How to make a simple railgun?

January 19, 2007 3:07 PM

How to make a simple railgun to demonstrate the principle to children?

Using only some batteries (maybe a car battery, but certainly nothing larger than that) and readily-available materials, how can I design a classroom-grade 'railgun' that demonstrates the basic principle?

It would seem to be quite simple, but a couple of minor attempts I made produced no results. I have very little experience with electricity outside of basic safety protocols and some electrolysis experiments.

There was a recent article in the news about railguns my nephew brought up and I thought a simple demo might help get him excited about his science classes.

posted by jjsonp to science & nature (12 answers total)

I've considered this in the past. It's really ferociously hard to get the timing right.

Powerlabs takes on the railgun.
posted by porpoise at 3:09 PM on January 19, 2007

Oh, here's an interesting article/guide.
posted by porpoise at 3:09 PM on January 19, 2007

My nephew made a basic Gauss rifle for a science project that worked well to illustrate the principle involved. No electricity required. I believe this is the guide he used.
posted by greatgefilte at 3:29 PM on January 19, 2007

To be clear, a Gauss rifle is not precisely a railgun, as the Wiki article points out, but it's a hell of a lot easier to make!
posted by greatgefilte at 3:32 PM on January 19, 2007

Your car battery is several orders of magnitude too weak to make this work. I don't think what you want to do is possible within the constraints you've set yourself.

The problem is that a railgun relies on the magnetic field created by current flowing through the rails, but since they're just straight lines, it takes enormous current to produce enough of a magnetic field to move an object. A car battery isn't up to the job.

OK, you say, then just use a tiny projectile, like a BB. But the other problem is that the rail gun relies on the projectile bridging the two rails to complete the circuit. There's no way you're going to make a BB stay in contact with both rails, and if it doesn't, then there's no magnetic field.

A Gauss gun is definitely a better idea. In that there's a coil, so your wimpy car battery's current is leveraged enough to actually do some good.
posted by Steven C. Den Beste at 3:44 PM on January 19, 2007

That is an awesome link greatgefilte. I'm pretty sure I have all those materials at home. I am so building one of those.
posted by quin at 3:48 PM on January 19, 2007

Yeah, go for the Gauss gun. The railgun is going to need heavy duty capacitors with at least a few hundred volts potential difference to ramp up quickly enough and something on the order of several thousand microfarads to a farad to provide enough current, and those are expensive and unsafe to have around curious children. The caps I used for my amateur railgun (second sweetest project I have been allowed to attempt for a college course) were lethal when charged, and made me nervous. I managed to get some sweet pulse capacitors on ebay, but eventually had to give up as my railgun project got too expensive. After about two months of work in my scant free time, I achieved a device that would jump metal or graphite projectiles a few centimeters along the rails before they fused to the rails. The projectiles (I think mass around 30g) moved too quickly for me to see them, which was great, but they didn't move far. I needed some sort of injection system so that the projectiles wouldn't fuse; using everday materials and a simple design your greatest problem when you have achieved a good enough current ramp-up is that the projectile is going to partially melt. I bought the necessary parts to inject the projectile, but my air compressor blew up and I was out of money and time for my semester project. Still, I almost built a friggin rail gun. If you are really serious about your railgun, I still have those pulse capacitors and will let them go for a reasonable price, with some lamentation and grieving (I'm not using them, but I love knowing that I have them). Four 600V, 6000microF caps if I remember correctly.
posted by Derive the Hamiltonian of... at 4:32 PM on January 19, 2007 [1 favorite]

Two simple guides for miniature rail gun, suitable to demonstrate the principle, both based on using a disposable camera (the camera flash circuit generates 300V from a single AA battery, and stores it in a capacitor, thus, most of the work is already done)

Make a rail gun.

Make a coil gun

There are other sites which detail larger gruntier rail gun projects, but I'd suggest starting off in miniature - the only way to go up to is get a bigger, meaner, capacitor bank, and that gets dangerous pretty quickly if you're inexperienced. Likewise, if you're inexperienced, you'll quickly find out, getting a few highly unpleasant belts off the caps in the miniature one, fortunately, unless you give it a path that crosses your heart (or brain), they should be harmless. Just unpleasant :)
Don't give it a path across your heart.
posted by -harlequin- at 4:48 PM on January 19, 2007

For the above projects, a good source of parts is online surplus store Electronics Goldmine

Here are strobe circuits from disposable cameras

Here are photo flash capacitors if you want to make a bank of 2-4 capacitors (I suggest not using more than that, as then you're starting to get into the area of burns and vapourising)
Here is another camera flash cap

They also have other things you'll have use for, such as like battery boxes, switches, project cases, etc
posted by -harlequin- at 4:58 PM on January 19, 2007

Possible tip for What To Do Next: (Though I haven't done any of this)

The cap bank will discharge rapidly. Almost instantly. That means almost all of your energy is exhausted by the time the slug is barely moving. If you fire your slug down the rails with some other mechanism (such as some kind of spring mechanism), your short energy pulse can be used to accelerate the slug faster, rather than be spent just getting it moving in the first place.

This is true for both rail guns and coil guns.

Whether it's "cheating" to have a launch mechanism that isn't the electrical rails probably depends on what you're trying to demonstrate.
posted by -harlequin- at 5:08 PM on January 19, 2007

10 Minute rail gun?
posted by mdpc98 at 6:41 PM on January 19, 2007

The scitoys thing is (as they point out) really more of a linear homopolar motor than a railgun, but it's closer to being a railgun than a coilgun is.

I n^th the idea of building a coilgun. A friend of mine built one for a middle-school science fair some time ago. His design: Take a glass tube, wrap coils around it at intervals, and connect the coils to a set of contacts (he used bolts in a piece of plywood). (The coils each have one wire going to a battery terminal and the other wire going to a contact.) Swipe a wire from the battery's other terminal across the contacts, to pulse the coils in sequence. If you get the timing just right, you can shoot a projectile (like a finishing nail) out the end of the glass tube. I kind of think the manual timing is a feature, if you're doing this to demonstrate the principle. It makes it a bit more interactive than just "push the button and it goes thunk".
posted by hattifattener at 7:08 PM on January 19, 2007