Howitzer Hybrid Potato Gun - Introduction

So you want to build a potato gun. Well, that's a worthy goal, but you must ask yourself, "What kind of potato gun?" If you're like me, you don't want to make a regular potato gun because that's what everyone else in the neighborhood has. (At least in Idaho) So what do you do? BUILD A SUPPED-UP CANNON OF COURSE!

In this blog, I'll show you how to construct a hybrid potato gun. Generally there are three types of potato guns:
1.) Combustion - Which is what most people make.
2.) Compression - Using compressed air.
3.) Hybrid - Which utilizes both compression and combustion.

More specifically, a hybrid potato gun compresses a fuel/air mixture, which allows more flammable mass to be fit in the chamber which is then ignited. In my design, I pressurize a propane/atmosphere mixture of up to 60 pounds inside the chamber before ignition.

 Utilizing this design produces extreme pressures that requires the gun to be constructed of steel rather than plastic. As such, the gun can be very heavy and unwieldy. So, in conjunction with making a hybrid potato gun I decided to make a gun mount modelled after a WWII howitzer cannon - which of course makes the whole thing about twice as cool.

This is about is powerful as it gets with this size of potato gun. The boom is like a thunder and I have only been able to see the projectile in a frame by frame video. It definitely packs a punch, and is quite satisfying to fire.

I found most of the parts for the cannon at a plumbing supply store in town and the rest at Home Depot. Then I pieced together the mount from parts I had laying around my home.

I purposely designed to cannon so that it could be made with no welding experience. Pretty much all the parts are either screwed or epoxied together. The only difficulty that comes with this is that you have to find and fix a lot of leaks before you can operate the cannon, but this is not hard to do.

Step 1 - Principles and Concepts of the Cannon

         Like I said before, a hybrid potato gun combines the features of a combustion and pneumatic potato gun. As such, it will obviously be more complicated than either of the two styles. A great resource for information is http://www.spudfiles.com/spud_wiki/index.php?title=Hybrid_launcher


There are 4 major components to a hybrid potato cannon:

1. The Barrel - The barrel is pretty straightforward - just a length of steel for the projectile to go down. I don't know if it had to be made of steel, but I figured better safe than sorry.

2. The Combustion Chamber - The chamber contains the gases as they are pressurized. It needs to be made of steel to withstand the pressures of ignition. It also is nice if it is long enough to accommodate the metering system.

3. The Metering System - This is the crux of the whole operation. The metering system allows precise injection of gases into the chamber which gives us the ability to get the correct stoichiometric ratios for maximum power. I suppose we could build a hybrid gun without one, but we would just be guessing when we filled it up, which might result in no ignition at all.

4. The Ignition System - This goes beyond the barbecue lighter used in other guns. Because of the increased pressures inside the gun, high voltages must be used in order to make a sufficient arc inside the chamber. This sounds hard but is actually really easy to accomplish.

Operation: First, a burst disk is placed between the chamber and the barrel. This disk will allow pressure to build inside the chamber, but will "burst" when the gases are ignited. Then, the potato gun is fueled via the metering system which injects just the right mixtures of fuel and air into the combustion chamber. The fuel mixture is then ignited by spark plugs connected to a high voltage source. The resulting combustion produces enough pressure to break the burst disk holding the gases inside of the combustion chamber and to shoot the projectile out at high speeds

Design Considerations :

•  One of the most important things to consider when building any potato gun is the chamber to barrel volume ratio. Experiments have shown that a chamber to barrel ratio of 1.5 : 1 will efficiently utilize all of the guns power.

•  The next thing to consider is the barrel length. The longer the barrel is the more time a projectile has to accelerate. However, the larger the barrel, the larger the chamber. So we have to strike a compromise and just decide on a practical and economical barrel length.

•  For the meter pipe I got the largest pipe I could at the longest length that would fit along the chamber. Since we will be using pressures to measure mass, a meter pipe with a larger volume means that you don't have to pressure it up as high when measuring fuel. This is what I would suggest.

•  When we incorporate multiple ignition sources we increase the power of the cannon. Igniting the fuel mixture at multiple points will create multiple pressure waves which will increase the velocity of the gases. I have built two ignition sources for now, but I have left room for two more.

- The next step will demonstrate how to build the barrel.