3-Stage Coil Gun - (Quest for 200m/s)

aarpcard, Fri Apr 10 2015, 02:02AM

Hey guys,

I've been a member of this forum for a while, but rarely post. The few times I've posted asking questions, you all have been extremely helpful. I just want to share my current project on here.

I've been building coil guns for 6 years now - each new design has evolved out of it's predecessor. Currently I'm on my 4th design. I've made a lot of progress, but I'm a looong way from finished. Ultimately my goals are as follows:

-200m/s projectile velocity.
-Projectile flight stabilization
-Some kind of portability (will probably require a backpack with an umbilical cord to the gun itself.)
-Failsafe, fully automated controls, and easy to understand controls.

Currently I have the first two stages of the gun built and firing. The second stage is not yet tuned, nor is the timing correct on the trigger. With just the first stage, I've achieved 30m/s. This is about 2.5% efficiency. I'm hoping to get to about 50+m/s with the second stage fully completed. I haven't wound the third stage yet.

Here's the business end of the gun. The first two coils are wound with 10awg magnet wire. The firing tube itself is 1/16th inch thick aluminum. The coils are bound on each side by 1/8th inch thick steel washers. The third coil isn't wound yet. You can see the optocouplers in between the coils.

The assembly on the back of the firing tube is a AC induction motor and a solenoid that act to spin up the projectile to ~50,000rpm prior to firing. This should mimic rifling and hopefully stabilize the projectile in flight.


Below is the reverse side of the firing assembly. I'm using KP100A SCR's for the switching. They're crap Chinese SCR's but at least they're cheap and work well when they're not DOA.


Here's a top down view of the whole system. The capacitors are rated at 520V and 1800uF a piece. Three are for the first coil, two for the second, and one for the third. In the bottom left you can see some of the logic circuitry. There the optical board, a temporary switch board, and an arduino with some glue logic for sensing the projectile velocity.



Below the assembly, are the dampening resistors. Each is 100mohms with taps every 10mohms. I haven't made the third resistor for the final coil yet. Each is built out of 16awg magnet wire.


Here's the main transformer providing the charging voltage. It's a 4kw PLC transformer wired in reverse. 120V primary, 600V secondary. In it's current configuration it can charge the bank in about 6 seconds.


In need to current limit the primary side of the transformer so it doesn't blow the breaker. I have 4, 200watt light bulbs in parallel in line with the primary on the transformer. You can also see the 100W, 5kohm wirewound dump resistor I have used for discharging the cap banks when I don't want to fire the gun.


Here's all the HV circuitry. The caps are are charged together and then a relay switches, separating them into banks of 3, 2 and 1 caps. Everything is controlled by relays and will eventually be automated by logic.


Below is the optical board. It takes the readings from the optocouplers and sends the firing signals to the second and third SCRs. I have delays built in for fine tuning the timing of each consecutive coils. Additionally, there is hardware hysteresis which requires that the optocoupler stay triggered for a tunable period of time before the board accepts it actually was triggered. This is to avoid accidental triggers from dust or other noise.

This is one of 4 logic boards that will be in the final gun.


Here's a close up of the projectile. It's machined from solid steel. It is 40mm long and weighs 41grams. The notches at the rear interface with a coupling on the rifling motor.


Right now I'm working on tuning the second coil. I'm also in the process of building a voltage monitor board which will read the voltage on the capacitor bank, output it to a display, and also control the charging of the capacitors based on the current back voltage. When the gun goes into firing mode, this board will isolate the capacitors from all other circuitry. Ultimately everything will be automated. There will be 3 buttons: Power, a dump switch for the cap bank, and the trigger.

Once I have all of the subsystems functioning correctly, I'm going to work on stepping up the voltage to ~1000V from 520V. Hopefully this will result in a doubling of my projectile velocity. In order to do this though, I'm going to have to make custom charging relays.

I'm also planning on building a DC-DC converter to power the system off of a car battery so I don't need to be tethered to the wall.

Once everything is built and working, I'll start working on making the design portable. It will most likely consist of a backpack housing the capacitors and other circuitry with a tether going to the firing tube itself in gun form.