Bomb defusings and controlled explosions?

February 20, 2023 9:29 AM

I know that unexploded WWII bombs are frequently found in the UK and Europe. I've read that these bombs are "defused." I've also read that they undergo "controlled explosions." Then I read that a bomb was "defused and later underwent a controlled explosion." What's the difference between these scenarios?
posted by swheatie to Grab Bag (11 answers total)

If you defuse a bomb, you’re removing or disabling the thing that was supposed to set off the explosive. But the explosive is still there. The concern with old bombs is that the explosive may have degraded in ways that made it unstable enough that you couldn’t just throw it away — the safest way to get rid of it is the just explode it, but at a time and place of your choosing (hopefully far away from people and stuff).

If the thing is sitting in a field, you can skip the defusing step. But if it’s in someone’s basement it’s best for obvious reasons to defuse it so you can more safely move it somewhere out of the way first before exploding it.
posted by goingonit at 9:34 AM on February 20, 2023 [12 favorites]

goingonit is right, and sometimes a "controlled explosion" can be something that just tries to set off or violently disassemble the fuse rather than the whole thing. Very often though a "controlled explosion" means setting off the whole thing in a safe manner (like in a big hole or covered in sandbags to redirect the force upwards).

The classic WW2 bomb would have a fuse screwed onto the nose that was supposed to detonate on contact, so defusing it involves unscrewing it - sometimes they use a rocket wrench.
posted by samj at 9:55 AM on February 20, 2023 [4 favorites]

GREAT answer: it's important to remember that some of these explosives are so unstable they are extremely dangerous to even move (there's an entire ship filled with 1400 tons of WWII explosives—the SS Richard Montgomery—in the Thames Estuary).

eta: thanks to samj for the link to the rocket wrench
posted by ivanthenotsoterrible at 10:09 AM on February 20, 2023 [1 favorite]

To make clear what samj alludes to: When they say "controlled detonation", what they're really referring to isn't the detonation of the explosives in the ordnance. They use a smaller explosive -- because they are more confident of the effects of its explosion than they are of the effects of a years- or decades-old explosive that was made before a lot of modern quality-assurance measures -- and blow that up. It then destroys the older explosive, usually but not exclusively by setting it off.
posted by Etrigan at 10:48 AM on February 20, 2023

Further to the comments above - this article is about a recent case in Great Yarmouth where a bomb was being worked on but went off (somewhat) unexpectedly, and explains:

Army specialists had been cutting the bomb using a technique that creates a slow burn of the explosives, and burns off that material.

Officers had said there was a risk of an unintended detonation. (...)

[The police said on Twitter]: "This was not a planned detonation & happened during slow burn work to disarm the explosives."
posted by penguin pie at 11:05 AM on February 20, 2023

One way to think about it is that a fuze basically is a controlled explosion, particularly for WW2 era munitions. The difference between the two scenarios is who has control of the explosion, and the effect has on the munition.
posted by zamboni at 11:55 AM on February 20, 2023

GREAT answer: it's important to remember that some of these explosives are so unstable they are extremely dangerous to even move (there's an entire ship filled with 1400 tons of WWII explosives—the SS Richard Montgomery—in the Thames Estuary).

For comparison, that’s almost two thirds as much as the tonnage of ammonium nitrate which caused the 1947
Texas City disaster.

9:12 a.m., the ammonium nitrate reached an explosive threshold from the combination of heat and pressure.[7] Grandcamp detonated, causing utter destruction within 2,000 feet and extreme damage throughout the port. The tremendous blast produced a 15 ft (4.6 m) shockwave, levelling nearly 1,000 buildings on land.[2]: 3  Among the buildings destroyed was a Monsanto Chemical Company plant, killing 145 of its 450 workers.

Flying shrapnel resulted in ignition of refineries and chemical tanks along the waterfront. Falling bales of burning twine from Grandcamp's cargo added to the damage, and her anchor was hurled across the city. Two sightseeing airplanes flying nearby were blown out of the sky,[8] while 10 miles (16 km) away, half of the windows in Galveston were shattered.[9] The explosion blew the almost 6,350 short tons (5,760 t) of the ship's steel into the air, some at supersonic speed.

posted by jamjam at 12:23 PM on February 20, 2023 [1 favorite]

A much longer answer might involve reading Service Most Silent: The Navy’s Fight Against Enemy Mines, a 1955 history by J.H. Taylor, reissued in 2008. It's an understated, stiff upper lipped adventure story about bomb-disposal in WWII. The principal voice is that of Cmdr John DG Ouvry [stay for the comments] who was the first to dismantle an unexploded WWII parachute-mine on 23 Nov 1939 and lived to be nearly 100. Many of his team died in harness, leaving only small fragments behind. Picking up the pieces of your colleagues was part of the job-description. Part of the conflict was the continuous change in the way bombs and mines were constructed - specifically to thwart safe disposal. S.O.P. was for two Effectives to trick about with the unexploded ordnance relaying each step taken by radio to colleagues at a safe distance. That way any progress to date could be captured even if the task was "not completed successfully".
posted by BobTheScientist at 1:11 PM on February 20, 2023 [2 favorites]

Highly recommended: Danger UXB, an ITV-produced show starring Anthony Andrews that chroncled a lot of the similar defusing activity that went on during the blitz.
posted by scolbath at 2:09 PM on February 20, 2023 [3 favorites]

I'll just mention this amazing resource I recently came across, that catalogs British Fuzes, Pistols and Detonators for WW2-era British bombs. And here are some examples from the U.S.

The vast array of schemes for safely arming the bombs (and keeping them disarmed until the right moment), delaying explosions until the right moment, actually detonating the main explosive, etc is downright mindboggling.

Just for example, in the type of fuze I was interested in, the arming mechanism is a little propeller thingy that has to rotate a certain number of revolutions before the detonator is armed. The idea is, you put the bomb in the airplane - it's not armed. You drop it out of the bomb bay - still not armed. It drops into the windstream, which turns the little propeller around 150 revolutions - which typically is about 10 seconds - now the bomb is armed and if it hits something, it will go off.

By the time all of this has happened, the bomb-dropping people should be far enough away to be safe from the explosion.

But if you drop it on the ground while loading it, the propeller will not have turned those revolutions and so it will not be armed, and so the bomb will not explode (well, if you are lucky and everything has worked as designed). Also if you drop the bomb while flying along at the treetops at 75 feet, not enough revolutions will be turned before it hits the ground, and again (if all goes as planned) it will not go off. This is a good thing, because this detonator is attached to a 2000 lb bomb and 75 feet away is not nearly enough for you to survive the blast.

Once it hits the target, there are a variety of fuzes & pistol mechanisms that come into play. Sometimes you want the bomb to explode instantly, but more often you want to delay it by some fraction of a second so that it penetrates into the building, or the armor, or whatever, before exploding inside of it. So you have everything from a 0 second delay to 0.0025 seconds to 0.1 seconds, 1 second, 10 seconds, and everything in between, including different methods of creating delay, from straight-up timing to different methods of detecting contact that would cause the main explosive to detonate at certain key points, or wait until the exact desired moment.

And then sometimes there were delays of a few hours or days or whatever. (These were apparently specially designed to catch the enemy's bomb squads as they attempted to deal with unexploded ordnance. Yikes.)

All extremely fiendish, but fiendishly clever at the same time.

Now imagine trying to safely deal with all this, after 70-80 years of rust, chemical changes of various types, and so on. Not fun.
posted by flug at 3:16 PM on February 20, 2023 [1 favorite]

I'm in Frankfurt, and there's generally a bomb find of some description once or twice a year. (Usually during construction.) The most spectacular one was the one in the river (there's video too). They originally planned to defuse this one, but the fuse was too encrusted, so they went for an explosion instead.

(Here's a photo of a defused bomb before it was transported away for a controlled detonation, in case you are interested what WW2 bombs look like. This one was found in 2020 in the Frankfurt Trade Fair grounds.)
posted by scorbet at 8:47 AM on February 21, 2023