Water Balloon Cannon Optimization
 

Hello everyone,

As a side project I am building a water balloon launcher with the support of my team. Currently I can launch balloons approximately 30 meters at aroudn 90psi and am looking for ways to optimize this distence. Here are the specs of the cannon:

-Air Chamber - 3ft 2"dia PVC pipe with value stem at end
-Valve - 24v 1" port spinkler valve
-Barrel - 2ft 3"dia PVC pipe
-Angle - 30-45 degrees approx.
-Pressure - 80-100psi currently
-Record Distences - 30meters approx.
-Control Mechinism - Wireless control system developed with a ATmel controller and a wifi point running dd-wrt.

Many of you might be concerned about my use of PVC pipe in this project for pressure applications. Let me assure you that we are being very safe about this. ABS plastic is unaviable locally in my area so we are working with the next best alternative. So far all of our testing has been done behind a .5" glass door approximately 10ft away from the device. For the final product we are going to drop the cannon into a 4-6" piece of drainage pipe in order to cetch any shratnal in the unlikely chance it explodes.

I'm looking to have a range of approximately 100-200 meters when finished. We have some pretty impressive ambitions for this project. We want to be able to do some basic balistic calculations and put the cannon on a two axis controller and motorize it so we can drop balloons on-command.

Any Suggestions? I know there are some physics geeks out there just dying to answer :D

Re: Water Balloon Cannon Optimization
 

from what ive read *ive never built one* the longer the barrel the faster it gets going. also have you tried a little bit of oil or lubrication on the water balloons?

thats what i would try

...Forest

Re: Water Balloon Cannon Optimization
 

The ideal angle is 45 degrees. Half of your energy goes to propelling it forward and half goes to keeping it in the air. You will get the maximum distance for this.

Well, having a larger valve would increase air flow. Try to find a valve that is close to the size of your barrel. It would be nice if your air chamber were as big as your valve but not totally necessary. Make sure that your air chamber has atleast 1.5 times as much space as your barrel can hold (I think that would help).

hope it helps, Vivek

EDIT: fimmel is right, if the barrel is longer, the balloon has a longer period of acceleration. The air chamber larger than barrel rule still helps though. Water lubrication will be fine as long as you just keep the inside slightly wet.

Re: Water Balloon Cannon Optimization
 

Not necessarily. Ignoring air resistance, you are right (I think--I'm trying to remember my physics.) But as soon as air resistance enters the picture, you need to tweak. In 2006, very few robots shot at 45 degrees; most shot at about 30 degrees. This is a slightly different case, though.

You might try adding a backspin, though I am not sure if that would help or hurt due to a non-uniform load.

Re: Water Balloon Cannon Optimization
 

Not sure how adding backspin would work though (maybe I am not thinking outside the box enough...).

Personally, I would tweak to find the best angle for your setup. I forgot about air resistance (air... way too overrated :))

-Vivek

Re: Water Balloon Cannon Optimization
 

I was told in a ballistics class during a science summer camp, you can best maximize distance by shooting at around a 38 degree angle. Whether or not this is factual, I can't say for sure, but it did work with popsicle stick mangonels launching marbles. Since you're launching water balloons at a higher velocity, I'd imagine that angle would shift some, but I don't know how to calculate, or even predict whether it would increase or decrease.

Finally, be sure not to pop the balloon. Calc AB at my school has a final project where they launch water balloons, and typically there are a lot of kids who break them without launching them. You need to be careful not to launch them too hard.

Re: Water Balloon Cannon Optimization
 

I was playing with a water balloon launcher at a party today. Long story short, the max range on the thing was about 10 feet, but I learned a few things.

Lubrication for the balloons is a must. Water doesn't work. It beads up and leaves most of the barrel completley dry. I suspect this is due to surface tension, thus, soapy water might work but we didn't try it.
Some type of cooking oil might be a good idea. It should lubricate the barrel as well as be safe should someone manage to ingest some of it or get it on their skin.

There should be some way of draining the pressure chamber. If the balloon breaks in the barrel, the water will drain into the pressure chamber.

Backspin seems like an impractical idea. The only reasonable way I could see to do it is to not lubricate a strip on the top of the barrel. This might also cause the balloon to break though.

Post pictures of this thing once you've completed any modifications. The wireless control system sounds pretty cool.

Re: Water Balloon Cannon Optimization
 

Okay, so far in summary I've heard the following ideas:

-Lubrication - Hmm this definately could cut back on the friction on the balloons.

-Sabot Modification - (didn't see this one but it's a good idea) Right now what I've done to avoid breakage of the balloons is I've been creating rudimentry sabots out of plastic drinking cups. Basically my procedure is to place the cup in the barrel as far as it'll go, draw around it with sharpie, and then cut it odwn to size so it'll fit. I think there is a lot of room for improvement with this. Once I've fired the sabot and balloon out of the cannon the sabot has cracked on every side. This is telling me that most likely air pressure is escaping around the sides of it. Maybe a stronger material is needed.

-Longer Barrel - This is true although buying more 3" pipe is expensive. Once I get paid again I might go for it.

-Better Value - Solinoid valves with large diameters do get pretty expensive. If there is a reasonably priced one I'll go with it. Right now my cannon does make a honking noise which I attribute to the valve not being fully open but using a manual valve would remove the electronic control (which is a very important safety feature at this point)

-Angle of Firing - Yes 45 degrees is the ideal angle without wind resistence... 40ish is with it.... of course I've been firing somewhere between 30-45 using whatever angle can be obtained with various pieces of deck furnature.

-Weight of Balloons - I hope to get a balance and find the ideal weight for my cannon, I'm not sure how this will affect range.

Here's my plan of action for now:
-Next weekend I plan to build a stand for the cannon with more percise angle measurements (we also hope to motorize this stand at some point to do some very cool balistics)

-I will experiment with different sabots and balloon masses

-I will incrase the pressure up to 120psi.

-I will try some different forms of lubricant.

Re: Water Balloon Cannon Optimization
 

Try using shaving cream as a lubricant. It should help improve your sealing as well.

The longer barrel should also help a lot.

Valve. Is there a reason it must be remotely operated? you could get a much larger pipe valve and a heafty spring. Have a safety lock on this mechanism.

Sabot- Use wadding. It has worked for centuries for cannons and muzzle loaders.

To get an idea of travel here is a little math.

E=F*D
E=1/2MV^2
D=Muzzle length
F=Avg. Pressure times area
(if your chamber volume is large relative to muzzle volume and your valve is not terribly restrictive, you can assume P= reservoir pressure)
F=90psi*7(apporx A of 3" pipe)=630lbs
E=15120 in*lbs
E=1700 J (used converter)
Assume a mass of .5kG (you should wiegh this)
V=82m/s
For a 45 degree launch (assume ideal physics for a minute)
Vx=Vy=58m/s
It will reach apogee when Vy=58-9.8t=0
therefore t=6 seconds
To calculate distance X=Vx*2t=696m

Translation is you are seeing nowhere near this performance. How to diagnose your losses.
First off weight the balloon. Re-do calcs with appropriate weight.
Second, time the ballon as well as measuring distance. From this you can approximate the lateral velocity and can then back calculate your muzzle velocity.
Looking at the volume ratios you have more muzzle volume than chamber volume this means P goes from 90 to about 50. This will give an average pressure of 70psi.

To give you an idea of how sensitive distance is to these numbers let's change the weight to 1kg, Pavg=70,

this give an E=12000 in*lbs
E=1355J
V=52m/s
Vx=Vy=37m/s
t(apogee)=(37m/s)/(9.8m/s^2)=3.75
Therefore Distance=37*2t=277m

My guess is you are getting a lot of blow-by and pressure drop across your valve. Try the shaving cream and that may double your 30m to 60m if you get a good seal. Work on optimizing your balloon fill to find minimum weight, but adequate size to seal well. With some optimizing you should be able to get 100m. 277 is quite unrealistc as it does not take into acount wind resistance and muzzle friction.

Last but certainly not least do your get a lot of kick (backwards motion of the launcher?) If the launcher is moving during the launch, this may dramtically reduce the energy going into the ballon.

Please be safe. 90psi is a lot of pressure. A 50 m/s water ballon is like a 100 mph fastball.

Would you stand in front of a MLB baseball pitch?

Re: Water Balloon Cannon Optimization
 

Thanks for the response! I apperciate the input a lot :)

I'm using a remotely operated valve because we have plans to completely automate the cannon. We are going to build a platform that allows us to control it's angle of launch and rotation. Also we are trying to develop a procedure that allows us to eventually develop hardware to automatically reload it. Right now this is the first prototype so we are looking at optimizing everything and finding the most efficient design before we try to implement something like breech loading Eventually we'd like to make it completely remote operated and see how accurate the cannon will be. Right now it looks promising. Using the same pressure, water balloon size, and angle with the test cannon I had balloons landing within a foot of each other.

To estimate the weight of the balloons I'm using (I don't have a scale yet...) I assumed 1ml of water = 1gram. THe balloons are between 2in and 3in in diameter. Doing the appropriate calculations the balloons will weight somewhere between .1kg and .25kg. Not quite .5kg. According to the calculations you've done I think this means that I should have an even further range.

Here's my revised plan of action:
- Use shaving cream as a lubricant and to create a better seal
- Abandon the sabot and instead use wadding to prevent the balloon from breaking on fire
- Use balloons that fit the barrel exactly
- Find a way to weight the water balloons.
- Use a higher pressure
- Start collecting more exact data.

And when I get my next pay check:
- Create a longer barrel

Re: Water Balloon Cannon Optimization
 

The honking noise in the valve is caused by the diaphragm not opening fast enough. In sprinkler valves the port that opens to let the diaphragm open is not very big. This is to prevent water hammer, but since you are using it for air you don't don't care about water hammer. When I make my cannons I usually remove the solenoid, epoxy the port, and cut my own larger port in the top of the valve. I then screw in a pneumatic fitting and use a KoP valve to fire it. This will get rid of the honking noise and increase the valve's performance.

If you decide you need a bigger valve you can use two of the 1" sprinkler valves with T-connectors to fake a bigger valve.

Re: Water Balloon Cannon Optimization
 

Yep, two sprinkler valves in parallel should get the same effect as a larger one.

Re: Water Balloon Cannon Optimization
 

I would not increase the pressure for safety concerns. Although you already stated that you know the dangers of PVC, increasing the pressure will not help. I've posted before that a kid at school was charging his PVC potato cannon and it blew up in his face at 40PSI. This was the same tank that was on our t-shirt shooter before I revamped it last summer. PVC is dangerous and should not be used for storing air pressure, and certainly not anything above what you are already trying! Please at least put duct tape or something around the tank. Make a wooden box or something to go around it when it explodes.

Re: Water Balloon Cannon Optimization
 

You could also try to use a gate valve if you find that the sprinkler valve is limiting your distance. With this valve we were able to shoot a t-shirt about 40 yards with a system that was under 60 psi. (it uses the kit compressor, too!)

We've shot tennis balls with it, and they go far. Never tried water balloons... hmm...

Re: Water Balloon Cannon Optimization
 

I am perfectly aware of the dangers of PVC pipe. I'm testing the cannon from behind a half inch glass door as I stated, doing the testing outside, and there is a large block of wood separating it from me. From the time the cannon has pressure in it to the time it's fired I am no where near the cannon. The valve and compressor are controlled remotely from my laptop and I don't feel like I'm in danger.

Re: Water Balloon Cannon Optimization
 

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 :cool:

Re: Water Balloon Cannon Optimization
 

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.

Re: Water Balloon Cannon Optimization
 

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.

Re: Water Balloon Cannon Optimization
 

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.

Re: Water Balloon Cannon Optimization
 

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.

Re: Water Balloon Cannon Optimization
 

Anyone feel free to correct me --

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.

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.

Re: Water Balloon Cannon Optimization
 

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 :p

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

Re: Water Balloon Cannon Optimization
 

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.

Re: Water Balloon Cannon Optimization
 

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!

Re: Water Balloon Cannon Optimization
 

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.

Re: Water Balloon Cannon Optimization
 

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?

Re: Water Balloon Cannon Optimization
 

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

Re: Water Balloon Cannon Optimization
 

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!

Re: Water Balloon Cannon Optimization
 

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.

Re: Water Balloon Cannon Optimization
 

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 :P

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.

Re: Water Balloon Cannon Optimization
 

RyanN and others:

Looking at potential tanks I'm having a hard time finding any with a 1" opening on them. Most of the ones I'm seeing have 1/2" openings. Where did you guys find your tanks?

Re: Water Balloon Cannon Optimization
 

We cut a hole in the top of our tank and welded a fitting to it. Any small leaks were fixed with JB Weld.

Re: Water Balloon Cannon Optimization
 

A weld *should* hold up but I'm not sure I would trust it. Although, if it was thicker steel like those tanks and you had a competent welder I would doubt a failure.

-Vivek

Re: Water Balloon Cannon Optimization
 

We put some reinforcement in place, and I didn't weld it, so I trust it. We've tested it to 120PSI, but we only charge it to like 60 or so depending on the distance. Once you get to 60PSI or so, the distance remains about the same due to air resistance.

For reinforcement I think we used some steel washers.

Re: Water Balloon Cannon Optimization
 

My quick hunch is that because the mass of a rigid sphere increases with the cube of its radius, and the area of the sphere increases with the square of its radius, and the effective front surface of the sphere only increases roughly in direct proportion to its radius; a large massive sphere will be affected less by air resistance than a small one made of the same materials.

This hunch ignores the sloppy oscillations and other distortions that will plague a flexible water balloon in flight.

This hunch also ignores the nonlinear behavior of air (a fluid) at various scales (sizes) and speeds. For many insects, flying through air is more like a human sized device trying to swim through oil. On the other hand, for a uranium rod shot from an M1A1's 120mm cannon, I don't think that air behaves much like oil behaves at human scales, other attributes will dominate at the rod's speeds and scales.

Because of the many difficulties involved in using "simple" physics to do more than roughly approximate a water balloon's trajectory, my suggestion is that for your wiggly water balloons, you use empirical data to predict trajectories. Make enough test shots to allow you to build up accurate heuristics and then use the heuristics. As an interesting academic exercise, figure out which of the equations folks have cited in this thread best explain the results you measure.

During the test shots, keep track of the wind at all points (at altitudes and along the ground) along your trajectory (for obvious reasons). Also track ambient air pressure, humidity, and perhaps temperature.

I suspect that the pressure and humidity will have noticeable effects on moderately light rigid spheres, I don't know if the effects will be noticeable when combined with all the wiggling a water balloon will do. If you publish your data, you can challenge us to explain it using equations derived from first principles....

Finally, big guns often use a sabot to increase muzzle velocity, not just to protect their projectiles from damage in the barrel (which is what you seem to be describing).

For a given propellant pressure (see others' advice on tank sizes, valves, etc.), with a wider barrel you get more force on the projectile than you do with a smaller diameter barrel. This is because of the increased surface area of the projectile. However if the projectile is a single isotropic lump, the increased mass of the projectile tends to cancel the advantage of wider barrels. You can try to counter this by making the projectile less dense, but then it is strongly affected by drag once it leaves the barrel.

The notion of a sabot (a shoe) offers an effective compromise. You wrap a sturdy low-density sabot around a small high-density core. The two stay together in the barrel but the sabot falls away after leaving the barrel.

At launch, the sabot fills up the wide barrel so that you get a lot of force from the propellant's pressure. The sabot falls away after the projectile leaves the barrel so that bulk of the kinetic energy of the total (core plus sabot) projectile is sent on its way to the target in the compact high-density core that was wrapped in the sabot back when the core was in the barrel.

Maybe a sabot made of stiff foam, or Styrofoam, or foam in a bigger plastic cup, would give you a good sabot of the sort I describe.

Blake

Re: Water Balloon Cannon Optimization
 

I have been building air cannons for the past few years and I've never had any PVC component explode (tons of leaks but never anything exploding) so I think you guys will be safe as long as you are under the rated psi. I usually run whatever is in my compressor's tank (nothing over 120psi) and if you care I can tell you the parts I use for mine.

A few months ago I tried the water balloon idea and I found that the overall force produced by the air hitting the balloon directly would cause it to explode in the barrel. I'm not sure if you are using a type of wadding to get around this but thats the main problem I faced. My projectiles now are usually the potato whenever available after dinner and foam inserts.

Lubrication wise.. I've noticed on a few projects where I've built my own multistage cylinders that PVC and oil do not get along. This is in the sense that if you have a tight fitting component against PVC and you add oil it actually makes it harder to move. The one lubricant that seemed to work was a white lithium grease.

As far as the opportune angle for the most distance I'd say run a regulator to make sure that the psi is constant and instead of a balloon shoot something where the weight is consistent like anything you can shoot more then once obviously. Whenever you fire it mark a flag where it lands and either increase or decrease the angle depending on what you are doing.

If you any questions feel free to ask.

Re: Water Balloon Cannon Optimization
 

Your logic of "it's safe because it hasn't happened to me" is very bad.

Just because one can go golfing during a thunderstorm and not get struck by lightening doesn't mean that it's a good idea, or that it'll never happen to them.

There's a reason groups like OSHA prohibit PVC from being used in any pressurized gas applications; rules like that just don't spring out of thin air. Obviously enough people over the years have gotten seriously hurt or killed from exploding PVC shrapnel, that it was deemed a serious enough safety issue to ban it.

Re: Water Balloon Cannon Optimization
 

If you have leaks, you have problems with PVC construction.