Introduction

This is a really fun launching device mainly due to the sound of the boom when fired! It doesn’t have much of a range when compared to potato cannons but it does manage to launch a tennis ball about a hundred feet into the air. It also doesn’t have much of a service life as the recoil during firing crumples the bottom-most cans against the ground which then have to be replaced in the field; its worth bringing some spare cans and duct tape.

Materials used

• 2 to 4 baked bean tins with stacking ridges on their base (standard top)
• 2 to 4 baked bean tins with a standard top and base (no stacking ridges)
• Tennis ball (or similar that fits into a tin)
• 5mm Ø drill bit and drill
• Duct tape
• Top cutting tin opener
• Nibbler, tin snips or saw to cut tins
• Long reach BBQ lighter/igniter
• A few bricks and/or something to hold the mortar in firing position

Construction of the mortar

If you can’t find any tins with stacking ridges then normal cans can be used, the ridges just assist with stacking the cans together. The photo below on the left is the base of a ridged based tin (one that stacks) and the photo below on the right is of a standard tin with the base cut out (one that does not stack).

The diagram below illustrates the general configuration of building the mortar. It is important to experiment with the number of baffle tins and barrel tins in order to get the best performance out of the mortar. The ridge based tins have baffle holes cut into their bases and the non-ridge based tins have both their base and lid cut out. Start off with building a mortar with four baffle tins and two barrel tins.

I made baffle holes in three of the ridge based tins by cutting out a circle in the bases using a drill to start off and tin snips to enlarge a hole. The hole in each of the bases is about 30mm in diameter. The baffles help to keep the mortar rigid and seems to effect the combustion rate of the propellant – again some experimentation is worth while.

I then cut the base and lids off two of the standard tins using a top cutting tin opener (not a side cutting tin opener!). These tins are stacked together to form the barrel section of the mortar.

The bottom-most ridge based tin needs a small firing hole drilled in the side of the can to allow an ignition source via a long reach BBQ lighter/igniter. A 5mm Ø hole about 25mm away from the base of the tin seemed to work well.

To join all the tins together I simply taped them together using a couple layers of tightly wrapped duct tape.

Last of all, the tennis ball needs some modification to increase its diameter to fit better (tighter) in the mortar barrel. A cricket ball, hockey ball or base ball may be a closer fit but are harder to find and are more expensive. I crudely modified the tennis ball by wrapping the tennis ball with layers of duct tape. To keep the ball spherical I wrapped the duct tape round in alternating crosses. I kept adding layers until the ball was a close fit in the mortar barrel yet still moved freely through it.

To use the mortar stack up some sturdy bricks that can support the mortar well. Place a modified tennis ball down the mortar and spray a very small amount of propellant through the firing hole. About 5ml of lighter fluid also works well but requires a little extra time to vaporise with some agitation while covering the hole. Ensure the mortar is placed securely and pointing away form buildings/people and position yourself next to the firing hole and using a long reach lighter ignite a flame at the firing hole to initiate combustion of the propellant inside – BOOM!