Water Balloon Cannon Optimization

[ab500]

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

[fimmel]

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

[vivek16]

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.

[EricH]

Originally Posted by vivek16

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.

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.

[vivek16]

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

[Ian Curtis]

Originally Posted by EricH

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.

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.

[Mr. Freeman]

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.

[ab500]

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.

[IKE]

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?

[ab500]

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

[MAteo9944]

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.

[vivek16]

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

[RyanN]

Originally Posted by ab500

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.

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.

[s_forbes]

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...

[ab500]

Originally Posted by RyanN

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.

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.