This Instructable will show you how to make a spud gun unlike any other before it; rather than using compressed air or combustion, it uses pressurized CO2 created by a chemical reaction to propel the projectile. This project is a very simple build, but it will require a bit of math and some chemistry.
Step 1: Materials
Note: you can use any size of PVC parts for this project. I just used 1/2" because it was available.
Step 2: Building
This is a very easy project to build. All you need to do is attach two ball valves to a section of pipe and add a coupling to the end of one of the valves. You also need another section of pipe to serve as the barrel; it should be a similar size to the other part we just made. You can use any size of PVC parts that you have because it doesn't need to withstand that much pressure. In the pictures above, I used 1/2" parts and the chamber is 16 inches. The chamber should be very long or wide because when the baking soda and vinegar are mixed, it will expand and produce CO2 and water, so there needs to be enough room for the CO2 to form and for the mix to expand. Remember that the larger you chamber is, the more baking soda and vinegar you will need/have the potential to use.
After that, you will need to make a projectile. Remember the chemical reaction we just went over? When we combine the baking soda and vinegar, it produces CO2 and a lot of liquid. It is important to keep this in mind because the projectile has to be able to hold up against all of the high pressure liquid. This means that the ideal projectile to use is a dart with solid feathering. I make feathering by wrapping duct tape around a nail. Sponge also works, but it is challenging to work with when it is dry. Any dart will work as long as the feathering isn't made out of paper or a flimsy material.
Here's the basics of how this gun works: the ball valve at the back is opened to pour in the baking soda and vinegar. When it is closed, the combination of baking soda and vinegar rapidly produces pressurized CO2 in the chamber of the gun. When the ball valve in the front near the coupling is opened, this CO2 goes into the barrel and propels the projectile. In the next steps we will go over the specific details of the chemical reaction and how to use the gun.
Step 3: Chemistry and Ratios
First, let's go over what is actually happening when baking soda and vinegar are combined; here is the chemical reaction: CH3COOH + NaHCO3 → CH3COONa + CO2 + H2O. This states that when sodium bicarbonate (baking soda) reacts with the acetic acid from the vinegar, it produces carbon dioxide, water, and some salty acidic stuff. This reaction produces so much CO2 that when it is confined to a small space, it will pressurize itself. This is what powers the gun.
In order to produce the most efficient reaction of the baking soda and vinegar, we need to know what ratios to use to mix them properly.
Here is the link I got my information from:
http://www.instructables.com/answers/What-is-the-... (DOES NOT WORK)
Unfortunately, this link got deleted. Basically, it said that for every half cup of vinegar, you need three teaspoons of baking soda. Depending on the size of your chamber, you can divide or multiply this ratio. Also, make sure to check the acetic acid content of the vinegar. It will usually say it somewhere on the container. The first link is for vinegar with 5% acetic acid, which is the average amount vinegar has. If your vinegar has a different amount, buy another brand. If you really don't want to buy more vinegar, you could also try experimenting with the vinegar you have to determine how much baking soda will produce the most bubbles in a certain amount of vinegar. The more acetic acid the vinegar has, the less vinegar you will need and vice versa.
After we know the ratio we need to use, we need to apply this ratio to the size of our chamber. A not so mathematical way to do this is to first, close the bottom valve and pour quarter cups of water down the chamber until it is full. Record how many quarter cups it takes, and then find 50% of that to calculate how much vinegar you will need. Then, apply the amount of vinegar to the proper ratio based on your vinegar's acetic acid concentration. Use the picture above as an example to calculate how much baking soda and vinegar you will need for your chamber.
Step 4: Operation and Videos
In order to shoot the gun, follow the diagrams above.
Here is a video of me shooting it. Keep in mind that it's very hard to be accurate with a ball valve, and the position in which you have to hold the gun makes sights very awkward to use and the point is moot. In the first part of the video, you can see the dart hitting a target from close range. In the second part of the video, you can see how fast the dart is going and the awesome trail of liquid the dart leaves in flight.
Step 5: Cleaning
A very important step when using this gun is cleaning, and it should be done after every session you use it. As the chemical reaction in step two states, the combination of baking soda and vinegar produces CO2, water, and CH3COONa. CH3COONa is basically an acidic salt, and every time you use this gun CH3COONa gunks up the chamber, barrel, and valves.
To clean it, take off the barrel and open up all the valves. Then, run water through the inside of all the components; a good way to do this is by filling up a cup with warm water and pouring it inside and outside the pipe. If the valves are sticky, run water all around those colored parts you turn and keep on opening and closing the valves until they will turn easily again. Dry off the outside of the pipe with a towel and open up both valves.
Step 6: How powerful is it?
This next step we will be discussing how powerful I have estimated the gun to be. I used a 1/2" chamber and barrel, so if you make yours with bigger pipe it will be a lot more powerful. I have noticed that with a dart, the gun has a similar range and velocity to the 3/4" gun I made my first Instructable on.
How much PSI can the baking soda and vinegar produce: approximately 60 psi
How far will it go: 100 yards
How fast will it go: 50 FPS
Penetration with a dart: one sheet of cardboard
I estimated the PSI based on the fact that it has around the same range and is just as loud as 60 PSI in my 3/4" gun. I figured out the range by firing it in the air at a 120 degree angle. I found the velocity by recording footage of shooting at a cardboard target 50 feet away and counting the time between when the dart was fired and when it hit the target.