Introduction: Rapid-Reload Vacuum Cannon: Magnetic Piston, No Burst Discs
Preface: This is a written guide as an accompaniment to a video I have produced on the same subject. To see this cannon in action you can check it out by viewing it through the embedded video above, or by clicking this link.
This project is a long overdue update to my vacuum cannon series. Specifically an update to my piston operated design that I introduced in a short series last year. This style of vacuum cannon is my own invention with the key feature being that it does not require an external vacuum pump. A vacuum is created inside the cannon barrel simply by pulling an air tight piston from one end to the other. When the piston is pulled all the way out the air tight seal is broken, and atmospheric pressure forces air back into the barrel at a velocity several times faster than the speed of sound. The speed of sound is usually the limit for how fast a pressure wave can propagate through a gas like air, but in a vacuum gas particles can travel as fast as their temperature will allow. If there's a solid object still in the barrel when the seal is broken it's accelerated to impressive speed.
This is the primary function of a vacuum cannon; to demonstrate the forces of atmospheric pressure in a spectacular way. It's an excellent scientific demonstration that never fails to please a crowd. It's also a ton of fun to use.
This cannon uses several components I show how to make in earlier projects, most notably an air tight piston constructed from 1 1/4" PVC fittings, pipe, and o rings.
Step 1: Design & Features
This new design I've come up with solves a lot of the issues I've had with my earlier cannons. A cannon that uses an electric vacuum pump takes a long time to reach a deep enough vacuum inside the barrel for it to fire. A piston operated vacuum cannon can fire comparatively quickly. The hard part is figuring out how to anchor the barrel down so that it doesn't move when the piston is pulled out of it. The first piston cannon I built was tethered to a tripod which used cables and stakes driven into the ground to keep it still.
For this design I wanted something that did not require being held down. To accomplish this, the piston is pulled with a crank which is mounted to the same board as the barrel. In this way the board is supporting most of the tension as the piston is pulled through the pipe.
The piston in my earlier designs was attached to the end of a long shaft which I pulled by hand. Since this new cannon uses a crank and rope I've just attached a bolt to the back of the piston so that I can snap on a steel carabiner. Being steel, this carabiner doubles as a tool for pulling the rope back to the front of the barrel after the cannon has been fired. I can use a magnet to drag the steel clip along right through the wall of the pipe until it comes out of the end where the piston can be reattached for the next shot.
Step 2: Sealing the Front of the Cannon
The front of a vacuum cannon needs to be sealed so that a vacuum can be created inside when the piston is pulled through the pipe. This seal also needs to allow a projectile to punch through upon firing, and so aluminum foil burst discs are a common solution. The trouble with burst discs is that they need to be taped to the barrel between shots which is a time consuming process.
I tested a number of different valves as possible replacements for burst discs without success. Mechanical valves can't open quickly enough and they get destroyed by the projectile. So for this cannon, rather than using valves or homemade burst discs, I put a 2" rubber fitting on the end of the pipe. I then take a disposable plastic cup and push it into the fitting like a cork. This gives me a quick way to make a seal on the end of the barrel, and the cup is held gently enough that it doesn't create resistance when the cannon fires.
If the end of the pipe is polished smooth the rubber fitting is not necessary. Smaller condiment cups can be placed directly into the end of the barrel and they work quite well to make a seal when the piston is retracted.
Step 3: Loading the Cannon
The PVC piston I have made for this cannon (which can be built according to the steps in this video) is modified with a magnet held to the front with epoxy. The purpose of this magnet is to pull any projectile loaded into the cannon along with the piston as it's drawn through the barrel until the piston pops out of the far end. When the piston is pulled all the way out the air that rushes to fill the barrel overpowers the magnet and blows the projectile down toward the sealed end of the pipe. In order to function in this way the projectiles must either be made of a magnetic material, or simply have a few steel washers glued to the back as I have done for the wooden plugs I use in this project.
Side note: these are the same plugs cut with the hole saw when making the wooden mounts for the barrel. They were a bit oversized but could be sanded down to be a nice fit in the pipe.
So to go over the loading process from start to finish, the rope is first fed through the barrel (using a magnet and the steel carabiner), the piston is attached and then pushed back into the pipe, a projectile is snapped onto the magnet at the front of the piston, and then a cup is placed into the rubber coupling to make a seal. The cannon is now loaded and ready to fire. Quick and easy.
Step 4: Firing the Cannon
Originally I used a hand crank to pull the piston through the pipe but I found it worked better to use a drill due to increased speed. The more quickly the piston is pulled the less it will matter if there is a small air leak as it will not have time to sufficiently fill the vacuum before firing.
The crank I am using for this cannon is the remains of a boat winch I purchased for another project. I've modified this winch to only use the housing and a single bolt to act as a pulley, which I widened with more of the wooden plugs left over from making the cannon frame (the wooden plugs didn't withstand the elements so I later replaced them with a piece of acrylic rod). Putting a nut on the end of this bolt allowed me to turn the pulley with a socket driver in my hand drill. This worked very well to fire the cannon.
I also added an eye bolt to the front of the crank which you can see in one of the above images. This is to help guide the paracord so it wraps evenly on the pulley. After repeated shots I also decided a cushion would be a good idea to absorb some of the impact when the piston springs out of the pipe, so I added a piece of a large pool noodle in front of the eye bolt. This should increase the life of the piston.
A vacuum cannon is powerful not because of the amount of pressure pushing the projectile down the barrel, which is only about 14 pounds per square inch, but because of the extreme velocity that air can enter the barrel. Because of this, the best projectiles are light weight so that they can accelerate quickly to take advantage of the fast air flow. A vacuum cannon can fire heavy projectiles like PVC end caps, but because they accelerate slowly the speed they achieve is only about on par with a regular air cannon. For supersonic velocities to be possible the projectile needs to be something like styrofoam or a ping pong ball - very light weight. Even so it would require a vacuum cannon much longer than this one to give the projectiles enough time to really hit their maximum speed.
There's still some experiments I need to work on to really get these cannons to achieve their full potential.
Be sure to watch the full video of this project to see this cannon in action! You can click here to be redirected to the video.