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Circular railgun?

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BigBad

Wed Jan 21 2015, 02:05AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600

I was toying with the idea of making a circular railgun. I figure if you bend the rails around in a near circle, and used a ball bearing for the projectile it could get up to a fair speed.

It might be possible to do it in the bottom of a jam jar and use aluminium foil for the contacts. You need rigid sides to minimise rolling resistance.

If there's a scoopy thing, perhaps I could use an electromagnet to time the release and fire it out the scoop when it reaches top speed by sliding it over a bit maybe.

Sulaiman

Wed Jan 21 2015, 09:02AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

the principle sounds workable
but in a glass jar ?!

how about something like
model railway track glued to the inside of a pvc pipe?

DerAlbi

Wed Jan 21 2015, 10:06AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
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A railgun is a device that is particulary known for its wear and tear of its rails. I do not think that you could use aluminum foil for contacting. the movement alone will produce plasma/arcs that will consume your foil in no time, dont you think?

Even with just placing a ball on 2 parallel wires will make the thing practically questionable. It will kind of float on its plasma and since the ball is not restrained down with pressure the contact will be horible. I dont know if you ever watched a trams wheels on the rails.. even with that pressure you sometimes see it arcing.

But yeah: interesting idea. you are practically limited by radial forces only and you could have a quite long acceleration distance in a good form factor. the increase of speed will actually put some pressure on the rails... hmmh.
First electromagnetic accelerator with an inherently spinning projectile

haha

Shrad

Wed Jan 21 2015, 10:17AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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totally imaginary concept but very related... if you have a torus in which the interior had two conductive zones acting like rails, and a bead of mercury which would act as the shell, would it accelerate and evaporate and form a rotating plasma in the torus?

I guess the conductive rails would have to be superconductive and contain nothing but the mercury

accelerating a projectile would be easily achieved in a circular accelerator, but for a railgun it would be really difficult to design a mechanical path disconnection which would be applicable at such speeds

dexter

Wed Jan 21 2015, 03:38PM

Registered Member #42796 Joined: Mon Jan 13 2014, 06:34PM
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first alot of force will be wasted in the radial direction
2nd the above looses depends highly on the radius - the smaller the radius the bigger the looses - to such extent that at a certain radius the projectile would stop inside the loop or even burst through the rails

also the projectile will move in a straight line after leaving the gurved rails - so no curve the bullet still remains in the fiction world :)

if you have the materials build it regardless of how counterintuitive it sounds

BigBad

Wed Jan 21 2015, 05:46PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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DerAlbi wrote ...

A railgun is a device that is particulary known for its wear and tear of its rails. I do not think that you could use aluminum foil for contacting. the movement alone will produce plasma/arcs that will consume your foil in no time, dont you think?

The thing is the current is a lot lower, with a normal railgun you need super high current to get decent speed before the end of the rail, with this you can go around lots of times at lower acceleration.

You're still going to need to wind up the voltage though, the top speed of a railgun is limited by the back emf of the rotor; it's actually a form of universal motor.

Sulaiman wrote ...

the principle sounds workable
but in a glass jar ?!

how about something like
model railway track glued to the inside of a pvc pipe?

PVC is really too soft, you'll get lots of rolling friction; you need something really hard like steel or glass.

wrote ...

first alot of force will be wasted in the radial direction

No, the radial isn't (directly) lossy. The friction force is lossy; which is proportional with the rolling friction (and the radial force.) For steel on steel or steel on glass, that's going to be pretty high.

wrote ...

2nd the above looses depends highly on the radius - the smaller the radius the bigger the looses - to such extent that at a certain radius the projectile would stop inside the loop or even burst through the rails

Yes you probably wouldn't want two rails with a gap between them; although if the ball bearing weighs a gram and the g-force is a 1000, then that's only a kilogram force; 10 N.

wrote ...

also the projectile will move in a straight line after leaving the gurved rails - so no curve the bullet still remains in the fiction world :)

I think that's rather the point if we can work out how to do the release mechanism! ;)

Shrad

Wed Jan 21 2015, 07:07PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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lower current times the number of passes required to achieve the wanted speed... as soon as friction damage from much more passes does not exceed one high power shot damage yes, but I don't think that would be the case

BigBad

Wed Jan 21 2015, 11:01PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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It's just a slip ring; they can have pretty decent life.

Shrad

Thu Jan 22 2015, 08:55AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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Posts: 780

I think that due to current flow, movement and conduction losses at the interface would induce more stress and ablation with multiple passes at a low voltage/power than one pass at high voltage/power

You can always use a kind of conductive lubricant like graphite, on rails in a material which produces a conductive oxidation layer

I could well be wrong as I have no practical experience in railguns ;)

BigBad

Thu Jan 22 2015, 05:48PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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There's going to be pro and cons but a lot of it is i^2 R heating of the slip ring, and i is low.

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