A Simple & Cheap Spud-zooka Fueling System

A picture of the gun etc.	A picture of the chamber

To build:

Steps 1 and 2 are optional.

1. Install a tire valve stem in your combustion chamber. You can get one for about $1 from an auto supply store. You will need to drill a 7/16" (or is it 9/16"?, unfortunately 1/2" doesn't work) diameter hole for the valve and insert it from inside the chamber (just like you would mount it on a tire rim).  The valve should fit very snugly. Even a cheap, all rubber tire stem should take 100 PSI, which is quite a bit higher than the 30-50 PSI that a combustion spud gun will generate.  If you have an extra couple of bucks you can get a truck tire stem which is metal and has a mounting nut.  These valves are also known as Schrader valves.
2. Remove the valve from inside the tire valve stem. There is a small tool for this which should be available where you bought the tire stem. Sometime you can get one that is also a valve stem cap.
3. Get a propane torch and unscrew the burner tip from the valve, this leaves you with the valve and a tube with a very small orifice (hole) at the end.
   • If you are going to use a butane refill cartridge you will need to come up with a pretty small ID tube to join the cartridge to the syringe needle.
4. Get a 2-3" piece of rubber hose with ID that fits over the tube on your propane torch.
5. Get a 60cc plastic syringe. Ours was $2 from a local farm supply store where they are sold for giving shots to cows and horses. Another possible source for the syringe is the cooking section of a department store like Kmart or a cooking supply specialty store. Sometimes they have 60cc syringes for basting turkeys and such. I don't mean the great big eye-dropper shaped things, it should look basically like the syringe in the picture below. While you are at it, also get the biggest, both in terms of length and ID, syringe needle you can find.

How Much Propane To Use:

To get the best performance out of your spud gun you need to use the correct amount of fuel. Too much or too little fuel will produce less power than the optimal amount of fuel. If the amount of fuel is really far from the optimal amount the fuel won't burn at all.

Since combustion requires both fuel and oxygen (or other oxidizer) the size of your gun's chamber controls the amount of fuel that is needed. The chemical equation for the complete combustion of propane (C₃H₈) in oxygen producing carbon dioxide, water and heat is;

This equation tells us that for every 5 molecules of oxygen we need 1 molecule of propane. We will produce 3 molecules of carbon dioxide and 4 molecules of water. When working with gases, the number of molecules is directly proportional to the volume. If we have one volume of propane it will contain the same number of molecules as one volume of oxygen (if both volumes are at the same temperature and pressure).

Since we are using air as the source of oxygen we need to take into account that air only contains about 21% by volume oxygen. Therefore, our ratio of propane to oxygen is 1:5, and to correct for using air as the oxygen source we have to multiply by an additional factor of 21% (0.21).

The equations above are accurate enough for most proposes. However, to be exact we need to take into account the fact that any propane introduced into the chamber displace the same amount of air (and oxygen). A more accurate equations is;

The Simplest Usage:

1. Load a spud.
2. Put the rubber hose on the propane tank valve and put the syringe in the other end of the hose. 
3. Crack the propane tank's valve and the syringe plunger will move back. For my 3"ID x 11" chamber it takes 53cc propane.
4. To inject the fuel remove the cleanout plug, slip the syringe needle between the cleanout adapter and cap, (point the gun down so the propane settles toward the front of the gun), squirt in the fuel and then quickly screw the cap on.
5. Point down range and pull the trigger.
6. Remember to flush fresh air into the chamber before your next shot.

A Little More Complex Usage:

1. Load a spud.
2. Put the rubber hose on the propane tank valve and put the syringe in the other end of the hose. 
3. Crack the propane tank's valve and the syringe plunger will move back. For my 3"ID x 11" chamber it takes 53cc propane.
4. Insert the syringe needle into the chamber via the valve stem and squirt in the propane.
5. Put the little black cap on the valve stem to close it.
6. Point down range and pull the trigger.
7. Remember to flush fresh air into the chamber before your next shot.

Questions, comments or experiences, send me an email.
Modified: July 3, 2007