13Joule Single Stage Coil Gun Learning Project (With Video!)
Coil Gun 1A (Click images to view full)

Introduction and aims.

Since this is my first coil gun, and also my first project to do with electromagnetic propulsion, the aim of this project is to construct a simple and working single stage linear electromagnetic mass accelerator to better understand the workings of this type of propulsion systems and to be a basis for my further higher powered projects.  This will be used to research and demonstrate the principles of magnetic acceleration, and the accelerator is to be kept as a test bay for different coil/projectile combinations. The use of a single stage keeps design parameters simpler and leaves more room for experimentation. Furthermore, this is my first attempt at building one and therefore, the power would be kept low. This coil gun would be built from a used disposable camera. Cheap and easy! A high voltage sign is printed to warn others from receiving a nasty shock!

Power Source

Being my first project, I shall start with cheap and simple stuff. Power source is 2 electrolytic capacitors fitted into a used disposable camera casing (which will house the electronics as well). Both capacitors are rated for 330VDC and store 120uF charge. This amounts to 6.534 Joules each or around 13 Joules in total. The capacitors are connected by wires fitted within the casing. Although total power is quite small, it will work. Furthermore, beginners should always start with less dangerous levels of high voltages and current. Starting small is good. However, it rarely charges to 330V, and almost always stops at 300V (i believe it is due to the lousy charger) and therefore 300^2 x 0.5 x 0.00012 = 5.4 Joules x 2 = about 11 Joules total energy.

Switching

This coil gun utilizes a spark gap as a switch. Noisy, loud, bright and scary, it works. Following projects would employ a solid state switch which would make the coil gun totally quiet. For now, the low ratings make this a feasible solution. In the photo, you can see the two red wires sticking out. Connecting them together makes the current flow through the coil. Switching is just connecting the two red wires together. Sometimes, the spark heat melts the two wires together and they have to be cut. Much energy is lost however, through this switching devices. Gives some people a nice scare.

Charging

Since used disposable cameras already come with their own charging system, I used the one which comes with it. However, since I added another capacitor, charging takes around twice as long and makes a nice high-pitched whine. It works :)

Coil form, Barrel and Projectile

Here both the projectile and the plastic pen tube around which the coil will be wound are shown. The projectile is a 2mm thick, 25mm long piece of metal. It fits just nicely in the pen tube (barrel). The coil form is a 3mm inner diameter pen tube with 2mm thick walls. Such thick walls make the efficiency low however, but it was the best barrel I could get with such limited resources. It is 13.5cm long. I used 0.6mm magnet copper wire. The coil is 4cm long, 5 layers and has an estimated 200 turns. You can see a comparison in the photo.

Measuring Projectile speed

I used two methods of calculating projectile speed, the distance/height method and the acoustic method.

Distance Method

I set up the gun and had a white backdrop with a ruler placed there too. I took a picture (with flash) of the projectile in flight. I then measured the distance it had travelled and the height it had dropped with the ruler in the picture, and calculated the speed.

The projectile travelled a distance of 143mm and had fallen 11mm. Using the formula Speed = d * SQRT(g / 2h) , where d is horizontal distance in feet (or meters) ; h is vertical distance in feet (or meters) ; and SQRT is the square root function. Taking gravity to be 9.8m/s², 0.143 * SqRT(9.8/2x0.011) = Speed = 3.018m/s

Acoustic Method
I placed a microphone by the coil gun setup. Using the sound card and microphone on my computer to record a shot. Then I edited the recording and then used the time scale on the audio software to measure the time between firing and impact. I did this two times and here are the results.

Test 1: Fired at 03.15216 sec, Hit at 03.23216 sec, 0.08 seconds for 25cm, (1/0.08)x0.25 = 3.125m/s
Test 2. Fired at 04.16375 sec, Hit at 04.23815 sec, 0.0744 seconds for 25cm, (1/0.0744)x0.25 = 3.360m/s

The average speed would be closer to acoustic test 2 for in the distance test and acoustic test 1, the capacitors were not fully charged.

Click HERE to watch a 492kb .wmv video (Windows Media Player required to play) of the assembled prototype firing at full power. (or you can right click, save target as). The video showcases two different shots on targets. A lot of work still needs to be done as far as efficiency goes, but at least it works and the goals have been achieved. In conclusion, this is a rather successful project. Enjoy the video!