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Short question topic

A place to ask general spud cannon related questions.
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Re: Short question topic

Unread postAuthor: Moonbogg » Mon Oct 07, 2019 5:04 pm

You solved the problem perfectly. He is trying to fuel the cannon but there is back pressure coming from the chamber, preventing proper flow of fuel and air into the chamber in the correct ratio. Well done. Wow, what an easy fix that would have been. I thought they had to build a whole new cannon, but simply adding a ball valve to the chamber and opening it could at least have allowed fuel and air to enter while displacing the burnt gasses. Does that sound right? That actually sounds like a really nice cannon design. Very easy to fuel for each new shot. That Hectmarr is a pretty smart guy I think :)
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Re: Short question topic

Unread postAuthor: hectmarr » Mon Oct 07, 2019 6:59 pm

Oh yeah! These guys have not analyzed enough the weapon ... :? Without introducing the ammunition, so that the burned gases escape and so that there is no pressure higher than the atmospheric inside the weapon, while you load the mixture, it can work, even without any valve . Press the valve of the bottle and let the mixture run for a few seconds through the inlet, where the hose is connected, and let out the barrel a little. Then, you enter the ammunition, press the lighter and boom! :bom: , assuming stoichiometry is correct, which I think is this way.
The solution is good, it seems simple and effective, but the operation of the weapon, in the video is incorrect. Likewise, nice weapon and system. :)
I would try it in a smaller gun size, but with a mixture of oxygen and propane, (without logically compression), and I think it should work, and with enough power. Safety is guaranteed because oxygen and propane are separated into two small bottles. This adds a little "spicy" to the weapon, for which we have a little experience. :twisted:
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Re: Short question topic

Unread postAuthor: Moonbogg » Tue Oct 08, 2019 12:14 am

Oh my god, of course you don't need a valve, lol. I thought they needed one, but I don't know why I thought their chamber was sealed and air tight. With no projectile, it's open. I'm looking forward to your oxygen propane fun. Keep us posted!
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Re: Short question topic

Unread postAuthor: hectmarr » Tue Oct 08, 2019 5:37 pm

A weapon more added to my wish list, but for the moment I am out of all this. Regards! :)
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Re: Short question topic

Unread postAuthor: wyz2285 » Sun Oct 13, 2019 1:04 pm

For a spring piston airgun, do you guys think it's better to have a short, tough spring that's not in much tension when it's not cocked or a long weaker spring that offers a more constant tension both cocked and not? I know for PCP hammers a sling shot hammer is better (short, tough spring that's not under tension)
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Re: Short question topic

Unread postAuthor: wyz2285 » Mon Oct 28, 2019 7:13 am

Can anyone explain why would increase weight on a spring airgun would increase the muzzle energy?
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Re: Short question topic

Unread postAuthor: Moonbogg » Mon Oct 28, 2019 6:51 pm

The weight of the gun resulting in an increase of muzzle energy? I'm not seeing it...
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Re: Short question topic

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Re: Short question topic

Unread postAuthor: wyz2285 » Mon Oct 28, 2019 6:52 pm

Right my bad, the weight of the piston. I was thinking about it while posting and ended up writing too fast...
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Re: Short question topic

Unread postAuthor: jackssmirkingrevenge » Mon Oct 28, 2019 7:15 pm

wyz2285 wrote:I know for PCP hammers a sling shot hammer is better (short, tough spring that's not under tension)


The main benefit of the "sling shot" hammer is the lack of hammer bounce, meaning the hammer valve can be tuned to open just once instead of having the valve flutter as the hammer bounces back from the initial impact then comes back to open the valve again at increasingly smaller intervals. This makes it much more efficient and consistent.

Can anyone explain why would increase [piston] weight on a spring airgun would increase the muzzle energy?


Note that this only works for a given projectile weight and barrel length. If the barrel is too short or the projectile is too light, then it might leave the muzzle before the piston has time to develop its full compression. A spring piston airgun is a dynamic system of which the projectile is a significant variable.

The heavier the piston, then the more momentum it has. Yes, the spring will accelerate it at a lower rate, but it will also be harder to stop, so the peak pressure inside the barrel will be higher provided that the projectile is heavy enough and the barrel long enough to keep the muzzle plugged for long enough for this to happen.
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hectmarr wrote:You have to make many weapons, because this field is long and short life
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Re: Short question topic

Unread postAuthor: wyz2285 » Tue Oct 29, 2019 8:10 am

What if I use a sling shot heavy piston? Could be more consistent since the piston probabaly bounces too.
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Re: Short question topic

Unread postAuthor: jackssmirkingrevenge » Tue Oct 29, 2019 8:26 am

wyz2285 wrote:Could be more consistent since the piston probabaly bounces too.


The piston does bounce, but by the time it hits the end of the cylinder the projectile would have left the muzzle.

This means that it won't really affect ballistic performance, just the feeling of "recoil".

This is a great little demo of the internal dynamics of a spring piston gun: http://www.arld1.com/images/swfs/rifled ... maller.swf
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hectmarr wrote:You have to make many weapons, because this field is long and short life
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Re: Short question topic

Unread postAuthor: wyz2285 » Tue Oct 29, 2019 8:29 am

Well I experimented with a caron fiber piston and a solid aluminium piston, the lighter one produces almost no recoil but is louder, while the heavy one produce noiceable recoil but is quieter...
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Re: Short question topic

Unread postAuthor: jackssmirkingrevenge » Tue Oct 29, 2019 8:37 am

What about the projectile velocity?
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hectmarr wrote:You have to make many weapons, because this field is long and short life
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Re: Short question topic

Unread postAuthor: wyz2285 » Tue Oct 29, 2019 8:42 am

Very unfortunalty the spring is pre compressed into different length with the two pistons, the heavier piston is compressing the spring slightly less. Taking that into consideratin, they produce similar power, around 0,2 J less on the heavy.
The sould could be duo to the 3D printed cup seal I used on the heavy, the almost hollow construction serves as an excellent absorber.
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Re: Short question topic

Unread postAuthor: jackssmirkingrevenge » Tue Oct 29, 2019 9:06 am

wyz2285 wrote:Very unfortunalty the spring is pre compressed into different length with the two pistons, the heavier piston is compressing the spring slightly less.


In that case you've added another variable so you can't really compare... I regularly use shims to adjust pre-compression on airsoft spring rifles and it doesn't take much to make a significant difference in power.
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hectmarr wrote:You have to make many weapons, because this field is long and short life
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