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New Hybrid design

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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New Hybrid design

Unread postAuthor: boyce123 » Thu May 31, 2012 3:16 pm

I am new to hybrids, and i havn't yet explored all of the posts and forums, but i havn't seen this design of a piston hybrid yet.

Image

The design is based loosely off of the coaxial combustion cannons. The metering would have to be seperate, along with a presurized venting setup. The only dilema i could see is a large and very heavy piston. How well would this design work?
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Unread postAuthor: jackssmirkingrevenge » Thu May 31, 2012 3:20 pm

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Unread postAuthor: boyce123 » Thu May 31, 2012 10:16 pm

i've seen the mini cannons, i was curious as if there were any inherent design flaws that would prevent it from being effective in a large design.
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Unread postAuthor: jackssmirkingrevenge » Fri Jun 01, 2012 12:00 am

No reason you can't scale it up as long as you build it strong enough.

The only dilema i could see is a large and very heavy piston.


This can be a big problem.

A piston doesn't have to be large an heavy, if it's a good enough fit then it can be say half the length of the chamber diameter, and shaped like a cup to save weight and also act like an umbrella seal and stop the combustion gasses getting into the pilot area.
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Unread postAuthor: boyce123 » Sat Jun 02, 2012 8:42 pm

I decided to make the design. I am currently making the chamber out of pvc, i have layered 3" copper pipe to 3" pvc to 3" expanded pvc and fittings to 4" pvc. I plan on making the piston out of a 2.5" pvc cap. The chamber is going to be relativly small, being about 6 or 7 inches long with a 1 1/4 galvenized pipe running through the middle of it. How many x's would be safe with the chamber?... i'll have pics posted later today, it'll be a while until i complete it as i don't want to make the meter quite yet.

input is very much appreciated
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Unread postAuthor: jackssmirkingrevenge » Sun Jun 03, 2012 1:21 am

Interesting chamber combination, what are the wall thicknesses of the pipes?

How are the endcaps going to be made and attached?
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Unread postAuthor: Bowman » Sun Jun 03, 2012 2:15 am

boyce123 wrote:I decided to make the design. I am currently making the chamber out of pvc, i have layered 3" copper pipe to 3" pvc to 3" expanded pvc and fittings to 4" pvc. I plan on making the piston out of a 2.5" pvc cap. The chamber is going to be relativly small, being about 6 or 7 inches long with a 1 1/4 galvenized pipe running through the middle of it. How many x's would be safe with the chamber?... i'll have pics posted later today, it'll be a while until i complete it as i don't want to make the meter quite yet.

input is very much appreciated


Why even bother with all the pipe layering? Just get steel pipe or something. This is what just regular steel piping can handle:

http://www.engineeringtoolbox.com/stain ... d_346.html

It might be a little more expensive but should hopefully save you a lot of heart ache.
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Unread postAuthor: MrCrowley » Sun Jun 03, 2012 2:44 am

I'm with Bowman on this one; galv. iron pipe surely can't be more expensive/harder to get than all the fittings for that layering.

After you've done all that layering, I wouldn't exceed a 4x mix personally. I'd be surprised if you can get the PVC piston to last long.
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Unread postAuthor: 8tonsemi » Sun Jun 03, 2012 7:51 am

So are you going to use a bumper in the gun or external type of bumper. What type of fueling are you using.
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Unread postAuthor: boyce123 » Sun Jun 03, 2012 9:55 am

I'm using all schedual 40 pipe, so the pvc thickness is around .68" (almost 2x the thickness of schedual 120 pipe), plus the thickness of the copper pipe. As far as the piston is concerned, i havn't really put that much thought into it yet. i have a wood lathe, so would it be worth it to turn a wood piston, or would concocting somthing out of pvc be more effective. I was planning on making an internal sping bumber surrounded with foam. If you look at the fueling on SB15's cannons, it would give you a good idea of how i'm going to meter, but i'll have the meter seperate from the cannon. would propane or MAPP work best in this scenario. As for my choice in pvc, i had the pvc and copper pipe laying around, so i figured i would do a little experimenting into hybrids without spending any money. I was going to hold out on buying all the galvenized parts until i got the concepts down and could easily figure out fueling, then i would go ahead and build a large steel hybrid. I don't really expect to use this cannon for very long, its more of a conceptual test for me. (i don't know if this is the same for everyone else, but at least where i live, pvc fittings are around $1-3, where galvenized fittings are around $6.)
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Unread postAuthor: 8tonsemi » Sun Jun 03, 2012 10:17 am

The fuel you use really depends on what type of metering you are useng. Have you tried getting in contact with mrc45 he will be able to give you a lot more help on fueling then me. I'm still pretty new around hybrids my self.Also have you ever made a burst disc hybrid because it's a good first step in making hybrids. Because a piston hybrid is very advanced I wouldn't rush into the whole thing. I wish you the best of luck with it. And If it succeeds make sure to post it :lol:

(edit) have you read the post about springs as bumpers. They crush under impact. So I would not recommend using a spring.
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Unread postAuthor: boyce123 » Sun Jun 03, 2012 11:34 am

i was acually thinking of a valve spring, but that would probably be to stiff, i am still in the designing/engineering stage, so it'll be a while before i decide what i do with the fueling. sorry for the lack of pictures... i can't seem to find my camera...
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Unread postAuthor: MrCrowley » Sun Jun 03, 2012 5:02 pm

8tonsemi wrote:The fuel you use really depends on what type of metering you are useng
I'm sorry but no, it doesn't :wink:

8tonsemi wrote:So are you going to use a bumper in the gun or external type of bumper
I'm not sure what you mean by an 'external' bumper.

@boyce123,
MAPP is generally seen as 'more powerful' than propane but in your case, I would just stick with propane since you're using PVC and all.

A spring bumper probably isn't a good idea unless your piston is made from a strong plastic or metal. At low mixes, the spring could probably survive but your piston will bounce in to the seat. A wood or PVC piston is not recommended, you may even have trouble getting a 100% air tight seal with wood and o-rings.

With a piston hybrid, metering is by far the easiest part. You could go with a manometric meter where all you need is a 0-15PSI gauge (with 1.5% F.S. accuracy or less), a 0-150PSI gauge, and a couple of ball valves. With MAPP gas, each mix is an extra 0.71PSI of MAPP, so regardless of chamber size I always fuel one of my hybrid chambers with 3.5PSI of MAPP gas for a 5x mix. It will be different for propane.

I'd suggest jumping in the deep end and starting with galv. iron fittings. If you're worried about metering, build a burst disk hybrid first; a piston hybrid will just give you headaches (not that a burst disk doesn't). With galv. fittings, you can't really screw anything up as you're not modifying them or tapping them or drilling in to them, the only thing you can really screw up is the piston; which you can rebuild. I doubt you'll learn anything that useful from this cannon as when it comes to building the galv. version, you'll need an entirely different piston due to the sizes of the pipes and the extra forces it has to take at higher mixes.

Galv. iron fittings are closed to $12USD for me here, so count yourself lucky :wink:
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Unread postAuthor: boyce123 » Sun Jun 03, 2012 10:24 pm

As i said, this is a personal concept test to step up to a larger galvenized hybrid that i plan on making. I measured the chamber volume today (filling it with water), and it came out to be 400 ml (or around 25 cubic in), to put size in perspective.
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Unread postAuthor: jackssmirkingrevenge » Mon Jun 04, 2012 1:40 am

I would think the pipe would hold up to 4-5x or so without significant danger, my concern would be the endcaps blowing out, especially given the diameter.

Exhibit A.
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