Tonight, my next cannon was born in my mind and my heart started racing. That means this is happening. Not right away, but this will become a reality. Here it is:
It's a machined aluminum, over-under, air-spring-actuated piston hybrid (like my last piston design). Chamber volume is about 300 cubic inches at 2-4X mix. Porting around 1.75". Going after an interchangeable barrel design; 2" for spuds and fruits, soda can barrel and an even larger size for something like softballs or medium-sized soup cans. Maybe a universal changeout barrel design for something small like golf balls as well. Going that small will complicate things though.
The question for now: Does spark placement matter much? Imagine an over-under design where the spark is placed at the end of the chamber near the bend rather than in the middle or at the other end of the chamber. My concern is gasses being pressurized toward the front end with nowhere to go and DDT=BOOM? Probably not. Another concern is bad performance. My understanding is that with a hybrid, it doesn't matter much due to the valve opening pressure dictating much of the performance.
Cooking up a little something else...
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mark.f
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A spark at either end gives about the same amount of runup distance, which is to say not much for a normal sized launcher. AFAIK multiple sparks would be even better by eliminating a lot of runup distance, as well as (slightly) improving performance. You can play around in HGDT though and it's not much.
Looks like the Venom, just better.
Looks like the Venom, just better.
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Moonbogg
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That image is the venom. I just used it to show the spark placement. The new cannon is way different. I'll have something to show before too long. I'm working on the piston housing.
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Moonbogg
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Design evolution is happening. I am wanting your thoughts on o-ring blow-out near the port. I show the piston open and closed. Going with aluminum piston and I want fine control over opening pressures and reliable sealing up until the opening pressure is reached. This will also make the cannon a very viable pneumatic with basically no compromises. Pilot pressures will be very low.
So, do you think that o-ring is likely to blow out? I might have to do an experiment where I test a tube with an o-ring and load it in the breach of a cannon and fire it to see if it blows out.
So, do you think that o-ring is likely to blow out? I might have to do an experiment where I test a tube with an o-ring and load it in the breach of a cannon and fire it to see if it blows out.
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mark.f
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What kind of O-ring? I would think you could climb the durometer scale until you found something that stays put.
EDIT: also something you could consider, but you would have to use a longer stroke for the piston to get the same flow and it would introduce some turbulence.
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Moonbogg
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Thanks for the reply, Mark. The ported sleeve is a good idea and I didn't consider that. However, what I am trying to avoid is running over the o-ring with any kind of holes to avoid shredding it over time. I read D-Hall mention you can machine a smooth step to increase the ID to avoid pressure on the O-ring, but I'll try to keep it as simple as I can and avoid any hole contact.
As you suggested, I'm thinking simply choosing the right o-ring will likely be the solution. The o-ring is sitting back and out of the main flow path, so hopefully that helps. If the o-ring was on the front of the piston, I'd have even greater concerns about it getting sucked out. I'm excited about this project. It has double the surface area for flow from the port as my last one and that thing can really chuck a soda can, so this new one should be pretty ludicrous. I'm going for ludicrous.
As you suggested, I'm thinking simply choosing the right o-ring will likely be the solution. The o-ring is sitting back and out of the main flow path, so hopefully that helps. If the o-ring was on the front of the piston, I'd have even greater concerns about it getting sucked out. I'm excited about this project. It has double the surface area for flow from the port as my last one and that thing can really chuck a soda can, so this new one should be pretty ludicrous. I'm going for ludicrous.