Small bombs, rapid fire

Threads related to water balloon launchers.
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cantab
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Re: Small bombs, rapid fire

Post by cantab » Fri Aug 15, 2008 8:19 am

Is there any particular reason to use multiple barrels? In real guns (eg gatling guns) it's to avoid overheating issues, but WBL's don't have that.
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aEx155
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Re: Small bombs, rapid fire

Post by aEx155 » Fri Aug 15, 2008 3:07 pm

cantab wrote:Is there any particular reason to use multiple barrels? In real guns (eg gatling guns) it's to avoid overheating issues, but WBL's don't have that.


I think the sole reason to use multiple barrels is to lower the chance of popping balloons. Since the only time they move (in the barrel) is when they're loaded and when they're fired, that leaves very little chance of them breaking.

I'm thinking that you could have a team of 2 - 3 people, one (or two) loading a set of barrels and one firing. That would have the advantage of ROF while keeping balloons safe. Probably you could use some sort of breech system if you wanted to lose the multiple barrels, but that would reduce the ROF.

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SSCBen
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Re: Small bombs, rapid fire

Post by SSCBen » Fri Aug 15, 2008 5:58 pm

Multiple barrels are easy to make, relatively cheap, and don't pop balloons. They are also heavier and bulkier.

What I'm thinking of is something that can be held by one person rather than something operated by multiple. Range-wise it'd be able to shoot maybe 200 feet. I might add some simple iron sights to it too to improve accuracy.

Once I find my camera I'll post up a drawing of my concept...

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Silence
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Re: Small bombs, rapid fire

Post by Silence » Sat Aug 16, 2008 2:16 pm

If you try designing a launcher or Nerf gun that uses magazines instead of multiple barrels, you'll find a brick wall when it comes to loading, creating a seal, and firing the balloons without popping them. Sabots don't help too much. Rotating barrels prevent parts from touching the balloons.

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cantab
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Re: Small bombs, rapid fire

Post by cantab » Sat Aug 16, 2008 2:23 pm

There are stock Nerf guns that use magazines. Though of course Nerf darts don't pop.
I work on Windows. My toolbox is Linux.
Arsenal:
Super Soaker: XP215, 2xXP220, Liquidator, Aquashock Secret Strike M(odded), Arctic Blast M, CPS1200, CPS2100, SC Power Pak, 3l aquapack, 1.5l aquapack
Water Warriors: Jet, Sting Ray M, Shark, Argon M, Tiger Shark, PulseMaster
Others: Waterbolt, The Blaster, Storm 500, Shield Blaster 2000, generic PR gun, generic backpack piston pumper (broken), 3l garden sprayer M, 10l water carrier:

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CROC
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Re: Small bombs, rapid fire

Post by CROC » Sat Aug 16, 2008 9:43 pm

Oh, but they do cantab, but they do. If enough air goes through a Nerf dart, the top will be forcefully removed from the gun, or the glue will magically wear out, and just the dart head will fly.
-Croc
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cantab
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Re: Small bombs, rapid fire

Post by cantab » Sat Aug 16, 2008 9:59 pm

Oh yeah...I've actually had that happen, though probably due to crappily glued darts rather than excessive power.
I work on Windows. My toolbox is Linux.
Arsenal:
Super Soaker: XP215, 2xXP220, Liquidator, Aquashock Secret Strike M(odded), Arctic Blast M, CPS1200, CPS2100, SC Power Pak, 3l aquapack, 1.5l aquapack
Water Warriors: Jet, Sting Ray M, Shark, Argon M, Tiger Shark, PulseMaster
Others: Waterbolt, The Blaster, Storm 500, Shield Blaster 2000, generic PR gun, generic backpack piston pumper (broken), 3l garden sprayer M, 10l water carrier:

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Silence
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Re: Small bombs, rapid fire

Post by Silence » Sun Aug 17, 2008 3:43 pm

Darts are also too soft in most cases. Many magazine-fed Nerf guns (both commercial and homemade) use complicated shells and ejection systems to make sure the darts aren't crushed. Without shells, you need long springs with low force constants and very tight tolerances.

aEx155
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Re: Small bombs, rapid fire

Post by aEx155 » Mon Aug 18, 2008 2:21 am

Kind of unrelated and related at the same time, but would using CO2 be acceptable here? It's not a water gun, and technically, since there's no chamber to fill (you could make it metal) you wouldn't have to worry about fracture.

Also, cantab's bit about multi-barrel guns having to cool down makes me thing that they would be able to "warm up" after firing, which would reduce the chance of breaking.

I know you could use HPA or bike-pumping really fast, but it was just an odd idea I though about.

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SSCBen
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Re: Small bombs, rapid fire

Post by SSCBen » Mon Aug 18, 2008 2:53 am

CO2 is a bad idea with plastic because the temperature makes the plastic brittle. It's also rather inconsistent unless you have very good equipment. I'd prefer HPA, especially given that they use the same threads and everything.

When I actually get around to making something like this it'll surely start off with PVC pipe but I'd love to get my hands on some CNC equipment and maching something... a simple WBL would be fine to start with.

Though, with CNC equipment I might be able to make a working bolt system, but I'd have to make parts fit together so that there's no seam, which might not be possible.

If I don't post a picture of my idea tomorrow, someone should send me an email to remind me...

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Silence
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Re: Small bombs, rapid fire

Post by Silence » Mon Aug 18, 2008 4:22 pm

HPA expands to maybe 10 to 200 times its compressed volume, depending on the pressure. Propane expands to around 300 times its original volume as a liquid. Carbon dioxide, about 850 times. So there are definite advantages to using liquid gas, plus the vapor pressures are lower than HPA. But vaporization is endothermic and can chill PVC even when deregulated.

Ben, if you *can* use metal for the entire launcher, then you could use propane or carbon dioxide instead and get a massive storage boost.

aEx wrote:Also, cantab's bit about multi-barrel guns having to cool down makes me thing that they would be able to "warm up" after firing, which would reduce the chance of breaking.

You mean, letting the PVC warm up after being chilled by the passing air? That's probably not a very reliable mechanic and it's pretty risky, if you ask me.

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SSCBen
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Re: Small bombs, rapid fire

Post by SSCBen » Sun Dec 28, 2008 5:50 pm

With a few weeks available to me and a desire to calibrate a computer model of a pneumatic gun, I think I'll work on this project. However, I am going to specifically not use HPA so others won't feel restricted and I can optimize the gun for energy efficiency. If I used HPA, the work's already been done to get the air at a pressure, so I'd have to optimize the gun to use less air mass. The two approaches yield different optimal setups. Optimizing for energy efficiency tells you to use moderate pressures with moderate sized air chamber. Optimizing for air efficiency tells you to use the highest reasonable pressures and tiny air chambers.

The goal is to build a small WBL that can shoot at least 6 rounds continuously in less than 20 seconds without recharging. The goal range is 200 feet. The pressurization method is a bicycle pump.

I'll need to first figure out the mass and drag coefficient of a 40 mm water balloon. This'll allow me to calibrate my exterior ballistics model so I can figure out what sort of muzzle velocities to design for. It'll also allow me to optimize the mass of a water balloon a bit, but I'll likely be limited in this respect. My quick math says that I'd have to cut the weight by a third to optimize the projectile and that would only cut the energy use by 10%, so a 40 mm sphere is fairly close to optimal as is.

Also, I had said earlier that CO2 is less consistent than HPA. I've since read that is a myth.

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Silence
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Re: Small bombs, rapid fire

Post by Silence » Sun Dec 28, 2008 8:24 pm

CO2's vapor pressure changes considerably with temperature, and HPA's pressure itself changes with temperature, too (per the ideal gas law). No time to compare right now, but I'll check later...

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SSCBen
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Re: Small bombs, rapid fire

Post by SSCBen » Wed Dec 31, 2008 5:50 pm

The differences between the two won't be too great from the empirical data I've seen on the subject. In fact, the data I saw suggested CO2 would be slightly more consistent (the standard deviation differed between the two by about 0.5% or something like that).

I've revised my plans a bit. The goal is to use as many parts I already own as possible. I've decided that 1.5 inch balloons are rather inconsequential so I'm going to use 2 inch balloons.

In terms of efficiency, while my computer simulation isn't as robust as I'd like, a few things that can be optimized are apparent:

- The pressure and volume to get a certain range. Based on my ballistic model calibrated with empirical data I should get a good idea of the required muzzle velocity for a certain range. I then set the muzzle velocity of the internal ballistic model to that and figure out acceptable pressures and volumes.

- The amount of "dead space" between the projectile and the valve doesn't quite seem dead. My model and GGDT both predict there's an optimal amount of dead space that can increase efficiency. I have no idea why this is, but I figure it's worth taking advantage of.

- The barrel length. Longer barrels seem to require slightly more energy. The difference is rather small but it can add up. I think I'll test barrels ranging from 12 to 24 inches and decide what is efficient while keeping the accuracy high.

- Some other things, like reducing the pilot volume, are obvious and not much more can be done than I will do.

Most of the remaining parts are coming in later today so I can start building and testing very soon. I'll keep everyone updated.

ben@sscentral.org / Please read this before emailing or PMing me

Do not ask me water gun questions by email or PM. Please post the question at the forum. Private questions and suggestions are welcome by PM and email. Also, I do not sell or buy water guns online.

aEx155
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Re: Small bombs, rapid fire

Post by aEx155 » Sat Jan 03, 2009 1:56 am

Just popped in to see what was new, and this was the only topic, so I'll just add my bit.

Are you planning to use sabots?

(Not quite the most beneficial thing, but I was just wondering.)

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