By bavetta 2 ratings I made a couple of air cannons out of steel pipe. Managed to shoot a potato over 250MPH and over 600 feet!

Check out more photos and video of the tests and construction here: http://www.crazybuilders.com.

We headed out to an airfield to test the air cannons out and the big cannon fired potatoes at over 250MPH and over 600 feet!!

Goals

Small Cannon

• As safe as possible
• Within budget
• Simple and fast construction
• Easy for me to carry
• Impressively powerful blast
• Cannon parts easily found at Home Depot

Big Cannon

• As safe as possible
• Within budget
• Simple and fast construction
• Transportable by truck
• As powerful as possible

Design

I kept the designs for both cannons very simple. Both designs have:

• Compressed Air Tank - Stores energy
• Valve - Turns on/off the flow of air
• Barrel - Channels the air and accelerates the projectile

Design: Small Cannon

Tank - I decided to use steel pipe both for tank and barrel for this cannon. The pipe can be found in home supply stores, has a very high pressure rating (~2000 psi) and is very durable. The steel pipe is appears safer than PVC pipe, especially in colder temps (see Blast Box experiment, below) - I think the safety is worth the small difference in price. I used black pipe over galvanized as it is slightly cheaper. The galvanized would hold up against rust better in the long run.

Valve - The valve I used was a standard 1-inch diameter PVC automatic sprinkler solenoid valve. They are typically used in automatic sprinkler systems. They can be found in local stores year-round in fair weather states, and seasonally everywhere else. The valve opens when you apply ~9 to 24 volts to the terminals of the valve. Check out the "Solenoid Valve: How it Works" page, linked to below.

Barrel - I used 2-inch diameter steel black pipe, same as what I used for the tank.

Design: Big Cannon

Tank - I found a large (17 gallon) tank on eBay which was owned by a tank supplier that was going out of business. It was actually supposed to be used for fire suppression gas/fluid (halon), rated to 150psi and had been recently hydro-tested to 300psi. It had a wide threaded opening on the end, which is great for getting maximum air flow (see "Flow Restrictions", below).

Valve - The valve I used came from McMaster-Carr - this was the big expense luxury item at $320. It is a 2" nickel-plated brass valve pressure rated to 150psi and with a very low resistance to flow (C_v of 53.0). Item #7944K28

Barrel - I used a 10-foot section of 2-inch diameter black pipe for the barrel - the same stuff I used for the small cannon.

Flow Restrictions

Fluid losses are most often dependent on the square of the velocity. So it is often in your best interest to keep fluid to a minimum flow rate to avoid pressure losses.

For our cannon, the narrowest section of pipe is a major factor that limits the maximum flow rate through the barrel. When the air goes through a narrow section it must speed up to get the same number of air particles through per time as the wider section. When speed of the fluid goes up, the fluid losses go up reducing the apparent pressure on the projectile. As the fluid speed approaches the speed of sound, huge inefficiencies result.

As it turns out, poorly designed pipe sections (with sharp angles, etc) can create sections in the pipe where the flow of air is restricted not by local pipe geometry, but instead by the flow of air through those sections. You can see this phenomena often in rivers: when the river turns a sharp corner the flow goes exceptionally fast toward the outer radius and much slower near the inner. One reason for this is due to the momentum of the fluid, which means without perturbation, the water would just go straight. Water (anything with mass) is a major slacker: it wants to take the easiest route, which means it goes straight until it is forced to turn. By making one section of the curve have a higher flow rate than the average flow rate, the same concerns with inefficiencies due to higher speed flow results.

So the lesson is: try to avoid narrow sections and sharp corners if possible.

Improvements

I've seen a method of reducing the valve opening time by drilling into the valve and threading an air release valve right into the sprinkler valve, above diaphragm in the valve. By opening this new valve, you cause the air in that area of the sprinkler valve to escape which pulls the diaphragm away from the high pressure side, opening the valve. Ya, that's the same thing the solenoid usually does to actuate the sprinkler valve, but the idea is to get a higher flow rate than is possible through the tiny hole that the solenoid exposes. If the valve opening time is significant, this sounds like a plausible way to increase the exit velocity, and I'd like to see someone test out a cannon with and without the modification. Done this, or have other modifications? Post some pics to the Forum.

Additional Info:

Solenoid Valves

Solenoid valves use the pressure of the fluid they are switching to actuate the main flow.

Pressure is released from behind a piston/diaphragm which then lifts to uncover a path for the main flow to pass through the valve.

PVC Pipe

So here's the deal. I've made a few PVC air cannons in the past. Well, recently I've have heard horror stories of PVC cannons exploding, and witnessed one explode on a cold January in New England. So, before I presented my take on the potato cannon here, I wanted to take a look at the safety of PVC air cannons. Is PVC a good material for compressed gas?

Turns out: no, and yes. Here is the evidence