Explosives

So, I went over to my local reloader today to use his 'library' and per his suggestion grabbed
Tenney L Davis's Chemistry of Powders and Explosives

Turns out like many things, it isn't so simple, for the following reasons. Legal code and Science don't mix. Our laws have rules about explosives, and this of course influences how likely a new substance is to be considered at the very low end of the scale as an explosive, and high deflagerates not explosives by some scientific defintions get classified as explosives so they fall under the correct existing laws. This happens a lot in biology where there is pressure to describe something as species vs subspecies due to our endangered species act only recongizes species.

Also, there is a scale of burn rate, and the division of low explosive to high explosive is of course quite arbitrary with the highest end low explosive being nearly identical to the lowest end high explosive.

Grain Dust is not an explosive
Grain Dust plus a saturation of oxygen in a confined space is an explosive.

Many of the items considered explosive require oxygen saturation and confinement to produce burn speeds of a level that reach the threshold for a low explosive. Hence gas in the tank of your car normally isn't going to be an explosive, but under the right conditions it can be. (and contrary to the movies of the 80's rolling it down a cliff isn't one of those) I mentioned 'blackpowder' (old school gunpowder) being an explosive and 'gunpowder' (new school smokeless powders)being not. That wasn't quite right. Both black and smokeless have oxygen present already right in the mixture. Black powder has a burn rate high enough to classify it as an explosive regardless of confinement, whereas smokelss powder only has a burn rate to classify it as an explosive when under confinement.


Of course, most confinements aren't designed to handle explosives, so frequently the confinement will break and only a small part of the potential explosive will actually explode, the rest just forms a nice bright fast burning fire.

The plane hitting the world trade center did have it's fuel explode, but only some, and that wasn't visible, and required the confined space of the tank, plane, and building. Once confinement had ended we saw the fireball but that was just fuel burning, and it burns hot and bright, but it is no different than throwing a bunch of dry wood the thickness of your fingers on a campfire. It burns really bright and the flames leap to the sky, but it is just adding fuel to the fire. "Rapidity of reaction distinguishes the explosive reaction from an ordinary combustion reaction by the great speed with which it takes place. Unless the reaction occurs rapidly, the thermally expanded gases will be dissipated in the medium, and there will be no explosion" (although in this case gas does not equal jetfuel, however it is those expanding gases that pushes the jetfuel out and into the air so we can see the nice fireball)

Often it is the absence of oxygen in sufficeint quantities that prevents something from being an explosive. Most explosives are a mix of fuel and oxidizer, as relying on atmospheric oxygen isn't very producive. Coal yields 5 times the the heat energy output per unit of weight as nitroglycerine, yet coal burning on an open fire is not explosive by any means. Burning it in a furnace and pumping bellows to add more oxygen, gets better results, and from wikipeida "When the wood or coal is immersed in liquid oxygen or suspended in air in the form of dust, the burning takes place with explosive violence. In each case, the same action occurs: a burning combustible forms a gas."

Somewhat of a sidenote, Tim, did you ever get to throw real grenades in your army days? I was told that they have very little or no 'fireball'. Most 'explosions' on TV, if the hero can avoid being touched by the fireball he is just fine, whereas real explosions, it is the fragments that get you and there may not be much of a fireball at all. (Absense of fireball makes sense, the fuel is extremely fast burning in an explosion, so there should be none to see, or it will be a flash)