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Theory Ideas If you would like to learn the basics of how a coilgun works, try Barry's Coilgun Design Site. Monday, April 26, 2004 I realized recently that I have a Windows PC now, so I went ahead and downloaded FEMM. I played around with it a little, modelling my coilgun, and the values were coming out as I expected; about as my simulator applet predicted. I modelled the coil-on-projectile idea, but in order to fit in the tube, the wire gauge would have to be so low that the field would be too small to be worth it. The force FEMM calculated was way below a normal coilgun's force, even with a huge mass of iron. I haven't written any Lua scripts yet, but I think soon I will write a complete simulator script to use FEMM to do what my simulator does now, but more accurately and taking into account the change in inductance (or at least flux and velocity) as the projectile moves into the coil. Sunday, April 21, 2004 In my last update, I was thinking about reversing the roles of the coil and the projectile. Shortly after writing that, I tried to try it; I never actually got to try it because the make-shift mini-coil I wrapped around one of my normal nail projectiles wouldn't fit into the barrel. A few days ago that got me thinking about the relation of mass to efficiency. I plugged some numbers into Barry's inductance applet and into my own simulator, trying different projectile diameters for constant coil inductance while keeping the coil closely coupled to the projectile. It turns out that for my coilgun, I could jump from my current 0.3% efficiency to possibly 3.7% efficiency if I would use a projectile with diameter 15mm. I could get a 2.8% efficiency if I used a more standard 9mm projectile, 28mm long (with a matching coil). Of course, the velocity is smaller, but the mass makes up for it in kinetic energy. There is an optimum diameter, though: a coil of constant inductance can't have too large an inner diameter, or it will lose magnetic influence (field) on the projectile. A large 40mm shell will have the same kinetic energy as my 2mm nails. My quest now is to find a good source of small caliber solid iron or steel slugs, and matching barrels. After that, I'll check out stabilization with elliptical coils, because rifling barrels is too much work for this project. Wednesday, December 18, 2002 From the news page... My coilgun is in storage for the moment, but I've been thinking about it and I have a great new idea that I may try much later: the force on the projectile is directly proportional to the amount of iron in it, and because it's all iron, that means the mass. So as the mass increases, so does the force. However, acceleration is directly proportional to the force and inversely proportional to the mass, so this "decrease" exactly balances out the increase caused by mass. Now, if we make the coil the projectile, and surround the center of the barrel with an iron mass, then we're dealing with two masses: the one that increases the force can be varied and very large, while the mass of the projectile, which decreases the acceleration, can remain the same. Explanations of all the math involved are available for each applet. Pulse Width and Shape After learning Java, the first applet I created was this one. It inputs data from your coilgun and graphs the current flowing through the inductor. I've refashioned it with new formulas and cleaned up the interface since then, and it's still a work in progress. RLC Calculator RLC Source Code NET - Formulas Used for RLC I plan to continue development; implementing the "Transistor Switch" option, improving the graph accuracy and speed with a Runge-Kutta or similar model, and providing extra exact data, such as the time the peak current occured and the time of zero crossing (all currently calculable at the MGC). HAPES The High Amperage Procedure Evaluation System is really just a useful multi-directional formula calculator. The segment titled "Energy in a Capacitor" can calculate the energy stored in a given capacitor, or reverse the equation and calculate the capacitance or voltage you need to acheive a certain energy level. The "Projectile Speed" segment can calculate the speed of a projectile given either experimental distance results or the mass of a projectile and its kinetic energy. Like the above section, it can also calculate any of the other values, but only if a logical combination of other fields are filled in. For example, it will only calculate firing range (distance) if it is given a height, and a speed (or mass and energy, which can be used to calculate speed). Experiment, and find out what works. The "Discharge Rate" segment calculates the approximate time in seconds it would take to drain a capacitor through a bleeder resistor. Note that this is very approximate, and should not be used to calculate high-current pulse times. Use the above RLC Applet for that. HAPES Calculator HAPES Source Code HAPES Formulas Kinetica Kinetica is a simple applet to show you how far a theoretical projectile (no air resistance) can travel after being fired at any height, velocity, and angle. Simply input velocity, initial height, and firing angle in metric or standard units, and hit "Calculate". It can also graph a relation of time to projectile height on the same axis (overlay). Kinetica Applet Kinetica Source Code Math - Gravitational Acceleration Electromagnetic Force Simulator The EMF Simulator is the crown jewel of my applet collection and represents many weeks of research and obsession. Input several bits of information describing your coilgun and hit the calculate button, and it will show you an RLC graph identical to the above RLC Calculator, and a graph of the velocity of your projectile with respect to time. Very useful for determining the optimum factors of coilgunning, such as projectile and coil length, voltage, projectile mass, and capacitance, among other things. EMF Simulator Applet EMF Simulator Source Code Rescued and available! Magnetic Force Equations