Well, not exactly a primer. I’ll just tell you everything I know, so you may judge the better for yourself what might be true and what must be false with regard to the Christmas Nashville Bomb.
One early and thoughtful commentary was this one, from a self-described “grunt” who has been in various war zones and seen first hand the aftermath of IED’s and VBIED’s. He has a credible command of the kind of evidence that is sought in the aftermath of such an assault. For instance, if propane was involved, you would expect to find identifiable remains of propane tanks. Also, in this very early post where he self-admittedly speculates wildly, he suggests the blast may have been directed in some way and offers other interesting guesses and insights. The photography is poor, but this individual also happens to know the neighborhood well. Hereinafter I’ll refer to this commentator as BC.
There’s an apparent error in his analysis, though. For one thing, to his eye the blast appears “too fast” to be what he calls “AMFO”. Later, he writes that he thinks it involved black powder. Anyone with my level of civilian training with explosives will roll his eyes at this.
But there’s a way for this analysis to be right. We’ll take that last. Let’s first explore why it seems to be wrong. You’ll get the best effect if you read it in a very plummy, professorial voice.
It may surprise you to know that gunpowder generally, both black and smokeless, is not explosive, at least depending whom you ask. A firearms expert will tell you cartridge powder does not detonate, it deflagrates. That is to say it burns very fast, but not that fast.
To detonate, to be thought of as an explosive by some people, a material (or mix of materials) must sustain a chemical reaction that propagates faster than the speed of sound in that material. A chemical reaction at that speed creates a shock wave.
Deflagration of gunpowder, in the bore of a firearm, can drive a pressure wave that is supersonic in air. The projectile gives a supersonic crack which is in fact a shock wave. But the reaction within the gunpowder is slower than the speed of sound in gunpowder. That’s why gunpowder-fired fireworks never make shock waves. You can make as big a heap of gunpowder as you want and light it off; big bang maybe, but no shock wave. Gunpowder doesn’t detonate. And indeed, there are no guns which use detonating solids as propellants.
One thing that sometimes does detonate is a fuel-air explosive. That’s when you get just the right mix of fuel and air and ignition. Getting the right mix of fuel and air and ignition is great in your engine, when the fuel-air mix deflagrates like gunpowder. But sometimes it is just right, and the result is a detonation in the engine. That’s not so good.
Fuel-air explosions were documented in the early Industrial era, when air in working spaces became saturated with coal dust, sawdust, fine fibers of cotton, etc. Even these seemingly innocuous fuels could cause a devastating explosion if mixed in just the right ratios with the surrounding air. The occasion was rare, but much feared, with especially miners taking precautions.
Later, military scientists developed fuel-air explosives for the battlefield. They learned how to use a primary charge of explosives to atomize a large amount of high-energy fuel and mix it with the surrounding air, followed by a second ignition charge. They achieved some of the most spectacular chemical explosions ever seen, delivered by rocket no less.
That all seems very impressive, but what’s not so impressive about fuel-air explosions is their detonation speed. Remember, detonation means a chemical reaction that travels faster that the speed of sound in its material medium. In a fuel-air explosive, that medium is pretty much air. The speed of sound in air is about five seconds per mile, so the reaction has to propagate a little faster than that. It’s faster than you can run, but not all that fast in the grand scheme of things.
Actually, the speed of detonation of fuel-air is comparable to the deflagration speed of gunpowder, and there exist “firearms” which use fuel-air mixtures. Your backyard potato gun, powered by propane or hair spray, is an example. The detonation is small and slow, and the potato has a large surface to face the pressure, and low sectional density compared to anything metal. In a word, the potato is light enough to be blown away before the pressure gets dangerously high.
The speed of sound in solid materials is much higher. One such solid material is PETN (pentaerythritol tetranitrate). Different sources list different speeds of detonation for PETN. Depending whom you ask, it’s anywhere from 17,000 to 24,000 feet per second. That’s on the order of eighteen times as fast as a fuel-air explosive. PETN is what Detonating Cord, a standard industry product, is made of. In theory, using “det cord”, an explosive event that originates here can be a mile away in a quarter of a second. Faster than the eye can see, faster than the mind can comprehend.
Det cord is used in industrial blasting, to set off a line of charges. In effect, all the charges go off simultaneously.
The king of commercial explosives is TNT (trinitrotoluene). TNT boasts high energy density, indefinite shelf life, and extreme stability. Nothing short of a shock wave will initiate detonation in TNT; not a hammer blow, not even a supersonic rifle bullet. You can light it on fire; it may burn hotter than you like, but it won’t detonate. You can melt it on the stove and cast it into any shape, which is how shaped charges are made.
TNT is sold in the form of “boosters”, small cylindrical charges. A two-pound booster is about the size and shape of a Progresso soup can. In one end a hole is drilled about an inch and a half deep, called the “cap well”. There’s a drop of PETN in the bottom of the cap well, to help things along. To “arm” the booster, a blasting cap, which may be started electronically or by a fuse, is inserted in the well and taped in place. The cap contains PETN and something else to start the PETN. When used with black-powder fuse, the cap contains lead azide, which is sensitive enough to start with a gunpowder spark, and powerful enough to start the PETN. Sometimes the TNT actually fails to start; the blasting cap blows the booster to fragments without causing a further detonation.
Finally, there’s ANFO (Ammonium-Nitrate/Fuel Oil). BC refers to this stuff as AMFO (maybe that’s what they call it in the military). Manufacturers mix up the ingredients and cast the product into tiny balls called “prils”. The prils are sold in bags like concrete or dog food. ANFO is used for large-scale blasting, where the cost of TNT would be too high. Like TNT, ANFO is very stable; only a shock wave will start it. A booster, suitably armed, is embedded in a sack of ANFO prils, and they all go up together.
I mentioned black-powder fuse, which they call “safety fuse”, before. The final consumable is a disposable device called a “pull-wire igniter”. This is a water-resistant cardboard tube which fits over the end of the fuse. The arrangement is weatherproof. On the back end of the tube is a piece of string. A firm pull on this string drags a squiggly wire, coated with friction compound, through a section of tube lined with something like safety-match material. The material ignites and starts the fuse.
So for most industrial blasting, the chemical sequence goes:
- sulfur (sometimes antimony III sulfide) and oxidizing agents (usually potassium chlorate) in the igniter
- black powder (sulfur, carbon, saltpetre) in the fuse
- Lead Azide in the cap
- PETN in the cap
- PETN in the det cord (for line charges)
- PETN in the cap well
- TNT in the booster
- ANFO in the sack of prils
A truckload of ANFO would make a huge explosion; that’s what Timothy McVeigh used in Oklahoma City. (BC says it wasn’t AMFO in Oklahoma; I don’t know what he thinks did happen.)
A gunpowder explosion can be every bit as devastating as a TNT explosion; you just need more gunpowder. But a gunpowder explosion can’t be like a TNT explosion; both the event and the results are dissimilar. When terrorists set off a truck bomb in the parking garage of the World Trade Center, investigators examined the wreckage of cars to determine what category of explosive was used. Dynamite would tend to shove or toss cars around; faster explosives would tend to shred cars.
BC makes the point that by estimating the speed of a detonation you can make an informed guess as to what material exploded. So I watched the video and here is my informed guess: commercial explosives were not involved.
The video shows a large, bright, sustained flash which appears reddish (the redness may be due to camera sensor overload). I glimpsed one video where the explosion seems to tower upwards as a momentary flame.
Maybe it’s different at larger scales, but I’ve seen two or four pounds of TNT go off, and it’s nowhere near that spectacular. If there’s a flash of light it’s so brief the visual apparatus can’t perceive it. For a tiny fleeting moment you can see what looks like a cloud of cruddy smoky air about twenty feet in diameter. That’s all gas that wasn’t there before, moving at shockwave speeds, many times the speed of sound in air. Before your mind can fully register the visual, a hail of tarry soot rains down from a shapeless cloud above. TNT is not as fast as military explosives, but it is way faster than you.
By comparison this thing in Nashville looks downright leisurely.
BC notes that the aftermath is lousy with soot. That would be consistent with TNT, which contains more soot than the mind can comprehend. But there’s a more likely explanation for the soot, as we’ll see.
Perhaps the strongest argument against industrial explosives is the fact that they are not easy to obtain for nefarious purposes in the United States of America.
This Anthony Warner character does not look like an International Kingpin or a World-Class Desperado. He’s a common ne’er-do-well, and people like that can’t obtain industrial explosives.
Legally, in the US, you are not allowed to touch any of the components I mentioned above. Not the fuses, not the igniters, not any of it unless you have at minimum what is called an Employee Possessor Letter, which you get by passing the same background check as for a firearm. You’re still not allowed to touch the stuff except when you’re doing company business on the clock. Clock out, crack a beer, pick up a piece of fuse…technically you’d be a Federal felon.
At all licensed worksites, blasting caps and primary explosives are kept in separate, secure facilities; things like huge steel boxes and fire safes in secure locations. And “separate” means Separate. The facilities are not adjacent, not near to one another. To make some daring escapade of it, you’d have to breach both facilities at once and get away. Good luck with that.
Sometimes you take explosives out in the field, you don’t end up using them, so you bring them back. There are rigid protocols about when and where to arm and disarm the boosters; armed boosters are allowed nowhere near the facilities. Every transaction at either storage facility, whether it’s taking out, putting back, bringing in a shipment: it’s all logged and witnessed to the specific number of items, what and who and when. Inventories, down to the last foot of fuse, are frequent. ATF reviews this information to the dot. If they spot an unreported discrepancy, they rake the Licensee over the coals.
An inside-jobber has been known to smuggle out a bomb or two by faking the books. One guy supposedly lit off a couple of boosters under traffic cones along a Parade route. I didn’t hear that he killed anyone, which is good. I also hear it didn’t work out for him, which is better. Commercial TNT has taggants in it. Investigators can instantly zero in on the manufacturer, the lot, the purchaser. They nailed that guy right away.
As for smuggling out a quantity, it is to laugh. Forget it. Unless you’re a major mobster or equivalent, you can’t get that stuff.
It is possible to make homemade ANFO. Anyone who knows the right mixture can obtain Ammonium Nitrate (artificial fertilizer) and Fuel Oil (Diesel). The mixture does not have to be cast into prils to be effective. But it’s not shelf-stable, meaning its efficacy degrades rapidly, and you still need a solid detonation to start it, which puts you back at square one. As a terror weapon, homemade ANFO is almost certainly ineffectual. The Feds probably figure that if someone has it in mind to make homemade ANFO, they would rather he did it than not, because then they can catch him.
So what’s left? There’s Tannerite. There, I mentioned it, now we can forget it. Tannerite isn’t a weapon, it’s a signaling technology with very limited application.
That leaves fuel-air and gunpowder. A large quantity of gunpowder would make soot. A largish container of fuel-air mixture, such as a trailer, could detonate, scattering gunpowder which in turn would burn at about the same rate as the initial detonation, prolonging the relatively mild shockwave and adding some heat.