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Registered Member #1480
Joined: Sat May 10 2008, 03:21AM
Location:
Posts: 4
Hi everybody, I'm working on a really simple railgun design for a class project. I'm using ~80psi air to kick projectiles (solid graphite or aluminum in cubes or little cylinders) into big copper rails. I'm using a single 10kV 70uF cap, although I've actually been firing at 7-8kV.
I've generally been charging the cap with the rails directly connected, then using the compressed air to push the projectile into them. I've also played around with a solenoid-based HV switch and tried to time it so the cap connects to the rails right after the projectile enters. However, I always see a large deceleration when the current initially starts flowing, and generally the slug leaves the gun slower than with just the air on. Anybody have any ideas why it's slowed down considerably by the initial spark? Here's a link to a picture of the setup:
Registered Member #191
Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720
1, you need like 4awg between the rails and your cap, at least. 2, are you relying on the cap screws to transfer current down to the rails? bad idea. 3, the gun doesn't seem to have tight enough tolerance. not even close actually.
Registered Member #1480
Joined: Sat May 10 2008, 03:21AM
Location:
Posts: 4
The cables connecting the cap to the rails are braided copper, which seem to work pretty well. The screw contacts do conduct the current down to the rails, but I don't think that's the problem - the cap discharges down to about 500V (from 8000).
Which tolerance are you referring to? The rails are milled flat and well aligned. I assembled the gun with the bar of graphite that I've been using for projectiles inside with a bit of paper wrapped around, so the gap between the slug and the rails is about the thickness of two sheets of paper. When I had it tighter than that the projectile tended to jam in the rails.
As I mentioned above, it is discharging from 8kV to 0.5kV, which by my calculations is about 2.2kJ of energy dissipated, or half a Coulomb of charge (in a small fraction of a second). Also, even if the contacts were a problem, it shouldn't slow the projectile down when it initially makes contact.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I don't know much about railguns but this is what I think you have wrong: The rails should be below the projectile so it actually makes contact with them just by gravity alone... It should make contact all the time with the rails. I think you're using a too high voltage, maybe something below 2kV would be better, but again I've never been into railguns.
Registered Member #690
Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616
Electroholic is right, your connections are not good enough. I don't think you understand exactly how much current you are trying to send through these rails. Every thousandth of an ohm resistance and every microhenry of inductance hurts you. A lot. I can already see some wear on your rails.
You absolutely need thicker wire, and it needs to be directly connected to the rails.
Plus, just because your capacitor discharges fully doesn't mean it discharges quickly.
Registered Member #191
Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720
sterculus wrote ...
Which tolerance are you referring to? The rails are milled flat and well aligned. I assembled the gun with the bar of graphite that I've been using for projectiles inside with a bit of paper wrapped around, so the gap between the slug and the rails is about the thickness of two sheets of paper. When I had it tighter than that the projectile tended to jam in the rails.
There is a gap between the rail and the projectile? how do you expect it to work then?
Registered Member #1480
Joined: Sat May 10 2008, 03:21AM
Location:
Posts: 4
But the biggest problem currently is that the current flow actually slows the projectile down when it first hits the rails. Making the current pulse narrower will just exacerbate that. For example, here's a plot of the velocity vs range in the gun with the cap vs just air:
The big drop in velocity for the red data, which is the projectile with power, corresponds to when I see the initial spark on the video.
Electroholic wrote ...
There is a gap between the rail and the projectile? how do you expect it to work then?
By ionizing air in the few thousands of an inch in between the rail and the projectile. When it was assembled to a tighter tolerance the slug thermally expanded (I think) and jammed.
Registered Member #690
Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616
This is the trouble with railguns; that everything needs to be *perfect* for you to get good results. Current flow is not slowing your projectile down, it must be friction with the rails or something else. The more current you have flowing, the more force on the projectile. Making the pulse more narrow may overcome the friction with the rails and help you.
Plus, there needs to be NO GAP between the projectile and the rails. Arcing will f*ck up your rails (apparently this has already began). If the projectile gets stuck when in good contact with the rails, then maybe you need rails that are machined better.
Registered Member #1480
Joined: Sat May 10 2008, 03:21AM
Location:
Posts: 4
Shaun wrote ...
This is the trouble with railguns; that everything needs to be *perfect* for you to get good results. Current flow is not slowing your projectile down, it must be friction with the rails or something else. The more current you have flowing, the more force on the projectile. Making the pulse more narrow may overcome the friction with the rails and help you.
Plus, there needs to be NO GAP between the projectile and the rails. Arcing will f*ck up your rails (apparently this has already began). If the projectile gets stuck when in good contact with the rails, then maybe you need rails that are machined better.
But I'm comparing against the projectile going through the gun with just the compressed air (no voltage on the cap). It's dramatically slowing down when the gun sparks, and there's no deceleration at that spot with just the air, so it's not friction with the rails. Look at the graph I linked to a couple posts up.
When it was put together tighter, it could fire the projectile through the rails with just the air on, which means that the rails were machined fine. However, when I then fired it with the power on it got stuck - I assume because the slug and rails expanded enough due to resistive heating to wedge. I don't really care if the rails get messed up from the arcing. I have spares and, as I said, this is a class project so it doesn't have to last a long time.
Registered Member #511
Joined: Sat Feb 10 2007, 11:36AM
Location: Somerset UK
Posts: 55
Hi sterculus
I built a small railgun two years ago and had exactly the same problem. The injector would shoot the armature through the rails with no current applied, but when the current is turned on the armature stoped dead. I never did find the cause or the solution, but here are my thoughts on the matter.
1. My railgun was a bad design and the propulsive force was probably negligible.
2. The arcs on each side of the armature somehow caused a drag force strong enough to stop it completely.
3. The tesla coil guys who use rotary spark gaps talk about large forces caused by the arcs, this is why they use large motors to spin the gap.
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Simple railgun help
