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430J Single Stage Coil Gun

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uzzors2k

Sat May 26 2007, 08:42PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I've finally finished my first proper coil gun. Its portable, single stage and has a bank capacity of 430 joules. The switch hanging out the side will be replaced soon, so just ignore it.

Specs

Bank Voltage: 430V

Bank Capacitance: 4700 ÂµF

Bank Energy: 435 Joules

Charger: Boost Converter MK-II, 15W

Coil Inductance: 80 ÂµH

Coil resistance: 0.18 ohms

Estimated Peak Current: 1.4kA

Projectile weight: 9 grams

Projectile speed: 24 m/s (poor measurements, still unsure of speed)

Projectile Energy: 2.57 Joules

Efficiency: 0.60 %

This is the loading mechanism. It is capable of holding projectiles in place, but not so tightly that it impairs preformance. Incase it's not clear I simply used a weak bar magnet. The coil is held together by zip-ties, and a steel coil former. The external steel should help contian the magnetic field, but it might be too thin. The coil and external steel gets hot after a few shots.

And here is a soup can with a slug on top. The coil gun is capable of firing 9, 11 and 17 gram slugs. As you can see they do great damage, pushing their way through things rather than cutting. Notice how much the bottom of the can is bulged out? The can was shot from about 0.5 meters distance. Measuring projectile speed has been problem. The coil gun makes a loud clank noise when firing, which ruins my attempts at microphone speed determination. I've built a simple PIC16F84A speed trap, but between poor code and the optical gates not triggering getting a reliable speed readout has been a problem. The two times it's measured the speed it gave me 22.3 and 23.9 m/s. The 23.9 m/s was when I fired without charging the bank to full, just half or one quarter of max. I would use BjÃ¸rn's SpeedTrap, but I don't have 7-segment displays or LCDs handy. So the actual speed and efficiency are still questionable. If it is correct then I have a very inefficient coil gun, but can 2.5J really pack that much of a punch? Does anyone have a simple and clever method for measuring speed? I think I might try a ballistics pendulum, but something less elaborate would be welcome.

More info and pictures at my site.

[Edit: Fixed image size]

Firnagzen

Sun May 27 2007, 05:37AM

Registered Member #567 Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147

Hm, I have a suggestion for measuring the speed, instead of using optical gates, have the can sit on a switch so that the switch is pressed down. (I assume that the can lifts up into the air when the projectile hits, thus releasing the switch.) Then you connect the firing switch and the 'can switch' to the PIC and measure the time. Well then, you have the time and the distance, you can go from there.

Bjørn

Sun May 27 2007, 07:42AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

I made the speedtrap so that we should get rid of the metal can speed index. If you don't have displays then ELFA has some nice ones

.

Measuring the range when firing horizontally (or timing vertically) is a simple method to check your measurements. It is always a good idea to use two completely different methods to verify your measuring device to make sure you don't waste a lot of time by getting the wrong results.

When it comes to detection of the projectile I have done accurate measurements at speeds over 200 m/s using aluminium foil as switches. Two sheets of aluminium foil with a sheet of paper between them sometimes can be used for many shots before shorting out permanently; if the projectile is small.

uzzors2k

Sun May 27 2007, 06:41PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Today I used 3 different methods to measure the speed. Take a vertical shot straight up, and measure the time it takes for the projectile to hit the ground again. Use a can and a microphone, the impact on the can is loud enough to be separated from the muzzle fire. And the '84A speed-trap, with tin foil gates. (great idea, btw.) The results are:

Average Speed            24.05 m/s           29.05 m/s           27.21 m/s
Energy                    2.60 Joules         4.64 Joules         6.29 Joules
Efficiency                0.60%               1.07%               1.45%

That's more like it. The heaviest and most efficient projectile is also the longest, approximately just as long as the coil. Tomorrow I'll see how much of the capacitor bank is really needed to achieve the same speed, and hopefully I can tweak the efficiency a little more.

UPDATE: I measured the speed relative to bank voltage, and found that the greatest efficiency was 2.65% at 200V. This was with the 11 gram projectile, and I have reason to believe the efficiency with the 17 gram should be over 3%. But how can I transfer that efficiency to full power at 450V? I thought the greater efficiency was due to the projectile moving slower and therefor absorbing more of the magnetic field, which should last for 2.9 ms. Following that logic I thought I could then cut my bank in half for a 1.7 ms pulse and get a higher efficiency but the same power at the full 430V. However the efficiency is only 1.4% at full power, which is not enough to weigh up for the loss in power. Given that max efficiency was at 200V, what can I do to get higher efficiency at 430V? Specs for simulation are 80ÂµH coil, 4700ÂµF bank and 0.2 ohms resistance. Any tips would be appreciated!

Update2: I think the projectile is saturating, and/or just getting too much power. I didn't use that many turns in the coil, a random amount actually. Does this sound plausible? If I get some more wire, a new barrel, bigger projectile, then I'll upgrade the gun and see if it works better.

Energy

Fri Jul 06 2007, 05:57PM

Registered Member #868 Joined: Fri Jun 29 2007, 04:48PM
Location:
Posts: 2

interesting, since i'm planning to build one with a 3300uF bank @ 500 V. This is about the same amount of energy also the voltage comes very close to yours.

so i am interested if you have an updates, planning to launch my project next week wen i am free of school.

uzzors2k

Fri Jul 06 2007, 10:12PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Sorry, no new updates. I think my next coil gun project will be a multi-stage type, much more portable, and with the same velocity and much greater efficiency. But I won't start on it for a long time since I have so many other projects on the go, so this project is closed. Since your bank specifications are similar to mine it will be interesting to see if you can make your gun more efficient. If I had the time I would try using thicker wire and more turns. Just a tip.

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