Improving my railgun / Electromagnetic Projectile Accelerators / Forums

Forums

4hv.org :: Forums :: Electromagnetic Projectile Accelerators

« Previous topic | Next topic »

Improving my railgun

previous 1 2

Move Thread

LAN_403

Dr. Shark

Fri Mar 31 2006, 10:52AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Ignitrons are very scarce, although you might have more luck on your side of the pond. At 10kV you will be fine with a sparkgap, but I've got the feeling that 12kJ 10kV is not really a good combination. You are probably better of with more energy or less voltage, as ultimately it is the charge transfered though the projectile that speeds it up.

On my railgun (which has been shelfed years ago, OK...) I am using both pneumatic injection and active switching. Common, it is really not so hard to pick up a few beefy SCRs on eBay, especially for a railgun powered off electrolytics.

dan

Fri Mar 31 2006, 12:29PM

Registered Member #223 Joined: Mon Feb 20 2006, 06:42PM
Location: Ottawa, Ontario, Canada
Posts: 125

At 10KV there would need to be some sort of active switching or the rails would have to be far enough apart. The projectile can be injected and then the spark gap can fired just as the projectile is fully inside the rail gun. (Optically triggered) In theory the higher the voltage the better since it allows higher and faster discharge currents and thus a stronger magnetic field is generated. This is what moves the projectile after all. However if you do not have enough energy for a given voltage level the discharge will be over before the projectile moves a few inches down the rails. Or if you have too much energy and not enough voltage the projectile will leave the muzzle before the capacitors have been discharged.

I'm not sure the exact setup the 'pros' use but I would guess it would be similar to what I described above.(injection + active switching) I mean building a several Mega Joule capacitor bank out of low voltage capacitors would be impractical. Also to discharge them fast enough (several 10's of ns) they would really need HV pulse capacitors.

SCR's are slow and will only lengthen the discharge time and probably reducing efficiency. Yes the spark gap will lose a few joules but I'm betting that it will more than make up for that. The basic operation of a rail gun may seem simple but there are literally endless design parameters that have to be taken into account. Just naming a few, pulse length, rail length, capacitor energy storage, charging voltage, injection speed (if applicable), projectile size & type(solid, brushed, plasma), stray inductance, â€¦.

As you may notice most of them are closely related to each other and the design of the rail gun must built around those factors. However all of this said the improvements all depend on your ability to obtain materials and your skill level to design and build it.

pulslaser

Fri Mar 31 2006, 06:11PM

Registered Member #156 Joined: Sun Feb 12 2006, 07:04PM
Location:
Posts: 23

Hi Dan,
if You want to discharge in ten's of nanoseconds you will not need any rails which are longer than 1 cm. In ten's of nanoseconds, if you assume a relistic speed, I think world best is arround 6000m/s in air, your projektil will be accelerated for 0.3 milimeter. Of course you're right high voltage -> high current -> high force. But high speed is also reached with lower but longer pulse.
And lower currents are much easier to handle. Of course if you have to spend some megajoule with low voltage the capacity becomes very large and the rails have to be very long, but this isn't mostly the problem of amateur builder.

McFluffin

Fri Mar 31 2006, 07:22PM

Registered Member #119 Joined: Fri Feb 10 2006, 06:26AM
Location: USA
Posts: 114

I got a chance to put some images online. Here is the general setup of the power:

You'll notice some tape on a capacitor on the right...the screw mount on it broke. Thats the voltage drouble and I figure as long as it still works, it doesn't really need to be on that good. The blue capacitors are the 350V ones and you can see part of the larger bank to the left. Its connected by two of the orange wires as are most things. The black and red probes go to my DIMM to measure voltage. Finally, the black and red wires coming in is from the power supply to trigger the SCR. The resistor network is in that bit of blue.
This is the last projectile that I used, which worked pretty well:

You can see that it got damaged pretty badly, but the damage doesn't go to deep. I'd like to try to make a brushed projectile to replace it.
Front view of railgun:

Top view after the last shot:

I also had a general question about osciloscope use. I have seen people trace out the voltage versus time on people's scopes for their capacitor discharge. How is this done? Do you need a digital scope to do that? I was thinking it might have something to do with external trigger on it, but don't really know. I have a Tektronix 2213 if that helps anyone(

As far as the triggering issue, I was under the impression that my SCRs were working pretty well. I wouldn't imagine that it would be worth it to try to construct a spark gap design using only 350V or so. In any case, this isn't a mega-joule design, so I'm not going to worry about some of the larger power design issues. I also don't have any (large) 10kV capacitors, so thats not really going to be an issue either.
As you'll see from the pictures, I'm on a steep learning curve for learning how to machine things. I am getting better very quickly, but things aren't as nice as I'd like them to be. I can get materials fairly easily. There is a scap yard near my house where I can get copper for around $4 a pound as well as polycarbonate(not too expensive I've been told, but I've never gotten any there). The only problem is that they close at 4:30 and I don't drive and get out of school at 3:15. So, the only way I can get there is to bug someone for a ride and try to get there quick.
Update:
Here is a picture of it firing!

previous 1 2

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob