Mechanical Explosives

The subject of mechanical explosives is an obscure one; in several decades of searching in libraries, I've found perhaps two or three paragraphs on the subject. For this reason, I decided to write a small article on mechanical explosives for the 0.0000001% of you folks out there who find this to be a fascinating subject.

There are basically three kinds of explosives: chemical, nuclear, and mechanical. Much has been written on chemical explosives as they are the most common variety. Chemical explosives work by some sort of chemical reaction, either by a very fast form of combustion (defragration), or by a shock wave causing decomposition of the explosive material (detonation). Nuclear explosives work by either the splitting of atomic nuclei (fission) or by the combining of atomic nuclei (fusion).

The most familiar type of mechanical explosives are what I would term 'thermodynamic explosives'. A good example of a thermodynamic explosive would be boiling water in a sealed container until the vessel explodes. During the era of steam-powered trains, there were many tragic examples of locomotives exploding when the steam pressure got too high and the safety valves didn't work. Any sort of transition from liquid or solid to gas in a sealed vessel can cause a mechanical explosion, such as the sublimation of dry ice or the heating of a material with a laser. There is even a type of detonating cap that works by a powerful electric circuit vaporizing a piece of metal so rapidly that it detonates the chemical explosive.

Another example of a thermodynamic explosive, familiar to those who live in a cold climate, is the bursting of pipes caused by freezing water. I recall an experiment where water was put inside a thick iron ball, and the ball was rapidly cooled by liquid nitrogen--the explosion was so powerful that fragments of the iron ball were embedded in a concrete wall. Even the heating of asphalt or concrete pavement on a hot day can build up enough pressure to cause a rapid buckling that could almost be considered an explosion.

The more obscure type of mechanical explosives are those that I would term 'structural explosives'. Structural explosives are truly mechanical as they get their power source not from heating or a phase change, but from elastic or kinetic energy. One type of structural explosion is the accidental shattering of a flywheel. Flywheels are used in industrial applications to store kinetic energy, and if a flywheel has a hidden flaw, it can shatter with enormous force that's comparable to a high explosive like dynamite or TNT.

The kind of structural explosive that is of the most interest to me are structures deliberately engineered to explode. One kind is the stick bomb, or what I humorously call 'xyloexplosive devices'. The simple stick bombs made with five or six sticks have been around for decades, and in the 1990s, I developed ways of making stick bombs of unlimited size. Because of the way the sticks are woven together, they are bent slightly out of shape. It is the bending of the sticks that cause them to store up elastic potential energy; the sudden release of the elastic energy causes them to jump in the air with a surprising amount of force.

Recently, I have been working on designs for other types of structures that explode, using springs or rubber bands for power sources. My latest practical development is what I call 'clever levers'. The basic unit for clever levers is comprised of a couple of sticks put under tension with one or more rubber bands. The units are combined in a chain in such a way that when each unit flies apart, it releases the next unit in turn. Since the amount of elastic tension is open-ended, depending on how many rubber bands are used, a lot of energy can be stored in such a device. In tests, I've made clever-lever chains that can shoot sticks several meters.

I've tried other exploding structures with less-than-optimal results. One idea that worked with limited success is for a chain-implosion device using rubber bands for a power source. I've also been intrigued by the idea of developing a system for building true three-dimensional exploding structures (as opposed to stick bombs and clever levers which are basically 2-D), but the engineering considerations are daunting. I'm firmly convinced that there's a lot of potential exploding-structure devices out there waiting to be invented by some clever person, and I've barely scratched the surface of this topic.

There's other intriguing ideas for structural explosives that I've heard about, but never actually seen. One consists of a large array of mousetraps loaded with ping-pong balls; throwing a single ping-pong ball into the array triggers (at least in theory) a chain reaction that mimics the chain reaction that occurs during nuclear fission. I've also heard about, but never seen, spring-loaded structures that fly apart when a string is pulled. If anybody has information, photos, or videos of such gadgets, please e-mail me at lunatim@infionline.net.

A final caveat: if you're going to experiment with stick bombs or mechanical explosives of any sort, you MUST wear safety goggles. If you're a geek worthy of your pocket protector, you should already realize the value of protective equipment. It's hard to be cool and sophisticated when you have a popsicle stick lodged in your forehead...

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