Ring launcher to coil gun conversion

[DrewScott]

I have a 0.7kJ ring launcher that will launch a copper ring hard enough to put a dent in the ceiling. It uses a pancake work coil that has about 8 turns. I wound a tube type coil with many more turns (I haven't counted but probably close to 100) since the two devices work on different principles. The ring launcher induces a current and magnetic field in the copper ring which repels the magnetic field of the coil. For the coil gun, the projectile has to be magnetized and pulled toward the center of the coil, and has a greater mass. My thinking was that I would need more turns on the work coil for the coil gun, but it will hardly move the projectile. Do I just have way too many turns and too high impedance on the tube coil? Is that the problem? The ring launcher is very powerful. I would like to make the coil gun just as stout.

[davekni]

MUCH more information would be necessary to make any useful comment.  You could copy a successful design.  Or you could take the longer and more interesting and educational route of designing your own.  It will require experiments and ideally simulation and modeling - at least some crude calculations.

At high accelerations, conductive (ie. copper or aluminum) projectiles such as your ring are more efficient than soft magnetic projectiles such as steel.  Steel saturates a bit under 2Teslas (20000 Gauss).  For short pulses, copper can repel much higher fields.  On the other hand, at lower accelerations, steel can be more efficient.

[klugesmith]

Good that you're trying to be engineer-ish about it, Drew.  There's a wealth of information about coilguns at this forum and others.  Just confirmed that https://www.coilgun.info still exists.

Remember that ring launchers and coil guns are not just about throwing electrical joules at the system.  I made that point to young Adam Munich about 10 years ago on 4hv, after fabricating some copper wedges for him to try in a contactor.  Design efficiency is sensitive to the length of current pulse, compared to the time scale with which coil-projectile geometry changes.  That makes it a nonlinear system, so efficiency with fully charged capacitor can be very different (in either direction) from that with partly charged capacitor.

For a given work coil size and mass, you can control the RLC discharge timing over a wide range
by choice of wire diameter -> number of turns to fill the bobbin -> coil inductance -> discharge time.
There's no general rule about more or less turns working better.  (Just as optimum grain size for gunpowder depends on the chamber, projectile, and gun barrel.)

A turns-optimized bigger & heavier coil can often do better than a turns-optimized smaller & lighter coil.  Less resistance for any given inductance, but that can eventually lead to underdamped discharges.

[DrewScott]

Thank you all for the info! Sounds like I have a lot more to investigate.