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5 shot hybrid repeater

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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5 shot hybrid repeater

Unread postAuthor: spudamine » Sat Jan 24, 2009 6:56 pm

This is my second cannon and my first foray into the world of combustion, she's not finished yet but I think there's enough material here to share. When it warms up around here enough to get back out into the garage I'll finish the build and post in the showcase. So far single rounds have been fired pneumatically using a burst disk and at a 1x combustion with no burst disk, 2x hybrid shots will be tested in the spring.

Description:
This is a hybrid cannon designed for automated fire at a 2x mix. I wasn't after a super powerful hybrid, just for something with a bit more punch than a standard combustion, what I really wanted to have a crack at was using burst disk cartridges.
The chamber and barrel are both made from 16 BAR PVC. There has been much debate on this forum about the use of PVC in hybrids, after digesting the bulk of it I decided this PVC would be sufficient, with precautions. So the chamber is completely sleeved in couplers giving a combined wall thickness of 12mm, this is set in an aluminium framework with galvanised steel sheet bolted around it to contain any shrapnel should the chamber fail, and it looks cool. The barrel and chamber are also held together in a rigid aluminium framework so the PVC is not bearing any of the weight or under any tension.
This framework is also necessary for the breech to work correctly. The chamber is 50cm of 110mm PVC, the barrel is just under 2m of 75mm PVC for tennis ball calibre.

Fuelling was originally going to be using a meter tube for the propane and a 6L tank for air, both regulated. control would be via 3 solenoid valves, one where each ball valve goes on a standard fuelling system. As solenoid valves are kind of expensive to buy I thought I would make my own for a laugh and made a manifold of 3 out of a solid block of aluminium. (If anybody hasn't guessed yet I really like aluminium.)

The valve actually worked, but to overcome the friction of my o-ring seals I had to make them really powerful so they get very hot and can't be left on too long. To simplify the fuelling and to avoid having to get 2 regs, I moved to a manometric system, filling the 6L tank with a 5x mix and then feeding this to the chamber via a regulator to set the right x, the solenoid and a check valve.

The breech is the complicated bit. In order to get automated fire, a rotary closure like a union would be too complex, a cam lock is a step in the right direction but would again have been too complex. Air rams would simply not have been strong enough to hold the breech closed even during fuelling, let alone firing. What I came up with was using two linear actuators to open and close the breech, consisting of a geared motor attached to a threaded rod. Hobby motors cost a fortune so I used a couple of rechargeable drills bought on the cheap with no batteries or motors. 12V drills, with the crappy switching transistors pulled out and overvolted with a 24V battery produce some serious torque. For threads I used lengths of M10 rod, a wedge had to be ground into the bases to prevent them being pulled out of the drill chucks. I could have removed the drill chuck and case but the torque settings on the drills were very useful, preventing the gun from pulling itself apart when the breech was closed. The combined set-up is very powerful, I had to use some additional steel box on the cannon to prevent the chamber from bending away from the barrel when the breech was closed and pressurised. With a very strong burst disk I have successfully pressurised the chamber to 150 PSI with only minimal leakage, at this pressure there is some serious force on a 3" burst disk! I have no doubt if you accidentally cought your hand in the breech when closing, it would snap your fingers like matchsticks.

The cartridges have a burst disk sealed onto the face using flexible silicone. The cartridge is dropped into the breech, the motors pull the barrel in and the cartridge is sealed between the breech and barrel, both of which have O-rings on the ends.
The magazine has not been finished yet, but will hold 5 gravity fed cartridges. Feeding will be controlled by one or two solenoids in the magazine. Another motor will be fitted under the chamber to extract fired cartridges, which will drop out of the bottom of the breech.


When finished the full reloading cycle will be:
1. Fire
2. Two motors open breech
3. Third motor ejects spent cartridge
4. Solenoid valve opens, purging chamber with fresh fuel mix
5. Magazine solenoids actuate dropping next round into breech
6. Breech is closed, ejector retracts
7. Solenoid valve opens a second time, refilling chamber to 1 bar
8. Fire (ignition by flyback transformer)

To achieve this I am currently working on a control board with a pic controlling about 10 high current relays. In theory a single button press should be able to fire 5 rounds off in under 1 minute, however there is a lot that can go wrong - at the moment my worst fear is the chamber melting...

For anybody who has actually read all of this here are some pictures and a video as a reward and proof that I am not just a rambling madman.

http://uk.youtube.com/watch?v=U0EUq9rGlBc
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Attachments
P1000186.jpg
A closer look at the breech area
valve.jpg
the diy solenoid valve manifold,with one valve complete, I hate winding coils.
P1000179.jpg
The 6L fuel/air tank, I had to drink a lot of cider to make this (9L in fact as I blew up the first one to see how much pressure it would take)
P1000184.jpg
The cannon so far (no It's not mounted on my bike before anybody asks)
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Unread postAuthor: LCTChamp » Sat Jan 24, 2009 7:14 pm

Wow great gun. Believe it or not I read everything. I hope the loading system works out. What are the specs of the chamber? You said it was rated to 16 bar. How much psi is that? The gun is really complex, and that is the way I like things. I can't wait to see it finished!!!
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Unread postAuthor: Ragnarok » Sat Jan 24, 2009 7:34 pm

Um... Bloody hell?
I'm not sure what more to say to be frank.

lctchamp wrote:You said it was rated to 16 bar. How much psi is that?

Too lazy to look up a conversion table or pull out a calculator?
It's 232 psi.
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Unread postAuthor: Big Boom » Sat Jan 24, 2009 8:27 pm

Nice cannon. I found it entertaining that after all of the effort you went through to prevent any stress on the barrel, the plywood board fell right onto the barrel.
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Unread postAuthor: starman » Sat Jan 24, 2009 8:44 pm

Really good work here spudamine! I still can't believe you wound your own solenoids, but I applaud your inventiveness... :wink: I like your aluminum framework and sheet metal "chamber guard" although in the event of DDT catastrophe (highly unlikely at 2X), it would just become another missile.

I like it a lot...sort of mirrors some ideas I've been kicking around actually. Why wait until spring to shoot 2x.. :?:
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Unread postAuthor: jackssmirkingrevenge » Sun Jan 25, 2009 12:27 am

Ridiculously overcomplicated, love it :) have you considered self contained cartridges though? You can channel your apprent love for fabrication into making the cartridges, which would then have sufficient power to operate a simple blowback reloading mechanism.
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Unread postAuthor: ALIHISGREAT » Sun Jan 25, 2009 3:29 am

Haha the cider bottle fuel containers are quality :D i bet you hated having to drink it all :wink:
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Unread postAuthor: Brian the brain » Sun Jan 25, 2009 5:15 am

yikes.
You´ve got some skills!

Love the aluminium parts!
You should check out Neospuds website for some inspiration on pneumatics....


Seems like you are able to build some good ones!
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Unread postAuthor: spudamine » Sun Jan 25, 2009 5:28 am

Thanks for all the encouragement guys, I need it keep up the momentum on this project.
@ Ictchamp, Thanks, I like complex to, spudders seem to fall into two camps. Those who admire simplicity, elegant design and clean lines, and those who just like to get stuck into a nightmarish mess of wiring, piping and metalwork. Although I can see the attraction of the former the latter is a lot more fun to build. As rags has stated the pipe used is 232PSI, however the sleeving couplers should up that significantly. That should offer some serious protection against pressure spikes. I'm only really worried about the PVC melting, I'm sure I heard somebody say somewhere that large PVC chambers can melt, and this is large.

@ Ragnarok, lost for words? a rare occurance indeed :D

@ Big Boom, yes a slight error there, however it goes to prove my point, on a normal cannon that could have caused some real damage as the weight of the board would have been transferred directrly to the breech via the barrel. On this cannon it was not a problem.
A larger error was actually made on the second shot , you will see I have moved the backstop to protect my bike from riccochets. Being a combustion newbie I wasn't suspecting such power from this shot, and as a consequence now have a perfect tennis ball shaped imprint in my garage door, it even started to peel two sections of steel sheet apart.

@ Starman, Thanks winding solenoids was really a pain, the first one was interesting the rest were a bit of a chore, although I improved the design by coating each layer in thermal epoxy as I went along. Not pictured there ia also a piece of malleable iron pipe which fits over the coil and really increases the power.
I hope not to have to see how useful the chamber guard is, the theory is any blast wave will be diverted out of the unshielded base and the shrapnel will be stopped by the steel sheet, or at least have a significant portion of its energy removed tearing it off.

@JSR, A man after my own heart, you can't beat overcomplication :D
Do you mean adding the fuel to the cartridge as well as the burst disk and projectile? I did think about this but couldn't come up with any feasible way of doing it. You could probably do it with a single use cartridge but that would be a lot of work for one bang.

@AlI, It was indeed an onerous chore.

@BTB, It's amazing what can be made with a pillar drill, a grinder and a power saw. Oh yes and quite a bit of time, I have been building this on and off for about 9 months now.
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Unread postAuthor: ramses » Sun Jan 25, 2009 4:57 pm

I would build a shield for the storage tanks, in case the mix in them ignites.

other than that, I love the muzzle brake, and the angle the first picture was taken at was pretty sweet, also. in the future, you could wind the solenoid on a lathe, drill press, or drill-in-a-benchvise.
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Unread postAuthor: daccel » Mon Jan 26, 2009 3:48 am

That's awesome! I don't think I would have the patience to work on something that complicated for so long.. so nice job getting to this point. Hope we see it finished soon :).

Ah, so that's the premixed reservoir you referred to the other day. If you don't want to build a shield perhaps just a long line so it's at a safer distance if it does go.

On the pre-filled cartridges, you simply need a burst disk holder on one end and a filling valve such as a schrader on the other. Though it's probably a lot more difficult to find a burst disk holder for a cartridge this size. Perhaps a threaded coupling and cap could be modified and some o-rings added to create one?
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Unread postAuthor: inonickname » Mon Jan 26, 2009 3:51 am

Awesome, love the homegrown solenoids :lol: . Though personally If I was building something like this I'd use cartridges. Great work
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Unread postAuthor: spudamine » Mon Jan 26, 2009 4:52 pm

I think the fuel tank is ok as is, I don't particularly want it to go off but if it does it will just make a very loud bang, and I'll have to do some more drinking. I never get that close to the thing ayway, firing is done remotely on the other end of a 20m command cable. I rather like the muzzle brake, that's one of the better things I have found with using PVC, you can make things like that quite easily, although I'm not sure if it will do sod all.
With the coil winding the actual winding part isn't that bad, I used quite thick wire so there's only about 300 turns and made a small hand winding jig. The problem I had was getting the gap down the middle the right size, I used a piece of the actuator as the spindle and wrapped it in PTFE tape to give a bit of clearance and get it out without the epoxy sticking it in. It's also quite hard to stop the ends sliding out and making the coil longer.
As stated earlier cartridges would make an interesting project but there are some real problems with making them this big,as daccel said, and that was the main reason I built this.
To get a similar power as with the present chamber I would have to go up a lot of mixes, making the burst disk holding a lot harder. There would also be a fairly good chance of the cartridges being destroyed if you used PVC, The only way around this would be to have the cartridge very closely fitted into the breech for extra strength, then you would not be able to use endcaps as you would need a flush cartridge. So then you're talking about metal cartridges and it all starts getting a little expensive at 3" diameter.
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Unread postAuthor: psycix » Tue Jan 27, 2009 11:20 am

The homemade solenoid is the most awesome part. Couldnt you make it smaller? (less friction)

I'd like to note that the sound in your video is awesome! A very low and loud bang without the mic going crazy. What did you record it with?
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Unread postAuthor: spudamine » Tue Jan 27, 2009 5:58 pm

Those solenoids seem to be really getting people going :lol:
They are far to big, the size was determined by several factors, the smallest O- rings I had, having a 1/4" (6.3mm) drill bit for boring out the holes which were a good fit for my 6mm bar material, and the next smallest stock size of Ali was slightly too small.
On the upside It made quite a novel valve with really good flow for a direct acting valve, although it only works within a narrow operating pressure band for a given spring setting. The spring force balances the pressure and the solenoid is just enough to tip the balance and open it.

the camera I use is an old Sony digital 8 camcorder, which wasn't exactly new technology when I bought it about 4 years ago. Bizzarly, with the current trend towards miniaturisation my big old camera still puts up a good fight, its got room for a decent mike and a 20x zoom lens. By all rights with the massive increase in CCD technology it should be completely obselete by now.
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