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My Railgun project

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Navrit Bal

Sun Oct 17 2010, 04:27PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

Hi everyone, I'm building a railgun at school now, the following are my components:

30F 15V (therefore 450C) PowerAcousitk capacitor (designed for car audio systems)
2 aluminium rails - 500mm long and 6mm height and width.
Current projectile is aluminium also - 6mm width, height and length.
The rail separation is 6mm.
I measured the resistance of the rails and projectile at 1.0 Ohms

Can anyone help with finding the force acting on the projectile when it is fired? (assuming no friction for this)

Steve Conner

Sun Oct 17 2010, 04:32PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Railguns aren't my specialist subject.

But if you think a railgun will work with car audio capacitors, and you think 1.0 ohms really is the resistance of your rails, I think you're probably beyond any help.

jaysun92

Sun Oct 17 2010, 04:36PM

Registered Member #2114 Joined: Fri May 08 2009, 11:58AM
Location: London, Ontario
Posts: 36

I'm no expert on railguns, but that capacitor wont work. it has too high of a capacitance, what you want is a high voltage and low capacitance.

hboy007

Sun Oct 17 2010, 04:41PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 373

N-Bal no need to ask people via pm. Shorting your meter will give you non-zero resistance reading anyway, so maybe you run some 10-20A across the setup and measure the voltage drop, that will give you more reliable readings.

30F at low voltages are no fun to work with. By applying Ohm's law and stuffing in the voltage in relation to the capacity (U(t) = Q(t)/C) you will find a differential equation that describes the exponential decay of voltage and current curves. It would be no great surprise to find that the decay times even with tempered bus bars (resisdance increases when bending copper bars because defects in the crystal structure are introduced) are prohibitive.

Railguns are usually run at several houndred volts. The same applies to photographic flash units.

badastronaut

Sun Oct 17 2010, 06:07PM

Registered Member #222 Joined: Mon Feb 20 2006, 05:49PM
Location:
Posts: 96

You can probably make it work by shortening the rails to 1cm or less and reducing the projectile size by half. Also, you probably need to add neo permanent magnets. Use the right hand rule to determine polarity.

GhostNull

Mon Oct 18 2010, 06:59AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

It won't work. You need a different capacitor. Railguns require a very high current pulse to work. A car audio super capacitor will not work. The voltage of your capacitor is too low and it most likely has a high ESR (internal resistances). Due to this you will not be able to achieve the required current for a working rail gun. What you need are some "Computer grade" capacitors. These store high amounts of energy at high voltage and have a low ESR, making them ideal for this purpose

Calculating force:

I can't verify if this is correct though. Not an expert myself either =S
Also take a look at Lorentz Force:

this is the driving prinicple of railguns

badastronaut

Mon Oct 18 2010, 11:58PM

Registered Member #222 Joined: Mon Feb 20 2006, 05:49PM
Location:
Posts: 96

You can make railguns work with currents as low as a few amps by using magnets and a rolling non-magnetic projectile.

Navrit Bal

Thu Oct 21 2010, 11:18PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

Its nice to see so many responses this quickly, but really disappointing about my capacitor being just wrong for this application.
Am I right in guessing that coilguns are also out of the question? - I would also like to make one by the way.

Coincidently, I have 2 N42 grade NdFeB magnets

which I am not using for anything.

Could you explain the 'rolling non-magnetic projectile' idea a bit more?

I believe it has 0.00015 E.S.R. (definitely advertised as low E.S.R.)

By the way, I'm not entirely sure about the resistance of my rails, I'm actually quite certain the readings are wrong!

Thanks for the responses nevertheless.

badastronaut

Thu Oct 21 2010, 11:26PM

Registered Member #222 Joined: Mon Feb 20 2006, 05:49PM
Location:
Posts: 96

Just have two rails with a magnet in the middle. It doesn't have to be anything fancy. An aluminum rod is placed on top of the rails so it can freely roll around. Then connect the rails to a battery, and the rod should roll up or down the rails, but only for the length of the magnet. You can also use a copper rod, and they can be hollow. Steel will not work since it will be attracted to the magnet and the friction and other forces will not allow the rod to roll.

This type of railgun is very safe and you can even buy premade demo kits.

Navrit Bal

Thu Oct 21 2010, 11:32PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

@badastronaut
Thanks for the suggestion, although I will only want to use that as a last resort because I am still trying to get some kind of 'mass launcher' instead of what you are suggesting...

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