Project Railgun

My friend David chatted me on Facebook this morning and told me that me he’d finally figured out something cool we could build together – “a huge catapult or something”. His anticlimactic message inspired me, however. For the past few years, there have been three things I have really wanted to build but have not for some reason:

  1. A barely-street-legal electric bike
  2. An antweight robot with an eggbeater weapon (the fairyweight has been taken care of!)
  3. A railgun

David’s mentioning of “catapult” gave me an amusing idea: what if we built an electric catapult of sort? What if we built a railgun? I looked on YouTube a few weeks ago and found this small, interesting specimen. It seems to use 3 120 microfarad photoflash capacitors charged up to 330v – a fairly cheap project.

If David and I decide to build this device, I will update this blog with our progress. This is a fascinating project for two high schoolers to undertake – we both took and did well in AP Physics last year but there are a huge numbers of variables that we’ll have to deal with and there is a large amount of danger considering the energy we’ll be storing. Still, it’s nothing we won’t be able to handle. After all, I’ve only been building combat robots for nine years!

Here’s what we’ll have to consider:

  • Rail material. I’ve read that copper is good but I have also heard of railgunners (I guess you could call railgun hobbyists that!) using aluminum and even graphite as materials.
  • Armature material. Obviously, making complicated armatures for a hobby railgun seems pointless since the railgun will either melt it or launch it in such a way that it will deform upon striking a barrier, so we’ll need some other kind of solution. I want to try metal BBs, but I’ve also heard of small railguns using everything from aluminum foil to nuts.
  • Capacitor capacitance and what voltage we want to use. I found a cheap source of photoflash capacitor chargers online (they sell for ~$5) that can charge capacitors to several hundred volts with the help of a power supply. We’ll have to do some calculations to see how large a capacitor we should use. There is a lot to consider – Lorentz forces on the rails (keeping our railgun from breaking), the massive losses (this Princeton railgun writeup states that good hobby railguns achieve an efficiency of 0.1%), how massive our projectile is, and how fast we want to shoot it.
  • How to activate the railgun. We will need to design some sort of large switch that will have four modes: fire, charge, discharge and disconnected. We will need to be able to activate it without shocking ourselves or anything else (and without spraying sparks everywhere). We also want to avoid welding the switch shut.

With all these worries and potential problems, I feel crazy even thinking about attempting something like this with college apps happening – it’s a near-certainty this project will get tabled come, oh, say, August 29th when I go back to school. I can’t wait to get started!

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