23 March 2012

Explosives 102

by Dixon Hill

Welcome back to my series on writing about explosives in your stories. I was hoping to make this last only through three installments, however I now think it will take 4 to 5. If you guys get bored and want me to change topic, let me know.

Additionally, while it’s true that I once held a Top Secret clearance and access, I was granted said clearance and access because certain people thought I could reasonably be expected to keep important secrets. I work diligently to avoid disappointing those people. Consequently, you don’t need to worry: nothing here is classified. And, my pics are all from open source material.

My goal in this series is not to make you an explosives expert; it’s to give you enough information that someone reading your work — assuming you write a passage in which a character employs explosives — thinks it sounds as if you know what you’re writing about. In other words, I want you help make your writing technically convincing.

What do you really do to make it go BANG?
A guy slips a metal tube with fuse on it into a block of something that looks like clay and then smooshes that clay into a crevice in a wall. Then he pours a line of gasoline from the fuse along the floor (evidently to give him greater get-away time). Finally, tossing the empty gas can aside, he lights a match, holds it to the gasoline — and runs like a chased rabbit!

Is that the way explosives work?

What about James Bond? He stabs a pencil into a similar clay-like block, twists the end of the pencil and runs down the hall to avoid the explosion. Does this work?

Well . . . maybe.

Time Pencils — ala James Bond — are quite real. As for the first scenario, however: I can’t see myself using gasoline like that unless I was woefully short of fuse. Even then, it would probably necessitate a decision about which was more important: the explosive going off correctly? Or my staying alive?

The answer to that one would probably be: the explosive going off correctly. If you’re using explosives, there isn’t usually a better way to do the job. And, in situations where I’ve employed them, if they didn’t go off many of my friends might have been in serious trouble. That being the case, I’d probably wind up wishing the explosion had taken me out anyway.

Down to Brass Tacks

One thing you need to understand, if you don’t already, is that explosives are set off by a chain of events usually thought of as the “explosive train.” A very small explosion sets off a larger explosive force through what’s known as “sympathetic detonation.”

Most explosives in common use today, are stable enough to require both heat and shock (or compression) to set them off. But, there are some explosives that require only heat, or only shock, or can easily be set off by either one (such as nitroglycerine, for instance—heat or drop it at your peril!)

So, to set off a very stable charge, such as C-4, you need to first set off a relatively unstable charge that’s snuggled up next to your C-4. This initiating charge doesn’t necessarily have to be large. But, it does need to be volatile enough to create enough heat and shock to set off the C-4. Otherwise, your charge fizzles and the bad guys laugh at you. So . . . what can you use to set off a stable charge like this?

Blasting Caps

There are primarily two types of Blasting Caps: Electric and (wait for it … wait for it …) Non-Electric (Surprised you, huh?).

A non-electric blasting cap can mean different things to different people, but there’s no question about what an electric blasting cap is, in most blasters’ minds, so we’ll start with that one first.

An electric blasting cap (similar to the one labeled “Solid Pack Electric Type Blasting Cap” in the picture below) is encased in a small, thin metal tube—closed at one end. You might think of this tube-like container as being sort of similar to a very tiny, extremely thin soda can that has the top cut off.



Most electric caps I’ve dealt with were probably about an 1/8-of-an-inch to 3/8’s-of-an-inch across, and 3 to 7 inches long. This metal tube is filled with an initiator (labeled “primary explosive” in the drawing), a detonator (labeled “output explosive”) and has two wire leads that run in through the top and down into the initiator.

Viewed from the outside, these two wire leads disappear into a plastic-looking (or sometimes sort of clayish-looking) substance in the top of the cap, which acts as an insulator, and as a “lid” for the blasting cap. Sometimes, these wires are completely bare. On other caps, they may be plastic-coated with only a few inches of bare wire at the far end from the cap.


Either way, there will be a “shunt” on the wire. The shunt is often a thin metal ring; it connects the two wires, completing the circuit, helping to ensure that a stray electronic impulse doesn’t excite the initiator inside the cap until the blaster wants it to. Military electric blasting caps often come in a small cardboard tube that has the cap’s firing wire wrapped around it (pic on right). Civilian electric caps often look like the picture below.

Now remember: a blasting cap has an initiator and detonator inside it — and those two wires sticking out of the cap, run down into the initiator. When an electric current is passed through the wires, quite a bit of heat is generated down inside the initiator (primary explosive). The initiator is a relatively unstable explosive that is extremely sensitive to heat. So, when the electric current heats it up, the initiator explodes. This explosion sets off the detonator (output explosive), which is a bit more stable, but creates a larger explosion through its superior size and/or higher RE Factor. When the detonator goes off, this nearly always creates enough heat and shock to set off the larger, more stable charge that’s going to do the real work for you — such as that C-4 we were looking at earlier. [Yes, I wrote “nearly always.” This is the reason that a good blaster always “dual primes” his/her charges, by using two blasting caps. If the first cap fails to do the trick, hopefully the second will get the job done.]

So, now you see the explosive train in action: a small unstable explosion initiates a slightly larger, more powerful explosion — which in turn sets off a whopper! But, what sets off the blasting cap?

The Blast Machine

We’ve all seen cartoons with a plunger, similar to the one on the right. But, would you be likely to run into one? Are they even real — outside of movies and cartoons, that is?

To answer the second question first: Yes. They’re real. The one to the right is an older model 50-cap blast machine. A Blast Machine is just what the name implies: a machine that creates a big blast or explosion when you use it correctly.

To explain in very general terms: a blast machine has a flywheel inside it, attached to a dynamo that generates electricity. In this case, when someone pushes down on that handle, the long vertical rod connected to it is driven into the case and starts that flywheel spinning. The flywheel spins the dynamo, and an electric current is generated. (You’ll hear a high-pitched metallic whirring when this is going on.) If the two loose ends of an electric blasting cap are connected to the metal terminals (the two screws with wing nuts toward the left side of the frame in our photograph), enough electricity is going to pass through there to set off that blasting cap.

Naturally, it’s not usually a good idea to be sitting right on top of your explosives when they go off. Consequently, blasters run “firing wire” — a thin, plastic-coated wire that looks similar to the cord you might find on a cheap table lamp — from the blasting cap wires back to the blast machine. (You don’t run it the other way, because — and, R.T., I think you will like this — you don’t want to hook up the firing wire to the blasting cap just as somebody trips and falls on the plunger of the blast machine you’ve already hooked your firing wire to. KABLOOEY!!)

Firing Wire

Firing wire comes in big spools, often with hundreds of feet of wire so you can be sure you’re far enough from the blast to avoid getting badly hurt. I’m used to spools of about 500 feet of wire — though the more you use the wire, the shorter it gets. Because, of course, the end closest to the explosion keeps getting blown off.

Firing wire has two ends, of course: one that’s attached to the spool, and one that’s on the end of the wire that is wrapped around the spool. The latter end is going to be attached to the blasting cap, so you can unroll your firing wire on the way back to the blast machine. This end, which gets attached to the cap wires, is called the “running end.” The other end — the one attached to the spool, which you’ll later attach to the blast machine — is called the “standing end.” And, please don’t forget, though we call it “Firing Wire” (singular noun), there are actually two separate wires inside it.

To hook the firing wire to the blasting cap wires, and then to the blast machine, the following steps are taken:

1. If the “standing end” wires, on the firing wire, are still covered in plastic, cut the plastic with a knife and peel the plastic back (trimming off any excess plastic) until you have bared at least two inches of both of the wires at this end. If your story character setting the charge is either smart or lucky, s/he will have wire cutters that have a wire stripper on them, and s/he can use this to more easily strip the wire.

2. Twist the two newly-bared wires, on the standing end of the firing wire, together.

3. Be sure you have at least two to three inches of bare wires at the “running end” of the firing wire.

4. Lightly twist the ends of the blasting cap wires together. (This is called “shunting the wires” and serves the same purpose as the factory installed shunt, which you are about to remove.)

5. Remove the shunt from the blasting cap wire.

6. Untwist the cap wire ends and twist-tie one of them to one of the two bared wires at the running end of the firing wire. (Your character, if s/he’s been well-trained will use a “Western Union Pigtail Splice” [WUPTS] to tie each cap wire to its respective firing wire.)

7. Quickly attach the other cap wire to the other firing wire end, again using a WUPTS.

8. Pound a stake into the ground about three feet back from where you’ve tied-in, pay out several feet of firing wire, then tie the firing wire around this stake so that the wire is slack between the stake and the cap wires. This will prevent you from accidentally dragging your charge back to the blast machine with you, when you unroll your firing wire. If you don’t have a stake, tie your firing wire to the trunk of a tree. Or, maybe wrap it around a heavy rock.

9. Pay out the wire by hand, carefully, walking backward so you can watch your charge and be sure nothing comes loose. If you want your explosion to surprise the bad guys, be sure to camouflage the wire as you go. Do this until you reach the blast machine.

10. Now hook the standing end of your firing wire to the metal terminals on your blast machine. If you’re using the plunger-type blast machine, be sure the plunger is in the “down” position before you hook-up. Otherwise, if somebody trips . . .

Common Blast Machine Types
Plunger-type blast machines are fairly rare in the U.S. these days. You’re far more likely to run into something like the 20 or 30-cap machine on the right. (That handle comes off incidentally, so you can carry it in your pocket and nobody can use the machine but you – unless they have another handle.) Or, you may encounter one like the 50-cap machine (below left), which I’m very used to.

And, just as their names imply, a 30-cap machine generates enough electricity to set off 30 electric blasting caps all at once, if they’re hooked in series, while the 50-cap machine generates enough to set off 50 caps at once. Some plunger-style machines are rated for 100 or even 150 caps, and these machines are still widely in use throughout the world, particularly in less-developed countries. I suspect that machine on the right is a 20-cap machine, but I'm going to call it a 30-cap machine because they look nearly identicle.

The 30-cap machine is operated by twisting the handle. It may take more than one twist to generate enough power to do the job, but don’t worry: Just keep twisting until a loud BANG! tells you it’s time to stop.

Though the handle can be removed and pocketed, there is also usually a chain attachment. This permits soldiers who’ve hooked up FUGAS, or some other anti-personnel devices around their perimeter, to keep the firing wires hooked to the blast machine, while providing a “safety” of sorts, by keeping the handle disconnected but attached to the machine by the chain. That way there’s less fumbling around if the bad guys try to overrun your base camp at night. And, you don’t have to worry that the guy guarding the blast machine might sneeze and accidentally knock the blast machine over, thus unintentionally twisting the handle.

The 50-cap machine is operated by rapidly squeezing the handle on the right side 3 to 5 times. This is because it takes a few pumps to get the flywheel up to speed. Along with the high-pitched metallic whirring that’s endemic to most blast machines, this one also tends to go “zing, zing, zing, ZING!” getting louder with each “zing,” until the explosion drowns out the sound.

The D-ring on the bottom right is the safety clip. Prior to hooking your firing wire to the terminals on top, compress the handle up against the machine, then use the D-ring to lock the handle in place. When you’re ready to fire, simply pop the D-ring down, freeing the handle, which will push out on its own.

The terminals have rubber coverings, and a spring-loaded top. To insert your firing wire, simply push down on the top of the terminal. This will open the hole in the terminal side, and you can slide your wire through. Release the terminal top, and the hole will close back up, trapping the firing wire between the metal terminal jaws.

Remember: you have two separate wires in the Firing Wire. You’ve hooked one of those wires to a terminal (doesn’t matter which one), so you now have to repeat this process with the other wire, connecting it to the other terminal. Then you’re all hooked up to fire.

The last blast machine we’ll look at is the one on the left, below. This is a Claymore Clacker, which is used to set off a Claymore Mine.

A Claymore is a very handy anti-personnel mine shaped sort of like a large soap dish. It contains a strip of C-4 inside the back, and a strip of metal that’s perforated into nearly-separated ball bearings inside the front. There is “wadding” between the C-4 and the metal strip. This wadding acts as a sort of shock absorber, preventing the C-4 from completely destroying the metal plate when it detonates.

The plastic casing of the mine has two sets of metal “scissors legs” attached. It is employed by driving the legs into the ground, then aiming the mine at head height at about 50 meters distance. Thus, the Claymore fires sideways.

When the C-4 goes off, the wadding is obliterated (along with the plastic casing of the mine itself) and the metal strip is shattered into a bunch of ball bearings that act as BB’s or shotgun pellets (the wadding helps keep those BB’s from being vaporized). The result is that the enemy is shredded by a sort of GIANT shotgun blast.

The Claymore comes self-contained, in a kit containing: the mine, 50 feet of firing wire (w/ electric cap factory-attached), the clacker (blast machine), and a small tool that may be used to test the clacker and cap-circuit (kit components in photo on right are missing the tester).

The Claymore Clacker, itself (close up on left), is basically a 2-cap blast machine. In the photo, the clacker is standing on it’s hind end. The handle on the right is the lever you depress in order to detonate the blasting cap. That black rubber bump between the handle and body of the clacker is the switch that closes the circuit so the electronic pulse can be sent through the firing wire. That black thing at the top of the clacker, in this photo, is a rubber cover. It protects the male end of what is essentially an electrical socket.

If you look closely, you may see a small notch cut into the clacker handle near the end. Near the bottom, right hand corner of the clacker body in this photo there is a small square-shaped D-ring. To set the clacker on “safe” simply flip the D-ring up, so that it clicks into the notch on the handle. This will keep the handle from being inadvertently depressed. To fire, just pop the D-ring back down, and squeeze the handle against the clacker body. It almost always works with just one pull, but occasionally likes to be pulled a second time before it fires the cap.

The firing wire for the Claymore (photo on right) has a female end with holes for two round (as apposed to rectangular, as you’re probably used to at home) metal prongs be inserted. This female end is also protected by a rubber cover, which doubles as a shunt. To hook the firing wire to the clacker, simply open both rubber covers (the one on the clacker and the one on the end of the firing wire) and plug the male end into the female end. Once that’s done, you can fire the cap by simply squeezing the clacker. In this case, there is no zing, zing — or even a metallic whine. Instead, there’s a just a dull “clunk” or “clack.” Hence the name “clacker.”

Claymore Clackers can be modified by cutting off the male-connector end of the firing wire along with 6 to 18 inches of the attached firing wire, and plugging it into the clacker. Often, this connector is then taped to the clacker body to keep it from easily working loose. Then the wires on the other end of the firing wire are bared, so that they may be attached to normal firing wire and hence the blasting caps.

A clacker isn’t a super-terrific blast machine; it’s just not powerful enough to set off caps at a great distance. However, it is often employed when you want to set off a nearby explosion very quickly and easily, with near-instant effect. If, for instance, your characters are cops using electric blasting caps to set off a charge that will blow open a door, give them a modified Claymore Clacker to do this job. It works very well.

Priming a Block of Explosives

You recall, hopefully, what I wrote above about “snuggling” that small (but relatively unstable) charge up to your larger (more stable) charge in order to set off the big bang.

The way you do this with a blasting cap is pretty simple: you make a hole in your larger charge and shove the blasting cap down inside it. Seems kind of stupidly simple, doesn’t it? The specific method used, however, depends on the explosive you’re trying to set off. And, since we’ll need to take them one-by-one, we’ll have to cover that in a subsequent installment.

In Explosives 103 (in two weeks) I plan to cover Military Non-Electric Blasting Caps, and civilian NON-EL blasting caps. Then we’ll touch lightly on unclassified Time Pencil info, and begin (if there’s time) looking at how to prime different explosive charges — probably beginning with dynamite.

After we examine how to prime a few kinds of common explosives, I’ll provide a short wrap-up and toss in a few extra tips that should lend a little “icing” of extra verisimilitude to any passage you invent about somebody using explosives.

If you guys don’t like this, let me know. I can always drop this subject and write about something else. I thought, however, that you might enjoy having a little “primer” you could refer to when (or if) it comes time for somebody in one of your stories to blow something up.

See you in two weeks,
Dixon

7 comments:

Neil Schofield said...

Dixon - this is absolutely fascinating stuff. I was riveted as I was with your first piece. But I'm just the tiniest bit nervous. Can you be careful with that stuff, please? One slip and you could blow this whole blog sky-high, killing all on board. And I'm sure Leigh hasn't thought about personal injury insurance.
Great article!

Janice said...

You tempt me to a character who likes to set things off!

Elizabeth Zelvin said...

Dix, are you SURE we can't try this at home? You're making me nervous too. ;)

David Dean said...

Wow, Dix, you SF guys sure learned a lot about blowing things up! The only explosives I was ever allowed around was the Claymore mine that you've included in your posting. It was largely idiot-proof and so I mastered its complexities after only several days of being yelled at by drill sergeants.

Louis A. Willis said...

I watched an episode of “Midsomer Murders” in which workers were using explosives in a quarry. When the operator of the blast machine pushed the plunger, nothing happened. The foreman at the insistence of the quarry owner persuaded the operator go see what went wrong. As he approached the area where the explosives were stuck in the side of the cliff, boom! He was killed instantly. As I thought about the situation, not knowing anything about blasting machines, etc., I asked myself why didn’t he disconnect the wires at the machine before approaching the explosives?

If he did, it wasn’t shown in the episode. We readers can benefit from your post on explosives to check if writers get it right.

R.T. Lawton said...

Dix, didn't know if Fugas (Foogas) barrels were still being used as defensive weapons at base camp perimeters. Foogas as I knew it was a 55 gallon metal drum of flammable liquid set into the ground with an explosive charge in the bottom. When set off, it acted as a giant flame thrower aimed toward any approaching enemy. It was a one-shot deal, but then fuel and barrels were cheap to make another one for the next uninvited guests.

Dixon Hill said...

RT: I've heard all sorts of stories about the origin of this stuff.

I've heard people say the name really derives from the FOOmp of the exploding GAS when it goes off. Others have told me it's an acronym: FUGAS (I think that's the one we stuck with in the Q Course -- though I can't be sure anymore). And the ways I've seen it spelled ... (lol)

Foogas, FUGAS, Fugas, Fuugass -- and, once, in Ghana: "Phiuguash!" if I recall correctly. Complete with exclamation point! lol (Seems to be quite a bit of phonetic spelling there; nice place though. I really liked it.)

But, yes, I'm talking about pretty much exactly what you're saying. And, as you so nicely pointed out: fuel and barrels . . . plus a few other odds and ends . . . tends to be a pretty cheap alternative to expensive (and sometimes hard to beg, borrow, steal or requisition) anti-personnel mines.