Some context as a chemist. Most explosives require DDT (deflagration to detonation transition) in order to explode. DDT occurs when the flame front goes supersonic in the material and creates a shockwave (simplification but close enough).

Low sensitivity is desirable because you want explosives to go off on demand, not randomly. Thus, most bulk explosives can be safely burned without detonation. Primary explosives have a low DDT threshold, meaning it takes far less energy and/or confinement to DDT. Thus, you can have a small primary, sometimes a secondary booster, to set off your bulk charge. The primer can be removed to render the weapon far safer. But not perfectly safe.

I don't know the exact composition, but it is likely a combination of TNT, and one of the hexagen/MX explosives (RDX, HMX) [1]. Luftwaffe used Trialen 105 in their 250kg bomb [2], which is TNT, RDX, and aluminum powder, so lets go with that. TNT is fairly stable, *MX are quite stable, but picrates are ornery. Over time they can break down and react with metals in the casing. Metal picrates are much closer to primaries in temperament. They are also far more shock sensitive, which is why (somewhat paradoxically) it's safer to evac and burn these old bombs out, then to try to relocate it. There are other ways in which unstable residues can be formed, not just picrates (e.g. TNT can release NOx over time and form metal nitrate or fulminate salts). You also have all the impurities from synthesis (such as DNT and mixed nitrates). Over time this forms a witches' brew of mixed nitrates that's typically much more sensitive.

(side note: this is why if you ever find old orndance, or old bottles of certain chemicals e.g. ether, you should stop immediately and don't try to move it, just evacuate and call a hazmat team)

What most likely happened is the old bomb corroded and formed a pocket of these unstable salts, and when the flame hit it, a DDT occurred which set off the bulk charge.

This is all well known to ordnance disposal folks, hence the sandbox cover and evacuation radius.

Edit: tweaked wording after some more research into bomb composition.

[1] https://en.m.wikipedia.org/wiki/List_of_explosives_used_duri...

[2] https://en.m.wikipedia.org/wiki/SC250_bomb

> It's just much, much harder to get a pile of AN to go off

Which is why Hezbollah is complicit in the Beirut port explosion of August 4, 2020. AN that's just sitting there for years doesn't simply explode for no reason.

There was a fire and the warehouse also contained a large amount of confiscated fireworks. Similar AN explosions have happened in several other countries too, with no resorting to conspiracy theories.

I'm reminded of the time I heard of the story of a US town blowing up a beached whale with TNT that didn't go as planned in high-school when we were on unit discussion the mechanisms of explosives. I think the teacher was there(?) as a kid and it...ignited...an interest in explosive engineering for engineering in him.

While you possibly heard about a different story, this one has made the rounds over the years: https://www.youtube.com/watch?v=V6CLumsir34

I would assume that the explosive was TNT. TNT actually takes some effort to detonate; this is why even artillery shells can be filled with it, an not explode during firing. TNT can also burn safely e.g if thrown into a fire in small amounts; you likely can even make a candle out of it.

The bomb likely had another, undetected detonator which was triggered, and it produced enough of a shock wave to make the TNT explode.

> The bomb likely had another, undetected detonator

That would be really poor bomb design. Bombs are designed to be stored inert near the airstrip, mounted to staged aircraft and armed quickly by inserting a primer/detonator. There would be one and only one per bomb. The detonator would certainly be removed first before trying to burn out.

Explosives usually become more unstable over time, sometimes forming pockets of primary explosives, which can detonate the bomb. See my sibling comment.

The Germans would sometimes use a timed detonator on their bombs, intended to kill emergency services personnel who moved in after the raid and were fighting fires, aiding the wounded, etc. If you haven't seen it, the show "Danger UXB" from the early 80's is good.

https://youtu.be/Mm-RQtPNvqY?t=268

Those crafty Germans must have timed this one to go off in 2023.

After I clicked reply and went to do a chore I thought to myself "I didn't talk about how it's been almost 80 years and it's unlikely to have been a timer." Doh.

You jest, but I believe some of these incidents have been caused by some sort of delay mechanism which gets stuck, until one day it is dug up or corrodes enough to unstick, at which point it goes off.

The British just sent two waves of bombers one a few hours after the first to kill off the ambulances and firefighters. Seems like the Germans once again beat the British at their own disgusting tactics with better technology.

Also do you have a source other than a post-war tv show? Im getting a lot of unrelated search results when trying to confirm that timed fuses were used this way. Typically timed fuses were used to make the bomb explode before reaching the ground so it wouldn't just blow dirt into the air but would actually collapse lots of roofs etc. mis-set timers could hurt rescue personnel but i havent heard that that was even intended by the Germans.

These were not proximity fuses, but fuses with clockwork timers designed to delay hours or days until they exploded. This turns them into area denial weapons. Militaries use them to this day (but with modern electronics) on targets like airfields.

Early "proximity" fuses like you're talking about (not the later radar VT fuses developed by the British & Americans, first used at the Battle of the Bulge) had mechanical timers with delays of only a few seconds to account for the planned height of the aircraft when dropped. Many of the men awarded the George Cross for their UXB work encountered them after they malfunctioned (If you ever have the chance to visit the UK Imperial War Museum, it's highly recommended.)

https://www.iwm.org.uk/history/10-incredible-stories-of-brav...

> Seems like the Germans once again beat the British at their own disgusting tactics with better technology.

Check your timeline. The German bombing campaign against the civilian population of major British cities started in early September 1940 (mom spent many nights sheltering in the family's Anderson Shelter from the German bombs). Retaliatory bombing in occupied Europe & Germany by the British didn't begin until the next year.

I was asking for a source that those were actually used on british cities. That article sounds like bombs that were supposed tk detonate not the ‘ambulance killers’ you say were used.

>When explosive devices failed to detonate

From wikipedia: The Royal Air Force began bombing military targets in Germany, such as docks and shipyards, in March 1940, and began targeting Berlin in August 1940.

Im sorry Churchill’s unpopular warmongering killed so many of your people.

Your commentary here and in your post above has been incredibly insightful. I was reminded of the 1967 USS Forrestal fire, when I had recalled reading that the ships captain and ordinance handlers were extremely worried about loading old bombs aboard the ship. Your explanations gave much further context to what was likely happening in that ordinance that gave those men such concern. Thanks!

https://en.wikipedia.org/wiki/1967_USS_Forrestal_fire

Thank you!

Well, poor or not, they did it, I found the American TM for german bombs of that era printed in the 50s and the SC250 definitely has a forward and after fuze pocket. Took 5, 17, 50, or 57 series fuzes.

Its wild that all this stuff from decades ago is online. You get used to not having any historical data about, well, historical events.

The type 17 fuze was a clockwork that could go 1.5 to 80 hours after landing. I could imagine something like that getting jarred into working again after a few decades...

Thanks for the correction! Yeah, I did some more digging and there are in fact some wacky designs. The single-fusing-element is more dominant in modern designs, but war is a crazy place.

(pdf warning)

https://www.fh-campuswien.ac.at/fileadmin/redakteure/News/Do...

Per BBC report[1]:

> Specialists started the process by using a robot to cut the fuse and trigger at about 17:30 GMT on Thursday, but abandoned this when water from the cutting process destabilised part of the sand barrier.

> A decision was then made to switch to a slow burn technique, which was being carried out when the device exploded.

[1] https://www.bbc.co.uk/news/uk-england-norfolk-64604115

This is what we were taught on seismic crews too. It is generally okay and safe to burn sticks of dynamite (TNT) but just make sure you don't whack them really hard trying to put it out. The energy required to make it explode was supplied by a blasting cap which took a small electrical voltage to cause it to pop, roughly a half volt if I remember right (40 years ago so no guarantees.) You didn't want to disturb the blasting caps but you could toss the sticks of TNT anywhere with no consequences.

> in my obviously completely inexpert opinion, I wouldn't have recommended

I'm pretty confident they had some of the best experts on the case.

The UK has some of the best bomb-disposal minds, the IRA campaigns on the mainland during The Troubles ensured that. Just look it up on wikipedia ... double-digit number of bombs planted in some years !

Over a thousand or about 60% of the Provisional IRA kills were British soldiers. So yea they got hit hard. I haven't seen any evidence that they actually improved their bomb disposal techniques though. The British Army kept getting successfully bombed into the 90’s.

Does defusing a brand new IRA bomb involve the same set of skills as defusing a decades-old Luftwaffe one? I would have thought it's a rather different problem.

Not entirely.

The IRA bombs are designed not to be disabled, whereas that's not a concern with a dropped bomb.

With the older bombs degradation of the explosive needs to be considered.

IED has progressed immensely since the internet and the best bomb disposal techs are the people who'd been doing it recently and they're mostly in the middle east.

For safety while handling, many explosives need both heat and pressure to explode. Famously, soldiers in Vietnam would burn C4 for heating water. However, explosives tend to become more sensitive over time, and there's always a chance that the explosive filler in this bomb wasn't an insensitive explosive anyway. Given the chance of it exploding when burned versus exploding when jolted, it probably made sense to burn it in place

There's a lot about explosives and explosives removal that is not obvious. Even more so because those techniques are classified both to keep other weapon designers from designing around your diffusal methods, but also to keep from elevating the "game" as it were of the public.

There's also a ton of stuff you don't learn until years later like how some old explosives cause salt crystals to form that mess everything up more. https://www.bbc.com/future/article/20150922-these-nazi-bombs...

What method would you have used in your common sense guess? The old cut the red wire method? Chat gpt says "call the police and leave the area" as the most effective bomb diffusal method. The experts then "diffuse the bomb" as the next step. So there you go.

No I believe it's a common approach. Without the fuse dynamites are inert and you can burn it without exploding it, since they are actually flammable. Of course, lots of steps can go wrong like incomplete removal of the fuse.

Doesn’t seem all that nuts to me. C4 can be burned without it going bang.

The rules must be different for 80+ year old explosives? Presumably every possible decomposition reaction is now present.

Some explosives, like C4, can actually be safely burned. To set them off, a detonator is required. Explosives from WW2 probably have similar properties, as it makes producing and handling huge warheads a much less dangerous activity. Of course, there might always be impurities in the material that makes them behave much more unstably, both from production and because of corrosion and chemical degradation over the ~80 years that have passed.

Those of us who've watched Looney Tunes have known this since our childhood!

https://youtu.be/lJKcdlj-Uiw

Clearly the experts do recommend it in certain circumstances.

Ah, yes, the good old HN classic, "I have thought about it for two minutes and come to the irrefutable and objectively true conclusion that the specialists who do this for a living and have decades of experience and technical know how must obviously be morons." Always so funny to read...

There’s nothing wrong with pointing out that a standard technique for disarming a bomb seems counterintuitive to a layperson. That’s the sort of observation that fuels curiosity and interesting discussion. I don’t think the OP was suggesting that it was the wrong technique.

I do agree that I see that comment a lot on here. However that seems to be a pretty uncharitable reading of the parent comment, as it seems they were more expressing surprise that burning the explosive is a legitimate means of defusing a bomb.

I think it was self-aware a joke referencing that very thing...

Whether it is or not, I would strongly recommend the OP - just like a striker who just scored a goal from a corner - immediately claim that is was 100% deliberate.

"Please don't sneer, including at the rest of the community." It's reliably a marker of bad comments and worse threads.

https://news.ycombinator.com/newsguidelines.html