Linear Induction Launcher

[Benbmw]

Hi All,

I am curious to know if anyone is aware of a hobbyist attempt at a Linear Induction Launcher (LIL). It seems to be one of the most viable schemes for an EM launcher for a variety of reasons.

• Reluctance coilguns suffer from limitation due to projectile saturation.
• Induction coilguns overcome this by operating on a non-ferromagnetic projectile, however they will still run up against some limitations due to the fact that the propulsive force is concentrated on the breech end of the projectile. Practically, material properties will limit the max non-destructive acceleration when you get to large launchers. Furthermore, significant efficiency is left on the table by not using the whole length of the projectile.
• Railguns are plagued by many fundamental and materials science constraints. They require immense physical pressure between the rails and armature which also means that most any projectile will need a sabot. This of course also results in severe rail wear. You cant easily take advantage of multi-turn coils and must handle mega-ampere currents through your barrel and associated pulsed power system. Detailed modeling requires very complex considerations at the armature-rail interface including plasma physics.

A LIL on the other hand seems to overcome many of these issues by giving the armature a "friction free" path while acting along its entire length. It does of course have its own complexities, mainly within the energy storage/driving system. It requires a traveling magnetic field with an increasing speed/frequency along the barrel. This means that ideally you have a resonant drive inverter of some sort for each coil, or an equivalent pulsed alternator/self-excited-induction-generator of some sort. There are ways to "share" the resonant driver, but at the cost of added complexity. Many forms of simplification in hardware (ie breaking it up into multi-coil sections of constant speed/frequency) make it difficult or impossible to maintain the phase of the traveling magnetic field, which is critical . At higher energies and muzzle velocities this involves some pretty serious silicone for switching.

It is something that I would like to tackle at a hobbyist scale at some point since I cant find anyone else who has.

Would love to hear your thoughts!

-Ben

[davekni]

The only experimental results I found in the links is 250m/s and below, less than what simple single-coil launchers achieve.  Any ideas for a simple practical design?  I wonder if ferromagnetic multi-stage launching would work better for hobby-level projects in spite of the ~2T field limit (linear version of a variable-reluctance motor).

[Benbmw]

The only experimental results I found in the links is 250m/s and below, less than what simple single-coil launchers achieve.

Yes, I suppose the proof is in the pudding, however if I am remembering correctly I believe the polytechnic institute achieved ~490m/s. Some advancements and refinement in thyristor tech since those attempts might allow for better implementation.

Any ideas for a simple practical design?

I can update later with some proposed simplified schematics/block diagrams. In general, you would have a lower voltage high energy capacitor bank charged to V0, could be electrolytics, that feeds a number of Thyristor H-bridges which are driving a series resonant capacitor and inductor. In this case the inductor is one of the drive coils and the resonant capacitor is rated for 3-4x V0 with a capacitance on the order of 10s-100s uF. This allows energy to be sucked out of the main energy storage bank feeding the H-bridge drivers which ring the voltage up to a few times V0 and can produce a relatively constant amplitude sinusoid (compared to a single capacitor ring down which would only provide a few decaying half cycles).

The coils would decrease in inductance from breech to muzzle which, given a constant resonant capacitor, would increase the resonant frequency of each successive coil. This combined with a proper trigger signal to each would produce “positive” and “negative” half sine pulses traveling down the barrel at an increasing speed. These pulsed would alternate as fast as the slowest coil would allow. Of course you would only energize the coils immediately surrounding the armature at any given moment, which would allow this “pulse rate” to increase with the increasing resonant frequency of the armature tail coil.

I wonder if ferromagnetic multi-stage launching would work better for hobby-level projects in spite of the ~2T field limit (linear version of a variable-reluctance motor).

This very well could be the case. I am very interested to know, and it would be cool to build up a hobby scale test stand of sorts to experiment with this scheme. Maybe even starting with just a resonant capacitor and single Thyristor for each coil; utilizing the damped sinusoid rather than a full H-bridge driver for each.

[davekni]

Quick crude calculations indicate ferromagnetic material is not worth pursuing for anything above 100m/s or so.  I do have some simple single-stage solenoid launchers for foam darts with a bit of steel wire inside.  Made these long ago for science lessons.  Just enough velocity to get across the room.