The Purple Pounder

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since 05/27/07

Behold the Purple Pounder.

This is my version of the ubiquitous Spud Potato.

I decided to take an engineering approach to design this shooter. Among the uncontrolled variables with the usual suspects are:

Uncontrolled mixture. Hosing hair spray into the combustion chamber isn't exactly precise.
Uncontrolled ignition. The Coleman lantern sparker isn't exactly repeatable, especially when wet.
Uncontrolled spud fit. Cutting the spud to about the barrel diameter and then cramming it in isn't precise.
Non-repeatable mixture from shot to shot. After the first shot the combustion chamber contains combustion products that impedes subsequent shots.

I identified several areas of improvement:

Separate the combustion chamber from mixture control. Make an explosive mixture before it enters the combustion chamber.
Optimum fuel. Hair spray (alcohol) isn't it.
Optimize combustion chamber size.
Optimize the ignition point. This assures most of the mixture is burned before it is discharged from the barrel.
Reliable and repeatable ignition.
Projectile diameter control.

Here is the igniter. This is a battery-powered gas grill igniter sold by Lowe's. The single AA battery fits under the button. It fires about 3 sparks a second.

The sparkplug from an oil burner. Experimentation showed that much better ignition occurs if the spark is nearer the center of the combustion chamber. This extended electrode sparkplug does just that. Note the wire soldered to the copper washer. This is the ground return to the igniter.

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This shot shows several important features.

The carburetor is a modified standard Bernz-o-matic propane torch. The air holes are experimentally opened to make the most explosive mix.
The sparkplug is located at the optimized location. I tried several other locations and selected this point as producing the most velocity. Unused holes were plugged with epoxy.
Gas injection port. Located at the far end of the chamber. This combined with the vent port (below) means that the gas flows through the chamber, flushing out spent gases from the previous shot.
Blow-apart fitting. The other joints are made up with PVC pipe cement. This one has no cement and is held in place by a few short sheet metal screws that just barely penetrate the pipe. This is designed to let the assembly blow apart before the pipe explodes.

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This photo shows the igniter fastened to the barrel with epoxy and the vent. The vent valve serves two purposes. One, it allows the chamber to be flow-through vented by the incoming gas mix, ensuring that each shot has the same charge. Two, leaving it open while ramming the spud makes it easier to ram.

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To ensure the spud fits the barrel perfectly we use the barrel to cut the spud. This photo shows how the end of the barrel has been cut to a sharp bevel. When a spud is placed over the end of the barrel and hammered down, the bevel cuts out a plug that just fits.

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An overall view of the gun. Note the folded combustion chamber. I chose to bend it 90 degrees so that the gun would not be so bulky and to improve the balance. Also note the slight curve in the barrel. This curve is elevation compensation of the same nature as the rear sight on a rifle. With this curve one can aim directly down the barrel toward the target and have the curve compensate for drop.

Here one can see the barrel curve even better. I curved the barrel by propping it between two concrete blocks and then heating it with a heat gun until it sagged the desired amount. A quick application of cold wet rags stops the droop.

Note the yellow cylinder of MAPP gas. I have found this to be the best of the commonly available fuels.

No engineering project is complete without test and measurement. What better measurement of a spudzooka than the muzzle velocity of the spud?

For that measurement we use a bullet chronograph. Such as the Oehler unit pictured here.

The chronograph measures a bullet's speed by timing its flight between two sensors located a known distance apart. In this case, optical sensors detect the bullet's shadow. There are three sensors or "screens" in this application because the chronograph makes two separate measurements and compares them as a quality control measure.

Here I am lining up for a shot.

The chronograph readout. Each shot is registered on the display and printed on the tape.

The best velocity I saw using an approximately 1/2 lb spud is right at 350 feet per second. This is in the low end of the range of velocity produced by spring type BB guns! Needless to say, very little spud remains after it hits something.

Conclusions

The engineering approach worked. The purple pounder produces a high and repeatable velocity, higher than any other I've tested. MAPP gas is the best propellant of commonly available chemicals that I tested. That it dispenses as a gas makes it doubly nice.

NOTE: The question of acetylene comes up very often. In a word, DON'T. Acetylene is much too energetic and too unpredictable to use in a plastic gun. I've used both acetylene/air and acetylene oxygen in a variety of devices. I have also burst a number of devices!

There is a more practical reason to stay away from acetylene. The pressure builds too rapidly for the spud-bullet. With MAPP/Air, the pressure builds relatively slowly upon ignition. This gives the spud a good shove without the shock that breaks it up. The pressure pulse from acetylene/air combustion rises so fast that it breaks up the spud. A spray of particles emit from the barrel. MAPP/air is plenty powerful enough.