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Woody's Hybrid Build

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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Woody's Hybrid Build

Unread postAuthor: Woody » Fri Jun 24, 2011 4:00 pm

Alright, so I've always wanted to make a hybrid but never fully understood them to I just stuck to combustions. Well I was playing around with my cannon the other day which was a breech loaded, pvc, stun gun ignition, on board fuel combustion. The threads for the endcap were burnt and leaking air so I decided to tear it apart and work on making a hybrid and so my research on here commenced.

I don't plan on turning heads on this forum, I just want to make a functional possibly hand held hybrid capable of shooting a golf ball or ball bearing with some umph behind it. On this thread I will keep yins updated on how the build is going and ask for your input.

So far this is what I have. The plan is to use that 2"x8" pipe as the main part of the combustion chamber. A reducer and 1.5" nipple will lead into the 1.5" union. I accidently picked up a black iron union, going to have to exchange that (don't want to deal with the rust). I may or may not order a 1.5" cam lock to replace the union. On the other end is a 2" end cap. Instead of having 4 reducers and bushings to get it down to 1/4", the plan is to drill a hole in the center of the end cap and tap that for a 1/4" nipple. That will lead into a ball valve and then a pressure gauge to a T with ball valves going both ways with an coupling for air intake and a barb that will go to the propane tank. Ignition will consist of a spark plug in the center of the 2"x8" pipe hooked up to the stun gun. I probably will not go any higher than 5x mix unless I devise a permanent remote ignition.

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Do yins think this setup will be adequate to launch a golf ball or should I go with a smaller union or cam lock. Also on that note, what do you guys think about cam locks vs union? I've been using a 2" cam lock on my combustion with a low pressure burst disk with great results.
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Unread postAuthor: Woody » Fri Jun 24, 2011 4:30 pm

Turns out that I do not have a 1/2" 18 tap (that is the right size for 1/4" piping right?), so I think I'm going to make a trip tonight and pick up a smaller spark plug and the tap for the end cap and spark plug. Found 2 sets of 1/2" 13 and 20, but no 18... go figure.
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Unread postAuthor: Fnord » Fri Jun 24, 2011 4:36 pm

Well, you'll need a non-standard barrel for golf balls (assuming you're going with a metal barrel). You can buy sch10 pipe online in steel or aluminum, but expect to spend $20-50 for about 3 feet. Golf balls need a bore diameter of 1.69".

You may also consider tapping the end cap for your ignition system. It's generally safer than tapping straight into the side of the pipe. Have a look at this custom spark plug you can make for such a design.

edit; Oh and which hardware store did you go to? Any decent store should carry at least 2"x1" reducers, and 1" bushings down to .5 or so. Maybe take a better look next time you're there?
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Unread postAuthor: Woody » Fri Jun 24, 2011 4:59 pm

I was also considering running a wire though a 1/4" fitting and epoxying around it running the other wire to the chamber, the spark plug just seemed simpler. I'm not too sure if I want two offset holes in the end cap. Is tapping the spark plug into the side of the chamber really weaken it that much? I could always thread it in with a generous bit of jb weld, or even weld it if necessary.

I went to home depo for the parts. There's a local guy that might have a better selection but he closes early. I would of had to to 2" to 1.5" reducer, 1.5" to 3/4" bushing then 3/4" to 1/4" bushing. I think tapping into the end cap looks cleaner.


Edit: Think I would benefit (strength wise) from using two 2"x4" pipes with a T fitting in the center which will have and plug with either a store bought or homemade spark plug tapped into the center of it?

Edit 2: I'm going to follow through with what I said above with the T. I think I will stick with using a spark plug. I don't want to compromise on safety, so I believe this is the right decision.
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Unread postAuthor: ramses » Fri Jun 24, 2011 6:38 pm

For the record, you need a 1/4" NPT pipe tap to tap 1/4" NPT pipe threads properly.
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Unread postAuthor: MrCrowley » Fri Jun 24, 2011 7:11 pm

Do yins think this setup will be adequate to launch a golf ball

With a decent barrel length, you could get supersonic muzzle velocities...so yes. The most annoying thing will be cutting the burst disks, I don't even want to think about how many layers of tin foil you'd need for a 10x mix. I'm thinking maybe over 200 considering I use 64 at 10x in a 3/4" union.
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Unread postAuthor: DYI » Fri Jun 24, 2011 7:21 pm

You may have thinner foil than we have standard over here then Crowley, 256 layers in a 2" union would hold ~2500psi for me.

Just to emphasize ramses' point, you DO need an NPT tap to cut pipe threads. They're tapered to allow sealing by deformation. Barring some more creative setup, I'd advise modifying the chamber with a central tee to facilitate ignition input. Model T sparkplugs have 1/2" NPT threads, so no jury rigging or modification at all is required.

Also, those caps are cheaply made and thin. You don't want to be tapping pipe threads into them on a system like this.
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Unread postAuthor: ramses » Fri Jun 24, 2011 7:40 pm

DYI wrote:Also, those caps are cheaply made and thin. You don't want to be tapping pipe threads into them on a system like this.


The bushings are really worth it. I used many close diameter bushings on my hybrid, only to find a 1.5-.75 bushing at Lowes when it came time to adapt the 2" barrel. I have 1/4" threads tapped in a 3/4" steel cap in the pilot of my hybrid piston valve, and frankly I'm scared to use it. It's worth the peace of mind to know you're not trusting 2-3 threads with your well-being. The shear area (by my guess is over 0.1 square inches, so it should hold the ~230 lbs on it at 1000 PSI, but...)
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Unread postAuthor: Woody » Fri Jun 24, 2011 8:06 pm

Well the cap I have looks to be about 3/16" thick, but if it is bringing up question then it must be a problem spot, so I will pick up the appropriate reducers and bushing. I have to go back to the store to take back things anyways. I am going to use a T fitting in the center of the combustion chamber for ignition. Thank you DYI for the information on the spark plug. That'll save me money on buying a special tap and give some more peace of mind. So I will probably make it to the store sunday night, so until then this project is going to be idling.

Also, What do you guys think about cam lock couplings? Would they hold up or should I stick with a union?
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Unread postAuthor: MrCrowley » Fri Jun 24, 2011 8:32 pm

Also, What do you guys think about cam lock couplings? Would they hold up or should I stick with a union?

I've been tempted to use the aluminium cam-locks myself, I know SpudBlaster15 has but i'm not sure if he used them at 10x.

Using them at a 10x mix and to hold burst disks seems a little risky to me. It would be pretty disastrous if you were holding the barrel and they failed.

Let me know how it goes :D

Then again I don't feel that safe using ABS as a barrel, I've been thinking of wrapping part of it in kevlar from an America's Cup boat sail to prevent hot gases burning my hand in the event the barrel does fail...
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Unread postAuthor: jackssmirkingrevenge » Fri Jun 24, 2011 11:01 pm

MrCrowley wrote:Using them at a 10x mix and to hold burst disks seems a little risky to me. It would be pretty disastrous if you were holding the barrel and they failed


Unless the expanding gasses can get between the male and female edges (which they shouldn't, otherwise the burst disk wouldn't seal) the cam lock connection really isn't under much stress.
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Unread postAuthor: Woody » Tue Jun 28, 2011 9:35 pm

It's late so I didn't feel like fooling with it too much, so I just loosely put it together as I picked up the rest of the fittings today. I'm going to rearrange the routing from the first ball valve to air in and fuel in. Going to need to pick up a few more pieces of pipe for that. Still need to get a spark plug and then I'll be good to go other than the barrel. I may buy a 1.5 - 1" bushing and switch to a 1" union now and then.

Here is how it sits now
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Unread postAuthor: Woody » Sat Jul 02, 2011 9:59 pm

Still have to grind the union for a flush mounting surface, setup ignition, grab a pipe wrench to tighten the big pieces a little more, and find some hose clamps for the propane hose. Oh, and I may try to setup a pull string remote ignition. Just to hit the button on the stun gun after it's armed.

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Unread postAuthor: SpudBlaster15 » Sun Jul 03, 2011 1:20 am

MrCrowley wrote:I've been tempted to use the aluminium cam-locks myself, I know SpudBlaster15 has but i'm not sure if he used them at 10x.


I did use them at 10x, but only as a breech loading mechanism on my piston hybrid. I would not suggest using them to hold a burst disk at over ~5x, as the clamping mechanism is much less robust than even the lowest quality malleable iron union.
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Unread postAuthor: jackssmirkingrevenge » Sun Jul 03, 2011 1:38 am

SpudBlaster15 wrote:I would not suggest using them to hold a burst disk at over ~5x, as the clamping mechanism is much less robust than even the lowest quality malleable iron union.


One must consider however that the stress is mostly on the burst disk. What the cam-lock has to do is hold the disk in place, it's not really being directly affected by the pressure. Once the disk bursts there should be no stress on the locking mechanism at all.
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