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Moonbogg
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Sat Jun 13, 2020 2:39 pm

I think I see what you are saying. The gas uses energy to move the piston, and once that work is done, that energy has been transferred and you can't get it back to do any work on the projectile. Would it be the same situation with a burst disc? It requires a lot of energy just to break the disc, but once it breaks, the projectile is gone. It's a good thought exercise. It gets kind of confusing because the gas is springy.
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hectmarr
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Sat Jun 13, 2020 3:48 pm

Yes, that's what I was thinking. My interest is to know how much of the total energy is invested in moving the piston, or breaking the disc. I see that it depends exclusively on the specific design, (areas, driving force, stroke), and the chosen method, (such as piston or rupture disc), to let hot air escape into the barrel.
Variables like damn dead spaces where compressed air by temperature can expand a little, lowering its pressure and losing energy, is another aspect that is undoubtedly negatively influencing the tests. The fact of having relatively long transfers, also dares, lost by friction.
I think a solution is to increase the amount of fuel, and the air age, (and consequently, the pressure of the hybrid), to compensate for these losses, especially when they are designed without any kind of calculation, just "by eye".
Ideas don't make holes. The weapons yes! ;)
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jackssmirkingrevenge
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Sat Jun 13, 2020 5:22 pm

I feel that such losses are probably not that significant. Pistons typically have a large surface so they do not require a high pressure to move. Added to that, their travel is usually very limited so the increase in chamber volume relative to the total is minimal.
hectmarr wrote:You have to make many weapons, because this field is long and short life
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hectmarr
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Sat Jun 13, 2020 7:21 pm

These are the two cases I was testing.
In case "A", the retention corresponds to an o'ring, and the power is much higher than the other case.
In "B", the "captive ball valve", when combustion occurs, the 6.35mm mole moves to the left and passes the flow through the transfer into the firing barrel. This green ball is the one that is thrown. The idea is to be able to make a very small restraint system to fire lead pellets or other ammunition. The two bb`s are the same. Different color is just for reference use. .
válvula de bola cautiva.JPG
sistema bola cautiva.png
sistema bola cautiva.png (5.36 KiB) Viewed 164 times
Last edited by hectmarr on Sat Jun 13, 2020 8:19 pm, edited 1 time in total.
Ideas don't make holes. The weapons yes! ;)
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Moonbogg
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Sat Jun 13, 2020 8:08 pm

Based on the sketches the first thought that comes to mind is that you may not have enough chamber volume to move the piston and also the projectile. Maybe I'm missing something, but if I reduced the chamber volume of my piston cannon to be very tiny, then I'd expect most of the energy to go into moving the piston and not much would be leftover. It might just be a volume issue. The chambers in the sketches look pretty small. I feel like I'm missing something here.
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hectmarr
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Sat Jun 13, 2020 8:26 pm

Moonbogg wrote:
Sat Jun 13, 2020 8:08 pm
Based on the sketches the first thought that comes to mind is that you may not have enough chamber volume to move the piston and also the projectile. Maybe I'm missing something, but if I reduced the chamber volume of my piston cannon to be very tiny, then I'd expect most of the energy to go into moving the piston and not much would be leftover. It might just be a volume issue. The chambers in the sketches look pretty small. I feel like I'm missing something here.
Sorry for my drawing, and for not clarifying that it is not at all to scale.
I have left constant the volume of the combustion chamber, (which in "A" works wonderfully), the same firing barrel length, the same ammunition, and the same execution pressure.
With the "A" system it has at least twice the energy of the trip than with the "B" version, which has the small valve incorporated.
I mean that the weapon is the same in all respects in "A" and "B". So my question.
In practical terms in "A" the ammunition cleanly passes two cans of soup. In version "B", only one side of one of these cans. So are the facts. :?
I have tried to raise the execution pressure of the weapon in version "B", to see how much more energy is needed to achieve the same results as in "A" on the target. It seems to make up for much of what I explain to you earlier. I increased it from 5x to 7x, and they are almost the same. I think that starting from the retention the bb, in case "B" it takes 2 bar at least, to say it in an informal, unscientific and simple way.
Ideas don't make holes. The weapons yes! ;)
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Moonbogg
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Sat Jun 13, 2020 9:39 pm

I wonder if the shape of the B version could also have something to do with it. At least you can solve it by making up for it with more fuel and air. The A version is more like a burst disc configuration. I'd like to have two identical cannons, one using a burst disc and the other using a piston, and then compare the performance of the two to observe any losses caused by the piston. That would take time, effort, and money though. Besides, you sort of just did that experiment anyway. It looks like burst discs win.
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hectmarr
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Sat Jun 13, 2020 9:57 pm

Moonbogg wrote:
Sat Jun 13, 2020 9:39 pm
I wonder if the shape of the B version could also have something to do with it. At least you can solve it by making up for it with more fuel and air. The A version is more like a burst disc configuration. I'd like to have two identical cannons, one using a burst disc and the other using a piston, and then compare the performance of the two to observe any losses caused by the piston. That would take time, effort, and money though. Besides, you sort of just did that experiment anyway. It looks like burst discs win.
Yes I agree with you. At least in this size, the rupture discs or equivalent, (like a retaining o`ring), use less energy than an o'ring`s used as a small piston.
Surely size influences all of this, so I would personally attribute a partial value, which is true for small sizes, and not necessarily for much larger or much smaller sizes.
Ideas don't make holes. The weapons yes! ;)
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