Blast Shield Experiment
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The Setup

The story goes like this. I have the great fortune for a co-worker of mine to also be a good friend - Chris. He was the one who got me acquainted with the Western Pyrotechnic Association. (WPA has an annual event called Winter Blast held during President's Day Weekend at Lake Havasu City, AZ. This year, the WPA also organized Do It By The River, held by the Avi Hotel along side the Colorado River in Laughlin.) It is at these events that we can fulfill our desires to construct and shoot off Class-B rockets. Chris likes to build whistlers, while I like pounding out black powder rockets.

Whistle rockets can be quite hazardous if they blow-up during construction. There is a lot of pressure applied to the composition, and it is common to place the motor in a re-enforcing sleeve (constructed with PVC pipe or some other suitable material) and a number of hose clamps. Eye protection and hand/arm protection is a must when pressing whistle rockets. It's also a good idea to have an additional barrier for your torso for protection. Chris uses a Lexan shield to protect himself in case of a catastrophic failure during construction. But there has always been a nagging question - would it really work? What material works best? Lexan? Acrylic? Wood? And how much Lexan - 1 sheet? 3 sheets? (1/4" thick sheets) To answer these questions, Chris devised and staged an experiment where different kinds of shields would be tested by purposely blowing up a rocket engine inside a square box made with different materials on each side. Three different materials were tested: Lexan, acrylic (i.e., Plexiglass) and oak. One side of the box was constructed form 1 sheet of 1/4" Lexan. A second side was made from 3 sheets of 1/4" Lexan. The third side was made from a 1" thick piece of oak, and the fourth side consisted of two sheets of 1/4" Plexiglass.

Now granted, there are a lot of other variables that need to be taken into consideration (size of the material, ease of escape of exhaust gases, etc.). We did this experiment back in 1999, when there was very little information about what would or wouldn't work. This web site was put up as more of an intuition tester, as well as to inform. We did this experiment as a crude but informative test. The results are only interpreted by us, you can draw your own conclusions.


Here Chris is holding the experiment. Now if you know Chris and myself, you would know that while we are not brothers, we sure look alike. I showed this picture to my wife, and she said "Did Chris take the picture?" She thought the picture was of me. Sheesh. Now before you get all over Chris for dirtying up one of Bob Weaver's shirts, YOU build rockets all day on the river bank. Dirty? Yes. Hot? You bet. Worth it? No duh.
Because the goals of this experiment was to see if a shield would hold up to an open blast, gaps were left between the different materials. Each side was securely fastened on the bottom, but clamped on the top to allow for flexibility (which is like the situation we face when dealing with rocket presses). 

The top and bottom sections were reinforced with steel plates held together by four threaded steel rods with heavy washers and bolts on the outside. A small hole was drilled in the upper plate to allow a fuse to be inserted into a whistle motor containing standard sodium salicylate whistle mix. The sacrificial rocket motor was enclosed on all sides.

Here is a close up showing three sheets of 1/4" Lexan on the left, and 1 sheet of 1/4" Lexan on the right. The fuse, which was long enough for a 30 second retreat, is just visible to the right of the motor. This shows the 3 sheets of Lexan on the right, and two sheets of Plexiglas on the left. The wood can be seen in the back, and the reinforcing sleeve with clamps in the middle. 
The top and bottom sections were reinforced with heavy washers, and a steel plate was located on the inside. The intent was to imitate a press, so the upper and lower sections were really beefed up.
The experiment was met with some curiosity by other WPA members and officers, including Frank Feher, Mike Carter (doing safety inspection - with the official WPA logo on the side of the loudspeaker), Mike Workman, and Keath Nupuf.
We then lit the fuse and retired to a safe distance. With a rather satisfying "POW", the experiment deployed.


So what do YOU think happened? Did any of the shields fail? Did the experiment hold together, or come apart? If any of the shields held, which ones? What did it do to the dumpster?

Click here to find out!

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