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Small stock mounted hybrid WIP.

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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Small stock mounted hybrid WIP.

Unread postAuthor: SpudFarm » Sat Mar 27, 2010 6:22 pm

Hello, I just started messing around in my bucket of fittings and realized that I could make a small bore hybrid pretty simple with few parts. So I started fitting parts together until I found something I was happy with. Then I used ballvalves to fire it as a pneumatic just to test the concept.
I chrony tested it to 112m/s three times in a row.


It will most likely be a 5x stock/pistol grip mounted gun with manometric metering from my large hybrid that I just got tired of draging around. It will have a 1/4" burst disk to start of with. But due to the small diameter It might be just as simple to make a neat little piston valve for it, this will also allow semi auto with small modifications.

I am posting this mostly because I need a ignition system that is small enough to be attached to the gun without throwing proportions out of the window.
It should also allow remote firing for high mixes but that is a piece of cake. I know I have had allot of ideas that never even got to the drawingboard but this is already far beyond that.

The budget of this gun is set to 300USD at the moment.

It will be fueled through the nipple where I now fill air. The meter will be external and be hooked up via a hose.

I think that is all you gun geeks need to know to give me some tips :P

My question will for now only be about a small, cheap and simple ignition system that can me used at a 10x mix. No BBQ ignitors! A coil and a camera cap will be to chunky and I can't get a stun gun here.
Oh and feel free to come up with a piston valve for a guy who have never built anything like this in ages.

Thanks!

[youtube]http://www.youtube.com/watch?v=-tmqfajjXKQ[/youtube]
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Unread postAuthor: chinnerz » Sat Mar 27, 2010 8:04 pm

looken good mate!!

My question will for now only be about a small, cheap and simple ignition system that can me used at a 10x mix. No BBQ ignitors! A coil and a camera cap will be to chunky and I can't get a stun gun here.


just find the largest cap you can fit, and get a few of them. charging may take a little while, but i think it will work.

i dont know how you are planning on finishing it,
i have no exp with combustion rifles, to this day i am still not too sure what a hybrid is...
Image
but will that work??

edit... i dont think i have anything going to the chamber... but you get the idea
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Unread postAuthor: SpudFarm » Sun Mar 28, 2010 9:18 am

chinnerz wrote:but will that work??


I doubt that I could get capacitors that has the power to create a pretty long spark. Works fine when fed through a coil but then it gets to chunky. Since there are more air resistance in a hybrid the ignition systen needs to be quite powerful.

Perhaps I have to go with an external ignition source with wires to my gun :/

You do know that a combustion cannon needs about 4% fuel/air to get the best possible combustion? in a hybrid you meassure lets say 40% of the chamber volume and inject it into the chamber. That mix will be to rich and need extra oxygen, that is fixed with injecting compressed air until the fuel/air ratio is right again.
Then you ignite the mix and the pressure rises all the way to the bursting point of your burst disk.
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Unread postAuthor: starman » Sun Mar 28, 2010 11:06 am

Well you know the spark generating choices as much as anyone around here. Have you considered the fusable link like Larda used?
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Unread postAuthor: SpudFarm » Sun Mar 28, 2010 12:57 pm

starman wrote: Have you considered the fusable link like Larda used?

I certainly thought about it, but the whole point of this gun is that it is going to be simple and fun (no horrible reload time) so I decided to look around to see if I found anything of interest.
I certainly did! A small coil that is compact enough to be hidden inside the trigger compartment of a rifle stock. I threw tougether a ignition system out of it and I am very pleased since the switch can be used as a trigger.

I made a video of it that turned out good enough, some small work is still needed on it though. I need more wires before I start making the system smaller. Now all that's left for a test shot is a union and a 1/2" brass endcap.

[youtube]http://www.youtube.com/watch?v=0s264RIIp1Y[/youtube]
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Unread postAuthor: SpudFarm » Tue Mar 30, 2010 11:31 am

Well, not much activity here :-p

I have now gotten a test shot with a union I made yesterday. I tried about 30 shots and got one off. Think the problem is the long hose for the propane, it worked fine for big hybrids but now I reach the correct amount of pressure from the propane before the fuel has made it into the chamber.
I will mount the tank directly to the meter later.

I also got a brass union today. My first time that I can trust a comercial union, completely sealed and the disk stays in there at 150psi+.

The spark plug is just a 3/8" brass endcap that I found. It has a stainless steel welding rod through it and shrinking tube as insulation. It is also filled with steel epoxy on the inside so I don't have to pull a welding rod out of my eye.

Other then that I just need one more part until I can test it while holding it. Hopefully I find a nice way to attach the coil to the gun.

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Unread postAuthor: JDP12 » Tue Mar 30, 2010 3:41 pm

Like it, looks very nice so far!
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Unread postAuthor: SpudFarm » Tue Mar 30, 2010 4:33 pm

JDP12 wrote:Like it, looks very nice so far!


Once our visitors leave I will do a test shot with a .22LR bullet. Will have to be done inside though so the target will most likely be some wood with a piece of styrofoam on to stop eventual ricochets.

Thanks!
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Unread postAuthor: JDP12 » Tue Mar 30, 2010 4:47 pm

Very nice.

have you considered a 3 way DCV for possible onboard metering? The meter pipe wouldn't have to be large at all, take a look at gorts combustion to see what he did-- same thing really, just on a smaller scale. Would eliminate the ball valves.

http://launchpotatoes.com/index.php?act ... roductId=3

That's the way I plan on configuring all of my meter systems in the future-- so simple. no valves to turn, just hit one button, automatic refill of the meter. All you need is a regulator on the propane/MAPP tank at the pressure the propane needs to be in the meter

On mcmaster there are quite a few examples-- here is a pretty cheap one- part no. 6464K12

it can only go up to 120 psi IIRC, but for metering purposes that should be plenty
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Unread postAuthor: SpudFarm » Tue Mar 30, 2010 7:47 pm

JDP12 wrote:Very nice.

have you considered a 3 way DCV for possible onboard metering?


I have had so much trouble with the volumetric metering system that I would like to stick to the manometric system.
If I decide to make it as a pistol it can have the meter I have got on it.

I did test shoot it now. 3x shots with .22 bullets. The first shots was a absolute hell since I thought I had metering problems. Then I took the union apart to vent the chamber for another shot and I saw smoke. What I learned is that two layers of ducktape can withstand a 3x mix in a 3/8" union.

Then I used one layer for the next shots and got three successful shots in a row, however I tried about 15more times and only got misfires.. I am losing my mind over hybrids that does not work, never know exactly whats wrong.

ANY help would be greatly appreciated. Look at the pictures and the video. I really need some advice before I melt the thing down and cast it into the worst looking mold I can find.

Image

Image

Image

[youtube]http://www.youtube.com/watch?v=ClKgDXWh-Ko[/youtube]


The power will go up with the mixes so don't worry.

I am sorry about my long reply.
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Unread postAuthor: SpudBlaster15 » Tue Mar 30, 2010 8:51 pm

SpudFarm wrote:I have had so much trouble with the volumetric metering system that I would like to stick to the manometric system.


Sound like you're having a lot of trouble with both. :wink:

The only thing that can really go wrong with a manometric system is the pressure gauge. Are you certain it is reading accurately, and hasn't been damaged?

Honestly, I'd go with a volumetric system for low mixes such as 3x. With such a low chamber volume and fuel pressure, a manometric setup has the potential to create significant error margins that might not even be noticeable on the gauge you're using.
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Unread postAuthor: DYI » Tue Mar 30, 2010 9:21 pm

Small, low mix hybrids tend to do poorly with manometric metering systems in my experience, even with sufficiently accurate gauges. Back in '08, I wanted a small system I could operate inside my house for relatively low energy ballistic testing. It cost me around $200, gave me no end of trouble, and never really worked. I got perhaps five shots out of it, likely on sheer good fortune.

I've got a few theories as to why it failed so completely. The main culprits were probably poor fuel mixing through the chamber, gases lingering in the hoses and other meter parts, and very slightly leaky fittings actively throwing off the mix during fueling. It used the same meter and gas sources that I had tested successfully at as low as 2X with a 2L hybrid, with zero failures to ignite. With larger chamber volumes, these small scale effects become insignificant and everything suddenly works. Higher mixes seemed to have a similar effect (I later got the tiny hybrid to ignite at 30x oxy/propane), although that may have been a consequence of the fact that I got frustrated enough to resort to an oxidiser/buffer mix that was roughly 90% oxygen :lol:

SB15's volumetric meter will likely work better than a manometric system here. They're tested and proven on small, low mix hybrids like this one. If you insist on sticking with this system, cut meter volume and hose volume as low as possible, eliminate even the slightest leaks, and use a very accurate gauge. Some people have tried venturi effect nozzles to improve mixing in small hybrids by increasing flow speed into the chamber. It would probably be worth checking into that as well.
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Unread postAuthor: JDP12 » Tue Mar 30, 2010 9:50 pm

I would go with a volumetric meter as well. Ive used it in a hybrid with a small chamber and gotten it to work reliably.

Something to consider would be using a syringe-- allows very accurate measuring of fuel gassses.

The above advice is assuming you go with a volumetric. I cannot comment on manometric as i have never used one before, never really understanding them and being content with volumetric.

My .02

i must add that im likin the aesthetics, a good balance of galvanised with brass... Hahaha.
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Unread postAuthor: ramses » Wed Mar 31, 2010 8:30 am

My manometeric hybrid doesn't like to ignite, although that's probably because of mixing. It's not really worth the trouble, IMO, unless you're fueling to 200x like larda.
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Unread postAuthor: SpudFarm » Wed Mar 31, 2010 8:34 am

SpudBlaster15 wrote:
SpudFarm wrote:I have had so much trouble with the volumetric metering system that I would like to stick to the manometric system.


Sound like you're having a lot of trouble with both. :wink:


I have never had trouble with this meter before. It even worked fine in a hybrid with about half the chamber volume.

I will try to get a 10x shot today. Should be easier to control the gauge at higher mixes.
Also, my propane gauge has been through ALLOT of abuse. The problem is that I can't find a new one. The website I bought it all from is closed.

It does not have a leak, It is completely air tight for a change. I might have to improve on the thread seal when I go to higher pressures though.

If it won't work reiable at 10x I will try to move the propane input right next to the chamber and the air on the other side to get it all in the chamber.

The diary will continue once I have killed myself, the target or the gun itself.
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