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Coilguns: voltage to capacitance ratio?

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Aussienitro

Fri Jan 22 2010, 01:38AM

Registered Member #2399 Joined: Mon Sept 28 2009, 09:29AM
Location:
Posts: 27

Hello,

I see many coilguns that have been made however nearly all of the have a bank of caps in the 300-500v range with high capacitance in the 1000's of microfarad range. Is there a prefered ratio between voltage and capacitance? I realise discharge amps would also be important however thats an unknown.

Any examples of higher voltage lower capacitance guns?

I'm holding a 3kv, 100uF cap and am curious to know how it would perform, any insights?

Turkey9

Fri Jan 22 2010, 01:53AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Reluctance coilguns (use attractive force) need slower pulses so you use higher capacitance. Inductance coilguns (use repulsive force) need a faster discharge so they use high voltage low capacitance. With voltage less than a couple hundred volts, the coils need to be low resistance to get enough of a magnetic field (think ohms law). Aside from that, it's easy to rectify mains and get the right voltage to charge caps at 330V.

You could make a good inductance gun with your cap. They usually get higher velocity for one stage too.

Andri

Fri Jan 22 2010, 02:15AM

Registered Member #1533 Joined: Wed Jun 11 2008, 02:13PM
Location: ReykjavÃk, Iceland
Posts: 46

I've never built a coilgun but, based on simple theory, I believe the following should answer your question. I hope to be corrected if I'm wrong in any way.

It's all about maximizing energy stored and minimizing cost.
Energy stored is calculated according to the following law:

W = 1/2 C VÂ²

As you can see the voltage is squared so, there is a greater benefit from higher voltage than there is from greater capacitance.

Your capacitor should hold a maximum of 450J which comes close to the energy transfered to a 9x19mm Parabellum bullet when fired. That's the bullet used in a Glock 17. You won't be able to transfer all that energy to the projectile from a coilgun, however. Also, that's an absolute maximum based on the numbers you gave us.

Aussienitro

Sat Jan 23 2010, 01:23PM

Registered Member #2399 Joined: Mon Sept 28 2009, 09:29AM
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Posts: 27

Arr OK, kinda the difference between a throw and a hit, both do the same things in different ways. Though I've never heard of an inductance gun before though, so that would be a kind of weoponised "ring launcher"?

I guess I'll try a classic coil first and see what happens, though I though for sure some out there have tried simular high V low F caps before. Maybe it just wasn't worth mentioning :P

Turkey9

Mon Jan 25 2010, 07:12AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Plenty of people have used High Voltage and lower capacitance. In that configuration, the pulse in a coil is too short to be used like a normal coilgun and attract a ferrous projectile. Instead, the fast pulse is used to create eddy currents in a non-ferrous projectile (like aluminum). These eddy currents drive the projectile away from the coil just like a ring launcher or a disk launcher.

If you do some research about RLC networks and Lorenz Law this will all make a lot more sense. It's all in the physics of electricity and magnetism.

cduma

Mon Jan 25 2010, 07:25PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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Posts: 300

In my totaly un-scientific opinion I would prefer higher voltages since that will have a stronger tug and end soon.

Aussienitro

Mon Jan 25 2010, 07:36PM

Registered Member #2399 Joined: Mon Sept 28 2009, 09:29AM
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Posts: 27

Thanks turkey.

I haven't been able to find much information on induction guns, plenty on launching a disk with a flat coil but little on ring or solid projectiles. Ring gun here but few details on construction

uses 5kv 40uF cap. Claims 200m/s , might be an exageration though the rings shown would lose velocity fast.

I found this page,

showing an induction gun built exactly like a reluctance gun, just with different positioning of an Al/Cu wouldbe projectile. But if it was made as in the picture wouldn't it just attempt to crush rather then propell the bullet?

A picture I drew :)

Turkey9

Tue Jan 26 2010, 12:48AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

That inductance gun built like the reluctance one wouldn't work. You're correct in thinking that it would just push in on the projectile. I've seen guns built like that but the projectile needs to sit just in front of the coil. Yet another reason to use high voltage on inductance guns: the force exerted on the bullet gets weak fast as it goes away from the coil. The pulse needs to be fast in order to transfer as much energy as possible as fast as possible.

That gun might reach 200 m/s but i don't know... they look really light so it might get that fast, but not for long.

When using a ferrite rod all you have to do is wind a solenoid shaped coil on it. Then the ring sits in front of that coil. The rod changes the field so that the ring is propelled away from the coil much better than a lot of other designs. It's called a Thompson Gun i'm pretty sure if you wanna know more.

Aussienitro

Tue Jan 26 2010, 03:03AM

Registered Member #2399 Joined: Mon Sept 28 2009, 09:29AM
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Posts: 27

Thanks Turkey,

To get a projectile to show good ballistics I've decided on the following pic, simply use a disk acting as a sabot to give the necessary surface area which drops away leaving the arrow to fly straight. I just went as far as ordering 8mm ferrite rod and 10mm OD 1mm wall Al tube. Pic shows it in its most basic form disk will probably sit over arrow tube and butted up agains Al fins, that should give a smooth glide path with out unnecessary weight.

Now I just have no idea what to actually use for the coil, never done this before so anyone chime in on what wire and gauge I should be looking out for?

Aussienitro

Tue Jan 26 2010, 06:31AM

Registered Member #2399 Joined: Mon Sept 28 2009, 09:29AM
Location:
Posts: 27

Moved discussion of "arrow gun" to new thread

, as it was straying way off topic.

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