I used to work in ordnance back in my United Tech days… mostly stage separation and solid rocket motor ignition systems. With those, deflagration and detonation are two rather different concepts that you *usually* tried to keep separate. In short:

Deflagration: something’s burning. A piece of solid rocket propellant, a pile of gunpowder, a handful of boron potassium nitrate pellets, once they are ignited, will burn at a fairly constant rate typically measured in fractions of an inch per second.

Detonation: something undergoing an energetic and *really* *fast* chemical reaction. Your det cord full of pentaerythritol tetranitrate is reacting at around 20,000 feet per second.

The speed of the reaction is the thing that separates the two… slow vs. fast. Detonation typically moves through the material in question at faster than the local speed of sound. You’d think that “a quarter inch per second” and “five thousand feet per second” are far enough apart that the two concepts will never meet, but reality is quite different. That slowly burning pile of gunpower on the tabletop? Light a match to it and it’ll merrily burn away for a few seconds. You’ll get light, smoke, sound and heat. What you won’t get is any meaningful sort of blast or shock wave. Now, take that exact same handful of gunpowder and put it into a sealed, sturdy container and light it off. As it begins to burn, the gas generated has nowhere to go. The pressure very rapids begins to rise. For many pyrogens, the burn rate is proportional to the pressure. So a quarter inch per second at sea level can get blisteringly fast at a few thousand atmospheres. As a result, that gunpowder that would have produced no shock wave now has a burn rate well in excess of the speed of sound. If the container can hold the pressure just long enough, the whole thing will combust and *then* the container falls apart, and you get a cheerful little blast. Congrats, you’ve built a pipe bomb. Expect a knock on your door from the ATF momentarily to tell you what you’ve won.

Some materials, like BKNO3, are largely insensitive to pressure. Their burn rate is pretty much constant where in a vacuum or in a thousand atmospheres. These materials are handy for igniters in rocket motors, since they’ll operate the same in nearly every circumstance.

The stuff they put in bombs, though, can be tricky. The explosive of choice for the Brits and the US in WWII to today, RDX (and the chemically related HMX), is reasonably stable and has about 1.5 times the power of TNT. It’s great stuff if you want to blow stuff up. RDX is a dry crystaline powder (like salt), but when mixed with TNT and a bit of wax you get a castable substance known as Composition B, still used in ordnance today. Somewhat similar to Comp B is “torpex,” a mix of RDX, TNT and aluminum powder. Now largely obsolete, Torpex was used mainly for torpedoes, but was also used in some of the biggest bombs of WWII like the Tallboy and Grand Slam.

Now: shove a detonator (a small device that produces an actual detonation shockwave) into a blob of Comp B and set off the detonator, and the blob of Comp B will happily take that detonation and run with it. Note: Composition B is not the same as C-4 “plastic explosive” which is mostly RDX mixed with a small percentage of other chemicals that serve as plasticizers, turning the dry crystals into something like modeling clay.

Anyway, back to Comp B or Torpex: if, instead of a detonator, you take a blob of the stuff and set fire to it with a match, you will get… a fire. A simple fire does not provide what Comp B needs to proceed from deflagration to detonation, so the fire will most likely remain subsonic. But, like gunpowder, if you contain the burning Comp B in a pressure vessel, the pressure goes up, the burn rate goes up, and at some point the Comp P gets frustrated with piddly deflagration and goes high order. Blammo.

Why do I mention all this out of the blue? Cuz why not, that’s why. I’ve been working non-stop on Illustrated Aerospace History Subject Book 2 for a while, taking a break now and then only to watch civilization being torn apart and cast down by the worst people the US has produced since the Commies and the Klan. So while you might think that watching scumbags block roads and assault passersby and tear down statues brings thoughts of deflagration to detonation transitions to my mind, it was actually a news report out of Poland:

Biggest World War Two bomb found in Poland explodes while being defused

A British Tallboy bomb with 2,400 kilograms of explosives was found in a river and was being defused when it detonated. The story is lean on the details I would have preferred, but it seems that it was being burned in place. This is a perfectly cromulent way to dispose of a bomb: drill a hole in it, set it on fire. If the hole is big enough, the gases escape and the pressure doesn’t rise. Hole *isn’t* big enough, you’ve made a pipe bomb. If the thing is underwater, the pressure of the water will actually make things more difficult if your goal is to keep things at deflagration. What the story leaves out is if that’s what the plan was. Another perfectly valid way to defuse a bomb is to just blow the damn thing up. When you do it that way you are pretty well assured that you’ll get it all, but there is of course the little problem of whether or not the blast is going to fark up the surroundings. In a river… seems like the best way to go is detonation. But as you can see in the video below, there is some nearby property, including some sort of observation tower, that could have been trashed but instead seems to get a bit of a light wash.

Well, back to the ol’ CAD machine…