Safety first: Lighting the mortar without a stand (like I did in the first shot of the video) is asking for trouble. A simple and fast stand can be made from PVC pipe and coat hanger as demonstrated in the photo. If no stand materials are available at least lean the mortar against something (bench, rock, etc.) so it isn't at risk of falling when you are trying to light it.
Tip: 3" PVC pipe will fit perfectly around Pringles cans with just the right amount of duct tape on them. A hole can be drilled through the PVC and cans for lighting. This provides extra structural reinforcement to the cans lessening the chances of a blowout.
Tip 2: Use a barbecue lighter, or like in the video a quick & easy torch (made by bending coat hanger around paper towel). This will put distance between your fingers and the bottom of the tube, in case of a blowout.
Generic safety stuff: Use adequate eye, ear, nut-sack
, and various other body part protection.
To prepare for firing: Set up the mortar in a large open area where you have little risk of hitting someone. Squirt about 1-2 teaspoons of lighter fluid down the barrel of the mortar, making sure to aim for the bottom can. Spray a little extra through the hole in the bottom, to help aid in lighting, or, if using hairspray, you can spray hairspray down the mouth of the barrel for a few seconds. Drop the tennis ball down the barrel. The mortar is now loaded, and from now on no one should pass in front of it.
To fire: Wait about two minutes after pouring, to give the chamber enough time to fill up with the evaporating lighter fluid (Pouring the perfect amount of fluid, and waiting till exactly the right time to light are the two things to master when it comes to tennis ball mortars). Light the barbecue lighter or torch, and hold it close to the hole on the bottom. If all goes well you should hear a loud bang, and see the ball sail off into the distance.