13 December 2015

The Objective

To mark the official launch of our website and channel, it's only appropriate that we launch the GoPro Hero 4 Session from our home-made pneumatic cannon! Here's our guide to building the cannon and other relevant components, as well as some pro(fuse) tips on avoiding certain pitfalls. That way you too can indulge in the ability to chuck a $350 $300 $200 camera though the air at high speeds.

Upon reaching this decision to squander our money in the coolest way possible, we immediately made a list of crucial goals that we had to meet in order to justify our obvious lapse of judgement:

  1. Cannon - Build the coolest-looking, most ergonomic air cannon that you can shake a ramrod at.  And do it in a $150 budget. In this case, 3D prints and carbon fiber stickers came in handy.
  2. Projectile - Design an enclosure for the GoPro which not only protects the camera, but also stabilizes its flight so we can pull some sweet footage.
  3. Launch - This is what really puts the victory icing on the challenge cake. We have to catch the projectile before it hits the ground, partially to protect the GoPro from pulverization, and also because it proves that projectile motion equations (and physics in general) are accurate.


The Cannon

The main attraction is an unruly amalgamation of PVC pipe fittings and Amazon Prime-able hardware. Not only is it sexy and cheap to make, but it boasts an extremely versatile selection of possible projectiles: tennis balls, potatoes, rocks, expensive cameras, or anything you can shove down its 2.5" barrel. It's a modern-day blunderbuss!

The cannon should be fairly straightforward to build, especially if you take heed of our mistakes. So before you venture off and start throwing parts together, we highly suggest you read through our 'Lessons Learned' section below and avoid some of these easily preventable moments of turmoil. You may notice that the cannon used in our launch video is a little different from the cannon built in our DIY video below. That's because we messed up on the compressed air chamber (but shhhhh, let's keep that between us).

Here's our How-To video on making the cannon:


Safety: As with any projectile launching device, whether it be a Nerf gun or a Howitzer, please be fully aware of your target and what's behind it. Know where your barrel is pointed at all times! Also, we highly suggest you load the pneumatic cannon before pressurizing it so you don't accidentally launch something while trying to stuff it down the barrel. And as a general rule, don't be stupid! (And if you do be stupid, don't hold us accountable for what happens)

Cannon Materials List

Trigger Assembly

  • 1" sprinkler valve (Make sure it has threads)
  • Air blowgun (Available here)*
  • 1/4 inch male to male threaded adapter
  • 3D printed trigger guard* (Download STL for free here, or buy it here.)

   *Trigger guard only compatible with the air blowgun above

Barrel Assembly

  • 2.5 inch PVC pipe (2 feet)
  • 2.5 inch PVC coupling
  • 2.5 to 1.5 inch reducer (no threads)
  • 1.5 inch PVC pipe (1 foot)
  • 1.5 inch PVC 90 degree elbow (Qty: 2)
  • 1.5 inch to 1 inch PVC reducer (no threads)
  • 1 inch PVC SCH40 pipe (1 foot)
  • 1 inch female to 1 inch male threaded PVC

Compression Chamber Assembly

  • Schrader valve (Attaches to a bike pump)
  • Pressure gauge with Schrader fitting
  • 4 or 3 inch* PVC coupling 
  • 4 or 3 inch* PVC SCH40 pipe (1 foot)
  • 4 or 3 inch* PVC SCH40 cap
  • 4 or 3 inch* to 1 inch reducer (threaded)
  • 1 inch male to male PVC SCH40 pipe (shorter the better)

*(stick with this size for the other pipes.)

Miscellaneous Launch Items

  • 3/4 inch pipe (2 feet) as a ramrod to stuff objects down the cannon barrel

Tools We Used

  • Drill
  • 23/64 inch drill bit
  • 1/2 inch drill bit
  • Pipe cutter or saw
  • JB Weld
  • 3D Printer
  • Teflon tape
  • Wrench
  • PVC primer
  • PVC cement
  • Dremel (for grinding off flash marks)
  • Spray Paint
  • Carbon fiber sticker (Amazon)
  • Soap water

The Projectile

Tossing the GoPro into our 2.5" barrel simply wasn't going to fly... GET IT? This is because the pneumatic cannon launches projectiles by forcing them out of the barrel with a high-pressure column of air. If the object in the barrel doesn't make a tight seal with the walls of the barrel then the air will rush past it and the object won't go anywhere! It'll just sit right in the barrel along with all your crushed hopes and dreams. With this in mind, we needed to create some type of enclosure for the camera which would provide such a seal and would also serve two other vital functions:

Flight Stability: An object traveling through the air at high speeds is subject to an enormous number of internal and external forces. Some of these cannot be controlled, such drag produced by nonuniform natural forces (wind, collisions with birds, etc). However, some forces and instabilities can be mitigated by following good design practices. Two of the most important design factors in flight dynamics are weight and surface area distribution. Any airborne object will tend to rotate about its center of gravity [the average location of its weight]. For that reason it's crucial that the center of gravity is farther forward than the center of pressure [the average location of aerodynamic forces acting on the surface area], otherwise the projectile will try to flip around backwards in mid flight! It can be a bit of an ordeal to calculate the center of pressure, so a good rule of thumb is to cram as much weight as possible near the nose of the projectile. Fortunately for us, we already planned on sticking the GoPro at the very front so it can glimpse all the action. You can read more about flight stability from our good friends at NASA; apparently they know a thing or two about "rockets" and the launching of these so-called "rockets."

We attempted to provide further flight stability by designing flutes which run along the length of the projectile to act as airflow straighteners; this design is the one being 3D printed in our main video. Check out the result [Click me].  Pretty disastrous... it's safe to say that design is not certified for living payloads, or really any payloads you still want at the end of the flight.

Ultimately we went with a more complex design and installed spring-loaded fins. Why spring-loaded, other than the fact that "spring-loaded" is a sick term and we wanted to say "spring-loaded" as many times as possible? Well mind you, the fins need to be flush against the projectile body while in the cannon barrel to provide that pressure seal we mentioned before. But once the projectile leaves the barrel, they're free to deploy and work their stabilizing magic. And by stabilizing magic we mean pushing the center of pressure further rearward on the projectile body. Here's our first test with the new fins [Click me]. Much better!

Impact Protection: On the extremely probable off-chance that we don't catch the projectile, it has to absorb a large impact force while keeping the GoPro safe. As such, the nose cone of the projectile has to create as much cushion possible. One could compare this to airbags or crumple zones in cars; an area which deforms or flexes under impact distributes the force over a larger period of time, meaning the peak force felt is lower than if it were a completely rigid object (like most 1950s cars / population control devices). You may hear enginerds throw the term "impulse" around; that refers to the momentum, or the force over time, of a collision. Momentum is conserved during impacts, which means if you can double the length of an impact, even from 0.01s to 0.02s, then the force felt at each discrete time is half of what it would have been otherwise! In engineering terms, this is what we call "super duper important."

Lessons Learned

Since we already made the mistakes, you don't have to! That's the beauty of institutional knowledge and the internet.  Anyway, here are some general issues we ran into when building the cannon. 

  • When prepping your PVC pipes & fittings, it is absolutely vital that you evenly coat the contacting surfaces with enough primer and cement. Get it to the point where there are no more white spots uncovered by primer and the cement is almost dripping. If you don’t do this properly, you will encounter air leaks and will have to buy new parts. And that's no fun. Trust us.
  • When applying Teflon tape to threaded pipes, be aware of which direction you're wrapping the tape versus the direction you'll be twisting the threads. You need to apply the tape in the opposite direction so they don’t come off while threading. A good rule of thumb is to start with the Teflon tape at the end of the pipe and imagine the threads as miniature highways. Would you drive along the highway or across it? Don't answer that, just drive along the highway like a normal person.
  • When painting, prime and paint the entire thing one dark color. Carbon fiber sticker can be pretty transparent, so any marks or printed lettering on your pipe will show through. Also, lightly colored spray paint will not cover up primer marks or printed lettering. Be patient between coats. 
  • When drilling use a center punch, or some method of scoring the part, to prevent the bit from “walking” or chattering.