Chapter 6: Coilgun Mark II

What if I made a single-stage coilgun, with capacitive discharge timing? How well can it work?

A coilgun is a great teaching device. As such, it illustrates principles of magnetism and stored energy. Rotary motors are highly refined creatures with a hundred years of development. It is difficult to understand how rotary motors work because they have several things happening at once: multiple poles and multiple windings are all doing their thing together. As the rotor turns, the contacts transfer electrical energy, and the rotary load interacts with the rotational inertia and magnetic fields that come and go. Through some massive oversight, the engineers pretty much ignore all the transients and study the motor as if it were steady-state! In a coilgun there is no such thing as steady-state. Everything that happens is a transient. I find this a very interesting puzzle to analyze.

Barry's Coilgun - Mark II

My second model, the Mark II, will launch small projectiles at a better speed with simpler construction.

It will be much easier to build, because there are no timer/counter circuits. On the other hand. it will take more fiddling to tune it up. The main goal: minimum part count.

Here's how to design the Mark II:

Hints and Suggestions
RLC Simulation, a Java applet that graphs voltage over time
RLC Analysis and what it means to coilguns
LC Time Simulation, a Java applet for choosing an inductance
Inductor Simulation, a Java applet to design an air-core coil
Wire Loop Simulation, a page to describe the math behind what will eventually become another interactive graphical applet

Here's how to build the Mark II:

If you've gotten this far, you need a way to measure the projectile speed using ballistics.

Now here's the results of my measurements and comparisons. I started at low voltage to tune the coilgun, then worked up to higher voltages and different projectiles.

How does the starting position affect performance?
How does the number of turns affect performance?
How does the projectile length affect performance?
How does the external iron affect performance?
How does the capacitor voltage affect performance?
How does the energy efficiency relate to capacitor voltage?
How does the firing tube material affect performance?

In summary, here is the Mark II coilgun:

it has the primary goal of minimum parts count
fires at 9 m/s
the projectile is 44.7 mm in length and 6.0 mmdiameter, weighing 10.03 g
the coil is 40 mm long, wound with four layers of 18 AWG copper magnet wire in 30 turns per layer
there is no external iron
the capacitor is 24,000 uF, and is charged to 55 volts for a total of 36.3 J

By the way, 9 m/s is equivalent to a flight of eleven feet out and two feet down. Not much danger... except maybe some nearby feet and ankles.