This was while they were attempting to disarm the bomb using the technique of "slowly burning the explosive", which in my obviously completely inexpert opinion, I wouldn't have recommended.
The bomb likely had another, undetected detonator which was triggered, and it produced enough of a shock wave to make the TNT explode.
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.
Edit: I'm completely mistaken! Some bombs in fact have multiple fuse pockets, so presumably, multiple fuses. Nonetheless, even with all fuses removed, UXB can still go off during disposal.
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.
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.
>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.
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...
(pdf warning)
https://www.fh-campuswien.ac.at/fileadmin/redakteure/News/Do...
> 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.
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 !
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.
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/dissolved 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. Sensitive, as in, workers have died when their shovel struck the bomb casing. There would not even be time for the clunk sound to make it to their ears before obliteration. Or even just...nothing [4]. Hug your loved ones, folks.
(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. [3]
Edit: tweaked wording after some more research into bomb composition.
P.S. I updated my notes on explosive composition. Turns out WW2 US/UK favored picrate mixes far more than the Germans (who favored TNT, RDX, ammonium nitrate, and aluminum), and as a result, allied bombs from this period are much more unstable than German ones. Also note, forming unstable species isn't necessary for DDT: e.g. the Halifax and Beirut explosions were caused solely by burning ammonium nitrate. It's just much, much harder to get a pile of AN to go off than rando metal nitrate salts.
[1] https://en.m.wikipedia.org/wiki/List_of_explosives_used_duri...
[2] https://en.m.wikipedia.org/wiki/SC250_bomb
[3] https://www.bbc.com/future/article/20150922-these-nazi-bombs...
[4] https://www.dw.com/en/wwii-bomb-self-detonates-in-german-fie...
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'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.