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Mid size railgun caps

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Mid size railgun caps

Unread postAuthor: inonickname » Sat Mar 14, 2009 9:00 am

Well I'm building a mid scale railgun (approx 60 cm, garolite and oxygen free copper construction, with at least 300 psi injection). 1/4 (square) bore.

After research upon powering them, I've come to the conclusion that 300-500 volts is best, and as many microfarads as possible. (capacitors are my only option from here)

The problem is, the parents aren't happy with me playing with large cap's, so unfortunately these are around my limit..

What I'm basically asking is if there will be implementations with using this type of capacitor, and will wiring (below 10) in parallel give me respectable amounts of power?

Keep in mind this is not full scale, only a medium size at 1/4" bore. I don't want outrageous velocities or massive amounts of power, just a visually impressive display with average power.

Thanks guys :wink:
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Unread postAuthor: Fnord » Sat Mar 14, 2009 9:49 am

You'd be better off making a coil gun or electrothermal gun with such low powers.

400V @ 470uf is 37.6 joules, so it'll take a lot to make a good bank. And ~$10 each is too expensive to buy in bulk.

When I bought caps from ebay I got around 4kj worth of caps for $50, shipping included. The equivalent bank made from these would cost over $900.

They are a little smaller than a soda can, but buying big caps is probably your only option if you don't want to spend a fortune.

If you still want to make a rail gun with less than 400J, look up "world's smallest railgun" on youtube and scale it up a little :P
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Unread postAuthor: inonickname » Sat Mar 14, 2009 7:16 pm

That's what I was thinking of- I'll have a look around ebay. Hopefully I can talk the parents into at least 5kj.

I researched the materials on mcmaster, and it would work out well below $50 but I think the postage would kill me, so I'm gonna have to look locally.

Thanks fnord :wink:

Edit: an hour of ebay brought up nothing. If anyone knows of cheap, good suppliers of caps it would be appreciated.

Looking at around 5kj worth.
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Unread postAuthor: rp181 » Sat Mar 14, 2009 10:33 pm

Look at these:
http://www.alltronics.com/cgi-bin/item/ ... -(Pkg-of-4)
I used 18 of these, all in parallel.
4 pack, each 312 joules for 25$. These are the ones i used in mine. Garolite is a waste of money and pointless for that low power (even the power i used). Polycarbonate would be cool since its clear.

If you are doing ~5kJ, I suggest starting with a .5" bore. It is much easier to work with. I plan on upgrading to a .25" bore, to increase velocity and lower copper costs.

If you still interested in a garolite bore, I have a extra .5" bore enclosure. It was machined close tolerance (.001", as good as you can get for laminates). It is a two piece design, 2 feet long (1 foot is injection). Cross section is 1.5" x 2". Pm me if interested.

EDIT: realized you where in AU. The ones i posted will still probably be cheaper. Add them to your cart, you can estimate shipping charges.
1/4 bore is actually quite large. I would use a 1/4" bore with a 30kJ system. Smaller to a limit) is better, it makes it difficult to machine though. I chose a larger projectile so i could test different things easily, as well as easier measurement of projectile velocity.

@Fnord I disagree with the value. You have no idea of there remaining life, or misuse, etc. Have you actually checked the capacitance?

@all:
When ever you buy large capacitors intended for pulse that are not new, You should reform them. You slowly add charge to the bank to full voltage over a long time (24 hours minimum). This reconstructs the oxide layer, and improves life (It does lower capacitance a little, its worth it though. It reduces leakage current). If the layer is fine, nothing will happen.
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Unread postAuthor: inonickname » Sun Mar 15, 2009 12:09 am

Thanks Rp, I'll find an Australian supplier for those and get around 16-20.

Garolite for this project would only total $15-20 from Mcmaster, but the insane shipping would mean that it wasn't worth it. We in fact have very thick sheets of polycarbonate, at least 10mm.

The bore will be dependent on the sizes of copper I can get- it could even end up smaller. It all depends on the sizes that are available.

I laughed when I saw the postage costs. For $100 worth in caps it's $233 in postage :oops: going to have to go local.


It should turn out that I will have access to a lathe to construct the injector (a sear held piston ie. basically 0 opening time.) and a mill for help with rail construction.

Already have enough stainless steel bolts and most of the injector materials for this. Construction shall begin on parts while I'm waiting to get caps.


Thanks Rp, that's very very helpful.


For once, my father actually had a partially good idea. He suggested that a small scaled railgun could be powered using an ignition coil from a car, charged by caps. Any input to that?
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Unread postAuthor: rp181 » Sun Mar 15, 2009 11:03 am

what grade garolite? G-7,G-10,and G-11 (what i used) are best. For mine, garolite would have cost past 200$.
What do you mean using a ignition coil? Do you mean pulsing the bank into the primary, and the secondary connects to the rails? If so, bad idea. The voltage will just cause arcing, and reduction in current, which is what you want. Don't spend too much time on the injector, you want to have the railgun do the majority of the accelration, or that is pointless. I just went with a burst disc, at 60PSI.
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Unread postAuthor: inonickname » Sun Mar 15, 2009 8:05 pm

rp181 wrote:what grade garolite? G-7,G-10,and G-11 (what i used) are best. For mine, garolite would have cost past 200$.
What do you mean using a ignition coil? Do you mean pulsing the bank into the primary, and the secondary connects to the rails? If so, bad idea. The voltage will just cause arcing, and reduction in current, which is what you want. Don't spend too much time on the injector, you want to have the railgun do the majority of the accelration, or that is pointless. I just went with a burst disc, at 60PSI.


I was talking G-9, but the sizes I was looking at probably were incorrect.

Given the low power of only around 5kj, the more injection power I can get the better. It won't have insane amounts of money spent on it though.
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Unread postAuthor: rp181 » Sun Mar 15, 2009 8:33 pm

5kJ is low power, but not that low power. Garolite is overpowered. Look at GP03 fiberglass from mcmaster. For me, it would have been 26$ for the required material, assuming no mess ups.
The "bigger the injection, the better" is wrong. You want the railgun to do the, well, gun part. Injectors suck period in railguns. As soon as the projectile enters the rails,the current rises quickly. Before the projectile is in the rails all the way, the current is already at a really high value. To get the projectile in enough, a extremely high speed (3-6km/s) is needed, where 5kJ is pointless.
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Unread postAuthor: inonickname » Sun Mar 15, 2009 9:53 pm

Well a 300 psi injection would require a fridge compressor setup which we all know is irritatingly slow (low flow) but at around 100-120 I could use the larger compressor and more readily available, off the shelf parts.

My uncle (who's a carpenter by trade) got us some leftover fiberglass sticks left over. There too thick to be workable, but I'm sure I can find uses for it..

Edit: If I don't have access to a mill, is there a better way of doing it than sandwiching the rails between fiberglass, then bolting together? Ideally the bolts wouldn't have current in them, but there isn't much of an alternative otherwise.

Edit:

Delrin?
http://www.mcmaster.com/#8662k21/=10r56y
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Unread postAuthor: Fnord » Mon Mar 16, 2009 10:29 am

The "bigger the injection, the better" is wrong.

But unless you have a huge switch or scr, an injector is still your best(only?) option. Yeah, unless you use insane pressures it will probably have little effect on corrosion, so I would just use the highest pressure that's convenient for you to produce.
Maybe the high pressure gas can help reduce arcing somewhat? who knows... there are probably still a good number of unknown factors at work.


@Fnord I disagree with the value. You have no idea of there remaining life, or misuse, etc. Have you actually checked the capacitance?


No , I have not checked the capacitance(I guess I'll have to though...).
They were sold as new(never been used), and they appear to be. No sign of corrosion on the terminals, and the only physical imperfection I can see is an extremely small dent in one of them.
By the way, they are the EXACT SAME kind as yours :)

Though I do consider it a lucky find for $50, even if they have been abused a bit.
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Unread postAuthor: rp181 » Mon Mar 16, 2009 3:57 pm

Pfft... All the caps i have i got for 50$, new (as far as new you can get from surplus....). 900$ is still abit much. A brand new single 2kJ capacitor costs 800$ from a brand name (EPCOS).

The injection does not help with the erosion, keep in mind when it arcs, it does so on a very small scale. I still disagree with use the highest pressure you can. 5kJ is low power (And it doesn't even sound like you will be able to get that high), it seems kind of pointless making a railgun if you focus on the injector =) I settle for 60 PSI (by the time it is fired, it leaks to 40 PSI from the shrader...), and it works the same as using high pressure co2.
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Unread postAuthor: inonickname » Tue Mar 17, 2009 2:50 am

Well, very good news! After some discussions with the parents I should be able to use up to 15-20 KJ, but they refuse to help me in any way with it, so it'll be a while.

If pressure did build up, it would make arcing more difficult but there's so much power in such a small space that arcing wont be stopped by such amounts of pressure.

I suspect using particular gasses would be nicer on the rails as far as oxidation goes, but I'd be more prepared to replace 2 copper rails every now and again than end up buying a bottle of (insert gas) and a reg..

Thanks Rp, I think it was mostly your advice that talked them into letting me go past a wimpy 5KJ. Though a condition of building was to get the electrician to check it pre firing :roll: oh well..

A buildup thread shall begin when I have the money for (and ordered) the caps.

Thanks heaps RP :D
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Unread postAuthor: rp181 » Tue Mar 17, 2009 7:16 am

Did you find a good supplier of caps? It is going to be hard to find a cheap supplier to make 20kJ affordable.

Tell you parents that the Electrical system is extremely simple, its mostly mechanical. Increasing the bank size does not increase direct danger, providing you keep the voltage at what it is, and you are not "forced" to endure the full discharge.

5 kJ isn't wimpy, its actually quite powerful=) When i fire mine in the basement (unfinished) It is loud enough to shake the above floor and the windows. Make sure you do it in a large space, or outside.

I did want to try to use a injection with sulfur hexafluoride, to see if it would help with the arcs.

Buying rails gets very expensive. It costs me 13$ for 1 set of rails (with some extra copper). That is another reason i wanted to switch to .25" bore, it makes copper cheaper.

Make sure you plan out your capacitor bank layout (especially for larger banks). The force may be large enough to repel the busbars, as i had happen on mine. I switched one bar around, so one comes out the left, and the other cable out the right.
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Unread postAuthor: inonickname » Thu Mar 19, 2009 3:30 am

I'm unsure of the supplier yet- will probably look at around inverter grade, pulse discharge/charge is far to expensive..

They know (or should know) it's simple, but I think the only way I've gotten around the capacity/power is that they don't know what to compare it to. For example, everyone knows what happens when you grab a severed mains cable? Just they don't realize what 20KJ at 300+ volts will do to you..and it's best to keep it that way.

Well, that's it. I'll do my best to keep the corrosion to a minimum.. Perhaps a non-oxidizing gas, aluminum projectile..et cetera. But it will just end up being part of the price you pay to own something like this.

Again, thanks Rp. I think I've set the gears rolling, though hardly fast enough.
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