MakeShift Challenge: Bomb in a Parking Garage: Most Plausible Entry

Dave’s Most Plausible Winning Entry
by Lee D. Zlotoff
April 15, 2009

Solution by Dave

Interesting scenario.

The first thought, of course, would be to run. One who is not trained in bomb disposal should, generally, never try to defuse a bomb. For that matter, even people who are trained in bomb disposal usually prefer to not defuse a bomb if they can safely dispose of it in another manner.

But, having said that, the first thing to do is to analyze the situation. The bomb appears to have an array of motion detectors. While their use is subject to some speculation (and, speculation is never a good thing to have while working with explosives), it would appear that their primary purpose is to initiate the timer when someone walks into the area. A secondary purpose may be to detonate the bomb if that person tries to leave the area.

It would be quite interesting to immediately observe the motion detectors to see if any of them have an indicator light on them which is illuminated. By observing this, one can presumably determine whether one of the motion detectors has been tripped or not. Assuming that it has, then the logical conclusion is that your motion has caused it to be activated.

An important point to note is that most motion detectors have a time-out where they’ll stop registering motion after a certain period of time. Thus, if you remain still, the motion detector may time-out. The question, at this point, is whether the timing-out of the motion detector will detonate the bomb (with yourself still in close proximity)? So, it may be safe to keep moving in the field of one of the motion detectors periodically so that it won’t time out (and possibly detonate the bomb!).

One quick option to consider is whether the time-out of the motion detectors will give you enough time to get away from the bomb, before the timed-out motion detector detonates it. A quick test would be to slide sideways until one of the other motion detectors is tripped, and then to slide back, while keeping the first motion detector tripped, and wait for the second one to time out. This may give you an indication of how many seconds it would be before the first one would time out if you attempted to get away from the bomb.

Unfortunately, the time-out value is too short for you to run a sufficient distant to get out of the basement parking area. You’ve considered slipping back into the elevator, but decide that the bomb is in too close of a proximity to be safe in there. You also have considered jumping behind your car and using it as a shield. Since the bomb isn’t tamped (i.e., the explosives are out in the open), most of the force of the blast would be directed away from the wall, and little structural damage would result to the wall or the support structure of the building (well, little compared to what a tamped bomb would do). But, the overpressure wave may still be such, in a confined parking area, to do you significant harm, even using your car as a shield. So you quickly reject this idea.

Next, consider the mercury switches, which are obviously in place to prevent removal of the bomb. Without the mercury switches, one option might be to toss the bomb off the roof of a building, letting it explode harmlessly when it hits the ground. Except, of course, that you’re in a subterranean parking garage, rather than on the roof. But, with the mercury switches, you don’t dare risk picking up the bomb.

OK, so we’ve burned through about 20 seconds so far …

The appearance of the device strongly indicates that the electronics are packaged in the small black box, with the larger brick-shaped object being some type of explosive, probably one of the “plastic” explosives, such as C-4, RDX, PETN, etc. This immediately suggests a course of action, as well as the structure of the bomb.

You know that most plastic explosives are a class of secondary explosives. These materials are quite stable and difficult to cause to detonate. In order to detonate these devices, a small, violent explosion is necessary. This is usually provided by a blasting cap. Electric blasting caps are small, metallic-looking cylinders with a couple of wires running into the end, and which contain a small quantity of a primary explosive, which are quite unstable and can be detonated by heat, friction, mechanical shock, or the like.

Thus, it is reasonable to assume that the bomb consists of an electronics package (timer, mercury tilt switches, batteries, motion detectors) attached to an electric blasting cap, which has been inserted into a brick of plastic explosive.

There are several ways which could be used to defuse this bomb. But most of those require some time and specialized tools, which you just happen not to have with you at the moment (you shouldn’t have cleaned out your briefcase recently!). However, there is still a way in which the bomb can be defused.

OK, check the timer. You’ve used about 45 seconds so far. Remember to keep moving so those motion sensors still record your presence and don’t detonate the bomb by thinking that you’re trying to run away.

The best solution may be to move close to the bomb and remove the blasting cap from the plastic explosive. (Careful! You don’t want to jiggle those mercury tilt switches yet.) Then, pull those magazines out of your briefcase, the ones you’ve been saving to read during those long, boring telephone conference calls concerning the software project you’re working on (yes, even the Make: magazine; you can always get another one), and wedge them between the blasting cap and the plastic explosives. No, don’t roll the blasting cap up in the magazines; just use the magazines as flat shielding between the blasting cap and the plastic explosive, ideally leaving a little room between each. Stick that hardback copy of Knuth’s The Art of Computer Programming in there, too. These will form a barrier to the shock wave produced by the blasting cap when it detonates, preventing the shock wave from reaching the plastic explosive, which will prevent it from detonating.

Make sure that there’s only one blasting cap (since sometimes two or more are used).

OK, you’ve used about 60 seconds so far.

Now go dive behind your car and wait. Either the timer will run down or the motion detector will deactivate, one of which will trigger the bomb, but instead of a huge blast, you’ll have a pop about the size of a goodly sized firecracker. Keep your head down, since there may be metal fragments from the blasting cap flying around for a moment.

While you’re waiting those few seconds for the pop, you think about other ways that you could have deactivated the bomb. Since some primary explosives are inactivated by oils, you could have grabbed a bottle of oil out of your car and poured it on the blasting cap. But, there’s a good chance that the oil wouldn’t have had time to work its way down into the cap and into the primary explosive before time ran out.

If you had had your tool kit in your briefcase, you could have removed the cover to the electronics package and turned the on/off switch to off. Or, if the bomb designer hadn’t thought to put an on/off switch on it, you could have simply pulled the batteries.

You also realize that you could have simply connected the shorting wires on the electric blasting cap back together. Since electric blasting caps are quite sensitive, even to static electricity and/or radio waves, they come from the manufacturer with a wire that shorts the two leads together. In normal use, the electric blasting cap is installed in the secondary explosive to be detonated, and then is connected to the detonator (plunger, battery, power cord, etc.), and then, as the last step, the shorting wire is cut. Thus, twisting the shorting wire back together would prevent the electrical signal from triggering the blasting cap. But this would leave the blasting cap in the secondary explosive, and it could go off at some time in the future (static electricity, police radios, etc.), so you’re glad that you’ve rejected this idea.

At this point, the bomb is safely defused, and you can then proceed to call the police bomb squad on your cellphone, so that they can come and take possession of the plastic explosive, and search for clues as to who the bomber might be. And, then, you can call and order a replacement MAKE magazine to replace the one that was turned into confetti by the blasting cap.

Afterward, you reflect that it was a good thing that the bomber used a plastic (secondary) explosive, rather than a primary explosive, such as nitroglycerin. You would have been sorely upset to have found a Mason jar full of a thick, yellow, oily material beside the detonator.

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