Rail Gun Designs

No Ideas here, but it does sound like a fun and interesting project. I'll be interested to see how things unfold.

I believe our representative from the PI (or RP) has some experience with these - if my memory serves. There has even been some discussion recently....

http://cr4.globalspec.com/thread/8666/Coil-Gun

OK, I was only sorta right

Something like this.

I see I made an error in my thinking, since the magnetic field strength is proportional to the current, not the voltage, what I want is not a high voltage capacitor bank, but a (relatively) low voltage high current capacitor bank.

Might want to talk to this guy:

http://cr4.globalspec.com/thread/8666/Coil-Gun

No details, speed is unimpressive, but it looks cool.

Coil guns and rail guns are completely different. coil guns utilize magnetic attraction, rail guns utilize the repulsive Lorenz force. Completely different mode of operation.

Glad you know the difference... Coil guns seem to have built-in inefficiencies.

But check this out. This was for another thread:

If you're interested I can tell you how Westinghouse powered their rail gun during the Star Wars period.

Interested? absolutely! Why, was it a small nuke or something?=D

Something I'm trying to understand. I've asked Mr. Barros but he has not replied yet. Mr. Barros is using a 3.2kV system. If the propulsion is based on current, not voltage, why use such a high voltage system? It seems to me that doing that just makes the system that much more difficult and expensive for (apparently) no good reason. Is there something I'm missing here?

I think the reason for the voltage is it helps the current "move" as fast as possible.

Anyway, Westinghouse used a huge, off-center magnetic weight (like the size of a car) that was spun about a shaft. This was contained within a huge cavity containing the coil through which the weight rotated. When they fired the gun, great, big steel breaks were fired at the rotating weight using high explosives, which stopped the weight from rotating almost instantaneously! This caused the field to collapse and create an enormous pulse of current - which was sent to the rails of the gun.

Of course, the whole system was self-destructive, it even split the rails! However, they were looking for not only a weapons system, but a way to put small payloads into orbit.

"I think the reason for the voltage is it helps the current "move" as fast as possible."

I'm not sure I follow.

BTW, every time I read your signature line, my mind inserts an "o" in the name, which gives a whole new meaning to the quote....

To put it simply, why use high voltage when current makes the field, back EMF. As the projectile accelerates the higher velocity produces a back EMF that cancels the voltage applied and now no current followed by no further acceleration. Now if one really could fabricate an ideal current source capable of fabricating any voltage required to counteract any load, then voltage would not be a problem.

Ah, ok, I see what you are saying. I was thinking about this this morning and I also realized that as the capacitor bank discharges, the voltage will fall, which since the circuit resistance is essentially the same (or higher due to erosion and arcing) the current will fall as well. So I would have to design the system so that I have the desired current at the end of the discharge cycle, not at the start and then extrapolate back to the beginning to find the initial charge voltage. If you throw the back EMF into the circuit that makes it even higher. I presume I will need HV/HC diodes connected backwards across the rails to prevent the back EMF from damaging the capacitor bank as well.

A consequence of this is you should find that your capacitor does not fully discharge. Keep that in mind when you design your capacitor charge circuit.

You're going to have a devil of a time doing this, so be careful. You say that you don't see the devil yet. Well he's right there, hiding in the details.

Yep, that is usually where he hides. He's a crafty bastard too.

I haven't done any significant electronic design in years and I'm really rusty.

Might you not start somewhere "safer"? This sounds like a dangerous project....

What's the fun in that? =D

I have a healthy respect for High Voltage systems, I've been bit by them before and it is NO FUN. I'm definitely going to follow Nikolai Tesla's excellent safety advice of keeping one hand in my pocket at all times....

...like that Captain in the film the "Caine Mutiny", always playing with his "worry balls"?

Heh, actually, the reason is to prevent the possibility of forming a circuit that runs from one arm, across the chest, and down the other arm. Such a circuit can lead to cardiac arrest.

I knew that having stood on pure rubber mats with my right or left hand in live switchgear on a rocking ship at sea.....the other safely in a pocket.....and two guys watching to make sure none of the MEs try and make contact!!!!

I was actually trying to make a small joke.....

Yeah I know.. I was yanking your chain too.

You got me!

And there is me thinking that you had no sense of humour!!!

In the words of Tommy Lee Jones in MIB....None that I am aware of....

ooh, just had a thought, what about a blank pistol/shotgun round for the injector? Hmm...

Would bronze do for the projectile? I know it's conductive and non magnetic. Al-bronze. K bronze, S bronze, Berrilium-Copper? All available as 20' bar stock. Anything else for that matter.

Sounds like fun!

I think using anything with beryllium in it would be a VERY BAD idea. These things vaporize the rails and armature (projectile) when they are shot. beryllium vapor would probably ruin your whole day (life actually). NOT SAFE.

Conductivity of the projectile is of paramount importance. That means you are pretty much restricted to gold, copper, silver or aluminum. And even then, you want them to be as pure as possible for maximum conductivity. aluminum is chosen because it's melting point is lower than the rails, it becomes the sacrificial member.

You have too much time on your hands.

Now I could see a sintered composite of tungsten carbide in a pure aluminum matrix as a projectile. Perhaps with a solid tungsten core.

Nope, depleted uranium is the way to go, sheathed in aluminum. The aluminum can be swaged over the DU chunk. Stand clear!

Mark

On general principles, I'd agree with you, but getting my hands on DU might be somewhat problematic....

Easy peasy ... a quick holiday to Iraq or Afghanistan with a small radiation counter will let you pick up the correct DU chunks everywhere there has been artillery. Larger chunks as the armor from all kinds of vehicles.

http://en.wikipedia.org/wiki/Depleted_uranium
:(
Mark

Yeah but then you have all the uranium dust to deal with too.... I'd just as soon pass, unless I really need to pierce armor or something.... but then the armor itself would probably be lousy with DU as well.

Interesting ideas.

Aluminum looks a nice idea if used as thick surface coating and if it melts then by wetting will provide better contact also. You still have to have high density for the projectile so inner core need to be heavy, non-toxic and also cheap. I think it is a nice idea to build these machines in small working model.

What have you got? Bad neighbours maybe?

Naw, I love my neighbors. They are quite literally the best anyone could hope to have. I'd say we're practically family, but I don't much like my family...=D

I just like things that go boom, I also like to be able to reach out and touch things at a mile or more.....

Besides, if the revolution ever does come, it might be a handy bit of kit....

yeah, those mexicans who trying to cross the border into texas. you need to arm yourself these days know.

Like these guys?

See! I just had my imagination running wild.

I believe the Infra cam was developed to watch the mexicanas crossing your border at night, am I right?

Maybe you need to hook up with this guy.....

Best of luck, but don't point it my way!

I'd lean more towards a superconducting coil gun myself. Something that would pump out steel ball bearings at serious velocity and a high rate of fire.

You could probably build a railgun with junkyard parts easier, though.

How about designing the railgun around a standard model rocket, then building the rocket out of aluminum tubing with an aluminum nose cone? The model rocket engines will inject the projectile into the accelerator, then continue acclerating it after it leaves the barrel. You might need to redesign the rocket nozzle to a De Laval type nozzle to maximize gas velocity. Can you say 'kinetic-kill weapon'? No illegal components, so the ATF can't complain.

For a timer, use a couple of laser pointers and old-fashioned photocells. Measure the exact distance between the points where the projectile will intersect the lasers. Write a BASIC or other program that starts a counter when the first photocell stops receiving light, then stops it when the second one stops receiving light. Figure the clock speed of the counter, and you have the time it took the projectile to go from A to B.

I'm pretty sure the muzzle flash would blind any optical sensor, which is probably why standard chronographs don't work.

I was thinking in terms of putting the photocells at the bottom of a piece of pipe or some such to shield them, hence the laser pointers.

I forgot to give you this link http://en.wikipedia.org/wiki/Rail_Gun after read it.

It is a good stuff but you'd need a bit of juice to run it.

I'd apply WAQAP principles ('Wrong As Quick As Possible', intellectual property of KrisDel^TM corporation)
Build a small test version, you will doubtless learn more from a small (safer) test rig than you will from a shed load of research.

It has the added advantage that materials and components will be cheaper and more readilly obtainable.
Also we'll get to see some results quicker....
Injector could be a simple spring or even a sharp tap with a hammer.
Del

I'm thinking a gas piston operated semi-auto mechanism for the injector that will load a new projectile into the breech with each shot from a gravity fed magazine..... I plan on building a "hot rail" design so it is just a matter of injecting the next projectile after the cap bank has recharged....(assuming the rails aren't damaged.)

This sort of gun does not scale down terribly well imho, you've got to have a strong enough current to generate enough force to move the projectile. That means that the rails have to be big enough to handle the current, but they can't be too big or the centerline of the field is too far away from the armature.

All that said, I don't plan on building a big one. Probably about the size of the one Sam Barros did, but with some tweaks. For instance, I'm thinking of using Argon as the injector gas. I'm thinking that it would form a plasma easier between the armature and the rail and make for more efficient current conduction than plain air would. It would also prevent the formation of nitrides on the rail surfaces.

I'm also thinking of altering the rail geometry at the muzzle to prevent a lot of arcing as the armature leaves the gun.

Sam used 32 6800uF 400V caps in a series paralell array, but those caps aren't cheap. I'm thinking lower capacitance caps and more of them. that should increase the amount of current they can instantaneously supply, if nothing else it makes it cheaper.

Neon may work better than Argon and shouldn't be too hard to get.

Make sure to tie a string on your projectile... Connect the other end to the rail gun. That way you can use it more than once.