Things That Go Boom (Not Bang)

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.

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About John Gilstrap

John Gilstrap is the New York Times bestselling author of Total Mayhem, Scorpion Strike, Final Target, Friendly Fire, Nick of Time, Against All Enemies, End Game, Soft Targets, High Treason, Damage Control, Threat Warning, Hostage Zero, No Mercy, Nathan’s Run, At All Costs, Even Steven, Scott Free and Six Minutes to Freedom. Four of his books have been purchased or optioned for the Big Screen. In addition, John has written four screenplays for Hollywood, adapting the works of Nelson DeMille, Norman McLean and Thomas Harris. A frequent speaker at literary events, John also teaches seminars on suspense writing techniques at a wide variety of venues, from local libraries to The Smithsonian Institution. Outside of his writing life, John is a renowned safety expert with extensive knowledge of explosives, weapons systems, hazardous materials, and fire behavior. John lives in Fairfax, VA.

26 thoughts on “Things That Go Boom (Not Bang)

  1. Another stellar post.

    I don’t suppose you take requests, or do you? 🙂
    Would you be so kind to post a primer on poisons? It would come in handy,

    Muchas gracias in advance.

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      • So glad you asked!
        I’m going to take my chances and ask you straight up:

        Say someone apparently committed suicide by overdosing on sleeping pills. Say a trail of clear vomit is found nearby the corpse.

        If the sleuth knew the approximate time of death, would he be able to gather something significant from the fact that the fluid was clear, as opposed to containing visible pieces of undigested pills?

        I do thank you in advance for you time and consideration.

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        • Ooh, that’s outside of my expertise. Intuitively, just shooting from the hip, I would think that a lot would have to do with how quickly the pills dissolve the the stomach juices. As I understand it, capsules dissolve faster than tablets, but I don’t know the significance of that vis-a-vis what a sleuth might deduce.

          I know we’ve got a couple of MDs among our regular TKZers. Perhaps one of them can take your quesiton?

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        • Hi NR.
          Sleeping pills aren’t poison. As for time of death, an ME or coroner could estimate TOD by liver temp to backtrack how much the body has cooled over period of time. Body rigor is also an indicator. Rigor starts to set in, usually first around neck area & arms, at 2 hrs. Lividity (blood pooling) can indicate if the body has been moved. Detectives know to look for this but ME can help with this too.

          I’m assuming the corpse hasn’t been in a room set at extreme temps (like excessive heat or cool) ti throw off TOD calculation.

          An autopsy will produce an exam of internal organs & lab work (which takes time to analyze, thereby delaying the final autopsy results) and that’s where a determination of CAUSE & MANNER of death. In case of a suicide, lab results will indicate what killed the person. Police will want to rule out homicide.

          I can’t tell by your question if you want the suicide to be a homicide. There’s a lot of backtracking detectives would want to do in order to determine if the dead person was suicidal. If sleeping pills wrte tainted, lab work & autopsy could cast doubt on suicide if the person actually died from whatever tainted the sleeping pills, for example.

          Dr Doug Lyle writes books on forensics for writers. He’s a mystery writer too. His FORENSICS FOR DUMMIES is in my library.

          Hope this helps. There’s a lot to think through police procedurals. Lots of research.

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          • Thanks a bunch!

            I own a number of books on the subject, including the Howdunit Forensics, the Crime Writer’s Handbook and a tome dedicated to poisons. Sorry if I phrased the question in such a generic way that it implied sleeping pills fall under the poison category, which, as you point out, they do not. And yes, the scenario is homicide in the guise of suicide.

            The minucia is what is driving me nuts. I’ve set a number of creative restrictions for my WIP, which I’m not prepared to let go of at this point, namely that the entire quasi-circadian novel takes place in real time, over just one weekend, and when only the preliminary autopsy report is available, but the results of the extensive toxicology exams are not.

            This means the sleuth, who’s no forensic expert, must rely solely on basic ToD estimation and witnesses testimonies to reconstruct and solve the case. It does seem daunting to pull off, but hey!, I’m giving it my positively best shot.

            So, going back to the original question: wouldn’t the fact that vomit – which evidently was expelled prior to death – was absent of any visible pill vestiges indicate something fishy?

            Would a preliminary autopsy report mention gastric contents?

            Finally, you wouldn’t happen to know a charitable expert I could reach over email to pop the aforementioned two questions, would you?

            Again, obliged!

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            • There’s a CrimeSceneWriters group on YahooGroups you can query & join. Great grp of experts on all things Crime Fiction. Wally Lind runs the group & is a retired CSI. They have various experts of all disciplines on there.

              Look into arsenic as a poison. The suicide by sleeping pills could be staged to cover up a longer term arsenic poisoning. Lividity could indicate the body was moved & staged. Arsenic may have telltale signs in fingernails & hair loss, but I can’t remember if this is true. Worth a check. Or maybe the poison of puffer fish from badly prepared sushi could show in stomach contents. Have fun killing your character.

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  2. Good information, but a lot more fun in your workshops!
    I got sidetracked when you used sea level as an example because I live at 9100 feet and everything is different when it comes to “normal” stuff like cooking and breathing, but I accept that bullets work just fine based on all the people who are using them up here.

    Now, off to reheat my coffee, because nothing stays hot up here, either.

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  3. Baloney, John. MacGyver has proved decisively that you don’t need detonation to create a bomb. You can put a gelatin cold capsule in a glass of water and when the gelatin dissolves, there will be a reaction that causes an explosion big enough to blow a hole in a wall.

    C’mon, man. Quit going out in the field and watch more TV.

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  4. Oh, my! I’m going to need a lot of Red Bull and Doretos so I can study for the Final Exam in Explosives.

    Seriously, this is great stuff. I always look forward to John’s posts.

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  5. Loved this! If I’m ever in the vicinity of your explosives seminar I’ll be sure to be there.

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  6. Good stuff. I’ve never really understood how explosions did damage. So the damage an explosion does to a person’s body is really the same kind of physical process as a sonic boom breaking glass? I guess I always assumed it was the shrapnel. But, of course, a 500-pounder landing on London doesn’t really create that much shrapnel (though it does put various other lethal objects into motion–boards, bricks, glass…).

    Interesting that the noise is really the epiphenomenon of the expanding gasses–the same, I assume, as the relation between thunder and lightening, or crackling and static? I’m guessing that the reason the sound of the explosion continues beyond the damage area is that as the gasses dissipate they are no longer moving at super-sonic speed, but still creating (sub-sonic) pressure waves?

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    • Eric, shrapnel is certainly an issue. High velocity chunks of metal and human tissue certainly do not get along well. But the pressure wave does damage as well. If you remember the awful pictures of the Murrah Building in Oklahoma City after the Timothy McVeigh bombing. The bulk of that damage was caused initially by the pressure wave of the ANFO explosive (ammonium nitrate and fuel oil), but as stone and glass break, those shards all become a form of shrapnel themselves.

      As for hearing the noise beyond the damage zone, you’re right; that’s just the residual pressure wave–which is transported through the ground as well as through the air. The farther you are from the source of the blast, the less the pressure–and it dissipates quickly.

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