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Third generation railgun project

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rp181

Wed May 21 2008, 01:04AM

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

Rather then SCR's, why dont you use an injector? This takes away the requirement of the SCR.

How do you determine the ideal rail length as to avoid extra friction/wasted EM energy?

ea6b607

Wed May 21 2008, 01:32AM

Registered Member #1320 Joined: Sat Feb 16 2008, 01:31AM
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Posts: 67

rp find the pulse length and calculate how far it will travel by the time the pulse is minimal. What gauge wire do you have connecting each parallel set of capacitor to the SCR. Also, with an SCR you can't set the projectile in the prime place. Would a large mechanical switch work better as it is possible to construct one with considerable lower resistance and nearly as fast of close time? Also, is there any reason why everyone always uses lytics (I did because they where cheap) or is there another reason?

Andyman

Wed May 21 2008, 04:45AM

Registered Member #1083 Joined: Mon Oct 29 2007, 06:16PM
Location: Upland, California
Posts: 256

If that really is 1/4 inch steel plate, I'd say the power of the projectile is coming close to a light sniper rifle round like .223 or .243. I don't think a .45 or a 357 magnum would quite pierce a plate like that.

badastronaut

Wed May 21 2008, 05:21AM

Registered Member #222 Joined: Mon Feb 20 2006, 05:49PM
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How much energy would those sniper rounds and magnum rounds have? How much energy do you think went into the steel plate?

Yes, electrolytics are used because they are cheaper. That's pretty much the only reason.

What is the prime place? With the SCR I can set the projectile anywhere I want. I used 4 AWG welding cable.

The pulse length is under 600 microseconds. This is too fast to have a mechanical switch close before current reaches too high of a value. Bouncing of the contacts are also a problem. Other than that, a mechanical switch is much more complicated and expensive to implement in my case. That might be different in yours. That switch would probably be more dangerous than the capacitor bank.

For rail length, you run simulations, or you can size your rails so short that the caps aren't discharged all the way when the projectile exits. Most people size their rails way too long. With the cap banks they use, they could make do with rails less than a half an inch long. Friction isn't that big of a deal since the magnitude of the friction force is dwarfed by the EM forces. Although friction is the reason why railguns don't work on the really small scale unless you use plasma armatures. You can make a small railgun with a rolling projectile, a 9v battery, and magnets since the rolling rod armature has little friction.

I don't use an injector because they are not necessary, they are more complicated, more expensive, and more dangerous in my situation. That might be different in yours. There's nothing wrong with using an injector. It will in fact increase efficiency, but the reason is mechanical, not electrical. The injector will impart a fixed momentum to the projectile, and the railgun will also impart its own fixed momentum to the projectile with no injection. With injection, the momentums add together, so the speed you get out is the initial speed plus the speed without injector. Simulations indicate that this relationship is approximately true because of rail resistance and other factors.

Another reason for not using an injector is that I would have to inject the projectile at like 2500m/s in order to have it fully inside the rails before the current reaches something like 5000A. Otherwise current will rise too high before the projectile fully contacts the rails and it will explode from the high current density. I suppose this is where you might use an inductor or even a saturable reactor, but this will require longer rails and the extra resistance will hurt performance though injection will help mitigate this.

Andyman

Wed May 21 2008, 05:53AM

Registered Member #1083 Joined: Mon Oct 29 2007, 06:16PM
Location: Upland, California
Posts: 256

A .223 ranges from 1.5 to 1.8 kJ and from 2750 to 3750 ft/s. The .243 is more powerful and is around 2.8 kJ and between 3 and 4k ft/s. 357 magnum is a smaller round and is from 600-800J at 1-1.5k ft/s.
If I were to guess, based on the steel plates I've shot up before, I would estimate around 1kj is going into that target.
The 223 goes through our 1/4 inch steel plate so it's less than 1.5kJ, but probably more than a 357 which would just dent the metal.

Gunner

Fri Jun 13 2008, 09:58PM

Registered Member #1397 Joined: Mon Mar 17 2008, 12:47PM
Location: Finland
Posts: 43

Can you say that is that possible to make a railgun that has 5kJ capbank and works with effiency 10-20% (500-1000J KE)?
I mean, without injector.

badastronaut

Mon Jun 16 2008, 04:15AM

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

Yes, and no. In theory it is possible to do, but with actual parts, it is too difficult. With 5kJ and ordinary capacitors and setup, expect efficiency to be an order of magnitude lower than what you desire or even less than that. If you do it really really well, then you could do slightly better. This is all considering without injection. Please give it a try and see how well you can do.

Gunner

Mon Jun 16 2008, 08:35AM

Registered Member #1397 Joined: Mon Mar 17 2008, 12:47PM
Location: Finland
Posts: 43

OK...but I think that I`m gonna do that with injector and multistaged because It`s hard to find good SCR for that...

badastronaut

Tue Jun 17 2008, 10:39AM

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

That sounds excellent! Please keep us informed about the progress of your project.

Gunner

Thu Jun 19 2008, 08:40AM

Registered Member #1397 Joined: Mon Mar 17 2008, 12:47PM
Location: Finland
Posts: 43

Yes I will but I have to decide which one I will do:Railgun or ETG...:P

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