My Proof of Concept (POC) Railgun

Disclaimer: This page is for educational purposes only!!! Although the device described herein does not constitute a threat to anyone and is not a weapon, the concepts presented could be used to cause harm to people or property. This device does not make use of very dangerous voltages or currents and thus poses less risk than plugging in a hairdryer after showering. I am not responsible for you hurting or killing yourself or anyone else because you decided to recreate the project I present here and especially if you feel the urge to increase the power of this device. If you do perhaps I'll see your name in the next Darwin Awards list.

To start off I'll explain how I got into railguns. Back in High School ('91-'95) my friends and I played a Role Playing Game called RIFTS created by Palladium Books and one of the most powerful weapons was a railgun. This was way before the movie Eraser and Quake 3. I got on the internet (primative as it was) and looked up railguns. That's when I found out that they were real and how they worked. From that point on I started taking apart old electronics like TVs, VCRs, Microwave ovens, and electric typewriters looking for components. I tried to teach myself as much as I could about physics, electronics, and railguns as I could but since I didn't take physics in HS and I didn't have much time in college, I didn't get back into it until Senior year when I finished college physics. By that time I had devoured everything I could find about railguns on the internet. Now that I'm done with school and unemployed, I have more time to get to work with my POC railgun.

My design isn't a true railgun because as most purists will tell you a railgun is simply two parallel rails that conduct large currents through a conductive projectile spanning the gap between the rails. An example of such a railgun can be found here at Railgun.org. I have spent a lot of time studying railgun theory and the problems inherent in their construction and operation. One problem is the need for rediculously large amounts of current required to generate the Lorentz force that propels the projectile. Generating this current requires very complex power supplies and expensive high voltage pulse rated capacitors. This current also creates a lot of resistive heating in the projectile and rails. With this much heat, the projectile will tend to arc weld itself to the rails.

Despite my propensity for being a purist, I am going to go against true railgun design and suppliment the magnetic flux density by using external magnets. I had pondered using electromagnets for a long time because in my experience they were much more powerful than permanent magnets. In my senior year of college my roommate Ben and I had taken apart a defective 200MB drive and inside I discovered the most powerful magnets I had ever come across. I did some research and found out that they are Neodymium Iron Boron (NIB) Rare Earth Magnets and are the most powerful permanent magnets known to man. Doing some web searching, I came across a website, called ForceField, that specializes in selling surplus NIB magnets to everyday people. Over time I have purchased 6 of their #5 magnets. These magnets are 1.5"x1.5"x3/8" 45 Grade NIB magnets and are so powerful that when all six are stacked together I have to use the edge of a table to slide them appart one at a time.

Before this experiment I tried a couple other POCs that worked enough to tell me I was on the right track. This model was my first more sophisticated model that actually started to actually resemble what I want to eventually build. I went to the Hobby Shop nearby and picked up four brass rods to make up the rails. I plan on using a penny for a projectile (you can't get much cheaper than that) so my rails need to be on a small scale. I couldn't find what I needed at hardware store. Each rail is made up from two brass rods and is 12" long. The smallest channel stock I could find was 1/8" so these make up the rail that the penny will slide down. To add structural support (you'll see the need in a moment) I place a 5/32" square stock rod outside of the rail. The structural rail is 1/32" bigger than the guide rail on purpose. I wanted to minimize the height of the rails as much as possible. A penny is less than a 1/16" thick and ideally I'd like the inside of the guide rail to be just a little bigger than that but as I said the 1/8" channel was the smallest I could get. In order to maximize the magnetic field strength between the rails I want to place one of the monster magnets above and below the rails and as close together as possible. Since the guide rails cannot bear the crushing pressure of the two magnets attracting each other, I have the structural rails 1/32" larger so that they bear the entire load.

Currently I have the magnets configured so that their North pole is pointing up. I have also put a layer of masking tape between the magnets and the rails to insultate the magnet's Nickel plating to prevent it from shorting the rails (that's the projectile's job). I will later replace this with some electrical tape or other insulator as appropriate for my power supply. The rails are spaced to allow the penny to slide down easily but so that there is no way for the penny to only be in contact with one rail. The structural rails are held in place by the pressure of the magnets and the guide rails are simply taped to the structural rails. Eventually I'll solder or braze them together. The bottom magnets are stuck to a piece of steel (the cover of my old VCR) mostly for structural support but also to help increase the magnetic flux through the rails. The upper rails are surrounded by air for now. Using the tools on this site I can see that without steel backing or yoking, these two magnets in this arrangment will have a flux density between them of 7440.1 Gauss or almost 3/4 of a Tesla!!! Trying to replicate this level of flux density with electromagnets without using superconductors would have crazy power requirements and add lots of weight to the system. I plan on yoking the two magnets together with soft iron in the final project which should increase the flux density up to 9583.3 Gauss!!! Having a flux density this high will allow me to keep the current requirements lower and thus simplify my power supply and help avoid rail welding.

How does it work you ask? Well to start off I grabbed a 10,000uF 35V capacitor that I pillaged from an old VCR a friend gave me back in High School and charged it up to about 7.5V with a 9V battery (~.28J). I first put the penny on the rails just outside the magnets. Then I attached the positive terminal to the right rail and the negative terminal to the left rail and as soon as the second terminal made contact there was a spark and the penny moved forward a fraction of an inch. Pleased but not satisfied, like a good American I went for more power! :) Having previously hacked together a power supply for this capacitor that would charge it up to 33V (~5.5J) I charged up the capacitor again, placed the penny so that the back end was at the back end of the magnet (thus placing the front edge at the center of the magnet) and applied the power. With a louder pop and a brighter spark (both at the capacitor lead and the penny's contact point) the penny jumped forward ~7/8" before coming to a rest. Now this may not sound impressive but the penny stopped moving because the current stopped flowing when the capacitor was completely discharged and I know that was the case because the penny was still in contact with the rails and had not welded itself to them. The edge of the penny had sustained some damage and so did the rail but the damage to the rail was minor.

Below is a POC mock up of my railgun design. I'm sure that I will have to modify it as I go in order to work around problems that crop up.

	This is the top view of my POC Railgun. You can see the rails, penny, and big Nickel plated magnets. The magnets have incurred some damage over the years because I let friends play with them or was careless with ferrous materials near them. You can see how small knicks in the plating on the right magnet have caused the NIB to oxidize beneath the plating, causing it to bubble. I'll have to coat them with epoxy paint or something. I have two more of these magnets that I'm not using in this model at this time.
	This is the end view of my POC Railgun looking into the breach. Notice the hollow structural rails. I'm thinking that if welding or rail warping due to excessive heat becomes problematic I might use that pipe to carry active cooling fluid like cold water or mineral oil. I had a problem with the penny dragging on the tape on the lower magnets so I used a piece of cardboard from one of the ever present empty Coke boxes that litter my abode to wedge the guide rails up off of the bottom magnet. Without Coca-Cola Classic my brain would grind to a halt and I never would have finished college or Grad School because I don't drink coffee.
	This is the capacitor that I used to power the test firings. As you can see I wasn't lying about the 10,000uF rating. I've never found another capacitor like this since. I've found .047F capacitors but they were only rated at 6V. The penny projectile is shown for scale (3/4") and you can also see the arc damage that it sustained. I'm not sure if the arcing occurred at the positive or negative rail. I'll try to pay attention next time.
	This is the kludged power supply that I put together one afternoon so that I could charge up the capacitor closer to its maximum rating. The top wall wart is rated at 12V and 100mA and the bottom one is rated at 12V and 200mA, yet they provide just over 33V together. My voltmeter measured one at 18V and the other at 15V (so much for the ratings). One came from a Sony Cordless phone that I smashed one night after it crapped out on me without warning (ask my friend Karin or my parents about that night). The other is from an AT&T phone (or answering machine) that I pulled from the recycle bin at my appartment one day.

In the first picture you'll notice the rather large gap between the pairs of magnets. This is as close as I can get them due to their strength. Perhaps by using iron yoking, steel framing, and/or some diamagnetic material I can reduce the repulsion. My other option is to use segmented rails such that each segment has the polarity opposite of the proceeding segment and flipping the magnets. This will cause adjacent magnets to attract each other, thereby elimitating the gap. The down side is that it will prevent the flux from being perfectly verticle because some of the flux will flow down to the lower magnet but some will also flow horizontally to the adjacent magnet. In the current configureation, some flux opposite of the desired direction is flowing through the gap between the magnet pairs as the flux flows from the North pole (top) of the top magnet to the South pole (bottom) of the bottom magnet. I kind of plan on going with segmented rails so that each segment will have its own capacitors to supply power. I'd like to keep the previous segments wired to the segments in front of it via some diodes so that should the projectile leave the segment before the capacitors completely discharge, the remaining current will be able to flow forward. This may not be necessary and may actually contribute to a back emf in the larger loop the rails will make with the projectile and thus provide a breaking force and thereby slow down the projectile. So basically I'll have to experiment with solid rails with one large power bank, segmented rails with seperate power supplies but identical polarity, segemented rails with alternating polarities, and both versions of segmented rails with the segments wired forward through diodes so that residual charge can move forward.

And now for some Links! Unfortunately some of my old railgun links are no longer live so I'm going to post the ones I have that still work and as a special bonus I'm going to post some links to coilgun pages. Cousin of the railgun, a coilgun is probably easier to build and has the added benefit of being totally silent. I might try to build one eventually but for some reason railguns appeal to me more (perhaps its the challenge) and since my time and resources are limited, I'm going with the railgun for now.