How to Build a Potato Gun
October 2003

Page 1: Design and Construction
Page 2: Comparissons and Field Tests

Introduction

This guide will show you exactly how to build a potato gun. These spud launchers are quite powerful and I take absolutely no responsibility for what you do with this information. In most places of the US it is legal to own these canons, and in my area of Canada they are legal as well - but check with your local authorities before building to make sure.

Step 1: Design and Planning

I really like the design of Spud Launcher I. I think it looks really cool, so I more or less duplicated the design for Spud Launcher II. This time around instead of blindly building I did a bit of research on the ideal combustion chambre volume to barrel volume ratio. Information on this is hard to come by but one source said 1.5:1 was ideal. I measured Spud Launcher I and it had a ratio of 2.1:1 and it was quite powerful. If I followed a 1.5:1 ratio the gun would totally loose the cool "canon" appearance so I decided to go with a ratio of 1.8 : 1. This means that the barrel is a bit longer and the chambre is a bit shorter than Potato Gun I.

I planned to use 4" ABS for the chambre and 2" ABS for the barrel.
Pi*4^2*21 / Pi*2^2*47
= 1.78 : 1

Step 2: Shopping

If you've got a basic gun design in mind, you can keep this in mind when you go to the hardware store and see what's avaliable. Here's a list of parts I bought for my gun:

- 13" of 4" ABS
- 48" of 2" ABS
- ABS Cement (the glue)
- Increaser 3 " to 4 "

- 3" to 2" Bushing
- 4" Threaded Adapter
- 3" Threaded Adapter
- 4x4x3" ABS T-Pipe (the handle)

- 4" Threaded Plug
- 3" Threaded Plug
- Electric BBQ Starter
- Push Button

I went to both Home Hardware and Home Depot, compared prices then bought what I needed. Most things were cheaper at Home Depot and they had better selection.

When doing your measurements, don't forget to include the length of the T-Pipe and anything else you include in your design. If I bought 21" of 4" ABS - my combustion chambre length would be closer to 3 ft.

Step 3: Getting Ready

Here's all the parts I bought laid out - basically how they'll be put together.

Here's a list of tools you may need:
- Drill - Glue Gun - Tape Measure -File

Step 4: Assembly

Warning: the ABS cement is corrusive. It won't burn your skin, but it's not a good idea to spill it on your clothes. If you want to try something fun, the cement does eat through styra-foam.

Use newspaper or something to cover where you're working. You don't want to get the glue on something important.

Take the glue and apply on the inside surface (less noticeable). Make sure you use lots of glue and get it ALL the way around.

Push the 2 pieces together, they should fit together easily.

The two pieces are joined, easy as pie. The glue only takes a few minutes to set, so try not to have to pull 2 pieces apart if you make a mistake.

Construct the Handle and Chambre Portion of the gun using the same technique.

		The glue and pipe are very easy to work with. The 3" threaded adapter is attached to the bottom of the T-Pipe "handle". The 4" threaded adapter is attached to the back of the T-Pipe. This will be unscrewed to put in the propellant.
Potato Gun I (Top) & Potato Gun II (Bottom)	Chambre Partially Assembled	Chambre Fully Assembled

If you have all the parts, to assemble the gun takes about 20 mins (not counting ignition installation). Once you have the chambre assembled you should install the Ignition System. It's much easier to work with a 2ft gun than a 6ft gun when installing the ignition system.

Ignition System

In Spud Launcher I, I used a standard push-button BBQ Igniter. (About $15cdn). I've replaced it 3 times - they wear out fairly quickely and do not gaurantee a spark every time. This time around I went the more expensive but RELIABLE route of using an electric BBQ igniter (about $25). These work with a battery and when the button is pushed, about 5 sparks / second are produced. Very effective.

	The inside of the electric BBQ igniter. I replaced the standard push button it came with a push button switch with longer wires. That way I don't need to reach INTO the gun to push the ignition switch =)
	The push button switch I'm planning to use is the black one in the bottom left of the picture. It's also the same switches I'm using in my Mame Project.

Drilling and Filing

Another reason I wanted to go with the electric BBQ igniter is that I could use a smaller switch rather than the large push button I was using before. This also minimizes the slight risk of the switch blowing out - literally.

A small hole is drilled. Easy as Pie. As Mr. Burns my Calculus teacher would say: "Done like dinner".

Assembling the gun took about 20 minutes - max. But getting this tiny switch to fit in that hole took more than an hour. I had to file and file and file some more to remove enough ABS to fit it in. Quite fustrating and time consuming. It took so long because of the ackward angle it was at. I got it in the end though. This was the most time consuming part of the project - aside from writing this article.

Fitting the Ignition System

The purpose of having the bottom of the handle be removable is because I wanted to place the BBQ igniter inside it and also be able to unscrew it to replace the battery when necessary. Also, by putting a threaded cap into the bottom - it creates a much sturdier fit than just gluing the cap in like I did in Potato Gun I. This became problematic after a fair bit of use.

		The trigger is now installed . Looks great in my opinion. The ignition cable is glued inside the gun as shown (far left).
A little glue is all it takes.		2 of the wires go to the trigger switch. The other 2 go to the ignition cable. It is the same one used in Potato Gun I. The igniter came with the parts required to make another cable, but it's much easier to use one that's already made.

The 3" with the electric igniter is screwed in and the ignition system is installed. When you hit the switch you hear a "click-click-click..." of the spark going. Very nifty.

Fitting the Barrel

This is just another glue-n-push and the gun is complete.