.

			Because induction requires a changing magnetic field, the projectile continually slips behind the portion of the wave at constant strength, requiring a more complicated field behavior to maintain inductive propulsion. In principle, a superconducting projectile (or sabot) with a critical field above the magnetic field strength in the bore could be accelerated without slipping, providing the benefits of inductive acceleration without the need for complicated field behavior.

	Induction does not saturate, such that induction coilguns can potentially be used at very high speeds where ferromagnetic coilguns would have too low of efficiency to be competitive.		Induction does not saturate, such that induction coilguns can potentially be used at very high speeds where ferromagnetic coilguns would have too low of efficiency to be competitive.



	An induction coilgun can launch a solid continuous slug of metal or other conductor. This has the disadvantage that the field can only penetrate into the driving jacket up to a skin depth. For projectiles thicker than a few millimeters, this concentrates the induced current in a thin section at the surface, leading to increased resistive heating and possible melting of the driver jacket. This can be avoided by using a wound coil for the driver jacket, which more uniformly distributes the current through the depth of the projectile<ref>I. R. Shokair, M. Cowan, R. J. Kaye, and B. M. Marder, "performance of an Induction Coil Gun", report SAND93-1358 (1993) https://digital.library.unt.edu/ark:/67531/metadc1280251/</ref>. For smallarms with bullet diameters of only a few millimeters, the skin depth may be large enough to allow acceleration over much of the volume of the projectile without using windings in the projectile.		An induction coilgun can launch a solid continuous slug of metal or other conductor. This has the disadvantage that the field can only penetrate into the driving jacket up to a skin depth. For projectiles thicker than a few millimeters, this concentrates the induced current in a thin section at the surface, leading to increased resistive heating and possible melting of the driver jacket. This can be avoided by using a wound coil for the driver jacket, which more uniformly distributes the current through the depth of the projectile<ref>I. R. Shokair, M. Cowan, R. J. Kaye, and B. M. Marder, "performance of an Induction Coil Gun", report SAND93-1358 (1993) https://digital.library.unt.edu/ark:/67531/metadc1280251/</ref>. For smallarms with bullet diameters of only a few millimeters, the skin depth may be large enough to allow acceleration over much of the volume of the projectile without using windings in the projectile.

Lwcamp: /* High speed */

2025-10-21T14:43:08Z

High speed

← Older revision		Revision as of 07:43, 21 October 2025
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	A projectile going at 2 km/s or more <i>will</i> explode. If it is a dense dart, it might punch through a considerable thickness of material in the process, but it will produce a significant blast in the process. Common media depictions of hypervelocity guns leaving nice neat holes with no collateral damage to nearby objects are not accurate.		A projectile going at 2 km/s or more <i>will</i> explode. If it is a dense dart, it might punch through a considerable thickness of material in the process, but it will produce a significant blast in the process. Common media depictions of hypervelocity guns leaving nice neat holes with no collateral damage to nearby objects are not accurate.

	At high enough speed, a projectile will not be able to go through ordinary air without being ablated away. Space debris encountering the tenuous upper atmosphere of Earth's mesosphere, generally at speeds of 7 km/s or higher, get heated by the ram compression of the air in front of them sufficiently that they are nearly or entirely consumed by ablation. In the dense air of Earth's lower atmosphere, the effect would be even more immediate and severe. You can expect that any projectile moving at around 5 km/s or faster ~~would only be able to go between 30 to 150 times~~ its ~~own length in Earth sea level air before being fully disintegrated~~.		At high enough speed, a projectile will not be able to go through ordinary air without being ablated away. Space debris encountering the tenuous upper atmosphere of Earth's mesosphere, generally at speeds of 7 km/s or higher, get heated by the ram compression of the air in front of them sufficiently that they are nearly or entirely consumed by ablation. In the dense air of Earth's lower atmosphere, the effect would be even more immediate and severe. You can expect that any projectile moving at around 5 km/s or faster will suffer rapid ablation and might not reach its target.

	=== Launch assist ===		=== Launch assist ===