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1.4kJ electrothermal gun

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Wizzup

Sun Jun 30 2013, 02:14PM

Registered Member #3302 Joined: Sun Oct 10 2010, 02:21PM
Location: Finland
Posts: 42

Thanks, I just uploaded a video of another outside shooting with a much larger projectile:

radhoo

Sun Jun 30 2013, 02:18PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

I watched both, nice! How is the barrel attached to the discharge chamber presented in one of the pics

Wizzup

Sun Jun 30 2013, 02:21PM

Registered Member #3302 Joined: Sun Oct 10 2010, 02:21PM
Location: Finland
Posts: 42

The barrel and the chamber have M10x1.5mm threads on them so I just put some PTFE tape on the barrel threads and screw it in.

Also I wrote it already in the video describtion but I bought 6 pieces capacitors from ebay today, they have same stats as my current ones so I will wire them for a bank of 4700uF at 1350V, thats about 4.3kJ. The new setup should discharge in the same time as the current one but will deliver 3x the energy. I cant wait for them to arrive so I can start building it.

Wizzup

Thu Jul 04 2013, 04:47PM

Registered Member #3302 Joined: Sun Oct 10 2010, 02:21PM
Location: Finland
Posts: 42

The new capacitors arrived today, I tested all of them and they work well. 6 pieces of them, 4700uF 450V each. I will get some copper pipe tomorrow that I will flatten for busbars and connect the caps for a nice bank.

EDIT: I will put a 47k bleeding resistor for each row of 3 caps in parallel so they will keep the bank discharged when not used and they also act as balancing resistors. I will not trust only those resistors to keep every row at balance but I will also put 3 separate voltmeters in place to measure the voltage of each row so that I can stop charging if one row is going near 450V.

radhoo

Thu Jul 04 2013, 05:27PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

I would use a regulated power supply to charge them, instead of measuring the voltage and stopping the loading process manually, the power supply should be capable of providing the selected voltage, and not more. Nevertheless these are `minor` details, looking forward to your new test! :)

Wizzup

Thu Jul 04 2013, 05:39PM

Registered Member #3302 Joined: Sun Oct 10 2010, 02:21PM
Location: Finland
Posts: 42

I know that a regulated power supply would be good but I don't really see a need for one right now, my charger is not that powerful and my bank is pretty big compared to it so the voltage won't rise that fast.

EDIT: Capacitors are now connected, I got some 1.5mm thick copper sheet from a local shop instead of flattened copper pipe and it became very nice in my opinion:

The new capacitors have attachment bolts on their bottom so the bank is attached to the wood under the capacitors so it is very sturdy. I will build a new switch tomorrow maybe because I dont really like the current one.

EDIT2: I got 30k bleeding resistors instead of 47k, I made them by connecting 2 15k 5W resistors in series and I have extra ones so I can change them to 45k if needed. I made a new switch that works in a different way than the old one and is much sturdier design. Also I put double cables everywhere to decrease the resistance and replaced many aluminum connections with copper.

Here are some pictures about the new setup:

bleeding resistors

bleeding resistor closeup

new switch

switch surfaces

switch surfaces top view

trigger

overview

side overview

chamber front

chamber back

If you want more pictures from some specific spots don't be afraid to ask.

I still need to make a safety discharge thing to this because the old one that just shorts the terminals isn't very good. I thought about a light bulb but I would need like 4 of those in series to prevent them from blowing up as my bank's max voltage is 1350V. Ofcourse the bleeding resistors will discharge the bank but it will take several minutes, I would like to have a better option than just waiting.

I did the first firing of the thing at 900V ~1.9kJ with a 6mm steel bearing, the result was very much like my previous shot on the aluminum but the crater was a little larger in diameter. I will have to wait for my 10M resistor to arrive so I can charge past 1000V as that is the max of my multimeter.

EDIT3: I made nice casings for the bank and charger from wood and lexan, they are now easier to handle and much safer. Now I dont have to worry too much if a diode or something blows up. Also for the safe discharge system, I ordered 2x 100W 15k resistors that I will put in parallel and discharge the bank through those. I dont know yet how I will switch the discharge system but Ill come up with something.

Here are some pics:

bank front panel

bank overview

charger front panel

charger overview

Wizzup

Sat Jul 20 2013, 04:02PM

Registered Member #3302 Joined: Sun Oct 10 2010, 02:21PM
Location: Finland
Posts: 42

I fired this thing with 3.3kJ energy and the chamber's nylon insulator couldn't take the pressure so it ripped out the nylon threads and the electrode hit the brick behind the ETG with some serious force...

Here is a video of the shot:

Even though the insulator blew out the projectile was accelerated to 225.7m/s regarding to the chrony so that is about 215J kinetic energy and about 6.5% effiency. That is pretty good in my opinion when you take into account that a lot of gas was wasted because of the insulator...

I will have to make a new chamber with longer threads for the insulator or make some kind of extra attachment thing for the current one to prevent it from blowing out.

I am considering buying a mini lathe, with it I could easily make a much better chamber and projectiles and lots of other stuff for other projects.

Goodchild

Thu Aug 15 2013, 08:56PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Wosa, how did I get over here? I frequent the Tesla coil forum the most but this has got my attention. Partly because I love electronics and also because I love real firearms in general.

What catches my interest is that even with your failed chamber you managed to get a projectile energy of nearly double what is seen with stock .22LR. (~160J) I imagine that you could approach 500J with a properly designed chamber and slightly more Q. This is entering the realm of 9mm pistol muzzle energies (~520J).

I can honestly say that this is the first hobby built electro style gun (that I have seen) that actually approaches the world of real firearms!
I may just have to build something like this in the near future. Great work, canâ€™t wait to see more!

Kizmo

Thu Aug 22 2013, 10:11AM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

I shot 8mm projectile through 4mm steel plate ;)

Wizzup: if you ever need more capacitors or parts for your ETG just contact me, i have huge stash of 3300ÂµF caps and i can machine pretty much anything.

Wizzup

Thu Aug 22 2013, 02:09PM

Registered Member #3302 Joined: Sun Oct 10 2010, 02:21PM
Location: Finland
Posts: 42

Very nice shot :) What was the capacitor configuration in that shot? I tried shooting a 2mm steel plate with the old 1.4kJ bank but my projectile tip just crushed and made a large dent on the plate, I guess I need to use harder steel to make it penetrate steel.

Before getting any more capacitors I need a new chamber that can hold the current pressures without blowing up so I have been looking for a small lathe to do the parts for my cannons, I am considering this one

as the working space of the lathe is 550mm long so I can do small and medium size parts with it and have some space to work. In Finland that same model costs about 1400 euros but just 900 euros if I buy it from Netherlands. You have experience with machining equipment Kizmo so do you think it is a good price for the machine or should I maybe look for an older used machine? I would use it to machine small parts such as ETG chambers and similar size parts from aluminum and steel and sometimes maybe a bit larger parts too.

I appreciate it that you could do the machining for me but I have a few other projects in my mind too that I would need a lathe in so I see it is better to get my own lathe. I will be interested in those 3300uF capacitors though when I make a new chamber so I will send you a PM here or in youtube when the time comes :) I can also send you my skype if you want to talk more.

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