Water Balloon Cannon Optimization

[JesseK]

In order to put enough backspin on a balloon to significantly attribute to its overall distance, one would have to consider how the balloon would react to the backspin while in flight. It's not hard to setup an experiment to show this; however for the experiment to be accurate, one would need to put a true tangental force on the exact edge of the balloon so that the rotational axis is the balloon's center of mass. Since the balloon is not a rigid solid shape, the water would be pushed to the outside of the balloon and eventually form two almost identical pertrusions out of the balloon (much like how galaxies spin). As time progresses, the balloon would look like a spinning dumbell. Eventually the forces on the middle of the dumbell would be great enough to break the balloon mid-flight, therefore sprinkling the target rather than drenching it.

At least it would make some cool rainbows though

[EricH]

Originally Posted by JesseK

In order to put enough backspin on a balloon to significantly attribute to its overall distance, one would have to consider how the balloon would react to the backspin while in flight. It's not hard to setup an experiment to show this; however for the experiment to be accurate, one would need to put a true tangental force on the exact edge of the balloon so that the rotational axis is the balloon's center of mass. Since the balloon is not a rigid solid shape, the water would be pushed to the outside of the balloon and eventually form two almost identical pertrusions out of the balloon (much like how galaxies spin). As time progresses, the balloon would look like a spinning dumbell. Eventually the forces on the middle of the dumbell would be great enough to break the balloon mid-flight, therefore sprinkling the target rather than drenching it.

At least it would make some cool rainbows though

Which is about what I figured would happen. Assuming you could get a decent backspin by shortening the lower half of the barrel or some other method, and the mouth of the balloon isn't in the way, you'd get the air burst effect instead of the shell effect. At least until they make stronger balloons.

[Mr. Freeman]

Originally Posted by RyanN

Please at least put duct tape or something around the tank.

I remember someone on this forum doing tests with PVC barrels to see what can contain them if they explode. They found that duct tape does not help in any way. It will not contain shrapnel.

[ab500]

Originally Posted by Mr. Freeman

I remember someone on this forum doing tests with PVC barrels to see what can contain them if they explode. They found that duct tape does not help in any way. It will not contain shrapnel.

From what I've read the best way to prevent shrapnel from spreading is to put the pipe in a larger pipe. Every other method is flimsy and only offers a certain degree of protection. Using a larger pipe will contain the shrapnel and let the air exit through the ends. If your cannon is too large to fit in a pipe then I'm sure a box of some sort would work. If we stick with pvc once we exit the testing phase we'll end up with some sort of cage like that.

EDIT
----------------
Here are some questions that I'd like to have some input on:
- Whats the best barrel volume/chamber volume ratio and how does the length of the barrel affect inital velocity (ie what is the ideal volume ratio and barrel length)

- If I could design a sabot that would stay in the barrel (maybe by hitting the mouth of the barrel with some sort of stopper slightly overlapping while letting the water balloon continue into flight) would it rob the water balloon of a significant amount of energy? (I know that there would have to be a way to deal with the additional pressure left in the chamber... but setting that aside for a moment)

- Would a heavier or lighter water balloon go further? Is there an ideal weight somewhere?


I'm taking physics next year so showing me how to do these types of calculations myself would be greatly apperciated also, I am definately interested in it.

[RyanN]

Originally Posted by Mr. Freeman

I remember someone on this forum doing tests with PVC barrels to see what can contain them if they explode. They found that duct tape does not help in any way. It will not contain shrapnel.

I know it couldn't help much. I thought it might at least keep shrapnel from flying as far or as much. Ever since I saw the remnants of our PVC barrel, I don't want it to happen to anyone else. A joint cracked and released the PVC pipe like a rocket, nearly missing the kids head and shot into the ceiling. He could have been seriously hurt or killed if his head would have been in the way. I'm not sure a 1/2" glass door will do much to stop a tank from flying through it. It may help with small shrapnel, but large pieces of shrapnel will likely go through the door if it has the chance.

As everyone here can probably tell, I'm completely against using PVC to store air. It is against OSHA regulation and it's against my common sense. It wasn't that the PVC pipe failed, it was that the even thicker coupling holding the PVC pipe together cracked and released the pipe like a rocket. It probably had less than 40PSI in the tank when it cracked. The expanding and contracting of the tank from many charges and discharges caused the PVC to weaken. Sure a brand new tank may be safe, but after using it a few times, the integrity of the PVC goes down and the tank becomes a ticking bomb. We only charged that tank to 40PSI and it took 3 years to blow (probably 300 or so charges/discharges). With 90PSI or even 120PSI it will weaken much quicker, and you are already reaching the limits of the PVC to hold air at pressure. PVC is simply not designed to hold air.

I would strongly consider finding a proper tank before attempting to even make it shoot further. Even though you feel safe, what about the people around you? Is there anyone around outside when you shoot it within 300 feet? The tank you have is like a grenade. You don't know when it will go off, and when it does, it will shoot sharp shrapnel through the air at extreme velocities.

For reference, we use an old metal freon tank. We only crank it up to 60PSI, but can safely crank it up to 120PSI.

[JesseK]

Anyone feel free to correct me --

... and how does the length of the barrel affect inital velocity (ie what is the ideal volume ratio and barrel length)

You have to think of the pressure in the cannon as a force. As long as the pressure differential in the cannon (while the balloon is in there) is greater than the atmospheric pressure, then the balloon will have some velocity when leaving the cannon. As long as the pressure in the tank is greater than the pressure in the cannon, the balloon will continue to accelerate. This also means that if the cannon is too long, the pressure will reach a peak value and then decrease, robbing your beloved balloon of its balistical distance.

Let's figure this out. You want 100m minimum right? Using the 3 kinematic equations and vector analysis of gravity vs distance (ignore air resistance for a second), figure out the Vi (initial velocity) at angle 45 degrees that the balloon must have to reach 100m. This must be the exit velocity of your cannon. Look these equations up.

From here, look at the available equations that deal with pressure, force, velocity, and mass. Velocity = acceleration * time (v = a * t) and Force = mass * acceleration (F = m * a). Also, looking up equations that show the relationships between two pressure states and their volumes in a closed system, you'll find Boyle's Law: p1 * V1 = p2 * V2 (p = pressure, V = volume). Note that it assumes 100% valve efficiency. What this really means is that if the volume of your barrel is greater than the volume of air coming from your tank, the pressure (therefore force) inside the barrel will be *less* than the pressure inside the tank. Practically, in order to facilitate proper pressure of both connected systems when the balloon is at the exit point, the volume of air of the tank must be at least 1.5 * the volume of the barrel. Finally, the unit of pressure (psi) gives away how to find the force from the pressure: F = p / A (A = cross-sectional area of the barrel).

There may be a better way, but I like holding a few things constant so I can get some starting #'s, then work from there. V1 and p1 are constant, and V1 > 1.5 * V2, so what volume of the barrel do I want? Well, the balloon (aka the ammo) has special constraints (too much surface tension will make it bust), so I want to keep its volume relatively low. This means a small balloon -- so pick a diameter -- the 3" you've picked seems to be ok. From here, figure out the length of the barrel -- V2 = L * A (L = length of the barrel & A = Cross-sectional area of the barrel) that is less than V1 / 1.5. When you plug this back into Boyle's law, you should have a theoretical working pressure, p2, inside the barrel.

From here you can calculate the acceleration of the balloon in the barrel: You know the force of the air from p2, and you know the mass of the balloon. You're almost there -- V = a * t -- so now you have to figure out how long it take for a constant acceleration to move certain distance. Since you're knowledgable about the kinematic equations by now, you can figure out the time, and therefore calculate the exit velocity of your balloon. If that velocity isn't at least as high as what you calculated back in the first step, you'll have to play with the constraints in the system to figure out what it takes for it to work.

Good Luck.

Would a heavier or lighter water balloon go further? Is there an ideal weight somewhere?

In physics you will learn that in an ideal situation the mass does not affect the velocity given a constant acceleration (aka gravity). However, since your cannon applies constant force instead of constant acceleration, acceleration is a result of the force:
F = m * a -> a = F / m
Therefore the smaller the mass, the larger the acceleration, giving a higher exit velocity.

As for air resistance and your water balloon, in order to increase the mass to add momentum that would easily overcome air resistance, you would have to increase the size of the balloon by adding more water. This would increase the surface area of the balloon, therefore making more air resistance. Hence, for air resistance purposes, size doesn't matter.

[Dick Linn]

Use the longest barrel you can. Potato cannon builders generally recommend that the air tanks are about 3 x the barrel volume. Matt and Vivek also made good suggestions about using a pneumatic release and parallel sprinkler valves. We have one here at home with three 1 inch valves in parallel, converted to pneumatic release.

I don't know to what extent the balloon will squeegee off any soapy water or shaving cream as the balloon proceeds down the barrel. A more waxy or greasy lube might work better. You can do some coefficient of friction testing and let us know

Occasionally you might find a surplus pneumatic dump valve for a good price. These are used to clear blockages in dust collection systems among other things. We have a couple of 1.5" (I think) aluminum body valves that were about $15. each on fleabay.

If using PVC, always use pressure-rated pipe, not DWV (drain waste vent). Learn to make proper joints (scuff, solvent, cement) and let them cure for 24 hours. PVC gets brittle when cold, and the ultimate strength drops off as it gets hot. http://www.madisongroup.com/failure/...eanalysis.html

[Doug G]

I've used a 3" x 4 foot barrel to launch 4, 1 lb water balloons, all at once to a distance of at least 50 yards. Using similar methods mentioned here but for storage tanks we use TWO scuba tanks, pressurized to about 80 psi and the only PVC part we use is the 1" Sprinkler valve. For lubricant I've found the best solution was to dump about a half gallon of water in the bottom of the barrell and then place water balloon(s) inside. When the cannon fires the incompressability of the water around the water balloons keeps them from popping on the way through the barrell. Using this method with LARGE storage tanks and a long barrell results in some incredible distances. The first time we tried this at my house, I figured the balloons might make it to my neighbor's house across the street (he was in on it), instead they flew over his tree, over his house, over the house behind his and hit in the street in front of that house. Incredible. Unfortunately the canon now sits on top of a robot and I don't dare let the student's launch water balloons until it's adequately waterproofed.

[IKE]

I second all the pressure vessel PVC concerns. Remember right up until it explodes, it won't explode. While simple this is a profound statement in that 90 psi has nothing to do with 120 psi if 100psi is what it takes to burst. Those advising you to be carefule are the ones that nearly killed themselves or others. Those that are dead can't give advice!

[JesseK]

Originally Posted by Doug G

For lubricant I've found the best solution was to dump about a half gallon of water in the bottom of the barrell and then place water balloon(s) inside. When the cannon fires the incompressability of the water around the water balloons keeps them from popping on the way through the barrell. Using this method with LARGE storage tanks and a long barrell results in some incredible distances. The first time we tried this at my house, I figured the balloons might make it to my neighbor's house across the street (he was in on it), instead they flew over his tree, over his house, over the house behind his and hit in the street in front of that house. Incredible. Unfortunately the canon now sits on top of a robot and I don't dare let the student's launch water balloons until it's adequately waterproofed.

Great idea! The water probably also helps with an air-tight seal around the balloon as it's fired so almost no pressure is wasted.

[ab500]

I think I'm starting to get a much better idea of what needs to be done to improve this cannon.

First off to all the people who come into this topic to discourage PVC pipe usage:

PVC pipe is not the final solution! We are planning to move away from it for later designs. This was merely the first prototype. In my opinion the way we were using it was perfectly safe. We are going to move to a more appropriate storage tank eventually but theres nothing like getting your feet wet first and PVC was the cheapest, most avaiable material out there. The barrel will probably remain PVC but I don't honestly see the material failing... there's a large hole in it where the balloons exit and I'm quite sure the pressure will force it's way out that way than through the side of hte barrel.

Now onward to the good stuff:
Doug - What a great idea with the water. I would have never thought of it and am kind of curious as to how you came across it. Good ideas all around, they will be a basis for our final design for sure.

Dick - Hmm.. Three in parallel.... that's not such a bad idea. I'm definately thinking about converting them to be pnumatically operated. Would just drilling the appropriate hole in the top of each one after epoxying the place the eletric soliniod used to be be and then connecting them all to one of the kit's pnuematic soliniods do the trick and provide for a quick enough release of the diaphram? How is yours set up?

JesseeK - I like the calculations. If I didn't have so many finals and the such I'd dig into them a little deeper. I plan to set aside some time once school is out and try to dive into physics.

I've started looking around for an air storage tank with a large port on it. 3" would be ideal, then I could have a three inch manifold with 1inch ports going to spinkler valves and get the full force although I'm pretty sure that without drilling my own holes not many tanks will have this feature. My friend is talking about using the air pressure of a scuba tank... as long as I can regulate it down to 100-120psi I'll be good with using it as a source of air for the tanks.

When you guys use large tanks do you use some sort of pressure rated flexible tubing or do you just mount the tanks with the gun so it all moves?

[vivek16]

I would use a bunch of tubes in parallel if thats the way you wanted to go. I learned the hard way that only having one 1/4" tube will not provide any oomph to your cannon.

Try to find 3/4 or 1" tubing and use atleast as much flow area as your barrel.

-Vivek

[RyanN]

Originally Posted by ab500

I think I'm starting to get a much better idea of what needs to be done to improve this cannon.

First off to all the people who come into this topic to discourage PVC pipe usage:

PVC pipe is not the final solution! We are planning to move away from it for later designs. This was merely the first prototype. In my opinion the way we were using it was perfectly safe. We are going to move to a more appropriate storage tank eventually but theres nothing like getting your feet wet first and PVC was the cheapest, most avaiable material out there. The barrel will probably remain PVC but I don't honestly see the material failing... there's a large hole in it where the balloons exit and I'm quite sure the pressure will force it's way out that way than through the side of hte barrel.

Now onward to the good stuff:
Doug - What a great idea with the water. I would have never thought of it and am kind of curious as to how you came across it. Good ideas all around, they will be a basis for our final design for sure.

Dick - Hmm.. Three in parallel.... that's not such a bad idea. I'm definately thinking about converting them to be pnumatically operated. Would just drilling the appropriate hole in the top of each one after epoxying the place the eletric soliniod used to be be and then connecting them all to one of the kit's pnuematic soliniods do the trick and provide for a quick enough release of the diaphram? How is yours set up?

JesseeK - I like the calculations. If I didn't have so many finals and the such I'd dig into them a little deeper. I plan to set aside some time once school is out and try to dive into physics.

I've started looking around for an air storage tank with a large port on it. 3" would be ideal, then I could have a three inch manifold with 1inch ports going to spinkler valves and get the full force although I'm pretty sure that without drilling my own holes not many tanks will have this feature. My friend is talking about using the air pressure of a scuba tank... as long as I can regulate it down to 100-120psi I'll be good with using it as a source of air for the tanks.

When you guys use large tanks do you use some sort of pressure rated flexible tubing or do you just mount the tanks with the gun so it all moves?

We use a flexible hose. I'm not sure if it is rated for the pressure, but it really doesn't hold the pressure as the valve is attached directly to the tank. We've never had an issue with it and are using ABS connectors to connect the hose to the barrel. Make sure your hose clamps are tight though!

[Lil' Lavery]

I haven't worked with water balloon cannons before, but I can give a few broad tips from working with other projectiles.

Match your barrel to your projectile:
There is no ideal projectile and no ideal barrel, the two are very dependent on each other. Barrel material, diameter, and length are all effected by what you're firing. You don't want excess pressure escaping around the edge of your projectile, so for a non-uniform object such as a water balloon sabot/wadding is favorable. Remember to take your sabot/wadding into consideration when doing any calculations later (they have a mass too!). The lower the kinetic friction between your projectile/sabot the better, as less force will be acting against it. Keep in mind you need enough static friction though to allow for adequate pressure to build behind your projectile to get maximum force (quicker valves does the same). Your barrels do not have to be uniform over their entire length either, which can be used to your advantage if properly planned.

Barrel length is key:
Your object will experience a "favorable force" so long as the pressure behind it is greater than the pressure in front of it. This force will cause the projectile to accelerate (F=ma). You want to find the point where the pressure equalizes in your barrel and the outside atmospheric pressure, and that should be the (ideal) length of your barrel. Afterwards you then need to factor in other forces (such as friction, inefficiency, air resistance, etc.) and adjust your barrel length properly. As soon as your projectile leaves your barrel (or once forces resisting it's motion are greater than those aiding it) your projectile will begin to decelerate. Look at JesseK's equations for a bit more detail.

Carefully chose your projectile:
In this case it's dealing with the mass/shape/material of your balloons, as well as your sabot/wadding. With practical limitations on pressure (and thus force) created by your barrel and other components, mass becomes an important consideration. In particular the mass of your sabot/wadding, as that mass will reduce your acceleration in the barrel, but will not resist the deceleration of the projectile while in flight.

[Daniel_LaFleur]

Originally Posted by ab500

I think I'm starting to get a much better idea of what needs to be done to improve this cannon.

First off to all the people who come into this topic to discourage PVC pipe usage:

PVC pipe is not the final solution! We are planning to move away from it for later designs. This was merely the first prototype. In my opinion the way we were using it was perfectly safe. We are going to move to a more appropriate storage tank eventually but theres nothing like getting your feet wet first and PVC was the cheapest, most avaiable material out there. The barrel will probably remain PVC but I don't honestly see the material failing... there's a large hole in it where the balloons exit and I'm quite sure the pressure will force it's way out that way than through the side of hte barrel.

I am very happy to hear that PVC is not the final solution for your tank. PVC gives almost no indication before it fails and when it fails, it fails violently when used in pressure applications.

What really concerns me is that the barrel will still be made of PVC. Your statement ("I'm quite sure the pressure will force it's way out that way than through the side of hte barrel") tells me that either you don't understand the gas laws or are just not believing they apply to your cannon.

Gas exerts it's force in all directions at once. So when you introduce the gas to your barrel the force that starts to move the balloon also is applied to the inside walls of the barrel. PVC is not designed to take that sort of shock load and will eventually fail ... catastrophically (sp?), and generally without warning. Only once the projectile (Balloon) leaves the barrel will the pressure be relieved from the inside of the barrel.

Sorry to be such a stickler, but PVC and pressure generally equal someone taking a trip to the hospital.


Now on to the fun stuff

For the firing valve, why not use a butterfly valve? Their CV is very high. The only drawback is cycle speed (which is not relevent here).

Also, any regulator you use (IE from SCUBA) will restrict your airflow. Your better off charging your tank to your firing pressure and then dumping through a large orifice valve directly into your chamber.