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Semi auto 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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Unread postAuthor: spudamine » Sat Jan 17, 2009 6:10 pm

spudamine - If that's how you like to get your thrills, so be it.. haha. But if you had a burst disk in a stronger storage tank, you could control the direction of failure which I'm sure your hands and face would appreciate.

Obviously the burst disk safety is the best solution, providing your disk is big enough, just a little trickier to implement. I certainly get no 'thrills' from my setup, its just the cheapest solution and if managed correctly the risks are minimal
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Unread postAuthor: Sticky_Tape » Sat Jan 17, 2009 7:13 pm

Why have an oring to seal the barrel? Why not just use a regular sealing face maybe that would help simplicity.
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You can tell how awesome a cannon is by the pressure used.
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Unread postAuthor: microman171 » Sun Jan 18, 2009 1:30 am

I guess when you think about it, he has a point guys.
To me, a hybrid seems simpler because you don't need the step of venting the chamber.
If you make the air\fuel ratio right to burn NOT including the air already in the cannon, then the problem is solved. The pressure equalises as the piston moves back, so you have a CO2 environment for combustion.

Add your air/fuel ratio and you have enough air to burn the fuel, you also have the CO2.

Make the cannon out of steel, and use a burst disk in the fuel/air chamber. That way you have a much safer cannon.

Good Luck - I think this will work, unless the CO2 changes things. Really it should change anything. The combustibles are at the correct ratio to burn at what ever pressure, in what ever environment.


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Unread postAuthor: sputnick » Sun Jan 18, 2009 11:03 am

Well you're right, I had no intention of bashing it, I'm just saying it sounds like alot of work for something that could be achieved an easier way.

Adding on to that... Why an air spring? wouldnt a metal spring be fine? assuming you found one with the right strength...

Of course the design is possible, especially the revised one in your video, but the question is if you really want to put all that time and effort into something you may never get to function properly.
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Unread postAuthor: microman171 » Sun Jan 18, 2009 5:15 pm

Yeah, I agree with the metal spring, With a few calculations you could find out how much force was pushing back on the piston, and how much you would need to keep the piston in place for a certain amount of time.

I'm surprised none of the bigger guys (Bigger Names) are here, this is new technology in hybrids.

I look forward to seeing this, it looks like it will be great!
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Unread postAuthor: daccel » Sun Jan 18, 2009 7:02 pm

A regular sealing face would work. I just thought if you were doing o-rings anyways.. but if it could be simplified to not need them then it would make sense.

A regular spring would work but then the piston wouldn't have a delayed return, so the chamber pressure might not drop enough. Unless there's another way of creating that delay. I'm not against using a simpler design, I just didn't think it would work without that feature.

Making a stepped piston like in the picture here would help increase hang time of the piston because you could use a weaker spring. Kind of like a pop off valve.

Edit: Now that I think of it, if you used that stepped design with a very small size differential between the front and back of the chamber, you could use a weak enough spring that it would allow the pressure to return to a very low level. Not necessarily zero, but that should only effect efficiency a bit and not prevent it from firing. You could also make it so the piston at the back of the chamber 'pops off' to expose the full piston face before the barrel is opened, to keep the opening speed high.

Edit 2: Wait, but then the chamber might start to refill with the large face exposed, preventing it from closing...
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Unread postAuthor: ramses » Wed Jan 21, 2009 4:44 pm

I would get rid of the check valve all together. I would think that there would be no build up of combustion byproducts, since most of the pressure would be down the barrel by the time the pressure behind the piston returned.

You could use a 555 timer with the "high" state switching your, with a serial capacitor, to limit the duty cycle on whatever ignition device and the "low" state, holding open open solenoid valve, allowing regulated air-fuel mix into the chamber. pulse width modulation would be helpful to maximize rate of fire, but you could do without.

go ahead and tear that to pieces.
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Unread postAuthor: daccel » Sat Jan 24, 2009 3:55 am

Pulse whaa? :P If I was more knowledgeable when it came to electronics I might go down that road but for now I think I`m going to stick with mechanical. Probably a schrader valve as long as I`m using a high enough mix to crack it open. But the electronic route certainly sounds like a feasible way to go about making it and would allow you to fine tune the timing of filling.

Anyway, this idea is still on the table, I have just had other priorities getting in the way of construction. Was thinking about eliminating o-rings today and came up with this. It works off the same principles but is just a different layout.

Here the chamber is actually within the piston, and it`s directly connected to the breech. Picture should be self explanatory, but ask if you don`t get something and I`ll try to do an animation tomorrow. It uses telescopic brass tube for all the sliding parts and barrel, I just exaggerated the gaps to make things more clear.

I wonder though if the chamber might expand and seal against the outer wall with a high enough pressure, preventing it from moving back? Perhaps there`s some way to actually harness that effect like in a real firearm, I just can`t think of a design that would at the moment.

Edit: Slowly getting some parts together, see pic. Yes, I know, small. Also contemplating trying to use those short steel pieces I have which happen to have a 5/8 ID.
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Unread postAuthor: daccel » Sat Jan 24, 2009 6:57 am

Any feedback on the new design? I have time this weekend to work on it, I just don't want to start if there's something I'm missing that would prevent it from working, other than it being an untested concept.

Here is an animation:
[youtube]http://www.youtube.com/watch?v=1g_dbd18Ta8[/youtube]
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Unread postAuthor: iknowmy3tables » Sat Jan 24, 2009 5:55 pm

I feel like this is making things a little more complicated than necessary, with these complications they should at least vent the chamber.

this is the best solution I could think of
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Unread postAuthor: daccel » Sat Jan 24, 2009 8:25 pm

Nice, that's a really good idea. The only problem I see is you'd have to restrict flow on the filling inlet or it could prevent the left piston from closing. I've already started building, but if I have problems venting I will probably try your idea.
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Unread postAuthor: daccel » Mon Jan 26, 2009 3:22 am

I don't think double posting in this case is against the rules/annoying, but let me know if it is. Just thought it would be more clear than a huge edit, plus it helps keep me motivated posting and makes a log of the project.

I attempted to make the fuel/air inlet by drilling between two washers and then cross drilling one and epoxying them together, only to realize it was going to be too difficult to get it to seal and have that many parts in such a small area.

I've redesigned yet again, and now have the fueling from the rear. It will seal similar to the breech/main valve so I know it will work doing it this way.

The breech I did manage to complete. No magazine yet to test feeding, but it seals with very low pressure and can fire an airsoft bb fairly well just blowing it open.
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Unread postAuthor: inonickname » Mon Jan 26, 2009 3:38 am

Wow, good to see you're actually putting this into motion and working on advice. Most people come up with a good idea then dump it, myself included. Kudos :wink:
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Unread postAuthor: daccel » Sat Jan 31, 2009 2:33 pm

Yes, that's usually me as well. :lol: But not this time.

Ok, this feels like a stupid question, but I'm having trouble visualizing the forces involved in my last diagram there.

I just realized the fixed area on each side of the chamber is the same. So would it actually move with combustion?

I could use smaller tubes on the back for fueling, but then I would have to buy some more parts, so I'd like to know if it would work as is.
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Unread postAuthor: daccel » Sun Feb 01, 2009 2:46 am

I decided to lay out everything I have so far to help me visualize how it's all going to come together. Going to be a tight squeeze!

Yeah, I'm making things more challenging by fitting it in the airsoft shell, but if I'm going through all the effort, might as well go all out.

The trigger, safety and magazine latch will remain and be utilized. Going to make a replacement magazine with the layout shown. Thinking about using a valve from a refillable butane can to seal between the magazine and gun.

A flashback arrestor was mentioned earlier in the thread. Opinions on whether the regulator and schrader will be sufficient for this? Regardless I will fit a burst disk on the bottom of the reservoir, but I'd rather not be in for that surprise.

Edit: Also, the barrel will be mostly in a suppressor with ports close to the end of it, because there isn't sufficient length in the shell.
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