Ping Pong Ball Mortar

The Ping Pong Mortar is one of the most gratifying projects I've ever made for the amount of time and effort it takes to create. The construction is straightforward, and the fireball-like combustion reaction can fire a ping pong ball over 40' – remarkably far for such a lightweight object!

Best of all, it's mostly made from recyclables, so you won't need to order any fancy materials. So go ahead and rummage through your neighbor's rubbish bin, and get started! Watch the video, or follow along with the step-by-step instructions.

This project is an excerpt from my book Launchers, Lobbers, and Rockets Engineer, and adapted for Instructables (published by Rockport). If you're into projects that go Bang!, then check it out!

And of course, as with any project that involve firing projects: Be safe.

• Only fire outdoors in open, uncrowded areas.
• Use with adult supervision.

• 4 paper towel tubes
• utility knife (or other tool to cut cardboard & plastic)
• duct tape (2 colors optional)
• plastic bottle with screw-on top that has a diameter of 2" (5cm) or less (like a vitamin bottle)
• Drill with 3/8" (1cm) bit
• Ping pong ball
• Hairspray
• BBQ lighter

Materials substitutions

• Paper towel tubes: if you don’t have 4 on hand, you really only need 2. Use strips of cardboard or another material for the bipod (part that holds the barrel upright)
• Drill: You just need to make a hole in a plastic bottle, so you might be able to use a sharpened pencil or other pointy tool.
• Hairspray: Hairspray is cheap and relatively nontoxic, however you can use other flammable aerosols. Do your research to learn how hazardous it is to combust your fuel of choice.
• Duct Tape: Substitute duct tape? Pfft! If you're thinking of making this and you don't own a roll of duct tape, now's a good time to get your first roll.

1. (Pictures 1 & 2) Cut a 2" (5cm) slit into one of the tubes and fold the edges together slightly.

2. (Picture 3) Insert the that tube into another one.

3. (Picture1) Carefully cut off the bottom of the plastic bottle.

4. (Picture 2) Slip the bottle over one end of the tubes

5. (Picture 3) Cover the whole thing with long strips of duct tape. The tape holds it all together, and increases its durability.

(Optional) Wrap the ends in red-colored tape for appearance.

6. (Picture 1) Apply more long strips of tape along the length of the two remaining tubes. The tape isn’t necessary, but it does add some durability, and makes it look much better.

7. (Picture 2) Pinch one end of each tube, and seal it closed with more tape.

8. (Picture 3) Tape the pinched end of each tube onto the side of the barrel, about ¼" of the way down from the end.

The angle at which you tape the bipod legs is up to you. For higher shot, tape them on it a way to angle the barrel upward. To transform this into a cannon-like launcher, angle the barrel only slightly upwards.

9. Drill a hole into the top of the plastic bottle near the neck. Make sure the hole is wide enough so the stem of your BBQ lighter can fit snugly inside.

How to fire:

1. (Picture 1) Drop the ball into the barrel. The ball should stop somewhere in the middle of the barrel because of the cut-and-folded section created in Step 1. If it doesn’t, see Tips & Troubleshooting.

2. Position the mortar FIRST. If you take too long to move and aim, the fuel will expire.

3. (Picture 2) Unscrew the bottle lid, and fill the back half of the barrel with a 1-second burst of hairspray. Don’t overfill it, or it won’t work. There needs to be enough oxygen for combustion to occur.

4. (Picture 3) Quickly screw the lid back on, reposition your shot, and insert the BBQ lighter. It should fit snugly. If it’s loose, the explosion may partially burst through the BBQ lighter opening.

5. Click the ignition and fire! You should hear a satisfyingly loud BANG! and see a small fireball (and ping pong ball) explode from the barrel.

6. After each shot, unscrew the lid and blow a gust of air through the whole barrel. This clears out the gaseous byproduct from the combustion reaction. If you don’t clear the barrel, these gasses will prevent another combustion from occurring.

What if the ball falls all the way through the barrel?

Poke two small holes near the middle, and insert a skewer or other thin, flame-resistance material through it. The skewer will prevent the ball from slipping past.

What if the ball doesn't shoot very far?

There are two things that could go wrong.

First, there may not be the right mixture of fuel (hairspray) and oxygen. If you ignite the lighter and nothing happens, try this: quickly remove the lighter, blow a gust of air into the hole, and reinsert the lighter to try again. Introducing a small bit of O2 may be enough to get the combustion started.

If it still fails, then remove the ball, clear the barrel, and restart the firing process.

Second, the paper towel tube may be too big. The ping pong ball should just be able to slide down the barrel. If there’s an excess of space between the ball and the inside of the barrel, the explosion will travel around the ball. We want the explosion to remain contained behind the ball, thus forcing it outward. You can decrease the size of the barrel’s inner diameter by cutting yet another paper towel tube down the size, and inserting it into the barrel (Pictures 1 & 2). Repeat until the ping pong ball just barely slides down the barrel, or even needs a gentle poking to get it into position.