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trouble deciding what to use

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rp181

Tue Dec 15 2009, 11:26PM

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

I am wanting to try the railgun with the projectile started from rest, but I am having trouble deciding what switch to use (SCR's are out of the question). Here are the pro's and con's I have come up with for the ones I am considering:

1)trigatron (how much energy do these really waste?)
pros: consistent flow of electricity, easy to trigger
cons: Loud, waste of energy[?], erosion

2)Mechanical blocks
pros: simple, minimal arcing
cons: electrode bounce, erosion

3)A large homemade relay
pros: less bounce, easy control
cons: I dont even know how to begin designing one of these that will work reliably, the coil will need a mini cap bank itself

4)air solenoid triggered
pros: simple, easy control, minimal bounce
cons: slow

5)barry's style contactor (the one for the mark V coilgun)
pros: supposed to "stick
cons: Not sure

6) none, keep the injector

Which do you suggest? Any ideas of items not listed on here?

For those who are not aware, the capacitor bank is 12,000 joules, 400 volts. Efficiency is priority!

Barry

Wed Dec 16 2009, 12:17AM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

A railgun starting from rest is an interesting approach. Do you have a way to avoid welding the projectile in place? The main reason people use an injection system is that the initial velocity overcomes the stiction.

Barry
I have a mind like a steel trap -- thoughts wander in and get mangled and never escape.

badastronaut

Wed Dec 16 2009, 12:20AM

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

There are no problems with welding at all. A gun that has problems with welding will never work to begin with.

If welding is a problem then using injection only turns it into a spark shooting air gun, not a railgun.

You can try to use a mechanical switch with a saturable reactor. Maybe that would help reduce sparking. MEOW

rp181

Wed Dec 16 2009, 12:33AM

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

I thought you always said you wanted to minimize resistance?
Saturable reactor's are interesting, but would cost too much for me, with the cable and iron.
If I did do that, what size of a core would be appropriate?
@ Barry: The navy railgun starts from rest, as do most professional research railguns. The main reason people use injectors is that it is the easiest switch, or because powerlabs said that.

badastronaut

Wed Dec 16 2009, 08:42AM

Registered Member #222 Joined: Mon Feb 20 2006, 05:49PM
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Yes I did say minimize resistance. If you wanted, you could build the saturable reactor out of superconducting wire. :)

You will still have hysteresis and eddy loss in the core, which will show up as a resistance.

I've never built a saturable reactor, so I don't know how much iron you need. The amount of iron you need is dependent on current, geometry, and amount of time and initial inductance you need. Higher mu is better, too. If you really wanted, you could do FE simulation.

You do understand why a saturable reactor will help with mechanical switching, including with injectors, right?

It could be as simple as finding a dead transformer from the municipal dump and rewiring the core with many parallel turns of heavy cable. Trial and error will help with number of turns.

You also don't want too many turns, otherwise your inductance will be too large and slow down the pulse too much. If you don't use crowbar diodes, inductance is bad for railguns in the power supply.

Maybe you could get a bunch of ferrite beads and put them all over your connectors. A faster switch means youll need less iron, so it's a trade off, just like everything in engineering. meow

You could also experiment in small scale by switching a car battery with a homemade relay and saturable reactor, just to see if the concept works. You'll want to monitor current with an oscilloscope to compare with and without the saturable reactor.

The problem with fast contacting mechanical switches is that the KE of the contacts must dissipate that energy somehow. Maybe you could use lead contacts to dampen the vibrations, assuming you have non arcing performance. Otherwise you'll have a hazmat situation.

Or maybe you could shoot (pneumatically) a tapered copper spike into a copper plate with a hole in the middle. Maybe you could use aluminum for the plate instead.

There are so many things wrong with the powerlabs gun...

rp181

Wed Dec 16 2009, 01:24PM

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

Ok, I get that. I think ile stay away from a saturable reactor, especially since you can't find anything where i live.
Im leaning toward a cone shooting into another cone right now, it would be easy to adapt my injector to that.

I was thinking of putting large ND magnets above and below the contacts for a mechanical switch, Do you think that would help by forcing the electrodes together? or just provide another "spring"?

Hon1nbo

Wed Dec 16 2009, 07:37PM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: North Texas
Posts: 1040

when you put "slow" for air solenoid triggered, in what aspect do you mean? - I figure that it would not be any slower than mechanical blocks, or that much slower than a relay in terms of energy transfer, unless you refer to reaction time from once the system is triggered, in which still should not be a problem unless trigger time is critical in your case

I recall seeing a pneumatic design for a large bank in which the electrode was stopped just short of touching the opposing electrode, preventing the welding of the electrodes (a small arc was necessary, but minimal)

As for the trigatron, I do not think that a significant amount of Energy is lost because IIRC the energy normally lost in a spark gap is due to the actual formation of the spark, not the actual transfer of energy from to the load, and the spark in a trigatron is not created by the Capacitor Bank in this case but by an external Trigger, and the Arc is already at a very low resistance once started so the Bank's energy just flows right through

for my bank I am building a trigatron as the primary switch, as well as a dual CM600 switching system for if I need more controlled lower-energy pulses, e.g. if I made a monstrous multi-stage coil gun which I just might)

there also is another option I thought of but never looked deeply into because I had never seen it done before and am a little iffy on the design... make a sacrificial switch. This would be a lower grade bit of silicon or some other device that when caused to fail, it would fail such that it becomes conductive. Most devices could become conductive on there own, hence why SCRs are popular - however, SCRs of this scale are expensive and if you blow it you are screwed. So I thought why not use lower grade SCRs or other transistors which only have to hold off the voltage, but once activated will pass the current like normal, which being above their capability would fry them - however, since they are low grade SCRs there would not be much investment in them... I figure this method would not last in the long term, for obvious reasons, but I think there might be some alternative to Silicon that will work in a similar way, that is obviously cheaper and still as easy to "reload". Maybe a Camera Flash Tube in a trigatron configuration? I assume such tubes are made to a reasonable degree of tolerance (so you could avoid electrode corrosion of a normal trigatron), and for small ones they are not that expensive so it might be worth it for at least testing

just some thoughts

-Jimmy

rp181

Wed Dec 16 2009, 09:43PM

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

by solenoid, i meant an air atuator, sorry about that.
The electrode stoping before the other seems inteesting, but it would be unrelible.

Badaronaut used sarifcial SCR's on his 20kJ railgun, but even thse had to be puk so the failre would be accepble, that meas I would hve to use arge studs.

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