Experiential Science: Spud Gun

LESSON PURPOSE

Present science in a fun format

Share the reaction details and vocabulary behind the science

Encourage you to try your own experiments with alternate variables

A spud gun is a weapon that can launch spuds at over 200 ft/s. It is a propellant based gun that uses any propane based aerosol as a propellant (most experiments use hair spray, for it is inexpensive and easy to use). The way it works is propellant is injected into the combustion chamber and ignited with a sparker, as the gas expands it pushes the potato up the barrel and out of the gun.

All spud guns have the same basic components:

Typically, the sharpened end of the barrel acts as a barrel knife, which shaves off the excess potato during loading.

The method for creating high-pressure gas is what differentiates the two main types of spud guns: combustive and pneumatic.

SPUD GUN BASICS

COMBUSTIVE LAUNCHERS

Combustion launchers are usually less powerful than their pneumatic or hybrid counterparts, but they are easier to make.

Combustion powered spud guns typically have the least complex designs, the four basic elements of which are:

In order to fire, the operator loads a projectile into the barrel, adds fuel to the combustion chamber (for example aerosols or propane), and triggers the ignition source (often using a piezoelectric BBQ igniter). The fuel then ignites, creating hot expanding gases, and forcing the projectile out of the barrel. Distances vary greatly depending on many factors, including the type of fuel used, the efficiency of the fuel/air ratio, the combustion chamber/barrel ratio, and the flight characteristics of the projectile. Common distances vary from 100 to 200 metres, and some cannons exceed 500 metres of range.

PNEUMATIC LAUNCHERS

Pneumatic launchers are considered a little more difficult to build due to the need of a completely airtight construction. These cannons have four basic components:

In a pneumatic spud gun, air is pumped into the pressure chamber. After the desired chamber pressure is reached, the pressure release valve is opened, allowing the gas to expand down the barrel, propelling the projectile forwards.

The filling valve is usually a commonly available type such as a Schrader or Presta valve but other assemblies to pressurise the cannon such as quick release connections with ball or check valves have been used.

The range of pneumatic cannons is more variable than the range of combustion spud guns due to the increased variation possible in the components. The maximum range of some high power pneumatic cannons has been said to be over 1000 metres (1km). Pneumatic spud guns are generally more powerful than combustion spud guns.

BUILD YOUR OWN SPUD GUN

3 sizes of combustion