ElectroThermal Cannon

Boom! The classic potato gun harnesses the combustion of flammable vapor. Show us your combustion spud gun and discuss fuels, ratios, safety, ignition systems, tools, and more.
headslammer
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Tue Jan 26, 2021 6:00 pm

Hello everyone!
I've been a lurker for a year or so now, having come here for help and research on multiple occasions on nearly all of my projects.
Over the last few years, I've made multiple basic hairspray cannons, a very temperamental metered propane cannon, and a large and rather powerful pneumatic cannon.

After making various new designs for a bigger pneumatic, I realized that the time and effort spent wouldn't be worth the marginal gains in performance I would see, so I scrapped that. After investigating hybrids for a short while, I found Larda's Electro-Thermal gun and instantly wanted to build one. So, here's my first attempt at a design for one, subject to much change.
Image
My idea is to use a combination of a locking lug system from a bolt action rifle and an interrupted screw breech with the obturator mechanism, as I intend for this to be a breech-loader.
Would love to hear some feedback on this, hopefully Larda and DYI see this at some point but they don't seem to be terribly active anymore.
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Moonbogg
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Wed Jan 27, 2021 4:30 pm

I'm not familiar with these as I'm more of a potato/soda can launching kind of guy myself. However, after watching Larda's videos of his shooting, might I suggest just buying a bolt action target rifle instead? :lol: Jokes aside, I haven't seen Larda in forever and I don't think anyone else around here has much experience with something like this. Good luck and keep us updated. Make sure to wear eye protection and a safety t-shirt.
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Wed Jan 27, 2021 8:09 pm

Moonbogg wrote:
Wed Jan 27, 2021 4:30 pm
I'm not familiar with these as I'm more of a potato/soda can launching kind of guy myself. However, after watching Larda's videos of his shooting, might I suggest just buying a bolt action target rifle instead? :lol: Jokes aside, I haven't seen Larda in forever and I don't think anyone else around here has much experience with something like this. Good luck and keep us updated. Make sure to wear eye protection and a safety t-shirt.
Yeah, I've considered just buying a gun but honestly, this is much cooler imo. The bolt-action-esque locking lug system I'm thinking of here is just me trying out different methods of preventing the rear of this thing from flying out. If/when this is done I'm going to remote fire it from the next town over lol
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jackssmirkingrevenge
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Thu Jan 28, 2021 2:47 am

Welcome to the forum!
headslammer wrote:
Tue Jan 26, 2021 6:00 pm
After investigating hybrids for a short while, I found Larda's Electro-Thermal gun and instantly wanted to build one. So, here's my first attempt at a design for one, subject to much
How come you discounted hybrids? From what I remember of Larda's work, his hybrid was significantly more powerful than the ETG and it didn't require a capacitor bank.

It seems to me that if performance gains are the object, hybrids are more easily scalable.
hectmarr wrote:You have to make many weapons, because this field is long and short life
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Sun Jan 31, 2021 7:22 pm

jackssmirkingrevenge wrote:
Thu Jan 28, 2021 2:47 am
Welcome to the forum!
headslammer wrote:
Tue Jan 26, 2021 6:00 pm
After investigating hybrids for a short while, I found Larda's Electro-Thermal gun and instantly wanted to build one. So, here's my first attempt at a design for one, subject to much
How come you discounted hybrids? From what I remember of Larda's work, his hybrid was significantly more powerful than the ETG and it didn't require a capacitor bank.

It seems to me that if performance gains are the object, hybrids are more easily scalable.
Funny you should mention that, after doing some more thinking and pricing, I've ended up back at a Hybrid design. Would you happen to have a link to Larda's thread on his? I find it difficult to find individual threads on phpBB based forums.
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Moonbogg
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Sun Jan 31, 2021 9:03 pm

How much pressure are you going for? What will you be shooting out of it and what velocity are you going for? If the answer is "maximum power to shoot a bullet-like thing", then there's really only one formula I can think of that makes sense: 4140 (or similar strength alloy), burst disc, big bottles of high-pressure oxygen, and a resistance wire that glows red inside the chamber to ignite it. Larda did a good job of it and it's a cool idea if you're into that kind of thing, but if it were me (and it's not) I'd rather buy a rifle (which I have). A simple rifle does the same thing (shoot something small at high speed) that the most powerful hybrid will do, but the rifle will be infinitely easier to use with a FAR better rate of fire, better at what it does, and all around more practical not to mention hugely more accurate. Also, a rifle is hugely cheaper. I mean, HUGELY cheaper, unless you machine the hybrid parts yourself and heat treat the alloy yourself. I imagine you'd have to buy 4140 annealed (or similar) and depending on the size, rough machine, heat treat and final machine.
If you can't legally own a rifle or don't want one, then ignore everything I said. At the end of the day, a high-powered hybrid is essentially a crappy home-made gun with a rate of fire that is put to shame by even the most janky black powder rifle or even a muzzle loading cannon. Hybrids are a pain in the ASS to operate in the higher powered category. Basically one shot every 5 minutes or something like that. It's a process that is dangerous and complicated and in my humble opinion, simply not fun. Again, if going for high-power, you'll need high-pressure pure oxygen. There's a perfectly reasonable chance of you killing yourself.
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Mon Feb 01, 2021 2:39 pm

Moonbogg wrote:
Sun Jan 31, 2021 9:03 pm
How much pressure are you going for? What will you be shooting out of it and what velocity are you going for? If the answer is "maximum power to shoot a bullet-like thing", then there's really only one formula I can think of that makes sense: 4140 (or similar strength alloy), burst disc, big bottles of high-pressure oxygen, and a resistance wire that glows red inside the chamber to ignite it. Larda did a good job of it and it's a cool idea if you're into that kind of thing, but if it were me (and it's not) I'd rather buy a rifle (which I have). A simple rifle does the same thing (shoot something small at high speed) that the most powerful hybrid will do, but the rifle will be infinitely easier to use with a FAR better rate of fire, better at what it does, and all around more practical not to mention hugely more accurate. Also, a rifle is hugely cheaper. I mean, HUGELY cheaper, unless you machine the hybrid parts yourself and heat treat the alloy yourself. I imagine you'd have to buy 4140 annealed (or similar) and depending on the size, rough machine, heat treat and final machine.
If you can't legally own a rifle or don't want one, then ignore everything I said. At the end of the day, a high-powered hybrid is essentially a crappy home-made gun with a rate of fire that is put to shame by even the most janky black powder rifle or even a muzzle loading cannon. Hybrids are a pain in the ASS to operate in the higher powered category. Basically one shot every 5 minutes or something like that. It's a process that is dangerous and complicated and in my humble opinion, simply not fun. Again, if going for high-power, you'll need high-pressure pure oxygen. There's a perfectly reasonable chance of you killing yourself.
Agreed, If I do go ahead with this, I'm not going to bother with small-bore, it would be 2" at least with a barrel to match, probably in the 84-96" range. Thankfully, as the chamber and barrel get bigger, the pressures during detonation decrease, making the range of materials I am able to use much wider.
At the moment, I am looking at some .25" wall DOM tubing, but am still doing some research. In terms of pressure, I'll go as high as the material can handle with an acceptable safety factor.
90% of the reason I do this isn't to make anything practical, I just do it because I enjoy designing and building things lol.
I might just go buy an old bolt action for fun, but I haven't really got a place to shoot it. At my friend's place, we can get away with a .22 and potato cannons but I doubt we could get away with anything significantly bigger/louder.
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jackssmirkingrevenge
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Mon Feb 01, 2021 6:43 pm

headslammer wrote:
Sun Jan 31, 2021 7:22 pm
Funny you should mention that, after doing some more thinking and pricing, I've ended up back at a Hybrid design. Would you happen to have a link to Larda's thread on his? I find it difficult to find individual threads on phpBB based forums.
Here you go: https://spudfiles.com/viewtopic.php?f=28&t=15524
hectmarr wrote:You have to make many weapons, because this field is long and short life
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Mon Feb 01, 2021 10:10 pm

jackssmirkingrevenge wrote:
Mon Feb 01, 2021 6:43 pm
headslammer wrote:
Sun Jan 31, 2021 7:22 pm
Funny you should mention that, after doing some more thinking and pricing, I've ended up back at a Hybrid design. Would you happen to have a link to Larda's thread on his? I find it difficult to find individual threads on phpBB based forums.
Here you go: https://spudfiles.com/viewtopic.php?f=28&t=15524
So he did use air and propane. My memory failed me. I thought it was pure oxygen and propane. I wonder what would happen at 200x with pure oxygen and propane. Probably just explode or something...
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Wed Feb 03, 2021 9:18 am

Moonbogg wrote:
Mon Feb 01, 2021 10:10 pm
I wonder what would happen at 200x with pure oxygen and propane. Probably just explode or something...
One would think DDT is more likely, but I would imagine that an adequately constructed chamber would be sufficient to contain it.
hectmarr wrote:You have to make many weapons, because this field is long and short life
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Wed Feb 03, 2021 3:03 pm

I'm pretty sure Larda holds the record for highest mix, at least the "public" record. 190X was totally insane.
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Wed Feb 03, 2021 4:12 pm

In my personal opinion, I believe that hybrids have the advantage of needing less air than tires, to achieve the same power. This is attractive. On the other hand, I think that the high "X" are one aspect of these weapons and not everything! The speed between shot and shot is an important aspect, so that you do not have to work so much measuring the gas, closing and opening valves, pumping air and all that. I think that in order to have a satisfactory experience shooting with a hybrid, the two must go together. For this reason and ventured into the systems they already know. I suppose that a hybrid like Larda's that can fire every 5 or 10 seconds, is a powerful and very practical weapon, as well as very dangerous, without a doubt.
Nice to have interesting dreams in the world of these things. :)
I am the author of the first semi-automatic hybrid, check this out :shock: ...viewtopic.php?f=28&t=26676
And of the hybrid that can shoot in the shortest time: :shock: viewtopic.php?f=28&t=27190
Single-pump autonomous hybrid . viewtopic.php?f=28&t=27038
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