Friday, October 4, 2013

Homemade Railgun Experiment

Yes, that's right. After gathering all the materials I built my first little railgun experiment.
Read the whole article after the break!

Let's first take a look at how a railgun works, a video about the whole topic can be found here.
Similar to a coilgun, a railgun works by magnetic propulsion.
A basic railgun consists of three parts: A set of parallel, metal rails, a huge powersource like a capacitorbank and a metal projectile. Here is a railgun's schematic for your convenience.
What happens inside a railgun? There's a huge current flowing through the rails and the projectile at the same time as shown in the illustration. Every current, or on a smaller scale, every moving charge creates a magnetic field. Take your right hand and form a fist - thumb in the current's direction - your fingers now indicate the magnetic field lines created by the current in your thumb's direction. Why is the projectile moving then?
According to Lorentz's law, every charge (red current) moving in a magnetic (blue) field experiences a force (green).
So basically the magnetic field emitted by electrons travelling on the rails propels the projectile. Before we will take on real railguns let's do a small scale experiment first.
These are two aluminum strips glued onto a piece of cardboard. Additionally I connected a 9V battery's plus to one and minus to the other rail. Our small scale projectile is a metal stick with two small, round magnets on each end. The magnets are just here to amplify the effect and can be ignored right now - the experiment also works without the magnets (proof).
Drop the projectile touching both rails and it will start accelerating towards the end of the rails.
What does this small scale experiment reveal about real railguns? First of all you can see sparks flying deteriorating the rails, just as in a real high power railgun. Secondly the projectile gets hot due to the current flowing through it and its electrical resistance - again a great similarity to the real deal.
And last but not least, the projectile has to enter the railgun with a initial velocity, because otherwise it would get welded into place not accelerating at all.

So what's the plan for a real railgun?
In order to achieve a high current through rails and projectile we need a huge, high amperage powersource: A capacitorbank. Since a railgun is basically a big short circuit, normal batteries and transformers wouldn't work well due to their inability to provide enormous high current peaks. The best solution for a low cost, homescale railgun is a capacitor bank. My capacitor bank consists of three 2200uF at 400V capacitors in parrallel:
Most capacitors already have threaded poles, which is very useful connecting many capacitors via metal sheets as you can see in the picture above. 
The rail setup consists of two metal rails sandwiched in between to PMMA acrylic sheets. Since the rails experience a lot of force during a shot, they have to be secured tightly with screws to the acrylic sheets. Here's my setup before and after screwing it together:

Now I had to decide wether to connect the capacitor directly to the rails, creating a "hot rail" railgun design, or to use some kind of super high current switch. 
I chose to stick with a "hot rail" design for two reasons: Firstly I simply did not have any switch or semiconductor that would withstand such high currents. Of course there are switches and semiconductors which would, but they were way too expensive for a small scale experiment like this. The second reason was, that the projectile needed to enter the rails moving anyways, so there was no need for a switch. The projectile itself would connect the rails acting as switch. 
So here you can see the "hot rail" design.
The next challenge was to find a mechanism which shoots in the projectile with an initial velocity. Most professional railguns use compressed air but I came up with a much simpler and cheaper method - a spring loaded lever:
  The mechanism works pretty straigth fordward. Once released the spring loaded lever pushes the projectile between the rails:
Let's take a closer look at the projectile. It is made out of metal and I chose a V-shape so it would be able to bend at least a little bit and compensate small unevenness.
The last step is charging the capacitors with the help of a little 400V DC transformer. The complete setup can be seen here:
Here is a slow motion gif of an acelleration at 400V.
This is what the rails looked like after an accelleration:

There's a video with a lot of footage firing the railgun experiment, of course:



 

20 comments:

  1. can yu pls make a schematic of your 400V dc charger and upload it?

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    1. what is there to ad you just hook up the transformer to the caps

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    2. Clearly you don't understand how transformers work. You can't simply hook a 9V battery to a 1:45 transformer and get 400V out.

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    3. No, he used an inverter circuit. Is a circuit that can turn the DC ino AC current, which can be trasformed and after there is a rectifier made of a few diodes. Just because he didn't ell anything about the inverter it doesn't mean that he didn't make it. The simplest inerter I know is made out of two identical NPN transistors and two identical resistors and a centre tapped primary transformer. http://4.bp.blogspot.com/-Os_dFBvh6fk/URcIUTS-AgI/AAAAAAAAAi4/vV-KOxCav3Y/s1600/IMG_2283.JPG

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  2. Would like to see the schematic/parts used in your charger? Thanks.

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  3. What happens if you coat the rods with something so you get the magnetic field, but not short out the circuit?

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    1. No short circuit, no current. No current, no magnetic field. So the answer to you question is, "nothing happens."

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  4. I'm curious what your final velocity is. Have you chronographed it, or done any penetration tests with an aluminum can or cardboard?

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  5. what about loading the projectile with a small solenoid that's spring loaded?
    Leave the "hot" rail, push a switch to activate the solenoid and "poof", dead rats all over the place.
    Use "C" shaped metal rails for guidance making for a smaller barrel and making sure the projectile stay on course.

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  6. Didn't understand directions, got balls stuck in the toaster again.

    In all seriousness though, great instructions. I'm currently working on integrating this design into an airsoft frame so it can be mobile, like your G36 Gauss rifle.

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  7. Hi how did you make the transformer that charged the capacitors?thx

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  8. where do you get capacitors rated at that high voltage/capacitance?

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  9. ebay has them for around 30-50 bucks a piece

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  10. Could you please give a more detailed video of how it is made and how it works

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  11. I also want to know a bit more about the wiring
    Two wires from the capacitorbank to the rails
    But what about the transformer

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  12. what kind of rails and the armature you use..?
    we are trying the same with aluminium, but they are getting melted when such a high current is allowed to flow. it creates a spark initially, but does not move forward.

    i think there is some friction acting in these.. you haven't mention about the frictional force in this. this frictional force is creating a lot of errors for us. how this frictional force should be overcome.
    please help.

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  13. Your spring trigger looks awesome, but how did you get the capacitors to fire at the exact moment that your spring did?

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    1. He didn't. Capacitors have already wired to the rails. So when spring fires, "bullet" comes in contact with those two rails, connects them. Current starts to flow, and the "bullet" accelerates.

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  14. Where can you get the capacitors? I havnt found any sites to order 400v 2200uf capacitors

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    1. http://www.ebay.com/sch/i.html?_trksid=p2050601.m570.l1313.TR11.TRC1.A0.H0.X2200uF+400V&_nkw=2200uF+400V&_sacat=0&_from=R40

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