According to the Times, the Times Square bomb was made up of a couple of propane tanks and five-gallon tanks of gas, and some “consumer-grade” M-88’s that fizzled and started the car upholstery on fire, thus alerting bystanders. (CNN also says that there was some “non-explosive fertilizer”.)
A “M-88” is a wanna-be M-80. Because I was a useless little shit as a kid, I’ve lit many M-80s, as well as a huge number of various wanna-bes, all of which were smuggled across the Mexican border in my and my brothers’ pockets. The M-80 has a thick fuse, is hard to light, sparks a lot after it is lit, and it tends to fizzle right at the base where the fuse enters the body of the firecracker. We’d often put the fizzled M-80 in a pool of gasoline in an overturned metal garbage can lid, and light that gas on fire. Even then, it would take a few minutes for the M80 to go off.
The reason that my brothers and I still have our limbs isn’t just dumb luck. The M-80 made a big boom, but it was remarkably weak. The most damage I’ve ever seen one do is to take a little bit off the top of an antpile. The “consumer grade” M-88 (we called them “M-100’s”) have roughly the same amount of explosive as a standard “Black Cat” firecracker.
As for the real explosive in this mix, propane, there’s a reason why we don’t hear about explosions at backyard cookouts — those cylinders are well-engineered. Mythbusters just aired an episode where they attempted to make a propane cylinder explode in a fire. It’s essentially impossible, since propane cylinders have pressure-relief valves, which vent the propane rather than allowing an explosion. Even after disabling this device, which involved some skill with metalworking tools, the Mythbusters crew had to heat the tank for some minutes over an extremely powerful gas burner to get it to explode. I can’t imagine a scenario where a burning car in the middle of Times Square wouldn’t be extinguished before even a properly modified propane cylinder would explode.
So, let’s review the bidding. Apparently, the masterminds behind this device lacked the experience of a ten-year-old boy, since they didn’t know that M-80s tend to fizzle and can’t blow up shit. And they must not get basic cable, otherwise they would have put a hell of a lot more gas in that car to get those tanks to explode quicker. But they are clearly public relations geniuses, because the attention they’re getting is way out of proportion to their bomb-making skills.