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finally getting started on rail gun project.. need some advice

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zephir

Tue Mar 30 2010, 01:18AM

Registered Member #1471 Joined: Tue May 06 2008, 03:17AM
Location: Irvine, California
Posts: 13

I've been collecting stuff for a rail gun for quite some time now, now its comes time to put it together and make something happen. Would this stuff work-

I collected 24 electrolytic caps each 400 volt DC and 3900uF

I found some old commercial-use microwave that I plan on removing the transformer from and using that to charge the caps. The microwave transformer will output around 2300 volts AC. I need some way to make that DC, but don't know of any cheap method that handles both high voltage and current (1-2 amps). And way to know the actual output voltage of the transformer, the 2300 volts is from the microwave capacitor's rating.

I figure 8- 400v caps in series and then parallel them. Should be something like 2400 volts and 1950uF maximum. Or around 5.6 kJ bank. (Joules = 0.5*F*volts^2)

I heard that rail guns unlike coil guns do not need a SCR because until the projectile meets the rails the circuit isn't complete and nothing will happen. Is this true and does anyone also do this?

I haven't figured out what the rails would be made of though, but I've read that some people used grooved rails, so I've been thinking of using two copper slabs, add a slight groove in the middle for rails.

Insulating material would be some Teflon and loads of cheap acrylic and polycarbonate (lexan and others). How much do I need to hold the rails together?

Injection I have some problems with, about what is the minimum speed needed for the projectile to be injected at anyway? Air guns (.177 and a .22) and pneumatic (spud gun) hasn't been able to shoot my projectiles much faster than 300-400 fps. Some time ago someone brought up the use of gun powder for injection, would that work instead?

And then projectiles I plan to use rifle or pistol bullets as projectiles. They're cheap, plentiful and very uniform. But what caliber is best, 7.62mm, 9mm, 11mm (.45), or 12.7mm (.50) and which works better, short or longer bullets?

What do you guys think?
Also, if I had a multistage setup, say instead of one set of rails, how about 2? splitting the cap bank in half, one half at 2400v 975uF and second at the same? would this be more efficient?

Turkey9

Tue Mar 30 2010, 06:51AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Don't use gunpowder as an injector, the residue will mess with the contact between your projectile and the rails.

If you're using bullets as the projectile, then you should groove your rails to get good contact. Also, I'd go with the lightest projectile that is also long for good contact and higher speeds.

As for the microwave, it contains all you need to create DC from your transformer. There is a high power high voltage diode right at the cap. You can find the voltage by using a meter... but you'll need a divider to allow normal DMM to read the voltage.

Really go wild with the plexi if you're going to use it, with that much energy the thing is going to take a beating. Use as many bolts as you can and the most material that is feasible (for strength).

I don't know for sure, but I don't think that electrolytics will like being seriesed to that extent. I'd at most go with 2 in series.

You might want to experiment with lower energies first, just you get to know how railguns work, and then go up to the level you're planning on.

rp181

Wed Mar 31 2010, 12:49AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

That is a fair amount of energy, That is between my V1 and V2 railguns. You can see mine here:

I've been collecting stuff for a rail gun for quite some time now, now its comes time to put it together and make something happen. Would this stuff work-

I collected 24 electrolytic caps each 400 volt DC and 3900uF

I found some old commercial-use microwave that I plan on removing the transformer from and using that to charge the caps. The microwave transformer will output around 2300 volts AC. I need some way to make that DC, but don't know of any cheap method that handles both high voltage and current (1-2 amps). And way to know the actual output voltage of the transformer, the 2300 volts is from the microwave capacitor's rating.

1)I would not use a MOT for the charger. Its heavy, High voltage, and just a waste in every aspect. I would go with a voltage tripler, that is what I used:
4261128896 6f46cdf990

That includes the capacitor bank, the 3 capacitors and diodes on the bottom make up the charger. R5-R8 are current limiters, each is 50ohm 225W (I can pick some up for you at a local surplus store, they sell them for around $11 each I think). All of the capacitors are the 400v 3900uf type, so their is no trouble there (note, there is an error in the schematic, half of the bank is not connected to the rail).

I figure 8- 400v caps in series and then parallel them. Should be something like 2400 volts and 1950uF maximum. Or around 5.6 kJ bank. (Joules = 0.5*F*volts^2)

2)In railgun's, It Is a BAD idea to series capacitors, parallel all of them. Series causes the ESR to go up, and reduces the yield. If you look up Badastronout on this forum, he has some very good explanations. You calculations are wrong, it is 7488 joules. 5.6kJ is strangely the power of my first version (18 capacitors).

I heard that rail guns unlike coil guns do not need a SCR because until the projectile meets the rails the circuit isn't complete and nothing will happen. Is this true and does anyone also do this?

3)You do not need SCR's, but you need a way to push in the projectile into the rails. People most commonly, including me, use a pneumatic injector. Look up "spudgun" to find more info about these, take a look at spudfiles.com.

I haven't figured out what the rails would be made of though, but I've read that some people used grooved rails, so I've been thinking of using two copper slabs, add a slight groove in the middle for rails.

4)This doesn't make sense. You say two slabs of copper for the enclosure? Take a look at my drawings:http://rp181.fortscribe.com/?p=237
I used garolite, but that is rather expensive and hard to machine. With polycarbonate, I would say minimum 1/2" thickness, 3/4" in high stress areas.

Injection I have some problems with, about what is the minimum speed needed for the projectile to be injected at anyway? Air guns (.177 and a .22) and pneumatic (spud gun) hasn't been able to shoot my projectiles much faster than 300-400 fps. Some time ago someone brought up the use of gun powder for injection, would that work instead?

And then projectiles I plan to use rifle or pistol bullets as projectiles. They're cheap, plentiful and very uniform. But what caliber is best, 7.62mm, 9mm, 11mm (.45), or 12.7mm (.50) and which works better, short or longer bullets?

5)As said, gunpowder is a bad idea. There is not minimum for injection speed. Go with a run of the mill pneumatic spud gun. Square projectiles are easier to work with in railguns, curving your rails will be hard. I would go with a .25" square bore, aluminum projectile. I used .5", but I wish I had used .25".

Also, if I had a multistage setup, say instead of one set of rails, how about 2? splitting the cap bank in half, one half at 2400v 975uF and second at the same? would this be more efficient?

6)Some one tried a 4 stage setup with ~8kJ, but Badastronaut (He has a huge understanding of railgun theory, wrote a paper but I can't seem to find it) said their was no real justification he could see for multistage. Go with single stage for now.

I have a ton of pictures here:

, and my website has a lot too. Ask if you have any specific questions.

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