Re: new caps & basic launcher testing rig
FastMHz, Wed Mar 29 2006, 01:01AM

That's 3.7kj worth of capacitors if you use all of them on one bank - you'll have a blast (pun intended)! I'm sure you've seen my post about my 3.5kj bank...And yes, you will get a launch with just one. Rings hit the ceiling with my bank charged to just 200v.

Re: new caps & basic launcher testing rig
Wilson, Wed Mar 29 2006, 01:33AM

wow, i really envy you guys in the US. Each one of those caps store 280J of energy at full charge, so assuming even 3% eff. you should be able to get a 20g platter moving at 20+m/s hopefully. It might be worth investing in some large SCRs in the long run, as you could probably explode most mechanical switches with that kind of energy ;)

Re: new caps & basic launcher testing rig
Quantum Singularity, Wed Mar 29 2006, 01:41AM


Well of course I saw you post I think I probably posted on your post too! But have you seen my 6kJ launcher? Or my plan this summer to try 12kJ? That will really be a blast (or a large explosion). But these new caps are for an easier / more portable / safer design. My big caps weigh like 100# each and I can barely lift them enough to get em in my truck. Plus the extra danger with 10kV. And I hope I can try solid state switching with these caps - like a scr - something I cant do with the big cap. I really hope to build a battery supply for them so I can make it really portable, atleast for a couple caps - maybe not a bank of all 13 of them.

I forgot to ask - since this is the first time I am trying electrolytics on a coilgun, will the caps survive the V reversal from the coil? Do I need to use clamping diodes to protect them? My big maxwells had V on them after I fired, I dont know if it was reversed or not cause as soon as I would fire I would quickly throw the rod across my discharge gap to make sure they were fully discharged. Then again that was probably mostly due to the spark gap & char buildup not conducting under a few kilovolts probably.

Re: new caps & basic launcher testing rig
FastMHz, Wed Mar 29 2006, 01:51AM

I saw your 6kj...nice stuff and very dangerous

I would definitely have a clamp diode...and if you use SCRs, you must, or you'll lose them for sure. I have a huge diode rated for 6000a pulses hooked in parallel to the coil, so backwards induced voltages are shorted. This protects caps and SCRs.

12kj...if you use any coil sized for HDD platters I think it might detonate...Now with these smaller designs and SCRs you have a lot more efficiency than with a spark gap...you might find similar performance with 3.7kj SCR switched rig to your 6kj spark gap one. I'm getting close to 10% efficiencies with mine and 3 SCRs in parallel.

I charge mine on 5ah 12v gel cell with a cheap inverter which powers a transformer from a 1950s era TV, which then charges my 1400v bank via a bridge reg and a center tap like a voltage doubler...

Re: new caps & basic launcher testing rig
Quantum Singularity, Wed Mar 29 2006, 02:17AM

Ok, wasnt sure about the clamping. I'll hold off until I can get some big diodes. There were a few from digi-key mnetioned in another post here from Rupidust I might look into, but 6,000A pulse rated diode I havent ever seen... is this something commonly available (if so where?)?

BTW, I probably wont be firing a 12kJ coilgun... on my 6kJ one I dont think I ever went more than 3kJ. But I do want to parallel both my big caps and try to push more weight with a better more rugged launcher design. And the coil I used was #8 awg and with 3kJ wasnt having any problems (the 12 awg was destroyed though). And the disks I am using are probably bigger than yours too. Everthing I have used so far has been the 5" disks from 'older' hdd's. I even have a super large 12" disk I want to try from some kinda mainframe /server machine that looked like a small washing machine.

Wilson, just to let you know, when I said 'simple spark gap mechanical switch' I meant like a couple of copper pipe peices with another copper pipe piece that slams together. Simple, cheap, and rebuildable. If I have the time I will make it selenoid & spring powered so it can be triggered by remote instead of by hand. Until I get an scr bank built it will suffice.

Thanks for all the words of wisdom everyone. Its been a few years since I have done any HV or EMP projects, and because I found this site a month ago I have a rejuivinated interest!

Re: new caps & basic launcher testing rig
FastMHz, Wed Mar 29 2006, 06:50PM

The diode is a Semikron and I got it surplus from someone on this forum a while back. It measures about 4" long by 1.5" wide and actually had two diodes in it.

Interesting that your 12awg was destroyed - I'm firing 3.5kj through 14awg. I have two 3.5" diameter coils of 14awg layered on top of each other and in parallel. It's completely epoxy coated which is probably what keeps it alive.

Your discs are bigger - mine are 3.5" in diameter.

EDIT: I noticed on your web site that your rings were cupped too...my 5/16" thick rings were also quite cupped and I need to hammer them flat repeatedly. This would indicate to me that the actual forces on the ring are around the outer edge...or may the magnetism in the ring itself be doing this?

Re: new caps & basic launcher testing rig
Quantum Singularity, Thu Mar 30 2006, 07:33PM

I am sure if the #12 awg coil I used was epoxy coated and securly mounted it would have probably survived just fine. Back then I had just used the duct tape method onto a piece of plywood. The #12 wire developed all sorts of kinks around the coil. Almost looked as though it was trying to form the coil into a square? My pulse length was probably a lot shorter and peak currents much higher, so I assume my coil was seing alot more di/dt which probably helped in its demise. When I tried the #8 coil it held up fine and didnt distort, so we used it. It appeared no different in performance but how do you really tell when things are going up high in the air? I am sure a chrono would have detected which one was better.

The were only 2 rings that were severely cupped with a single shot, one is the one in my avatar and I dont know how much power went into it. Your setup is probably more efficient as my 2-3kJ shots only slightly cupped the ring (like for example the darker ring pictured probably underwent 2-4 shots to become cupped - I dont remember for sure. The other one is the strangely warped one, it went from slightly cupped (even) to that odd oppositely bent form in one shot (~7kV), and thats the shot that the coil severely distorted. 5/16" is also alot thicker and probably takes more force to bend! Mine are all like 1/8" I am guessing.

About the cupping, I cant remember for sure which way it cupped (away from the launcher on the outside or inside). I think it was the outside - so placed in its original launching position would look like a bowl. I am no expert but this is what I have surmised what happens to bend the disk. The magnetic force is purely repulsive, correct? So the disk should just fly away rather then bending. The force holding it back is obviously inertia and air pressure. Inertia alone wouldnt cause it to bend unless there was a substantial amount more magnetic repulsion around the outer parts of the disk compared to the inner parts - this is possible. But the air pressure created must be huge. I have found in my launcher that over 3/4 of the force is excerted in the first say 3 inches. Accelerating a flat disk several square inches to a few hundred feet per second in a matter of a few inches is an enormous force of air pressure against a flat object. I dont know that much of physics to calculate the air pressure seen. Now then, there is one key thing that can help determine what is truly going on and I think you might have this. When you make a somewhat aerodynamic body above the disk, like a rocket, you have eliminated air pressure as a cause. But to determine better, the housing should place equal force on the entire surface of the disk (not just a tube that touches the outer edges). If it meets this criteria, then any resulting warpage must be the result of inertia or other magnetic forces at play. So does your rocket evenly distribute the weight accross the disk? And does your disk bend as much as with nothing on it, or not at all? Sorry for the long post but I think this is very interesting.

Re: new caps & basic launcher testing rig
FastMHz, Thu Mar 30 2006, 08:42PM

Excellent concepts in your post! My current rocket has the disc mounted to a piece of plastic which is then mounted to the base of the rocket. The plastic bends a little I'm sure, and the disc is severely cupped after just a 1.8kj launch. But the base of the rocket is solid great stuff, so it's not just the edge with pressure. These are 5/16" thick discs, much thicker than a HD platter but still cupping.

I'm in the process of building a new rocket and will be testing it with both a 5/16" disc and a new 1/4" I just cut out. I'll see what I can do to distribute the load as evenly as possible.

I also just built a new dual layer coil which is wrapped even better than my previous, and using a better fabrication method both sides of each layer are epoxied. My current dual layer coil suffered a bit of damage when the 1/4" disc went up 5 feet and landed edgewise...cracked the surface

Re: new caps & basic launcher testing rig
Quantum Singularity, Fri Mar 31 2006, 03:33AM

What I was thinking of making that would definately disperse the weight evenly on the disk is a solid wood rocket out of large round stock (like a dowel) and turn it on a lath and cut a roundnose shape to one end and leave the other end perfectly flat. I figure for good ballistics the length should probably be double or so the width. Do you happen to have a lathe? I probably wont get around to high power testing for a few months yet. I am not 100% sure the best way to attach the disk to the base of the wooden slug though, but even if its not attached at all it should probably be ok, it will just seperate in the air at some point.

To protect your coil maybe put a piece of 1/8" polycarbonate sheet on top it? You could even epoxy it to the top for extra strength, but then it couldnt be removed or replaced.

Re: new caps & basic launcher testing rig
FastMHz, Fri Mar 31 2006, 07:38PM

A lathe is one thing I don't have A solid wood projectile is a great idea but it may be too heavy. My great stuff rocket weighs 199g. Maybe a light weight wood like cork or balsa but stronger. I'm not sure what qualifies though...

I have my current ring screwed to a piece of plastic using three counter sunk screws, and then the plastic is screwed into the center of the rocket which is a pine core. It held up well except for the cupped ring.

My new coil has an extra coat of epoxy on it...I thought about plastic or even cardboard but that would reduce the magnetic coupling by quite a bit due to inverse square law...those pesky laws always get ya

I haven't launched anything for a few days...today's weather is great so I'm going to see about testing some things.

Re: new caps & basic launcher testing rig
Quantum Singularity, Fri Mar 31 2006, 09:06PM

Yeah, you want the disk to be as close as possible for max coupling but if your using mag wire I dont think 1/8" away will make very much difference, though I dont have effeciency and velocity measurements so I could be wrong, On my previous HV launcher setup I need 1/8" anyhow to keep it from arcing to the projectile and bypassing the coil. I just used regular insulated wire to prevent this rather than mag wire, but mainly because it was alot cheaper. When I start doing it again, especially with low V I'll be using mag wire. I have a small spool of #13 wire which I'll use on my electrolytic setup.

As far as the wood being to heavy... well its only too heavy if your needing some kinda minimum speed. But to be honest, using smaller disks (3.5") a peice of round wood 3.5" diameter and say 6" long will not weigh very much. I would even put money on it saying with the proposed bullet shape wood would go faster and higher than a flat disk alone. As shown on my website and in other posts I have launched a much larger peice of wood before, not aerodynamic at all, just to see what the extra weight would do. In my particular setup the efficiency had to be several times greater with the chunk of wood atop the disk. I dont have numbers, but launching straight up in the sky, if something goes 1/2 the hieght yet ways 5 times more, it is alot more efficient. I attributed this to the extra weight holding the disk down longer in the magnetic field. I am assuming the disk alone flies off the coil long before the peak pulse power is reached. Like I said I dont have any measured data but with my setup it all makes sense atleast. There is some light wood I have, I think its basswood maybe but I dont have a big enough peice to turn to 3.5" although I could probably get some as it came from a tree in my parents backyard. But I will just use pine, or whatever I can get cheap thats light and turns well. I dont mind the extra weight. You could even cut it in half and hollow it slightly, as long as you leave about 2" thick base I would think it would hold up... Got to get some diodes so I can start experimenting again!

Re: new caps & basic launcher testing rig
FastMHz, Fri Mar 31 2006, 10:28PM

You're right about the weight helping sometimes...I think I'll glue two 2x4s together and manually shape them into a cylinder and then sharpen it. I may cut it in half and hollow it a bit...perhaps add a payload, such as an egg and foam to see if it survives Then again it may shatter into a shower of splinters...

Re: new caps & basic launcher testing rig
Quantum Singularity, Sat Apr 01 2006, 03:04AM

Well dont forget 4"x4"s are easy to come by if you dont want to mess with putting 2 2"x4"s together. Then again maybe that would make a convenient way to open it to put in or remove cargo. If you dont have a wood lathe and dont want to sacrafice hours and hours I wouldnt worry about trying to shape it into a round nose, I bet a chisel cut on every edge would nearly as aerodynamic. I dont know the geometric term but it would look like a 4 sided pyramid for the tip. I myself would use a table saw or compount miter saw (if its big enough) set maybe to 30 degrees for a leading edge. Or if you were really good on a bandsaw you could follow the dome shape on each edge, that would give you a rounded pyramid shape whatever the heck that is called. I might have to find that out.