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Railgun projectile geometry

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Andyman

Fri Nov 19 2010, 12:08AM

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

I've looked through the past 10 pages of threads or so, and answers I found in a search were inconclusive, so I'll risk asking an answered question.
Besides the obvious challenge of machining, is there any other disadvantages to using a round projectile and round rails for a railgun? Are there any major advantages over square rails?

Jasonr

Fri Nov 19 2010, 04:45AM

Registered Member #167 Joined: Mon Feb 13 2006, 06:41PM
Location: Waterloo, WI
Posts: 54

By "round" I assume you mean cylinder, and not a ball? If you are suggesting ball that is a horrible idea because of the amount of contact area.
I do not know why cylinder would cause much problem, other than why bother?
Jason

Turkey9

Fri Nov 19 2010, 06:01AM

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

I don't know how much this would effect the projectile, but the current density wouldn't be uniform. The field at the top of the projectile wouldn't be the same as at the center for a round projectile. The forces might still be uniform but i don't know.

Apart from that I'd imagine that the increased surface area in contact with the rail would cut down on erosion of the rails. If you have the want, why not do it and see what happens? Then if other people have this question too, they can look here for the answer

Andyman

Fri Nov 19 2010, 07:52AM

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

Jason, i do mean a cylinder.

Also, for a railgun it's more desirable to have higher capacitance as opposed to higher voltage, correct?

Turkey9

Fri Nov 19 2010, 05:58PM

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

I think you're right... You want a really fast pulse but the inductance is so low that you get a short one almost by default. You want a lot of current though which higher voltage would do. I think around 400V is the happy medium there.

Andyman

Sat Nov 20 2010, 08:33PM

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

Ok, I talked to my physics professor briefly, and he was saying that with a square projectile, there will be magnetic field distortions at the corners, which will cause it to twist. (Probably that would mean it'll have more friction against the rails, and reduced efficiency.) With a round projectile, the field distortions are reduced, but it will still twist. Since it's round, it can spin freely without digging into the sides.
Now, whether current density is still a problem, I'm not quite clear on that..

I worked as a machinist for about 1/2 year, and my (ex-)boss said I'm free to use any of the machines/tools whenever I need, so machining the parts won't be a problem.

Neuuubeh

Sun Nov 21 2010, 10:18AM

Registered Member #3411 Joined: Sat Nov 13 2010, 08:25PM
Location:
Posts: 33

Well, there are some roundbore railguns the "big" guys use (60mm if im not mistaken). You can have a look for the kind of projectiles they were playing with in 1988 ->

I got curious, but couldnt find much - what rails does a round bore railgun use??? Concave ones? How would you want to make those :S.

I still do think square would be that much easier to fabricate really well, both projectile and bore - and it could be easier to make really stiff without going offboard with investments.

In case someone is a member at IEEE -

might give ya some answers

Andyman

Mon Nov 22 2010, 12:20AM

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

A simple ball end-mill would easily do the trick.
Thanks for that pdf!!

rp181

Mon Nov 22 2010, 03:54AM

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

I think thats the main reason people dont. It would be hard to make a accurate round rail. Maybe not hard, but you would need tools.

Artikbot

Mon Nov 22 2010, 12:56PM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

I was thinking about pressed copper sheet.

You know, get a round bar of desired diameter, get two sheets of copper and just press it down to the bar.

It would be for sure the easiest way. Machining the PTFE to do the housing for this hypothetical round halves can be 'easily' solved by making them epoxy-cast instead of PTFE.

If the sheets are pressed with adequate pressure, they can end up having a quite nice shape.

I guess it's the easiest way...Unless you really want to perforate a square bar and then cut it apart in two halves.

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