The railgun was fired using solid aluminum and plasma copper armatures. The aluminum armatures were designed to be functional; efficient projectiles. The front of the projectile is made more aerodynamic than a flat square face, or "V" shape. The center of mass is well towards the front of the projectile. These properties allow for a relatively stable flight.
The projectiles also have a groove that splits the projectile into two legs at the rear, this is referred to as a "C" armature. This is fairly important, yet I have not seen this used on amateur railgun sites with the exception of Jason Rollette's. The fins allow the projectile to be wedged such that it makes good contact with the rails. More importantly once the current begins to flow the armature legs are pulled with even more force towards the rails. This is due to that fact that in the leg region the rail current and armature current are parallel and in the same direction.
The plasma armature projectile is probably the easiest type to implement. It is also the most inefficient. The voltage drop across the plasma armature us generally around 1000 Volts, where the voltage drop across solid C armature is usually around 50 volts. The plasma armature also requires a minimum energy just to convert the metal film into plasma. That minimum energy may be significant for small amateur railguns, resulting in partial conversion into plasma and poor performance. In most cases the energy converted to heat across the 1000 Volt drop of a plasma armature will be the greatest loss in a system using plasma projectiles. The use of a plasma armature also requires injection to prevent excessive damage to the rails while the metal is ionizing. Solid armatures do not necessarily require injection, assuming your power source can get the projectile moving before it welds to the rails. Another consideration for an amateur railgun builder is whether or not they can build an injector strong enough to withstand the plasma pressure backfiring. After firing a plasma projectile that blew my injector apart I would strongly recommend using solid projectiles.
The aluminum projectiles each have a mass of 10.7 grams They are based on the standard "C" armature. They are ½ inch square with a pointed tip, and ¼ inch slot in the middle. 40 were made from ½ inch square aluminum bar stock. It took about 14 hours to make 40 projectiles like this. Although I imagine a real machinist could have done it at least twice as fast. These were made on a bridgeport EZtrack 3-axis CNC machine.
The projectiles that use a plasma armature consist of a short piece of ½ square nylon with the same point as the aluminum projectiles. These projectile are slightly long than the aluminum projectiles and have a mass of 7.2 grams. Thirteen nylon projectiles were machined over about 2 hours.
On the back of the projectiles a square piece of copper foil 2 mils thick is attached with some super glue. I estimated in rather crude fashion that the energy needed to turn the copper to plasma is about 70 joules. Even if this incorrect by quite a bit this amount of energy is a small fraction of what the capacitor bank can store. So the fact that the copper will turn into the desired plasma is certain.
My current stock of projectiles is just waiting for more testing. Two aluminum projectiles were lost, one was fired and recovered. I'll probably never use the plasma armature projectiles in the current railgun. To use a plasma projectile I would want a railgun with a bolt action, that is capable of withstanding around 60,000 psi.
visitors since June 10th, 2005.
