mark 2

The Mark 2 is a massive air cannon, featuring a 9' long, 2" diameter barrel, and 10' of 4" pipe for a chamber. Gating the airflow is a self-made piston valve built into the 4" cross, that can open to the entire 2" diameter of the barrel.

This cannon was built for power, which it had in quantity, but its design is somewhat awkward and inefficient. The chamber is far too large for the barrel, with almost 4x the volume, meaning that most of the energy of the compressed air went to waste. The piston valve, though effective, was not reliable enough. The tuning fork arrangement of chambers as well as the barrel between them requires substantial support; I never built a full structure for it.

piston valve

The most important feature of this cannon is the primary valve. It consists of a sliding piston that is sealed against the barrel by air pressure when the cannon is pressurized. To fire the cannon, a sprinkler valve (much like the primary valves of the Mark 1) dumps the air in the small volume behind the piston. This allows the air pressure on the other side to push the piston backward before the pressure can equalize, forcing the piston backward and exposing the end of the barrel.

Diagram of the piston valve, showing its operation.
A 3D rendering of the piston valve assembly. The sprinkler valve (not shown) goes on the end of the protruding piece on the far left. A couple of rings of vinyl tubing help cushion the impact of the piston against the end of the assembly.
This is the piston. It was made from a 3" PVC coupler, a wooden disk, and some rubber. Later, the wooden disk was bolted to a metal disk on the other side of the inner stop, and a fender washer was bolted on top of the rubber to keep it from blowing out.
The other end of the same piston. The hole is there to allow air to pass and fill the chamber; it is covered by a rubber flap on the other side that does not permit air movement in the other direction. I suspect that the gap between the piston and the cylinder was responsible for more leakage than this hole could ever have been.


In operation, this cannon fired mostly potatoes; the barrel has a chamfer on the end to cut potatoes to size for a tight fit in the barrel. When shot from this cannon, even a potato has astounding destructive power. Unfortunately there are not many photos of this cannon in action; what's here is all there is.

The cannon, set up to fire at a concrete barrier. The cardboard tubes were there to help contain a potential explosion, but more to placate some bothersome people worried about safety. Fortunately no explosion ever occurred, as the cardboard would probably do little to nothing to contain it.
A potato shot at 40PSI or so. You can see it flying toward the concrete barrier, where it will become a cloud of potato mush.