Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Simple railgun

  1. Feb 12, 2012 #1
    I'm currently working on a project for projectile motion and my teacher wants to see us launch a round ball about marble(maybe smaller) sized into a ring clamp on a retort stand somewhere around the room. She suggest we all use "gravity" (a simple ramp that lets a ball slide down and shoot up) but I'm looking for something cooler and potentially more precise.
    I read up on rail gun and don't know how to start it.
    I know the fundamental principles behind it and how it works but
    accord to the formula i found on wikipedia http://en.wikipedia.org/wiki/Railgun#Railgun_design
    I'm not even close to getting 1 newton. In order to do that I need like 100+ amps. I'm wondering how do people get 100 amp through the conductors. I know it has to do with capacitors and I've messed around with microelectronics before (arduino, small scale pic projects). Can someone fill me in on how to generate such force. I'm not looking for 10 N, 1 N is probably all I need.
  2. jcsd
  3. Feb 12, 2012 #2


    User Avatar
    Gold Member

    How do people get 100 amps through the conductors? For a serious rail gun, 100 amps is far too low. You need to be talking of values on the order of 10^6 amps for a really effective rail gun (which means that this is not suitable for your class room project).

    There are a couple of ways to get these seriously large currents. The obvious way is to discharge a very large capacitor bank simultaneously. For this we are talking about many farads (that's right, integer farads) of capacitance. This has been done in a number of places and it works. It is extremely expensive to put together such large cap banks, and they do wear out with time, but they do work and they are relatively simple in concept.

    The more interesting way is what is called the pulsed alternator. As the name implies, this is an alternator, an AC machine, which is brought up to speed with but with no magnetic field in the the machine. It has a rather massive rotor for a large moment of inertia and the ability to store a lot of kinetic energy. The armature windings (on the stator) consist of only a very few turns (4 to 8 turns total) so as to be very low inductance. With the machine spinning, the field current is energized (from an on-board battery or capacitor) to magnetize the field. This generates potential in the armature windings as the rotor turns. When the external circuits are closed, allowing current to flow out of the machine, each phase in sequence dumps magnetic energy out of the armature circuit. These current flows are passed through a set of diodes to rectify and switch them, so that they are utilized one by one, to combine into one time varying DC current of several mega-amps out of the machine. As the discharge is happening, the rotor rpm is dropping because kinetic energy is being converted directly into electrical output. Typically the speed drop is about 15% of max rotor speed by the end of discharge.

    One of the leading research centers for this sort of equipment is the Center for Electro-Mechanics, (CEM), The University of Texas at Austin. They have done some absolutely amazing work with such machines and with rail guns.
  4. Feb 12, 2012 #3
    there's lots of documented builds online, so now's a good time to work on your google-fu

    i had like a 200 word write up, but then decided not to include it, since it doesn't cover any safety concerns. as you learn more about the things i'm listing, you'll also learn the safety concerns involved with each one.

    capacitor banks
    high quality rails
    aerodynamically efficient projectile
    optional coolant

    good luck!
  5. Feb 12, 2012 #4
    thank you for your comment oldEngr63. I was still wondering about scaling issues. Building rail guns with 10^6 amps is a bit overkill for this simple project. My idea is kind of hobby/fascination based. I know these railguns can be scaled up into much more powerful weapons but what about scaling down. Scaling it down to show simple projectile motion and such. Is it possible?
  6. Feb 12, 2012 #5


    User Avatar
    Gold Member

    You still need dangerous amounts of current in order to generate enough force to move anything of interest. Thus they do not scale down very well. With only 10^5 amps they fire things like 1 cm cubes of lucite (which is not much mass at all). Of course they do get them up to some pretty high velocities, but the point is, it takes more current that you can safely deal with in a class room setting to move anything.
  7. Feb 12, 2012 #6
    lol. Thank you for your explanations. Safety is still very important to us. Would coil guns be much safer to build? Although less interested in coil guns, it's still a linear motor of sort.
    Also just for fun, are supercapacitors used in railguns?>
  8. Feb 12, 2012 #7


    User Avatar
    Science Advisor

    Probably the most reliable method would be the "actuator'.

    If you have a piece of iron partly inside a coil, and the coil is activated, the iron will be pulled to the centre of the coil.

    This is the loud "clunk" noise you have probably heard from washing machines.

    This movement is rapid and applies good force to anything connected to it. It is usually spring loaded so that the iron rod returns to the start position when power is removed.
    It can be used to control liquid flow, to move levers, operate an electrical switch or anything else that needs a lot of force controlled by an electric current.

    So, in this case, you would have a setup like that but attach an extra rod to hit the ball with. This should be non magnetic and possibly aluminum or plastic.
  9. Feb 12, 2012 #8


    User Avatar
    Gold Member

    Coil guns do not require as much current, but they are still complex and require a lot of switching circuitry. They are not easy to build and operate.

    I know that there has been an interest in super CAPs for rail guns, but I don't know where that stands now. I have been away from all of that for about 8 years now, and the technology marches own without us.

    The actuator recommendation is a very good one, and I suggest you follow up on it. That is a very simple electromagnetic machine, but you can learn a lot from building one.
  10. Feb 13, 2012 #9


    User Avatar

    Staff: Mentor

    Sorry, rail guns are a bit dangerous to be discussed here on the PF.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook