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critique my design.

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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critique my design.

Unread postAuthor: hudd » Fri Dec 05, 2008 10:43 pm

I want to make a canon that will fire a nerf mini vortex rocket adbout 60-70 yards. here the plan

3in.x6.5in air chamber> 1in. modified sprinkler valve> 2in.x24in. barrel.

If i did the math corectly then a 12g c02 would charge that air chamber to roughly 120PSI.

so will it work?
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Unread postAuthor: jitup » Fri Dec 05, 2008 10:52 pm

You better use a regulator or that thing will blow up in your face. 12 grams get over 900 psi inside if they are unregulated. Talk to Brian B if you have questions about unreg. C02. he and every one will tell you to use steel and do not even attempt if it is your first project
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Unread postAuthor: frankrede » Fri Dec 05, 2008 10:57 pm

jitup wrote:You better use a regulator or that thing will blow up in your face. 12 grams get over 900 psi inside if they are unregulated. Talk to Brian B if you have questions about unreg. C02. he and every one will tell you to use steel and do not even attempt if it is your first project
Thats 900 psi in a small container, in large chamber the pressures will be drastically lower.
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Unread postAuthor: Ragnarok » Fri Dec 05, 2008 10:58 pm

jitup wrote:12 grams get over 900 psi inside if they are unregulated.

Inside the 12 gram. Let that gas out into a large chamber, and you won't have anywhere near that.

He's right on the money - in a chamber of that size, a single 12g CO2 bulb will pressurize the chamber to 120 psi, there or thereabouts - it'll change a little with temperature.
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Unread postAuthor: hudd » Fri Dec 05, 2008 11:01 pm

so would that shoot a rocket 60 yards or so or would i need more volume.
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Unread postAuthor: jitup » Fri Dec 05, 2008 11:09 pm

oops! :oops: I miss under stood what he wrote! ignore my post I thought you were trying to use a 3 inch long 1 inch diamiter chamber! look were reading to fast gets you looking like an @ss
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Unread postAuthor: Ragnarok » Fri Dec 05, 2008 11:11 pm

hudd wrote:so would that shoot a rocket 60 yards or so or would i need more volume.

I reckon that should do that quite nicely... but at what angle?

If you want to use a low angle of 10-15 degrees rather than 30-40 degrees, I'll have to run some numbers (and ask some numbers too) to work it out.
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Unread postAuthor: hudd » Fri Dec 05, 2008 11:15 pm

i want it to be a relatively flat trajectory. I tried to download the ggdt but it wot work for some reason. so yeah if you could run the numbers for 10-15 degrees.
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Unread postAuthor: Ragnarok » Fri Dec 05, 2008 11:49 pm

hudd wrote:so yeah if you could run the numbers for 10-15 degrees.

The main problem is I don't know what your projectile weighs, so I can't do any modelling. If you drop a projectile weight in the thread, then someone will be able to help. I've got to go do other things, but there are plenty of other people who will be prepared to do it instead.
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Unread postAuthor: hudd » Fri Dec 05, 2008 11:52 pm

ok. Im really not sure what it weighs. Ill have to look into that.
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Unread postAuthor: MrCrowley » Fri Dec 05, 2008 11:56 pm

I can send a nerf about 50-60yards at 30-40psi with a fishing line attached, although with a far powerful launcher, you'll be using 120psi and still a somewhat decent valve.

I'm sure it's possible with that configurating, though you'd probably need to aim a minimum of 15 degrees.
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Unread postAuthor: psycix » Sat Dec 06, 2008 3:44 pm

Ragnarok wrote:He's right on the money - in a chamber of that size, a single 12g CO2 bulb will pressurize the chamber to 120 psi, there or thereabouts - it'll change a little with temperature.

Yep.
Do not forget that the CO2 may freeze your chamber.
I assume your 3" chamber is made out of PVC, so I advise to install a popoff valve.
And a pressure gauge woulnt hurt either, ofcourse.

Imagine that you fill up your chamber with CO2 up to 120 psi. The CO2 is freezing.
You wait some time, and the CO2 gets heated up from subzero temperatures to the temperature of a hot summer day.
What happens to the pressure? Indeed, it goes up.
What happens to the PVC? It COULD fail.
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Unread postAuthor: Ragnarok » Sat Dec 06, 2008 4:10 pm

psycix wrote:The CO2 is freezing. You wait some time, and the CO2 gets heated up from subzero temperatures to the temperature of a hot summer day.
What happens to the pressure? Indeed, it goes up.

Might be worth doing the maths next time. It's a fairly minor 15 psi rise if the ambient temperature is ~90 degrees F.
I think 130-135 psi isn't too worrying, don't you agree? ;)

Filling a PVC chamber from a 12 gram isn't anything like the same mine field as can arise with filling from a larger regulated tank, because you're injecting a set number of moles. People need to remember that in future when people ask about 12 grams.
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Unread postAuthor: grumpy » Sat Dec 06, 2008 4:26 pm

a nerf rocket weighs 36 grams, if you are going to use this at a field i would reccomend you do as psycix said, it will be more readily accepted if they see some safety features built in.
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Unread postAuthor: goathunter » Sat Dec 06, 2008 4:51 pm

Howdy hudd,

From my experience with the nerfs I'd shoot for around 80 psi chamber pressure. When you jack up the pressure the stresses on the nerf are pretty tough. At about 140 psi. you'll kill the nerf in 2-3 shots. 120 psi, you're sitting good but you'll see the nerf do some crazy things sometimes. The nerf design is meant to be thrown, so it flies the best at slower speeds. I've seen in my launchers an average of about 170 fps at 80 psi. It is a fair balance between accuracy and speed.
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