After my move from Brazil to study in the US I had to leave behind all of my ongoing projects, including the 3kJ single stage coil gun and my 7kJ multi stage coil gun prototype, both of which were incomplete (the multi stage coil gun was crushing the glass coil forms I was using as barrels and the 3kJ gun had barely been started). Not counting small prototypes I built as a kid, this is my 4rth Coil Gun project (I have worked with 1.5, 3 and 2.6kJ systems before), and although this research is nothing new to me I have come up with some theories over the years as my science and engineering education progressed and am hoping to test these ideas on a new gun platform with the objective of creating my most efficient and powerful gauss gun / linear coaxial magnetic accelerator yet.
The basic platform used here (busbars, barrel, SCR clamp and polycarbonate structure) was built in one afternoon when I arrived at the machine shop and found that the non metals center was closed so that the tables could be re-varnished. It is being built entirely from left over materials from the Rail Gun research and the Solid State Can Crusher.
Project Description and Goals:
Coil Guns, (also known as “gauss guns”, or “coaxial accelerators”, or “linear electromagnetic accelerators”) are extremely easy to make, a fact that explains their popularity on the Internet and as a science project in general. Basically, all one needs to build a coilgun is to wrap a couple turns of wire around some type of tube and run electricity through that wire with a piece of iron inside the tube. However, as with most things, when it comes to making a coil gun that performs well, that is, converts a significant portion of the energy stored in the capacitors into kinetic energy, even the best designs -the ones on this web site included- achieve at best a meager couple of % efficiency. Although a comparison with conventional electric motors and transformers, which can perform at efficiency levels as high as 90% and above, is not appropriate since they are completely different devices, some governmental institution coil gun designs have been quoted as achieving efficiencies as high as 26%, which makes it very obvious to me that there is something seriously wrong with the current amateur designs.
It is clear that the problem lies with a very low degree of power coupling between the coil and the projectile, which can be remedied by a smaller coil/slug ratio, thinner barrel walls, and a tighter fitting projectile, however several other factors are probably playing a role, such as the permittivity and magnetic saturation of the projectile material and the general fine tuning of the device. Solving the first problem can be achieved through the use of a more optimized coil/projectile design, although this will bring with it a power switching problem. Solving the second issue will now be possible through the use of a chronograph, which will make it very easy for the efficiency to be calculated and changes in efficiency quantified. I also hope to gain access to specialized magnetic materials so as to maximize the energy transfer potential of my Coil Gun.
Energy Storage: When Mr. Parler from Cornell-Dubilier provided me with 40 capacitors for my Rail Gun research I kept the 3 leftovers as possible replacements in case some of the capacitors from the bank were destroyed from over current. These capacitors have been sitting around for almost a year now and I decided that it was time to put them to some good use. The exact specifications for this 3-capacitor bank are (as measured): 2019uF, 22.81mOhms ESL. The bank can be charged to a peak voltage of 1300V, limited by the SCR being used. The stored energy on the bank is 1706Joules at that voltage, which I have found to be an appropriate value for a single stage (WARNING: It only takes 16Joules to kill a human by electrocution: high energy capacitor banks should only be handled by professionals!). The capacitors are inter connected through 1/16inch thick oxygen free pure copper bus bars that I had left over from the Rail Gun project. It is very interesting to note that at only 2.7 kilograms and measuring 23x16x8cm (9 1/4×6 1/4x3in) this capacitor bank stores 19% more energy than my first coil gun capacitor bank, which contained 10capacitors the same size as these, 30% more energy than one of the stages used in my 7kJ gun, which weighted just as much, and 38% less energy than the 3kJ coil gun capacitor bank, which weighted almost 7 times as much containing 20 non pulse rated capacitors!
Coil Barrel: Although for a future multi stage gun it would be highly desirable to have transparent barrels so that optical sensors could be easily moved along the gun for optimization experience has thought me that glass and most plastics are too soft to maintain good structural rigidity as a thin tube when a solenoid wound on them is pulsed with more then a couple hundred joules. Metals are ideal having a high strength to weight ratio but they suffer from severe eddy current losses in coil gun duty. My solution was to utilize a very thin (.001 inch wall thickness) brass tube (brass has the second lowest coefficient of friction of any metal, second only to Bronze.), and slot it with a 1/16″ mill bit so as to eliminate eddy current losses. The current barrel has an inside diameter of 5/16″ (.79cm), a common size for finding metal rods for, and also larger than my previous 1/4″ diameter barrels, in the hope that if magnetic saturation is indeed one of the causes for poor performance this will help alleviate it. In order to fully eliminate the eddy loss problem it may be necessary to insulate the projectile.
Coil: The coil was wound on a precision turned stainless steel form, slid out and then inserted into the slotted brass tube. By winding the coil outside of the coil form I can ensure that the turns are tight and accurate without deforming the coilform. The coil is 8cm long (3 1/4″) and 2cm dia (3/4″) and consists in 115 turns of AWG12 polyurethane insulated (Class A Type T-1 105C) magnet wire.
I currently have the facilities to manufacture most types and sizes of high precision/high performance coils and coil barrels for Coil Guns and other applications. If you are looking for a small quantity of custom coils contact me and we may be able to work out a deal.
Power Switching, triggering and protection circuit:
Since I am already foreseeing power switching problems with this gun as higher and higher coupling coefficients are experimented with I decided to use the largest solid state switch I could possibly get my hands on: a 1000Ampere, 1200V SCR. It will handle a 14kA, 1300V pulse, which may be just what is needed for a projectile to be fully accelerated in a distance of under one inch. I am currently looking for similar units; if anyone knows of a good supplier please E-mail me.
This SCR is mounted on a custom machined aluminum heat sink as the previous one I was using for it was unnecessarily large and heavy. Contact me if you need a custom machined SCR clamp.
The SCR will be protected from the CEMF produced by the collapsing magnetic field in the coil by two large MOVs and an ultra fast 2000A peak diode. The trigger circuit is still being designed but will be capacitive in nature.
Capacitor bank Charger: Again here I am using the same one as the Rail Gun (what do you expect from a device built in one afternoon!): Variac feeding a MOT with a full wave bridge rectifier. This crude prototype will have only one resistor which will serve as both current limiting for the PSU and as a discharge device. The temporary rail gun charger will eventually before a permanent charger for my coil gun experiments after the rail gun is switched over to a solid state high speed charger.
Here is a photo of the device are it currently stands, with the coil unconnected and minus charger, trigger and SCR protection circuit. Check this page back often as it will be continuously updated until the gun is ready, and click on the image for a higher resolution one.
The gun has been completed, and it works! I finalized the design so that I could show it on my presentation on electromagnetic accelerators -which mainly focused on the Rail Gun- for “The Daily Planet”, on The Discovery Channel. It works well, with only a few bugs to sort out; mainly the coil is moving backwards with the recoil of firing and this is disconnecting it and wasting energy. Also the back EMF is sufficient to blow all the diodes on the power supply and charge up the capacitor bank in reverse to over 100 volts, so a back-emf protection circuit is a must. I expect some more high power tests and videos soon, including chronograph results and optimization. For now, enjoy this low power test video (capacitors charged to +- 1kJ).
You can see my older Coil Gun Research on the PowerLabs Gauss Gun Page.
For a more spectacular kind of linear electromagnetic accelerator be sure to check out the PowerLabs Rail Gun Research