The drama about the Times Square bombing event is continuing, and now a suspect is in custody, thanks to science and forensics. But it looks like the suspect’s grip of science wasn’t so hot: His bomb probably wouldn’t have been as deadly as he hoped.
The investigation has revealed that the SUV contained an extremely jury-rigged device: simple alarm clocks, a “birds nest” of wiring, eight bags of fertilizer, jerry cans of 10 gallons of gasoline, M88 firecrackers, and three propane tanks. The intention was clearly to create a multi-staged explosive device that resulted in a fuel-air explosion of catastrophic power, blowing bomb and vehicle shrapnel into the surroundings and killing, maiming, and causing huge amounts of damage–pure terror, at the heart of New York. The small amounts of data that the police have revealed to the press so far indicate that the bomb partially detonated, with some of the M88s going off, resulting in smoke that gave away the bomb to that astute T-shirt salesman.
But could it ever have actually gone boom? Let’s look at it in individual parts.
The firecracker explosives used are a popular low-grade “toy” explosive in the U.S. A quick Google reveals a horde of Web data on how to use them for fun, and how to mod them. To see how powerful a single one is, check out the video below:
The M88 has just one fiftieth of the explosive content of an M80 “cherry bomb,” which is illegal in the U.S. An M80 has about 3 grams of explosive inside, while a stick of dynamite has about 140 grams of pyrotechnic chemistry inside. This means a single M88 has about 2,300 times less explosive power than a stick of dynamite–assuming they have the same bomb chemistry, which they don’t: Firecrackers have flash-like explosive content, designed to burn very quickly with a literal flash and bang. And there’s a question of trying to amass M88s together to act like one big chunk of explosive–the geometry of the things, surface area and so on, all play into how they may “explode.” Basically, it’s really hard to make any sort of bomb like this.
Gasoline burns well, but exploding? Not so much. Despite what you’ve seen in the movies, and even those dire warnings of sparks from your cell phone and the detonation risk at a gas station, it’s really hard to make gasoline explode–you’ve got to get pretty precise mixture of gas droplets and air.
If you’ve ever felt uneasy handling domestic gas tanks, if you’re a camper perhaps, then you need to relax. By all means treat them carefully, but they’re not as dangerous as you may worry. To prove this, Mythbusters even attempted to recreate a scene from James Bond, where Bond fires his 9mm at a gas tank to generate an explosion big enough to blow his attackers away. Their results? You can see the clip at this link.
Suffice it to say that you need a surprisingly powerful round from a rifle to penetrate a propane tank. And then when you do, the resulting gas leak doesn’t explode, or even turn into a deathly plume of flame–even when you use burning hot tracer rounds to pierce the tank. This is not a strictly scientific test, of course, but the Mythbusters team found you needed to use high explosives to penetrate and detonate the gas inside a propane tank…and even then the resulting explosion, while impressive, is far more concerned with fast-burning flames than the serious damaging percussive power of destructive explosions.
Fertilizer bombs are notorious and have been used by terrorist organizations for years, thanks to the relative ease of access to ammonium-based fertilizer. But actually fashioning a bomb out of the stuff actually isn’t that easy, thank heavens. First up you need a detonator and a liquid fuel. The detonator sends a compression wave though the fertilizer, decomposing the molecules and releasing gaseous oxygen, which is then ignited by the detonator’s own explosive energy. Getting the ratio of fuel to fertilizer is tricky, and then it’s hard to achieve a dense-enough packing of fertilizer so that the initial explosion doesn’t just disperse the rest of the fertilizer into a cloud of dust.
Put all these explosive elements together and what’ve you got? A horrible explosion, or a non-explosive fizzle, a kind of failed Rube Goldberg of a bomb? The latter, obviously.
Times Square image, via Newsweek