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I am considering building a small-scale experimental coil gun because I have an idea of how a magnetized projectile could be rotated while it is traveling through the coil(s), similar to how a bullet is rotated while it is traveling down a rifled gun barrel.

Before I start on this experimental project, I first want to find out from someone within the Electrical Engineering.SE community if creating a slanted magnetic field is even possible. I am neither an engineer nor a scientist, so I have a limited knowledge of Electromagnetism and of magnetic fields.

The idea I have is to create a slanted magnetic field within the coil(s) which I believe should rotate the magnetized cylindrical projectile (or a cylindrical permanent magnet) as it is traveling through the coil or through multiple coils.

I have created two drawings, shown below, which illustrate how I believe a slanted magnetic field should be created using this insulated stranded wire arrangement. I am planning to use insulated copper wire, which is enamel-covered 20 AWG 'magnet wire'.

enter image description here

enter image description here

I want to note that the first drawing shows just an exterior magnetic field, but there should also be an interior magnetic field.

Also, the hollow space between the wires is necessary because these wires will be wrapped around a PVC pipe and the projectile will be traveling through that PVC pipe. There will be a copper ring at each end of the coil and the ends of the wires will be soldered to those rings. A separate wire coming from each of these rings will be connected to a battery/DC power source.

I do not know if this slanted magnetic field will rotate the magnetized projectile as it travels through the coil(s), but it seems to me that the slanted magnetic field should create a circular torque upon the magnetic field of the magnetic projectile, and this circular torque should cause the projectile to rotate as it travels through the coil(s).

Can a slanted magnetic field be created with this insulated stranded wire arrangement?

EDIT

I thought about the issue brought up by user 'Andy aka' concerning the magnetic flux and I think this design change should resolve it. All the wires will be straightened out for a certain length at each end of the coil before they are soldered to the two copper rings.

enter image description here

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4 Answers 4

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One trick you can try to get the rifling you desire is to put "rotating" stages between the linear stages. Effectively, you would have a motor stator in between the linear stages (look up 3-phase motor stators on Google to get familiar with what I'm talking about). The basic rotator design would be 3 coils spaced 120 degrees apart with the magnetic flux pointing in the radial direction of the tube. You would then have a (modified) motor driver activating these coils in sequence. The projectile is essentially the motor rotor.

For clarity, the coils in the rotating stages are sequenced independently from the linear stages and, if power permits, would be left on before, during, and after the launch. In other words, you leave this static rotating field on the whole time independent of the linear stage sequencing.

However, all the rotating stages can be synchronized with each other and run from a common driver.

enter image description here

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  • \$\begingroup\$ Are you suggesting Lorenz force to induce rotation? Any amount of flux going radially in, will also have to go radially out again (div B = 0). As a result there will be no net radial flux in any of your rotation stages. I am still uncertain whether this means that there will be also no net torque \$\endgroup\$
    – tobalt
    Commented May 17 at 14:50
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    \$\begingroup\$ @tobalt What is the matter of doubt? A rotating magnetic field is a trivial thing. en.wikipedia.org/wiki/Rotating_magnetic_field \$\endgroup\$
    – MrGerber
    Commented May 17 at 15:03
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    \$\begingroup\$ @tobalt I don't think you understand how this works the way you think you do. The projectile indeed will be rotationally symmetric, specifically around its rotating axis. Every motor designed has a rotationally symmetric rotor (high-speed ones are also balanced as well). I'm not demanding nor expecting the need of a sabot. Even I did, it too, would be round and symmetric. The projectile only needs to be ferrous (steel) for the linear stages. Not a requirement for the rotating stages. No, two coils at 180 deg won't generate torque and therefore no spin. \$\endgroup\$
    – MOSFET
    Commented May 17 at 16:28
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    \$\begingroup\$ @tobalt It would be much more interesting if you can show an example of a motor rotor that is not rotationally symmetric considering EVERY SINGLE MOTOR ROTOR is designed symmetrically, not just for electrical reasons but for mechanical reasons as well. I just browsed about 100 google images, and as expected, every single one is symmetric. TBH, it's absurd to think a non-symmetric design would make more sense. \$\endgroup\$
    – MOSFET
    Commented May 17 at 18:41
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    \$\begingroup\$ @MOSFET, I think your idea is worth pursing and I plan on doing some coil gun experiments based on it. I have come to see how my idea for rotating a coil gun projectile will not work. \$\endgroup\$
    – user57467
    Commented May 19 at 12:54
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I first want to find out from someone within the Electrical Engineering.SE community if creating a slanted magnetic field is even possible. I am neither a engineer nor a scientist, so I have a limited knowledge of Electromagnetism and of magnetic fields.

No this won't work.

Your picture shows current in wires sloping upwards from bottom right to top left but, what you appear to have forgotten are the wires behind that slope from top right to bottom left that must form lines of flux that join to the lines of flux from the wires at the front. You can't create a helical shaped magnetic field without (for example) using ferrites forming a helix shape.

The idea I have is to create a slanted magnetic field within the coil(s) which I believe should rotate the magnetized cylindrical projectile (or a cylindrical permanent magnet) as it is traveling through the coil or through multiple coils.

This is the elephant in the room because you are expecting a magnetic field inside the wire arrangement. By virtue of Gauss's law, a tube that carries current (even if the tube is in fact a bunch of wires forming a circle) forms no magnetic field inside the tube. It's a well-known phenomena.

Here's an image I made some time ago: -

enter image description here

It shows a tube of current (in and out of the page). The current is nominally 1,000 amps but, that's not important. The important thing is the field colour within the tube; it's blue and indicates that the magnetic field is zero. You can get the student edition of QuickField for free if you want to try it yourself.

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  • \$\begingroup\$ Andy aka, I am not familiar with ferrites and how they work but I will read up on them. Also, are you saying that there won't be any magnetic field generated by this wire arrangement, neither on the exterior nor in the interior of the coil, or are you saying that there will be a magnetic field but it will not be in a helical shape? \$\endgroup\$
    – user57467
    Commented May 16 at 21:01
  • \$\begingroup\$ Ferrites won't help you with a coil gun because they conduct the magnetic field away from the projectile. They can bend the field but, in doing so it doesn't work with a projectile. I'm sorry but your idea won't work with any known tech that I know of. There will be almost zero magnetic field on the inside. Research "magnetic field inside a tube carrying current". \$\endgroup\$
    – Andy aka
    Commented May 16 at 21:08
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    \$\begingroup\$ If the current is helical, there will be a magnetic field inside. The strength of the field will depend on the helix angle or pitch. A single layer solenoid is an example a helix (with a fine piutch, low helix angle). \$\endgroup\$ Commented May 17 at 11:32
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    \$\begingroup\$ Before people waste time trying to find loopholes in this answer and creative workarounds using elaborate wire arrangements, it's worth pointing out that the field OP desires is ruled out by the laws of physics. Such a helical field would have nonzero curl on the centerline (that's what makes it helical). Maxwell's equations say that the curl of the magnetic field is determined by the sum of the time derivative of the electric field and the current density at that point. The current density at the centerline is necessarily zero, and there's no time-dependent electric field in the setup either. \$\endgroup\$
    – TooTea
    Commented May 17 at 12:01
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    \$\begingroup\$ @user57467 sorry but that won't make a difference because you still have a tube of current. Also, you must not use information given in answers to initiate any form of question iteration that invalidates answers already given. Yes, it's good that your edit makes this clear but, you are treading on unfirm ground here. It's nearly always better to raise a new post with a new proposal because it opens the problem up to the whole floor. \$\endgroup\$
    – Andy aka
    Commented May 17 at 12:17
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I first want to find out from someone within the Electrical Engineering.SE community if creating a slanted magnetic field is even possible.

  1. Making a transient helical magnetic field is possible.

  2. Even though possible, it may not help you to create spin in your projectile.

  3. The mechanism you propose to create a helical magnetic field is not feasible.

A magnetic field with helical lines of force is a superposition of a field with circular lines of force with a field with straight lines of force axial to the circles.

As pointed out by TooTea, a magnetic field with circular lines of force has a non-zero curl. According to Ampere's Law of with Maxwell's correction,

$$\nabla \times \vec{B} = \mu_0\left(\vec{J} + \epsilon_0 \frac{\partial \vec{E}}{\partial t} \right)$$

\$\vec{J}\$ is the current density. Since presumably there will be no current through the bore of your coil gun, this will be zero. This leaves only option of creating a time-varying electric field. The fact that this electric field must be time varying is the reason why the circular field, and hence the helical field cannot be a steady state field.

I can think of at least three mechanisms for creating a time varying electric field which lies along the bore of the coil gun.

The first is to use a resistive solenoid. We usually want conductors to have low resistance. However, low resistance also implies low voltage drop. Since we want the voltage to drop across some length of the bore, using a resistive solenoid will create such an electric field. If the current in the solenoid is time varying, the electric field will also be time varying, and the circular component of the magnetic field will be created.

The second is to use two rings around the centerline of the bore of your coil gun. They form a capacitor. Have a circuit to charge this capacitor. While the capacitor is charging, a time-varying electric field will be created, and consequently the circular component of the helical magnetic field as well.

The third method is to use a toroidal coil (preferably without a magnetic core) such as a Rogowski coil. Feed it with a time varying current, and there will be a time varying electric field, and consequently the circular component of the helical magnetic field as well.

Now, we also need a magnetic field that is linear and axial to the bore of the rail gun. This is produced by a solenoid coil with an axis the same as the axis of the rail gun. Note that the twisted multiwire hollow core cable that is illustrated in the question is an example of a solenoid coil, although not a very efficient one. Contrary to Andy aka's answer, the center of that cable will have a magnetic field within it. However, the field will be weak because the pitch of the turns is very long, so there are few turns. However, the twisted insulated wires are NOT equivalent to the "tube" used in his magnetic field simulation. In the twisted wires case, the current does not flow only axially, but also circumferentially. In the case of the tube, the current only flows axially.

Once again, although these mechanisms can be used to create transient magnetic field with circular field lines, and consequently when combined with an axial field, to create a field with helical field lines, it is not clear how this will impart a spin to your projectile.

It is also not clear why you would need to have an axial component to your "rifling" section at all. If a helical magnetic field will impart spin, wouldn't a circular one impart spin just as well?

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  • \$\begingroup\$ Your answer is a lot for someone like me to take in since I am not an electrical engineer nor a scientist. I will have to take some time to understand what you are proposing. I was thinking that perhaps a slanted/twisted magnetic field, created by this unorthodox wired solenoid, would wrap itself around the magnetic field of the cylindrical magnetic projectile/permanent magnet, and this 'magnetic wrapper' would create a 'circular torque' on the projectile's magnetic field, resulting in a rotating projectile. \$\endgroup\$
    – user57467
    Commented May 18 at 21:16
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    \$\begingroup\$ @user57467 I think MOSFET has the right idea. I think you want a magnetic field whose lines of force are rotating around the axis, rather than one whose lines of force are pointing in a helix, but which are static. \$\endgroup\$ Commented May 18 at 22:28
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EDIT: As I seemed to understand the issue the same way as Andy, my statement that the answer is explained very well by Andy, the answer to your question is no. It's still no, the field would not be slanted - it would be coaxial to the coil - a poor solenoid. See answer from Math Keeps Me Busy.

The core of my answer is anyway not to detail the physics, but to provide links to the X of this XY problem - how to impart spin on a EM-gun projectile. And the comment I originally posted were:

But the core problem you're trying to solve have been worked on for many decades already. There are several patents to look into regarding electromagnetically imparting spin on the projectile of a rail or coil gun.

Electromagnetic projectile launcher with magnetic spin stabilization by Dan R. McAllister: patents.google.com/patent/US4449441A/en
Method and apparatus for spinning projectiles fired from a rail gun by Willaim F. Weldon: patents.google.com/patent/US5189244A/en Projectile for round bore electromagnetic launchers with spin produced or prevented by electromagnetic means by Robert M. DelVecchio and Emmanuel Aivalotis: patents.google.com/patent/US4741271A/en

Are some examples.

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  • \$\begingroup\$ Andy's explanation is incorrect. Contrary to Andy, a hollow core cable consisting of twisted insulated wires will have a magnetic field inside. It is just a solenoid with an unusually long pitch. \$\endgroup\$ Commented May 19 at 13:27
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    \$\begingroup\$ A nit. OP asked about a helical (i.e. "slanted") field, not a rotating field. A rotating field is probably what he wants though, and the OP's proposal does not provide that. \$\endgroup\$ Commented May 19 at 14:30
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    \$\begingroup\$ @MathKeepsMeBusy True. And I seem to have missed the point about the conductors being isolated - I think I agree that that approaches a solenoid, but the field would be very weak. \$\endgroup\$
    – MrGerber
    Commented May 19 at 14:40
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    \$\begingroup\$ @MathKeepsMeBusy Edited my answer to clarify. \$\endgroup\$
    – MrGerber
    Commented May 19 at 14:45
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    \$\begingroup\$ @user57467 Yes, I know your intent was to create a static helical field. But to solve your actual problem, I think you will need a rotating field per MOSFET. How exactly would a helical field interact with your projectile to create a torque? (That is a rhetorical question. Comment sections are not for extended discussion.) \$\endgroup\$ Commented May 20 at 0:16

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