From my extensive viewing of Mythbusters and Burn Notice*, plastic explosives are very safe because they require heat and shock to trigger, i.e. a small explosion. Either one alone will not cause detonation. Electricity will not cause detonation.
Merijeek said:
Plastique is soft and moldable like modeling clay or play-doh. It may be thicker than play-doh (have not handled any myself), but it is pliable. It wouldn’t take a knife.
longhair75 said:
Right, the simplest bomb is some sort of trigger, and a signal line to the blasting caps. Cutting the signal line or removing the blasting caps from the explosive will greatly reduce the resulting explosion. (Not sure how bad blasting caps actually are.) Typical scenarios including hiding the wires and blasting caps under the detonator so that they are not accessible, and confusing the actual path with decoy lines. Booby traps like motion sensors and such make diffusing complicated.
I always wonder why you can only cut one wire, red or blue, and the wrong one will make it explode. I suppose cutting both wires simultaneously could complete the circuit when the cutters go through. But if I understand correctly, that end already has a complete circuit with the blasting cap, the open end is the signal source end. So why can’t you just cut both wires? Of course if the wires are hidden under the motion sensitive detonator that won’t work, but Hollywood never seems to do that.
Umbriel2 said:
I’m curious what kinds of complications can be put in to cutting the wires. I get vibration sensitive switches (i.e. liquid mercury) so any motion completes the circuit. I get putting wires to the case so opening the case triggers the bomb. What else?
*What, Burn Notice goes to incredible lengths to be accurate in how an out of work spy and a former IRA terrorist can make plastic explosives out of household chemicals. Okay, I probably shouldn’t rely on TV fiction for reliable information on explosives. It’s mostly Mythbusters, who have explosives experts and discuss how explosives work when busting myths about explosives, like detonating plastic with some random wires and a microwave.
I work on fire suppression systems that use blasting caps enclosed in a device that seals the fire suppression gas in the pressure vessel. The circuit is very simple. We have a power source that charges a capacitor with a diode in series with the cap. Upon initiation, the circuit reverses polarity and allows the capacitor to discharge through the blasting cap. The resultant small explosion ruptures the blast diaphragm and lets the gas out into the protected area. Cutting one or both of these wires will stop the blasting cap from detonating.
It is possible, with fifty or so bucks worth of readily available items, to build a bomb that will detonate if it is moved or when a 9 volt battery contained within goes flat.
Nope. Blasting caps usually require an electrical charge, not heat. You’d have more problems with the fumes from C4, which are rather toxic.
The bomb was thrown together by someone who isn’t a professional, from scavenged materials. There couldn’t have been anti-motion circuits because it was stuffed into a backpack and jostled quite a bit. In fact, I seriously doubt you could rig such a circuit that would accelerate the timer when a wire is cut.
Sure, they’re normally set off by an electric current, but aren’t they usually composed of a primary explosive that will go pop just from ordinary ignition?
Not until most of the C4 is burning. If you light it from the far end of the block, it will negate most of the explosive power of the bomb. But there was the time factor. In the 3 minutes they had until detonation, their best bet was to pull the caps and cut the wires, then toss the sucker into a sealed compartment.
Yeah well. . . it’s Hollywood man. I’ve never watched Lost, but I know if they tried to even look at it crosseyed, it would have detonated and started a Genesis Wave or something.
Tripler
Just get some swim fins ‘n’ inflatable water wings. That’s yer best shot.
Ok, no way to support a conclusion, but few actually deployed bombs I’ve seen any description of had any of the tamper proofing mechanisms that are in almost every bomb on TV. The closest to that are bombs which are only triggered my motion or a trip wire, like IEDs in Iraq, and then they usually wouldn’t have a timer. So, assuming you aren’t already reduced to smithereens when you first discover the bomb, in the case you describe, yank out the prongs, or the wires or the ‘blasting cap’. If you can’t do that cut one of the two wires. Do it right away, because only James Bond villains are dumb enough to leave an exposed countdown timer letting you know how much time you have left. If there is such a timer, look for the self destruct button for the bomber’s hideout. It will be well marked, and have, at best, minimal tamper proofing. Really tough bombs are the ones with chemical ignition. Touching it may set it off. Not touching it may not help. Praying may do some good, at least in keeping you occupied while you wait. If there are lots of wires whose purpose is not obvious, you may be in trouble also. If has the countdown timer, then pull as many wires out as you can just before the timer runs out. If not, consult the game theorists in that thread about some pointless game, then do the opposite of anything they agree on. If you have no course of action left, consider how much equity you lost from your house, your daughter’s boyfriend, and other predictors of the quality of the rest of your life. You might want to short all the wires together in that case.
In the precise case you describe, do nothing, and the director will yell ‘Cut’!
IANA explosives expert, but have a little background in volatile substances. IANA EE, but have a pretty strong background in electrical systems and electronics. I did work in Hollywood for a while. Also, my father told me to avoid submerging in submarines with bombs on board.
The IRA used to make some pretty complex bombs, with microswitches, optical triggers and movement sensors - I have seen images/diagrams in a book (but google is not helping, this time). They could be armed with a removable external link, and once armed, would probably go off with the timer or tampering. Given that they usually issued warnings, the anti-tamper stuff was partly to threaten the police/bomb squad and deter investigation/disarming.
This is why controlled detonation is usually the first port of call - easier and safer.
However, this level of skill is rarely shown, and most bombs are pretty simple (timer, battery, detonator), because a complex bomb is harder to get right every time, and you only need to get it wrong once to become a Darwin Award.
I still havnt seen a whole lot against my ‘tear off as much explosive off the brick as you can in 3 minutes’ plan, with the last thirty seconds or so involving tossing it in an empty room.
That’s correct. The usual components of a modern blasting cap are:
Bridge wire. Like a bulb filament, that becomes hot when a current runs through it.
Ignition compound, which combusts fiercely as a result of ignition by the bridge wire. Lead styphnate is the classic example.
Primary explosive, which detonates from the heat and pressure of the burning ignition compound. Lead azide is the classic example.
Base charge, a small quantity of secondary explosive that boosts the detonation of the primary explosive. PETN or RDX are commonly used.
On application of heat, components 2 or 3 have a good chance of lighting off and causing the cap to fire. A sufficiently violent impact may also trigger component 3, which is why John McClane’s improvised “drop it down the lift shaft” device in Die Hard may well have worked.
Re the OP - the “open bomb” with wires and detonator and timer exposed, so beloved of movies, is pretty much a fiction. If a bomb is open, that’s 90% of the battle won. A real booby-trapped bomb will be a closed box and can have tilt sensors and acceleration sensors. A simple light detector within the box will make it extremely hazardous to open. That’s why bomb disposal teams use robots, and knock suspect devices apart by remote control with an explosive-driven water cannon.
Also re the OP - C4 is only just sensitive enough to be triggered by a normal commercial no. 8 detonator. Even a badly-seated detonator can cause a hangfire, blowing the C4 brick apart without detonating it. Just pulling the detonators out should be good enough to make the device safe. Cutting the bridgewires should also work, although shorting them may be safer.
Of course, C4 being the squidgy stuff it is, there could be a completely concealed timer, detonator and battery within the block and all the external gubbins could be a dummy, if you wanted to make a nasty life-or-death puzzle.
My wife has run blasting caps (mistakenly left in the pockets of my work clothing) trough the washer and dryer without setting them off. I have never tried to trip one by hitting it with a hammer, but cutting one of the wires (doesn’t matter which one) will always disarm it.
IANEOD, however at it’s most basic, a bomb seems to have the following components:
Main charge - something like C4, Dynamite, Symtex or whatever
Detonator - ususally a blasting cap or detcord
Some sort of timer or remote device
Battery
Wires holding it together (usually one red and one blue)
So for something simple like this, I would imagine simply smashing the timer, disconnecting the battery, pulling out the wires or blasting caps should do the trick. In fact, that’s pretty much all Army EOD and bomb squad guys do except they do it with a robot armed with a shotgun or a watercannon.
It gets a little trickier if you have dummy circuits, backup batteries, mercury switches to detect if the device is moved (sounds cool and exotic…they are just the same switches your home thermostat uses).
Also, if you ever come across an “undefusable” bomb like the binary explosive bomb they used in Die Hard With A Vengence, you don’t need to spend hours staring at the complex wiring. The bomb was innert as long as the two containers of explosive weren’t mixed. There is no reason they couldn’t just drain one of the tanks or blast the electronics with a shotgun unless maybe there was a smaller conventional charged designed to go off and kill anyone who tampered with the device.
That pretty much covers your OP, Merijeek. As usual, the producers screwed it up. No hidden switches, no traps. All Sayid had to do was take it apart and they could have sailed away to safety.
