So here's how you do it.
The raw materials you'll need to buy or collect are as follows: three Pringles tubes, a good-sized roll of gaffer tape, some long matches, a tin of lighter fluid and some tennis balls. You will also need a three-foot long 'blast tube' to stand the mortar in, just incase you overcook it and blow the thing up. Something like 100mm PVC waste pipe is ideal. More on this later. It's probably a good idea to have goggles and ear-defenders for all involved, although we made do with keeping the kids at a safe distance and getting them to stick their fingers in their ears. Your call.
Step 1: Step 1: Prepping the Tubes
Step One: Select one of your Pringles tubes and set it aside. With the other two, cut a 1.5 inch diameter hole in the dead centre of the metal base (see image). The hole should NOT be big enough for the tennis ball to pass through. The cutting part is tricky, but you should be able to manage with a decent pair of scissors and strong hands. I found the best technique was to mark out the circle on the base and punch a hole in the centre of it. Get the scissors started from this hole and 'wind out' in a spiral until your spiral joins the circle. It's the only way to get the angle of attack right.
Two things worth noting here. While cutting you will occasionally make that hideous nails-across-a-blackboard noise. This is unavoidable, so you'll just have to man (or woman) up. Second, you should file the edges of the hole smooth so you don't accidentally lacerate yourself or your kids on the jaggedy edges. Plus, it will ensure the mortar fires better.
Step 2: Step 2: Joining the Tubes Together
The finished effect should be a three-foot long, gaffer-taped tube, open at one end. When you look down the tube you should see that the holes you've made in the bases align. These are the two 'baffles' and are essential. I don't understand the physics, but the mortar doesn't work without them. Your tennis ball will sit on the uppermost baffle, rather than at the bottom of the tube. If your ball rolls all the way to the bottom, you didn't follow Step One properly. Start again.
Step 3: Step 3: Creating the Firing Hole
Step 4: Step 4: Creating the Firing Tube for Safe Firing
Step Four: Make your blast tube. I'm going to assume you're using PVC pipe, but anything that doesn't deform or shatter under small-scale explosive force should do. So for example don't use a plant pot, a wooden box or a flimsy plastic storage box. Watch Master and Commander for a good idea of what happens when wood explodes. I used a section of metal pipe that I'd previously employed as a smoker. Drill a hole near the bottom of your blast tube, so that when your mortar is resting inside it, this hole aligns with the firing hole of the mortar. The lit match needs to go through here.
Step 5: Step 5: Firing Your Mortar
We warmed up by watching some footage on You Tube, starting with a German 88 and some British First World War field artillery to explain the difference between a gun and a howitzer, then moving on to Afghanistan for infantry mortars, then the biggest mortar in the world (a truck-mounted Russian beast). This is not an essential step, but it is interesting.
Create your firing position by finding a good place to prop your blast tube. A stack of bricks or rubble is usually ideal for this, and the blast tube should be weighted or anchored to stop it flying back and hitting someone. The angle of the mortar should be just off vertical and firing into space devoid of people and animals. And greenhouses.
Prime your mortar with lighter fuel. You'll need between 2.5ml and 4ml, depending on the weather. Use the lesser quantity if it's a warm day and more if it's damp and freezing cold. It's worth noting that different lighter fuels have different volatility and hence will give a different sized bang with different quantities. You'll need to experiment to hit the optimum quantity for your set-up. This is why we have the blast tube. You can either drip the fuel in through the open top or do what I did and use a syringe to inject it through the firing hole. Then quickly...
Load your mortar by dropping a tennis ball into the top. The ball should rest snuggly on the top baffle, making a relatively air-tight seal.
Shake your mortar to get the chamber good and full of explosive vapour. About 10 seconds should do it. You'll need to keep the ball wedged in place while you shake (a stick is usually sufficient for this) and makes sure that all the lighter fuel doesn't run out of the firing hole.
Place the mortar in the blast tube, being careful to align the firing hole near the base of the mortar with the firing hole near the base of the blast tube.
Fire. Strike a match and poke it through the aformentioned holes. If it goes right, there will be an enormous bang and the ball will launch a good 40 to 50 ft in the air. If it goes wrong, there will be an enormous bang and the ball will either not move or flop limply onto the ground about a foot away. Tweak your system and try again.
This is an awesome way to goof off with the kids and spend time doing something frivolous and a touch dangerous. Just remember to think 'what could possibly go wrong?' then ensure that it doesn't.