|With the energy considerations being what they are, every
bit of efficiency in the production of driving flux and volumetric
efficiency in a coil driver will be important. This set of stuff is just
some observations on how one might do a better job on winding a
coilgun's coils and arranging those coils for best efficiency.
One problem has shown up in all the coils I've seen pictures of. The inner coil wire is just pulled up at one end of the coil. It doesn't have to be that way. You can wind the inner wire up to the outside of the coil in a flat pancake spiral at one end of the coil, then proceed to wind the rest of the coil in the normal manner. This technique gets both coil wires to be accessible at the outside of the coil, and produces a much more mechanically sturdy coil.
Obviously, for coils you are doing experimentally, you would not want to do this, but for "production" coils, you would impregnate the coil with some kind of goop to make it mechanically solid. A good choice might be epoxy, perhaps even epoxy impregnated with zinc oxide or aluminum powder to help get heat out faster. Air cored coils are not the best for cooling.
Getting iron into the flux path is a huge improvement in getting to higher flux densities. There are some ways to do this with fairly standard hardware. Just putting large diameter iron washers between coils helps a lot, but it's not perfect. The iron is a conductor, and you get eddy current losses in the washer. A better way is to use the same washers, but split them across a diameter. With this done, you paint them with shellac or varnish, and then stack them with every other one rotated 90 degrees. Two or more washers like this can be stuck together and they will hold one another together mechanically while preserving the split to cut eddy currents. Cutting into quarters, eighths or more would be better from an eddy current standpoint, but it gets very difficult to do mechanically. One or two cuts is probably the best compromise for hackers.
Another thing to worry about with iron is how you get the flux around the outside of the coil. An iron pipe either between washers or as a cylindrical shell does this well. However, you then have to figure out where the external wires go. If you file or cut some reliefs in the iron washers, you can make wiring channels. Keep the coil radius less than the bottom of the reliefs and this will give you an easier way to wire.
Another thing that the slotted washers is useful for is position sensing. You can obviously get a beam of light down one side of the slot in a washer. Slit metal barrels are already useful for preventing eddy current losses, so if you align two slits on a barrel diameter, you can use optical sensing. However, LED/phototransistor sensing is subject to interference from all the electromagnetic hell that's going on when you fire one of these things. Better to run two optical fibers, one an emitter and one a sensor. The fibers can get the send/receive electronics for position sensing out to an electrically quieter place. Stacking three split washers at angles to each other gives a protected inner channel for the optical fiber. For mechanical sturdiness reasons, you might want to pour the slot of a split washer full of epoxy or other goop with a fiber already in the slot to make a special sensor washer, or a three-washer flux-and-sensor assembly. The optical fiber can exit the coil assembly through the wiring reliefs.
I can hear the moaning now - "How could I ever cut those reliefs into washers with only hand tools?" It's not that hard. Get a stack of your selected washer blanks. Run a length of threaded rod through them to align them and tighten nuts down on the rod to keep them in place. Then clamp the stack into a vise along the axis of the stack. Your stack of washers will be perhaps two or three inches thick. Use a hacksaw to cut multiple kerfs that approximate the reliefs, then smooth the relief with a file. Yeah, it's a lot of work, but it *can* be done, and you can get pro results this way. Your choice.
Here's some idea of how the whole beast might be assembled. The coils and inter-coil flux washers are stacked and aligned on a slotted metal barrel. This assembly is set into the inside of an iron pipe cut lengthwise. You may or may not need a full shell; it makes for a nice, sturdy package, though. It will probably take some prospecting to come up with large diameter washers that just match the inner diameter of an off-the-shelf iron pipe, but it can be done.
I have some offbeat ideas about the relative dimensions of the coil and projectile. Below is a diagram of a typical coil and projectile, and a graph of the force on the projectile as the center of the projectile moves to and past the coil. I've only shown one side of a cutaway coil. As mentioned and calculated at several other web sites, force rises slowly at first, then more rapidly as the center of the projectile approaches the center of the coil. It then reverses very quickly to try to hold the projectile in the middle of the coil. This is the position where the reluctance of the whole coil/projectile setup is minimized, and the M-field will try to hold the projectile there. The negative force on the projectile (also called suckback in some places) causes some fancy timing problems. You have to keep the coil current on till very close to the middle, then turn it off FAST to keep from eating up your hard-won velocity.
What the coil's field "wants" is to have the central bore filled with iron, and it does the best it can, getting the projectile in the center.
Look what happens to that if you have a very narrow coil compared to the projectile length. Now the center of the coil fills up with iron when the leading edge of the projectile reaches the far edge of the coil. There is very little change in reluctance as the projectile moves from first-fill to centered in the coil. The force goes negative, but not all that much, as the projectile passes center point, and only gets a lot of suck-back as the trailing edge of the projectile starts to un-fill the center of the coil.
What has happened is that you now have a lot more time as the projectile is passing through the coil to turn the coil current off without shaving off velocity. Turn off time is now much less critical. Instead of a very tiny time, you have a significant fraction of the projectile's transit time to turn off the coil current.
There are some advantages to winding lots of narrow, high coils. First, if you activate several of them together, you can approximate longer coils - the flux distribution is the same as the flux between coils cancels. You still get the less critical turn off times. Another real advantage is economic and engineering practicality - each coil is fired from a much smaller energy reservoir, so both the current switches and the individual firing capacitors are much smaller. Many small caps and switches are going to be much cheaper than a few monster caps and switches. Finally, there is easier control. With a fiber optic sensor between each narrow coil, sensing the position of the projectile is finer. You get a signal when it crosses every inter-coil boundary.
Using that information is fairly simple. To run a gun like this, you need a coil turn on signal and a turn off signal. Since you get an optical signal from both the leading and trailing edge of the projectile, you can make the coil current drivers turn on and off with signals from a multiplicity of set/reset flipflops. The flipflop turning a coil ON is set when the projectile leading edge is one or two coils earlier in the array. The flipflop is turned OFF when the projectile leading edge is one or two coils later in the array. It's self timing, and as long as the inductive time constants and driver circuits can actually manipulate the coil currents fast enough, it's very high performance - the projectile itself tells you when it needs coils turned on and off. The logic chips are very cheap.
There's more to do with recovering the coil current back into the reservoir without losing so much of the energy in the coil, but I'm tired of typing.
Think about it....