Water Balloon Cannon Optimization

[ab500]

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?

[RyanN]

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

[vivek16]

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

[RyanN]

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.

[gblake]

Anyone feel free to correct me
...
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.

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

[DeathMagnetic]

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.

[artdutra04]

Originally Posted by DeathMagnetic

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.

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.

[Dick Linn]

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