Large Bore Vacuum Cannon
Step 6: Firing the Cannon
My first tests with this cannon were with no projectile loaded, and only a single layer of foil acting as the burst disc on either end. In the ...
In this project, I'll be building a 4 inch bore vacuum cannon. This sort of cannon can fire large projectiles at high speed, and requires no explosive fuel or compressed air tanks to do so. Instead, the weight of Earth's own atmosphere is the force responsible for accelerating the projectile. With a vacuum pulled inside the cannon, breaking a seal on one end is like blowing the hatch on a submarine while it's still under water. Air explosively fills the barrel, pushing any object in its way until the momentum of those objects is great enough to punch through the other end.
Be sure to watch the video tutorial/demonstration embedded at the top of this page, and written instructions will be included in the following steps.
Parts/items required for the cannon:
- 4" PVC pipe
- Brass hose barb
- Vacuum pump
- Aluminum foil
- Duct tape
- 3" PVC end caps (ammunition)
Parts required for the stand:
- 2x4
- Wood screws
- Large zip ties
Step 1: Cannon Assembly
Because there is no valve or combustion chamber required, the design of a vacuum cannon can be very simplistic. Mine was made from a 10-foot-long, 4 inch diameter section of PVC pipe (sch.40, pressure rated). After covering a portion of the ends so that there will be a nice clean surface that I can later make a seal against I gave the pipe a coat of paint.
About 6" from one end two holes are drilled, just barely large enough to thread in brass fittings for attaching vacuum lines. I will be using two separate pumps to make drawing a vacuum inside the cannon a faster process, but one is adequate to get the job done. If using only one vacuum pump only one hole and fitting is necessary.
One nice thing about working with PVC is that it's soft enough for metal threads to carve their way through the plastic, which usually results in a perfect seal with no need for a tap and die kit. If the fittings end up leaking (possibly due to an incorrectly drilled hole) they can be removed and wrapped with teflon thread tape and screwed back in place.
Step 2: Building the Stand & Mounting the Cannon
To keep the cannon from rolling around I threw together a simple wood base made from a 2x4. Three pairs of holes are drilled along the length of this base for tethers to be inserted that will secure the cannon. After painting the stand, I inserted large zip ties through the pre-drilled holes and set the cannon on top. I found the three sets of zip ties to be enough to hold the cannon securely, though I did have one snap after about 20 shots.
Step 3: Finishing the Cannon Ends
With the cannon secured I can expose the ends. For this piece of pipe to properly hold a vacuum, the edges of the cut ends need to be rounded over and sanded very smooth. Rough sanding was done with a flap disc on an angle grinder, then finer grits of sandpaper were used by hand.
Step 4: Burst Discs
A vacuum cannon uses what is called a burst disc on both ends. The purpose of a burst disc is to be strong enough to hold pressure while the cannon is being emptied of air, but weak enough they they will burst when ruptured manually.
In this case the discs are made of four layers of heavy aluminum foil, roughly cut into a circle and pressed with a 4" coupling over the end of the pipe to properly form them into shape. The coupling is only temporary, to help pull the foil tight and fold the edges over. To hold the burst discs in place while the cannon is used the foil is secured with duct tape.
Step 5: Vacuum Pumps
As I mentioned in an earlier step I am using two vacuum pumps with this cannon.
One is an inexpensive electric pump that I purchased online, and it does an ok job of drawing a weak vacuum quickly, but it really slows down once it has to work harder to get the last bit of air out. The strength of a vacuum uses a unit of measurement called inches of mercury, with 30 inches being about the maximum that is possible under Earth's atmosphere. At around 15 inches I start using a manual high volume vacuum pump that I built myself in a project several years ago (seen here). This manual pump is hard work, but it does a great job pulling a strong vacuum when electric pumps tend to give out.
Step 6: Firing the Cannon
My first tests with this cannon were with no projectile loaded, and only a single layer of foil acting as the burst disc on either end. In the project video, you can see a clip where one of these single layers burst discs’ ruptures all on its own as it cannot withstand the pressure. I found that four layers made for the best reliability.
Even with the proper amount of foil it's important never to stand in front of, or directly behind a vacuum cannon that is under pressure. It doesn't take much for a burst disc to rupture by accident, and the cannon could potentially fire backwards if the forward disc were to rupture first.
The steps for firing this cannon are to first load a projectile (in my case a 3" end cap), then form and secure a burst disc to both open ends of the pipe. The vacuum pumps are then used to pull anywhere between 24-30in/hg of vacuum within the cannon, and when ready, a hammer ruptures the rear disc to send the projectile flying.