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Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: DYI » Wed Dec 05, 2007 4:16 pm

Some people here seem to have a very dim understanding of recoil. Big game rifles like the .700 caliber Nitro Express have over 11 000 ft/lbs of muzzle energy, and no shock absorption, and can be fired from the shoulder by an experienced shooter. And due to the fact that the gun will likely weigh over 50 pounds and be strapped down onto something, the recoil should be easily manageable (unless he goes with my design, detailed on page one, but I doubt he will due to cost issues). Also, I think you meant exerted, not "exhorted".

Air can't even be compressed to 17kpsi at room temperature, due to the way that all of its component gases liquify before 10kpsi. At the 5kpsi he apparently intends on using, a type K copper barrel could be used for small bore (due to the insane safety factor on copper pipe), but seamless steel hydraulic tubing would be easier and safer. Most of us won't even get a chance to use 5kpsi in a gun, so we may as well encourage him and teach him the proper safety procedures.
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Unread postAuthor: Infernal Maveric » Wed Dec 05, 2007 5:54 pm

ok I got some of the numbers wrong (I suck at Mathematics) and hence the reason why i don't know too much about formula for recoil/etc. I now know that only 5k PSI is being used, which in the hands of a "pro" is still deemed dangerous, having a newb to spudding use 5k PSI on a gun on which they were going to use a PVC barrel, the danger increases damaticaly. I might have been harsh/wrong, but I don't want to have some kid killed because of people encouraging him to build things that are out of his/her depth.
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Unread postAuthor: cwazy1 » Wed Dec 05, 2007 6:27 pm

So what i've noticed is that this guy just dissapears and then re-appears, he starts the topic with saying that he is getting a very high pressure cannon. then his next post in his topic like "adjusts" to what everyone is saying.

i do not believe this guy has such a settup, if he does, well this proves what soemone said to me "why do the noobs get the really cool stuff" lol

but please, supply some pics and/or videos so we can inspect it and make sure its safe and then give you pointers. the last thing we all want is for you to die and cause bad criticism on spudguns.
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Unread postAuthor: DYI » Wed Dec 05, 2007 7:23 pm

He has posted the oxygen bottle cannon a while back, so the 600 psi claim is feasible. I somehow doubt that he is a "kid" for some reason.

I'll believe the 5kpsi cannon when I see it. I agree that what he is trying to build is somewhat beyond the knowledge he has shown so far (can sprinkler valves take 17kpsi? and can I use a PVC barrel?), but the design that he has talked about so far, although the oxygen bottle isn't rated for that pressure either, so he'll have to use the HP pipe he referred to. Assuming that the pump is real and that he has a 5000 psi hydraulic QEV (which I'll believe when I see pictures), then the design should be OK. The ~3 kilograms or so of gas leaving the muzzle at who-knows-what-velocity should make for some interesting recoil though, regardless of the projectile.
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Unread postAuthor: Ragnarok » Wed Dec 05, 2007 7:59 pm

Actually, it's important that the recoil should at least be estimated before it's fired. I meant to do it before, but I forgot.

A couple of kilos of gas at 100 to 200 m/s as is likely will create huge recoil except in a launcher that weighs a couple of hundredweight.
The recoil from the projectile could practically be ignored.

1 kilo of gas leaving the muzzle at 100 m/s, which is the lower limits, so a best case scenario - even in a colossally heavy 20 kg launcher... 250J of recoil, higher than even the .577 T rex (215J), that famous cartridge we have all seen recoiling people across the room on Youtube.
That's a launcher twice what it's likely to weigh, half the mass of gas, at half the velocity, and the recoil is still going to be too high.

With the more likely inputs, it's 8000 J - about 5x more recoil energy than the M16 has muzzle energy. So, this launcher CANNOT be handheld. In fact, I'm no longer sure it can exist. With the higher mass and velocity for the gases, even if you cast it into a concrete block that weighs more than I do, the recoil is still going to be unstoppable.

Having assessed, it I'm going to post my verdict in huge red letters:
THIS LAUNCHER SHOULD NOT BE MADE. THE RECOIL WILL PHYSICALLY KILL ANYTHING LESS THAN 200 YARDS BEHIND IT WHEN IT FIRES.
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Unread postAuthor: Fnord » Wed Dec 05, 2007 8:21 pm

huge red letters wrote:THIS LAUNCHER SHOULD NOT BE MADE. THE RECOIL WILL PHYSICALLY KILL ANYTHING LESS THAN 200 YARDS BEHIND IT WHEN IT FIRES.


It might not be made today, tomorrow or in the next five years, but something of this power will be made eventually. I know I don't want to be standing anywhere near it, but saying it shouldn't be built is like saying we shouldn't try to go to the moon.
Maybe this person shouldn't attempt it, but someone eventually will (no offense pretzelperson. It's just that you haven't necessarily proven that you know what you're doing yet. Please prove that you do so we at least won't be worried about you hurting/killing yourself).

Now, if you'll excuse me, I'm going to go try rocket-jumping up on to the roof to clean out the gutters.
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Unread postAuthor: Ragnarok » Wed Dec 05, 2007 8:42 pm

_Fnord wrote:It might not be made today, tomorrow or in the next five years, but something of this power will be made eventually.

I don't doubt it, but it's going to have be done more subtly. Very high pressures in really large volumes done with relatively dense gases are not a good mix.

Using helium or hydrogen would increase projectile energy, but reduce recoil, because of the less dense gas. Less volume would also help, reducing muzzle energy a little, but massively cutting air mass.

You can't mix 5kpsi with a 3 to 4 litre chamber and still have it usable.

A couple of kpsi in a small chamber and done with a light gas would be manageable though.
I honestly think that power needs to come from a super light gas. Either naturally light like helium or the heated gasses of a hybrid. A 10x hybrid would be pretty impressive, but the low gas weight would help keep recoil down.

And I noticed I made a mistake. The recoil is 8 kJ, not 2 kJ, I forgot to alter the gas velocity.
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Unread postAuthor: DYI » Wed Dec 05, 2007 9:47 pm

That would be some impressive recoil... The 200 yards is a bit of an exaggeration, but I think you meant that. One idea could be to build a very sturdy steel frame around it, and attach wheels, then have a ramp behind it, like old artillery did. Then just add weights until it was 200 kg or so, and it may be controllable.
Either that or you'll have 400 pounds of concrete and steel flying through the air...

I don't think we'll have to worry about him trying to fire it handheld. And if he does try it, well, then we won't have to worry about him at all anymore.

I didn't really calculate the mass of the gases, I just kind of took a wild-assed guess based on the calculations I did for the dialogue on page two. I'm building a simple prototype mostly to see the recoil forces involved before I make the BB gun, and I'm using a similarly sized chamber, with a spring-loaded ball valve instead of the piston valve planned for the final version (mounted on a sturdy stand of course). The recoil should be similar, due to the mass of gases being the same, right?
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Unread postAuthor: Ragnarok » Wed Dec 05, 2007 10:05 pm

Ok, that was an exaggeration, but even if you set it in a concrete block for a total mass of 100 kg, the recoil would still be 800J - about twice the muzzle energy of a .44 special.

Ok, that's only 9 mph recoil, but that's an impractically heavy launcher. Cutting that to 50 kilos, it'll come back twice as fast again. Having been hit in the chest with a 5 kilo lump of square cornered iron doing only a few mph (thrown by a total half wit - I probably would have been severely injured, if not killed by it if my reactions hadn't been working to allow me to catch and slow it before it hit home - and my right toes would now be flat if I wasn't wearing steel toe caps that day - I wasn't quick enough to keep hold of it), I can tell you getting hit by a 50 kilo lump of concrete doing 18 mph would be seriously unpleasant, if not fatal - you're at M16 levels again.

Recoil is dependent on both momentum, and launcher mass - I wrote up a fairly comprehensive guide on it recently, it's probably worth a read if you're working on a super high power launcher.
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Unread postAuthor: DYI » Wed Dec 05, 2007 10:24 pm

Well, I'm not trying to kill myself with recoil, just get the performance closer to the weight:power ratios that firearms have. The use of helium in the handheld design should significantly reduce the recoil (7x less density than nitrogen, so the test chamber which will use nitrogen will be about 7x smaller).

Putting even 2kpsi in a launcher the size of the SCTBDC would be a really bad idea - the stand could get ripped apart.

Pretzelperson probably won't even build this, and we seem to know more about this launcher right now than he does.

And Rag, your near-invunerability and sheer luck remind me of someone from my highschool; he's been crushed, ran over, set on fire, smashed into things, and nearly electrocuted so many times that it's hard to believe he's still alive, yet he's still in relatively good shape and only walks with a slight limp, despite having broken both knees twice. Except he seems strangely human compared to you...
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Unread postAuthor: Ragnarok » Wed Dec 05, 2007 10:44 pm

It's bizarre - I still work fine after everything, I just pick up a number of scars, if anything. I'm so used to it, I wouldn't be too surprised if I lost a finger at some point and it had grown back completely a week or two later.
Still, I've got a severe challenge on the luck front from one person I know who completely broke his neck during rugby and survived just fine with no damage to the spinal cord. That is exceptionally rare.

It's a bit of a running joke amongst people that know me well enough that I might be the first step of the next stage of human evolution.
If that's the case, the next species in the chain will be rather prone to sunburn.
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Unread postAuthor: Novacastrian » Wed Dec 05, 2007 11:44 pm

How about using two guns strapped together in opposite directions and fired at the same time, they would cancel out each others recoil :D (Just be careful of what is behind you) :lol: :lol:
No really, it WOULD work!
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Unread postAuthor: LikimysCrotchus5 » Wed Dec 05, 2007 11:59 pm

Well yea because the forces would equal out but who would want to spend double to makw two?
I would? anf it would work :D :P
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Unread postAuthor: jackssmirkingrevenge » Thu Dec 06, 2007 12:38 am

Novacastrian wrote:How about using two guns strapped together in opposite directions and fired at the same time, they would cancel out each others recoil :D (Just be careful of what is behind you) :lol: :lol:
No really, it WOULD work!


What, you mean like the Davis gun?

Come on guys, it's not like large pneumatics have never been built before - have we forgotten the Zalinski Dynsamite gun? 15 inch calibre, maximum projectile weight of 1,150 lbs using a maximum of 2,500 psi :shock:

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Unread postAuthor: Infernal Maveric » Thu Dec 06, 2007 6:42 pm

jack, ti's all fine and dandy when it is built for/by the military, but when someone with a very limited amount of knowledge tries to build a cannon on that scale that things will go wrong.
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