Let’s talk a bit about explosives, shall we? After such a warm reception to back-to-back posts on guns and bullets, it seems like a natural progression. This post is going to address some of the technical concepts behind what explosions are, and why they do so much damage. Two weeks from today, I’ll go into more detail on the proper applications of explosives.
Understanding the essentials of overpressure. All day every day, each of us carries on our shoulders (and the rest of our bodies) the weight of our atmosphere. All of that nitrogen and oxygen and water vapor has mass, after all, and it exerts a pressure at sea level of 14.5 pounds per square inch (psi) of surface area. Take that number literally. Every area of 10 square inches of surface area is carrying a pressure (think weight) of 145 pounds; every 1,000 inches of surface area is bearing the burden of 1,450 pounds. Like that. Scientists and engineers refer to the pressure of “one atmosphere” (14.5 psi) as 1 “bar”. Twice that pressure would be called two bars, and so forth. Any pressure that exceeds one bar is called “overpressure”. The greater the magnitude, the greater the resulting damage.
One way we experience relatively harmless overpressure every day is sound. When we speak or clap our hands, we send waves of pressure through the air that our ears register as sound. The decibel scale, then, is actually a measure of overpressure, wherein every three-decibel increase represents a doubling of sound pressure. When someone whispers, he crfeates an overpressure of about 13 decibels, which is measured in microbars (millionths of one bar). As noise increases in intensity, the pressure increases geometrically. We start seeing glass breakage at 163dB. At 195 dB, we reach a one-bar overpressure the equivalent of an additional atmosphere of pressure. Ear drums will almost certainly rupture at that level. A Space Shuttle launch exerts about 215 dB at its surface.
My point here is that sound and pressure are the same thing. It’s an important concept to keep in mind when we talk about explosions, because the practical definition of an explosion is the rapid expansion of gases that creates an audible boom. A latex balloon goes pop when you stick a pin in it because the expanding flexible surface of the balloon has trapped gas under pressure. As soon as the pressure vessel fails, the gas instantly reconverts to atmospheric pressure and the suddenness of it all creates a ripple of pressure that we register as an explosion. If you stick a pin into a Mylar balloon, however, there’ll be no pop because there’s no expansion.
Still with me? Okay, here we go.
A gunshot makes a loud boom because the combustion gases which propel the bullet down the barrel are under tremendous pressure until they get to the opening at the muzzle, at which point they instantly expand and reduce to atmospheric pressure, disturbing all the still air that was surrounding it. A suppressor (“silencer”) works by dissipating those pressures through baffles in the barrel of the device to the point that they are nearly reduced to ambient pressure by the time they are released to the atmosphere. Thus, no bang.
Why the Speed of Sound Matters. The speed of sound (767 mph) is essentially the speed at which air molecules can move out of each other’s way. When anything moves faster than 767 mph–whether it’s an airplane, a bullet or super-heated gases–air molecules stack up on the leading edge of the speeding mass because they can’t get out of the way and they create more pressure–sometimes a lot more pressure. And as we discussed when talking about bullets, since nature abhors imbalance, as soon as the speeding mass passes by, it is followed by and equal yet opposite negative pressure (a “rarefaction” wave). When this pressure fluctuation is caused by a speeding jet, the resulting explosion is called a sonic boom, and it is often powerful enough to shatter glass. When it’s caused by munitions or certain other events, we call the resulting explosion a blast wave, and it is often powerful enough to reduce buildings and people to vapor–literally.
An explosion whose blast wave travels faster than the speed of sound is called a “detonation”. If the blast wave travels at less than supersonic speed, it’s called a “deflagration”. To put that in perspective, TNT detonates; napalm deflagrates.
The military and international community refer to detonable explosives as Class 1.1 explosives (“Class One, Division One), while the American civilian community refers to them as Class A explosives. Deflagrable, or mass-fire, explosives are referred to as Class 1.3 (Class One, Division Three) or Class B explosives respectively. Most fireworks are Class B.
Primary vs. Secondary Explosives. Blowing stuff up requires trade-offs. For example, you want it to go bang on time every time, yet you never want it to go off unexpectedly. Given these constraints, how do you transport your boomers from here to there and not yourself become humidity in the process?
The solution is to make the main charge of deployable bombs relatively hard to set off. For example, you can shoot a block of C-4 explosive with a bullet and it won’t explode, but you can cut off a chunk and use it to safely start a fire. Similarly, if a bomber crashes on takeoff, the bombs it carries will not explode. (Both of the above examples ignore the presence of gremlins, who so often prove us engineering types to be full of it.) These main charges are called “secondary explosives” because in order to get them to explode you need to hit them with a “primary” detonation wave. That’s what blasting caps, or detonators, or initiators, are all about.
Primary explosives are highly energetic, stupidly sensitive explosives that will go high-order (detonate) on impact or in response to a tickling charge of electricity. The primer in the back end of a bullet is a primary explosive. So is the active ingredient of a blasting cap. When those babies go off, they send a supersonic wave of energy into the secondary explosive, thereby causing it to detonate.
We’ve all seen those old newsreels of a B17 squadron during World War Two dropping bombs out of their bellies, and then the flicker of explosions way down there on the ground. What you don’t see is the progression of events that made those explosions possible. On takeoff, none of the bombs is yet capable of exploding because the fuses have not yet been activated. As they fall, however, a tiny propeller spins off the nose of the bomb and in the process arms the fuse. When the fuse encounters the proper conditions–altitude, in the case of an air burst, or impact in the case of an impact or penetration explosion–the fuse triggers the primary charge which sends a blast of energy through the secondary charge and the bomb goes off.
Okay, that’s it for tonight. By now, I figure you’re either bored to tears or totally jazzed. Either way, I’ll be back with more explosive material in a couple of weeks.