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Proplusion, Springs And Psi

basically what is above

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#1 2nd Sniper

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Posted 10 January 2008 - 09:42 PM

As I said in my last post, I am looking to branch out with homemades. I also feel the need to better understand the physics behind spring powered and air-pressure powered guns. I was jsut wondering if I could get any.. formulas, etc, in order to explain how to use spring. Also, a ratio of psi to velocity... yeah, if that makes any sense just hit me up here. It would be appreciated.

Thanks, 2nd loser
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#2 Peter

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Posted 10 January 2008 - 09:54 PM

F = -kx

where f is force in newtons
k is the spring constant (N/m)
and x is the displacement in meters.


Using that equation tells you how much force the spring will give if you know it's constant.
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#3 keef

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Posted 10 January 2008 - 09:56 PM

HAHAHA 2ND LOSER! Jaykay

Boltsniper.com has the barrel formula in the SCAR or FAR section. It shows how big the barrel should be if you plunger is whatever size big. I'm sure Captain Slug's homemades have some formulas or stuff like that. What type of homemade are you trying to make? You should start with a SNAP 1.2 springer by Carbon, or a BAMF by cxwq. If you do the BAMF, just make sure you mod the sprinkler valve. You can find out how by going to a potato cannon site (spudfiles.com, spudtech.com). If you want the instructional video PM me and I'll upload it for you.

Good luck,
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#4 2nd Sniper

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Posted 10 January 2008 - 10:08 PM

groovy, thanks
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#5 CaptainSlug

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Posted 10 January 2008 - 10:16 PM

You can't develop a ratio of PSI to velocity because that's not enough factors to make a formula that will tell you anything applicable.
You're missing supply volume, passage area (or restriction), projectile weight, and then you would have to have a barrel length to match.

I'm sure Captain Slug's homemades have some formulas or stuff like that.

Nope, I'm terrible at math. And since there are so few formulas to go around concerning the performance and design of pneumatic weapons I've had to figure all of this out through practical experimentation.

What I have learned after 6 months of tinkering
Minimum supply volume needed at 35psi to fire a dart to 100 feet (assuming the connection from the supply to the barrel has no restriction) is 4ci. 5-7ci will yield better results or comparable result if the system involve restriction.

Any passages inbetween the supply tank and the barrel need to be 1/4" diameter (or equal in terms of area) or larger in order to not restrict the flow rate of expansion.

There are no off-the-shelf directional control valves usable for Nerf.

Trigger pull load should be under 5 pounds and have as short of a travel distance as possible.

Edited by CaptainSlug, 10 January 2008 - 10:23 PM.

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#6 TAiLsChaser

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Posted 10 January 2008 - 11:13 PM

Sadly, I tried some of the things he said and he does appear to be right. I've gotten some lower psi's to "move" a dart but that's about it. However, I can say that Part one is complete.
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#7 Flaming Hilt

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Posted 10 January 2008 - 11:27 PM

Force does equal the spring constant times the distance, but knowing that doesn't really help at all... as CS said, you also have to take into account... well, all that stuff. And I'm not sure but I don't think you usually find k values with your springs, just part numbers.

CS, by, "ci," you mean in^3 (cubic inches), correct?

I'm in physics so I can help with all that barrel length whatnot if you really want, but I think CS is right... the best way to figure stuff out is just to tinker. Plus, we don't live in a perfect world, so even if you did figure everything mathematically, it still wouldn't be exact.
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#8 bobafan

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Posted 10 January 2008 - 11:42 PM

There is just so many variables, It would be very difficult to make such an equation. There is the inertia of the plunger assembly, the friction on the catch, the friction of the plunger head on the plunger tube, the compression of air before the dart moves, the dead space, various air restriction devices(such as tubes), the fit of the dart on the barrel, the inertia of the dart, and any leaks you have, just to find one variable, the legnth of tubing. And that is just off the top of my head.

The best way to find the ideal legnth for any particular gun would be to guess and change the legnth in one inch incriments until it works as well as you like.

General guides can be made for different streanths of guns, but exactness is difficult.
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#9 CaptainSlug

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Posted 11 January 2008 - 07:26 AM

CS, by, "ci," you mean in^3 (cubic inches), correct?

Yes
Around 7ci at 35psi with a very nonrestrictive trigger valve will get you Titan level performance (without the pump plugged).
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#10 Peter

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Posted 11 January 2008 - 08:28 AM

Force does equal the spring constant times the distance, but knowing that doesn't really help at all... as CS said, you also have to take into account... well, all that stuff. And I'm not sure but I don't think you usually find k values with your springs, just part numbers.

CS, by, "ci," you mean in^3 (cubic inches), correct?

I'm in physics so I can help with all that barrel length whatnot if you really want, but I think CS is right... the best way to figure stuff out is just to tinker. Plus, we don't live in a perfect world, so even if you did figure everything mathematically, it still wouldn't be exact.

If you're in highschool physics, nothing is done with compressible fluids. Also, f= -kx will and does work, it will return the amount of force that the spring pushes back with, not the amount of force going on the dart, friction has to be taken into account. You can just as easily find a constant using feet and pounds if you want.
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#11 jwasko

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Posted 11 January 2008 - 10:37 AM

You could always use Boltsniper's handy little empirical formula for the length of a barrel. It gives a rough estimation of the barrel length of a springer:
volume of plunger chamber = 4*volume of the barrel
Working from that equation:
lenght of barrel=0.25*(length of pull)*[(radius of plunger chamber)^2]/[(radius of barrel)^2],

Where length of pull is how far you pull back the plunger.
According to Boltsniper, this won't give you the exact barrel length; however, it can at least give you a good starting point.

PS: Here is the page where Boltsniper placed the formula.

Edited by jwasko, 11 January 2008 - 12:29 PM.

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#12 Carbon

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Posted 11 January 2008 - 11:18 AM

Boltsniper's writeups are excellent primers on homemade construction in general, for both construction techniques as well as the underlying physics.
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#13 sn1per

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Posted 11 January 2008 - 03:51 PM

Thanks, 2nd loser

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sn1per I appreciate your humor, that made me laugh literally out loud.

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#14 Flaming Hilt

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Posted 11 January 2008 - 04:39 PM

If you're in high school physics, nothing is done with compressible fluids. Also, f= -kx will and does work, it will return the amount of force that the spring pushes back with, not the amount of force going on the dart, friction has to be taken into account. You can just as easily find a constant using feet and pounds if you want.


Yes, I know that, but what are you going to do once you know how much force it takes to pull the spring back? You can determine how fast the air will be pushed out, and how much that will accelerate the dart based on the length of the barrel (assuming you've already calculate that the plunger stroke will produce enough air to fill the barrel). And yes, I can incorporate mew into that as well.

No, we don't work with compressible fluids, but simple kinematics and some common sense can take you a long ways. That being said, if you know about compressible fluids, you could probably find a more accurate answer than myself, but like I said earlier and like CS has kinda been saying, that's all in a perfect world. I'm betting my percent error with your answer as the actual answer, and your percent error with the [/i]actual[/i] answer as the actual answer would be about the same.
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#15 DTReaper

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Posted 11 January 2008 - 06:05 PM

You are all forgetting a key factor in the speed of the plunger and that is friction it will slow down the gun since F=MA the speed and weight are important factors to consider when making a spring gun so a lighter yet still strong plunger is a good way to improve ranges in your spring guns. However i agree with basically everything else that they say.

-DTR
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#16 Carbon

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Posted 11 January 2008 - 08:08 PM

Not forgetting....ignoring. We don't need that level of complexity, we have toys to discuss.

Joking aside, the coefficient of friction of the dart and barrel is even more important than plunger friction (you're just seeking to minimize that friction, anyway). The dart's friction is what allows the initial pressure to build, but not be so resistant to moving that range suffers.
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