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silent c:b ratio

A place to ask general spud cannon related questions.
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silent c:b ratio

Unread postAuthor: C0rPs3 » Mon Dec 10, 2007 3:54 pm

hi,
i'm planning to make mini pneumatic.
and i was wondering what c:b ratio would make a pneumatic as silent as possible :?:
as i want to avoid making a silencer.
thank you
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Unread postAuthor: Ragnarok » Mon Dec 10, 2007 4:00 pm

That depends on the pressure and valve. We need a lot more information than that to tell you anything.
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Unread postAuthor: jackssmirkingrevenge » Mon Dec 10, 2007 4:01 pm

Ideally for 100% suppression without a suppressor, for a given chamber volume and pressure, take your pressure and divide it by atmospheric pressure (14.7 psi) and multiply the result by your chamber volume. The result is the ideal volume for your barrel, divide this by the area of your bore and you'll have the barrel length.

If you follow this formula to the letter however, you'll either have a ridiculously large launcher or one with very low power. It depends on the maximum size you're willing to live with and the sort of power you're especting.
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Unread postAuthor: Ragnarok » Mon Dec 10, 2007 4:08 pm

@JSR: That won't account for gas decompression, you'll lose power from a vacuum behind the projectile.
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Unread postAuthor: jackssmirkingrevenge » Mon Dec 10, 2007 4:16 pm

Naturally, the formula was simplified and besides, if you've managed to achieve a vacuum in the barrel, then either your barrel's poking into the next county,you have a chamber the size of a gnat's knacker or you've filled your chamber by putting the schrader to your lips.
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Last edited by jackssmirkingrevenge on Mon Dec 10, 2007 4:20 pm, edited 1 time in total.
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Unread postAuthor: schmanman » Mon Dec 10, 2007 4:20 pm

nice sig, rag.

I was going to make fun of your sig, but I didn't. that would have been mean, and for no reason.

as for the ideal c:b ratio for a pneumatic, just make a suppressor for your gun If you are looking for low decibels and relatively high performance
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Unread postAuthor: Ragnarok » Mon Dec 10, 2007 5:54 pm

schmanman wrote:I was going to make fun of your sig, but I didn't. that would have been mean, and for no reason.

Well, oddly enough... I did see something... don't worry, I've had enough deep arguments across the interweb that mild fun is just water off a duck's back.

Actually, talking about that sort of thing, I've had my new avatar in the pipeline for a while, I've just forgotten to change it over.
I've been using the last one for over a year now (on several forums), and it needed a change. It's a Grey Wolf taken by some national geographic photographer - I've always liked wolves.

I also like Top Gear. I've actually got enough of clips of it on my laptop I'm going to need an external hard-drive soon - and then I'll be able to download every single episode! (Except for a couple that aren't available on the net). Now that would make me happy.

@JSR: Well, a compromise would be a pressure at the muzzle of 1 to 2 bar (15-29 psi). 3 bar (44 psi) would still be reasonably quiet.

Now, I know I've mentioned HEAL far too much, but last week, I bought a dB meter to find out exactly how loud the ~11 bar pressure that remains at the muzzle actually is. I'm expecting at least 100, probably closer to 120 (at 1 metre parallel to the muzzle).

That should be interesting, because as far as I know, no one has yet put numbers on how loud spudguns actually are.
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Unread postAuthor: schmanman » Mon Dec 10, 2007 7:23 pm

ok, I'm glad you can take a joke. :)

I understand talking about your gun a lot, dont worry about it.

people who complain should quit whining over some words.

I did the same thing forever about the swat, and I continue to talk about it and bring it into discussions , and probably will continue for a good long time.

It, like the heal, is quite an achievement (not to sound stuck up), and I am proud.

some will say it's just a scaled up cannon, but there's more involved than most would guess with building large cannons. at that size, pushing fittings together becomes a task in itself. and if you f**k up, your down and out $90 or so. the thickness at some of the joints is 3/4's of an inch to an inch.

the designing of the valve is harder too. you cant just grab a doorstop or a lego and pop it in the valve and have it work.

ingenuity is my friend.



jesus freaking christ.... enough about the swat.....
(adds this to annoy some people)

*bangs head on 1 1/2 inch thick granite countertop, makes a sicking thud*

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Unread postAuthor: Ragnarok » Mon Dec 10, 2007 8:05 pm

I can completely see why such a large cannon is a difficulty. Through necessity, my launchers have to be small - I can't use large cannons in my back garden, I have limited storage space, etc... and I've grown pretty attached to my handheld pieces, to the point where I haven't gone over a 1" bore in a long time (well, strictly 26.3 mm)

I may build some slightly larger pieces in future, but there's only so much energy that can be hurled around in a small space and be kept contained, and more velocity attracts me more than more mass.
Eventually things get so fast they can't be seen, but that just means you can claim it was even faster than it really was... and a loud bang helps keep people convinced.

Actually, HEAL is only one of almost 30 launchers I have built (if you include gauss guns, catapults, trebuchets, onagers and crossbows in the count), and in many cases, I could refer to those, most of them no longer exist and no photos were ever taken.
It's probably best to talk about a launcher of mine that people know about, and as I've only ever put two of my launchers on any forums, and one was a refinement of the other...
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Unread postAuthor: schmanman » Mon Dec 10, 2007 8:11 pm

the swat was my first.

I have built about three strictly for me.


for others, I have built five or six.

I have a small launcher (bb rifle), had a medium launcher, and have a large launcher (swat).

have you seen my bb rifle? there's a link in my signature.


wow, can anyone say "threadjacking"!
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Unread postAuthor: Novacastrian » Mon Dec 10, 2007 8:17 pm

I was able to achieve a damn near silent shot with a combustion (around 250ml) and a 2meter long barrel of 12.7mm. Since this gun operates at around 65psi i suggest you just use similar pressure. The power went down a bit but not much, probly like 10 percent. It has enough power to pierce with a 880mm long barrel, witch i way too short, at 2meters it put the c:b ratio at just about 1:1
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Unread postAuthor: Ragnarok » Mon Dec 10, 2007 8:17 pm

Well, my first launcher qualified as no more than dangerous junk. However, I can't help but wonder what would arise if someone started as well as you did with the SWAT, and had advanced as much as I had between my first and HEAL.
That would be... actually, there isn't a word for it.

I'm liking that BB "rifle".
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Unread postAuthor: schmanman » Mon Dec 10, 2007 8:18 pm

thanks.


I'm going to bed......

g'night!!
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Unread postAuthor: MrCrowley » Mon Dec 10, 2007 8:26 pm

Ragnarok wrote:Now, I know I've mentioned HEAL far too much, but last week, I bought a dB meter to find out exactly how loud the ~11 bar pressure that remains at the muzzle actually is. I'm expecting at least 100, probably closer to 120 (at 1 metre parallel to the muzzle).

That should be interesting, because as far as I know, no one has yet put numbers on how loud spudguns actually are.


I managed to get 114dB next to a race track when I was doing some timing with F5000's, and it there was actually a 90dB limit on the track but oh well. So yeah I think you could get around 100-120dB but maybe a bit higher then ~11BAR, at 300psi i'm sure you could easily.

You should also place it a bit further past the barrel, I remember when standing inline with the barrel and stadning in front of it had a noticeable noise difference.
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Unread postAuthor: Ragnarok » Mon Dec 10, 2007 9:09 pm

MrCrowley wrote:I managed to get 114dB next to a race track when I was doing some timing with F5000's, and it there was actually a 90dB limit on the track but oh well. So yeah I think you could get around 100-120dB but maybe a bit higher then ~11BAR, at 300psi i'm sure you could easily.

You misunderstand. The 300 psi pressure has fallen to about 11 bar/160 psi at the muzzle due to decompression and cooling.

I intend to take sound readings for directly ahead (if I can be sure of not hitting the meter), 45<sup>o</sup> to the side, 90<sup>o</sup>, 135<sup>o</sup> and directly behind - but 90<sup>o</sup> to the side will probably be the first reading.

And although it's interesting to know, I don't see this as a contest to see how loud it is. Ideally, I'd like a nice quiet cannon, but in order to get a good amount of power in a reasonably sized launcher, there has to be that waste energy.
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