# Coilgun Tube Size

What size of tube should you choose? What size of coil? Does it even make any difference?

## Choosing Tube Material and Dimensions

Your projectile travels down a tube inside the coils. It provides an air gap, which allows motion. (In fact, an air gap exists in all electromechanical machines. It is where magnetic forces are converted into motion.)

Your tube cannot be a ferrous material such as iron, because it would guide the magnetic flux away from the projectile. Bad idea.

Your tube must be some non-ferrous material, such as aluminum or copper or bronze, or even paper or plastic. Note these materials conduct current, which is not a major problem. However, it does cause two effects:

1. Internal eddy currents will arise during periods of changing magnetism; there are no eddy currents during steady-state dc voltage. Periods of changing magnetism are extremely brief during coil switching. We neglect it entirely in this application.
2. The projectile itself is magnetized and rushing through the tube. A conductive tube will have induced eddy currents from the moving projectile, which tends to slow it down. This is (I hope!) negligible.

So a non-conductive tube such as paper or glass might be slightly preferable, but it's not that important. (Right? Send me any comments on this subject.) Whatever it is, obviously you need a smooth inside surface to minimize friction. Duh.

Now, what about tube size? A popular approximation for the B-field (magnetic flux density) in a solenoid is directly proportional to the coil current I and to the number of turns per unit length n. The formula does not depend on the coil radius. Why not? Because it assumes the coil length is significantly longer than the coil radius. If that's not true, then your B-field will be compromised by fringing effects.

Fringe effects are what happens when the magnetic flux disperses at the ends of the coil. This tricky to compute. The path of the flux lines depends heavily on the parts geometry. If you really want to know what happens, it might be easiest to just sprinkle iron filings on a flat piece of paper near the coil and energize it. Can someone recommend a computer modeling application for magnetic flux lines? I'd sure like to hear about it! I found a free student's edition called QuickField by Tera Analysis. It's fast and relatively easy to setup, and has lots of interesting colorized field plots. You could also try Finite Element Method Magnetics by David Meeker (another free download).

The bottom line is to make your coil as long or longer than its diameter. Otherwise you may notice nonlinear effects, such as your projectile sticking to the side while inside the tube.

It is essential to performance that the tube should be only as large as needed to barely accommodate your chosen projectile.

## Spin Stabilization

Also - if you have a slightly elliptical coil, and a slightly elliptical projectile, the projectile will align itself with its major (widest) axis on the coil's minor axis. If you turn each succeeding coil's minor axis a few degrees, the alignment of the projectile will put axial spin on the projectile and stabilize it during its trajectory after leaving the tube.

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