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Practical Effects Air Cannon

A non-pyrotechnical explosion simulator for film.

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Half of a two-part project, the other part being "OBA on a Toyota."​

A friend of mine is making a short film which requires some simulated explosions. Since he doesn't have an explosives lisence and we will be shooting some scenes on public property and in generally public locations, we need a solution that is both legal, cheap, portable and, last but not least, effective.

Several months ago I watched the extra bits at the end of "Gods and Generals."  One of their locations was historic Harpers Ferry, which, being a national monument, precluded the use of pyrotechnical effects.  Instead, they used what basically amounts to an air cannon to send a cloud of styrofoam debris and dust flying into the air.  No fire, but it looked pretty convincing in the movie.  So that little idea lodged itself in the back of my mind to sit and stew until the opportune moment.  Lo and behold, the moment came several months later.

So here's the basic idea:
Take an air tank, attach it to some sort of a cone or tube which gets loaded with the simulated debris.  In between the two, have an air hose of some reasonable (but not too long) length.  Electric trigger for reliability's and discharge speed's sake.  Oh, and try to do it for under $200USD.

So here goes nothin'...

  • Shanghaied, or, rather, Shenzhened

    matthewjohn.erickson02/23/2014 at 19:12 0 comments

    In the initial design, I was planning on using 1" hose as the discharge line for the debris cone.  I ran into 2 problems: 1.) 1" solenoid valves are EXPENSIVE, and so is all the hardware involved with that tube size. 2.) ​1/4" solenoid valves are cheaper, but once you add in all the fittings for ganging 4 of them in parallel, you guessed it, it gets EXPENSIVE.

    Time for plan B.  After purchasing the tank of CL, I realized that the tank outlet is only 1/2" NPT.  Thus running a 1" line out of it makes little sense.  Yes, I realize that a 1" line will result in less pressure and velocity drop as compared to a 1/2" line, but I figure I should still be able to get enough through a 1/2" line to do the job.

    So I went online to look for 1/2" solenoid valves.  Wound up digging around on Amazon for them just out of convenience's sake.  Found plenty in the $20-$35 range, which was great for the budget.  Winning!!

    Only one problem.  They ALL were shipping from either Hong Kong or Shenzhen, which meant about a 15 day wait time at the least.  Only problem is that we start shooting in a week and a half.  Crap.

    After spending a couple days being really depressed about this and trying frantically to come up with another solution, I decided to work on the other half of this project, being installing OBA (on-board air) in my 1986 Toyota pickup.  One of the locations we're shooting on is waaaaaay up in the mountains.  No electricity, no air compressors.  Since I don't feel like standing there with a friggin bike pump thrusting away for 20 minutes every time we do another take, I decided to move up the timetable on the air compressor project on my truck.  In the process of hunting components for the OBA project I came across a solenoid valve. 

     In 1/2" inch size.  

    With twice the orifice diameter of the other ones.  

    And it ships from the US!!!

    Less than $40. Ordered it yesterday and it ships on Monday.

    Whew.  Now I just hope I'm not burning up all my good luck before we even get out the camera.

  • The first step is always the hardest.

    matthewjohn.erickson02/23/2014 at 18:48 0 comments

    After hunting around on Cragslist for a week or so hunting for the perfect tank for the perfect price, I realized time ​was starting to be of the essence and that I had to get started gathering the materials since the final design would be strongly influenced by what materials I could actually get my hands on.

    Contacted a couple listings, got a response back from one, met up a day later and bought myself an air tank.  $15 for a 5-gallon tank in good working order.  In retrospect, that's about as close to perfect tank/perfect price as anyone is likely to get.

    Boom.  First step taken.

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