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Air Pressure Question

If you cannot answer my question, please do not post.
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#1 Maniacal Coyote

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Posted 09 May 2015 - 10:40 AM

Before I begin, I want to make something clear: this is not a Concept thread; this is a physics inquiry thread. I am only posting this here because I know some genius out there will know the answer.

Concept threads tend to sound like, "I'm planning to build a nerf grenade, does anyone have any advice? Here is a shitty diagram to illustrate how little I understand physics and fabrication". You may notice that there are concept threads that squeak by under the radar because they have some value. If someone puts a ton of work into making a CAD diagram, or tries to do something new and fails, and they want to share that, that's great. We just ban concept threads as a general rule to keep things simple.


What is the minimum tank volume for 30 psi air needed to launch a JSPB Magnum 100+ feet?

Please, no hate. If you do not know the answer, please do not post. I need a firing tank size for a project that I am working on, and I do not need trouble.

 


Edited by Aeromech, 23 November 2015 - 01:12 AM.

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#2 Borgador

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Posted 09 May 2015 - 12:15 PM

Barrel length and diameter will greatly effect the size of the tank needed. What is your barrel size?

Assuming the blaster is fired level and 3 ft off the ground
u= initial velocity (AKA muzzle velocity)
s = distance (100 ft)
h = height (3ft)
g = accel of gravity (32.174 ft/s squared)

u = s / ( sqr root ( 2*h/g))
u = 100ft / (sqrroot (2*3ft / 32.174ft/s^2)
u = 231.6 ft/s^2

Now you need to find the volume of air, at 30 psi (pounds per square inch),required to accelerate a JSPB Magnm (~1g) up to the required muzzle velocity over the lengh of your barrel.

Of couse all of this is ignoring friction.

Edited by Borgador, 09 May 2015 - 12:17 PM.

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#3 CaliforniaPants

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Posted 09 May 2015 - 01:52 PM

for comparison: Ice Nine's recent homemade uses an air tank thats about 8 cubic inches, which is near the size of a UMB, little smaller than a big blast running at 30psi and works very well

If possible, since youre set on 30 psi (good safe choice), make a bunch of different sizes with specific volumes and see what works best. nerf can be outrageously inconsistent sometimes, so testing is key

Edited by CaliforniaPants, 09 May 2015 - 01:55 PM.

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#4 Maniacal Coyote

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Posted 09 May 2015 - 02:07 PM

I am planning on making something similar to Flaming Hilt's (TH)^2, but using some form of hopper and substituting a pressure regulator for the ball valve.

I will also have a rather large reservoir tank holding a crap ton of air at high pressure. That might affect the measurements a little bit.

I will probably use a 12" barrel with ARB 1/4 rifling.


With these in mind, would I need much of an actual tank or could I get away with a couple inches of 3/4 inch between the regulator and the check valve?
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#5 CaliforniaPants

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Posted 09 May 2015 - 07:06 PM

I am planning on making something similar to Flaming Hilt's (TH)^2, but using some form of hopper and substituting a pressure regulator for the ball valve.

I will also have a rather large reservoir tank holding a crap ton of air at high pressure. That might affect the measurements a little bit.

I will probably use a 12" barrel with ARB 1/4 rifling.


With these in mind, would I need much of an actual tank or could I get away with a couple inches of 3/4 inch between the regulator and the check valve?

please specify "high pressure". is your vessel rated for high pressure? what about the rest of your tubing and parts before the regulator? how well does the regulator flow at your desired "high pressure"?

and if you're actually going to be regulating your pressure down to 30psi the firing tank advice i gave above still stands. the pressure of your reserve tank doesnt matter compared to your firing tank unless they're running at the same pressure.

lolrifling
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#6 Maniacal Coyote

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Posted 11 May 2015 - 09:45 AM

please specify "high pressure". is your vessel rated for high pressure? what about the rest of your tubing and parts before the regulator? how well does the regulator flow at your desired "high pressure"?

and if you're actually going to be regulating your pressure down to 30psi the firing tank advice i gave above still stands. the pressure of your reserve tank doesnt matter compared to your firing tank unless they're running at the same pressure.

lolrifling


"High pressure" = Somewhere under 120 psi. I will be using PVC connected with both primer and solvent cement. The main tank will either be 2" PVC or (if I can find the pressure rated shit) 3" PVC and look something like Buffdaddy's mortar.
I do not know how the regulator will work at ~120 psi, but at worst, I can use it at around 60-75 psi.

How long of a piece of 3/4" Schedule 40 PVC pipe should I use as the firing tank?
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#7 Draconis

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Posted 11 May 2015 - 12:09 PM

Just do the math for the volume you want, dude. 3/4" Sch40 should be just about 20mm ID, so: (target volume)/((20mm/2)^2*pi)=(Length of pipe)
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#8 Maniacal Coyote

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Posted 11 May 2015 - 12:14 PM

Just do the math for the volume you want, dude. 3/4" Sch40 should be just about 20mm ID, so: (target volume)/((20mm/2)^2*pi)=(Length of pipe)


Thanks for the inside diameter and formula. That's good for later projects too.
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#9 CaliforniaPants

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Posted 11 May 2015 - 06:57 PM

"High pressure" = Somewhere under 120 psi. I will be using PVC connected with both primer and solvent cement. The main tank will either be 2" PVC or (if I can find the pressure rated shit) 3" PVC and look something like Buffdaddy's mortar.
I do not know how the regulator will work at ~120 psi, but at worst, I can use it at around 60-75 psi.

How long of a piece of 3/4" Schedule 40 PVC pipe should I use as the firing tank?

over a hundred is a lot for PVC, please make sure to paint it to prevent any UV damage and maybe add padding to keep it from being damaged. if you drop it hard DO NOT keep using it, shit can go bad real fast.
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#10 TantumBull

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Posted 13 May 2015 - 11:36 PM

over a hundred is a lot for PVC, please make sure to paint it to prevent any UV damage and maybe add padding to keep it from being damaged. if you drop it hard DO NOT keep using it, shit can go bad real fast.

This. A lot of people don't realize how large of an effect small imperfections in a material can have on it's ability to not fail. I'm not sure its necessary for this and there's a lot of math involved, but you could derive the critical crack length at which point the tank needs to be tossed. Plastics behave fairly well with small surface cracks so it should be visible.
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#11 Daniel Beaver

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Posted 14 May 2015 - 09:49 PM

I'll say it a third time, just so no-one misses the point: don't pressurise PVC to 100psi, keep it down below 40psi. While you can normally pump it up to 100psi without issues, understand that PVC piping is not manufactured with any sort of quality control on rated pressure, so it's not at all unusual to have batches that would rupture at 100psi. And PVC is particularly nasty when it fails, it sends sharp pieces flying everywhere.

Edited by Daniel Beaver, 14 May 2015 - 09:49 PM.

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