I don't understand explosives. That bomb in Exeter.

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Boustrophedon
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I don't understand explosives. That bomb in Exeter.

Post by Boustrophedon » Thu Mar 04, 2021 9:13 am

I have tried to follow online lectures about explosive detonation, but the thermodynamics is beyond me, it was never my strong point. See https://en.wikipedia.org/wiki/Chapman%E ... _condition. It gets more complicated in the ZND detonation model.

I would seem that it is possible to cause deflagration of the explosive in a large bomb without it detonating, which causes no shockwave and all the attendant damage. Alford Tech do this regularly in the third world, in de-mining and explosive ordnance disposal. Watch the video, fast forward to the bangs, and notice the gentleness of the explosion and the general lack of a f.cking huge crater. The second one is 1000 lbs. Notice too that there are lumps of un consumed explosive that can be collected and that metal bits of the bomb are confined within a small area and that large parts are intact.

https://www.youtube.com/watch?v=xAmraOE ... AlfordTech

Contrast that to the "controlled" detonation of a 2000 lbs bomb recently in Exeter.

https://www.bbc.co.uk/news/uk-england-devon-56250008

There seems to be a difference that is hard to explain.
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Re: I don't understand explosives. That bomb in Exeter.

Post by Martin_B » Thu Mar 04, 2021 10:15 am

I know a little about explosions, but mainly gas explosions (deflagrations, rather than detonations). The science behind explosions can generally be well modelled, if not well derived from first principles, as there are some complicated thermodynamics involved, but both detonations and deflagrations are common enough that the effects from them shouldn't be got as wrong as seems to be the case in Exeter.

I suspect that:

a) the video from Alford Tech shows disposal of smaller (250 lb & 1000 lb) bombs, possibly with some degradation of the explosive, but as it's a YouTube clip from Alford Tech you would expect them to put their best-case disposal outcome into their promotional material rather than the cases where the disposal left a huge crater in some poor farmer's field.

b) if Alford Tech are disposing of bombs dropped in Laos they probably have much more experience of disposal of that type of bomb than a UK bomb disposal squad has of disposal of WW2 bombs. A UK bomb disposal squad is probably more used to dealing with IEDs which tend to be smaller and with more advanced explosive material. So I can imagine that there could well be issues of not fully understanding the explosive effect of large quantities of old explosive.

c) the bombs being disposed of in Loas are rural, and so the damage possible to nearby structures is minimal. The bomb in Exeter is in an urban setting, and from the look of it, could well of been rural in WW2 but has been built-up since then such that the buildings have encroached within any blast radius which could be set up for a 2200 lb bomb. I am reminded of the Buncefield explosion which caused a lot of damage to nearby buildings and it was said that the owners of Buncefield should never have allowed such operations in a built-up area, but Buncefield was a rural site when constructed and had a 2 mile exclusion zone. Subsequent local planners had allowed development to encroach and when the Buncefield owners had tried to stop development they were ignored. The BBC article says that debris was thrown up to 250 m, but doesn't say how big that debris was (eg, pieces of shrapnel or just bits of dirt).
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Re: I don't understand explosives. That bomb in Exeter.

Post by shpalman » Thu Mar 04, 2021 10:28 am

I've heard about WW2 bomb disposal which mainly consisted of flushing the explosive out with hot water, and once it was out, it could be (relatively) harmlessly burnt off.

What are the differences in initial conditions which mean that a thing will either explode or burn when you set fire to it and why?

Yes I know it's complicated. If it were simple I would already know the answer.

(I didn't do the relevant "shockwaves..." minor option at Cambridge unfortunately, but the thermodynamics of shockwaves isn't the point - what causes something to burn in such a way that a shockwave forms in the first place?)
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Re: I don't understand explosives. That bomb in Exeter.

Post by Martin Y » Thu Mar 04, 2021 10:36 am

Another news report I watched (can't recall where sorry) had a reporter showing us large pieces of sheet metal he claimed had been placed "over" the bomb and which had been hurled considerable distances. The piece he was kneeling beside was probably over half a metre wide and several metres long. He indicated he could scarcely budge it as it was so heavy. Then the shot cut to another such piece sitting on the roof of a house at a jaunty angle. (Photo in this report: https://www.bbc.co.uk/news/uk-england-devon-56277388)

That outcome struck me as probably being pretty goddamned far from whatever the engineers had in mind.

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Re: I don't understand explosives. That bomb in Exeter.

Post by snoozeofreason » Thu Mar 04, 2021 10:40 am

Martin_B wrote:
Thu Mar 04, 2021 10:15 am
The BBC article says that debris was thrown up to 250 m, but doesn't say how big that debris was (eg, pieces of shrapnel or just bits of dirt).
Heavy girders, used to hold down the sand that was meant to control the explosion ended up travelling 100 metres, so it wasn't just shrapnel and pieces of dirt. You can see them at about 3:30 in the video this ITV report.

https://www.itv.com/news/westcountry/20 ... y-ww2-bomb

My mum's flat was just outside the cordon, but close enough to hear the bang. She was evacuated to Brixham during the war to avoid the bombing - obviously came back a few decades too early.

ETA: Ninja-ed by Martin there.
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Re: I don't understand explosives. That bomb in Exeter.

Post by Grumble » Thu Mar 04, 2021 11:20 am

Martin_B wrote:
Thu Mar 04, 2021 10:15 am
b) if Alford Tech are disposing of bombs dropped in Laos they probably have much more experience of disposal of that type of bomb than a UK bomb disposal squad has of disposal of WW2 bombs. A UK bomb disposal squad is probably more used to dealing with IEDs which tend to be smaller and with more advanced explosive material
Not sure about that, possibly true but I believe it’s fairly common to dispose of WW2 bombs still. Happens every single day, more or less, in London (source: episode of Coast about 10 years ago)
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Re: I don't understand explosives. That bomb in Exeter.

Post by Fishnut » Thu Mar 04, 2021 11:36 am

Grumble wrote:
Thu Mar 04, 2021 11:20 am
Martin_B wrote:
Thu Mar 04, 2021 10:15 am
b) if Alford Tech are disposing of bombs dropped in Laos they probably have much more experience of disposal of that type of bomb than a UK bomb disposal squad has of disposal of WW2 bombs. A UK bomb disposal squad is probably more used to dealing with IEDs which tend to be smaller and with more advanced explosive material
Not sure about that, possibly true but I believe it’s fairly common to dispose of WW2 bombs still. Happens every single day, more or less, in London (source: episode of Coast about 10 years ago)
I can't remember where I saw it so take this with a grain of salt but I'm sure one article said they don't see the Exeter type of bomb much and the last one was a couple of years ago.
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Re: I don't understand explosives. That bomb in Exeter.

Post by dyqik » Thu Mar 04, 2021 12:32 pm

shpalman wrote:
Thu Mar 04, 2021 10:28 am

(I didn't do the relevant "shockwaves..." minor option at Cambridge unfortunately, but the thermodynamics of shockwaves isn't the point - what causes something to burn in such a way that a shockwave forms in the first place?)
In the case of WWII and most later aircraft bombs, a different explosive in the detonator forms the shockwave that causes the main charge to detonate.

The point of disrupting a bomb is to separate the bulk of the main charge from contact with the detonator, so that the shock wave traveling from it doesn't fully reach the main charge. However, many bomb designs have multiple detonators, or they are buried deep in the main charge. Or they may be shaped or constructed in such a way that the disruption needs/creates a shockwave sufficient to detonate the bomb anyway.

Also, the main charge can become more unstable over time, so that it can act as its own detonator.

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Re: I don't understand explosives. That bomb in Exeter.

Post by Boustrophedon » Thu Mar 04, 2021 1:02 pm

shpalman wrote:
Thu Mar 04, 2021 10:28 am
I've heard about WW2 bomb disposal which mainly consisted of flushing the explosive out with hot water, and once it was out, it could be (relatively) harmlessly burnt off.

What are the differences in initial conditions which mean that a thing will either explode or burn when you set fire to it and why?

Yes I know it's complicated. If it were simple I would already know the answer.

(I didn't do the relevant "shockwaves..." minor option at Cambridge unfortunately, but the thermodynamics of shockwaves isn't the point - what causes something to burn in such a way that a shockwave forms in the first place?)
From what I do understand there is a total energy thing going on, if the volume of explosive detonated, as a small part of a large block, is too small then there is not enough energy released in that volume to expand the shock wave and it dies out, and the rest of the explosive just burns. So you can use PETN which is sensitive, to make detonation cord of 1/4" dia, where for TNT the minimum diameter is well over an inch and why in big bombs the detonator is in a booster charge of something like PETN. (Also why the IRA couldn't get ANFO to go bang for years.)

TL:DR you need a big shock wave to start with.

As for melting it out, I have seen film of this, but the detonators had to be unscrewed first to gain access and to stop them exploding from the steam. In the Exeter case the detonators were too corroded to remove or even identify.
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Re: I don't understand explosives. That bomb in Exeter.

Post by dyqik » Thu Mar 04, 2021 1:08 pm

To add to that, the point of the shockwave is to make the main charge all go bang at once in a coherent way, so that it builds a bigger but slower moving coherent shock that can do large amounts of damage.

The detonator shock has to be fairly fast moving to do this, and also has to be powerful enough to spread and still retain sufficient peak pressure at the periphery of the main charge that it can ignite the explosive there.

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Re: I don't understand explosives. That bomb in Exeter.

Post by Martin Y » Thu Mar 04, 2021 2:16 pm

Fishnut wrote:
Thu Mar 04, 2021 11:36 am
I can't remember where I saw it so take this with a grain of salt but I'm sure one article said they don't see the Exeter type of bomb much and the last one was a couple of years ago.
One of those news reports quotes a bomb disposal person saying they hadn't dealt with one for 3 years. Now I don't want to get all Dunning Kruger about an expert's field but 3 years doesnt sound all that long. And I doubt bomb disposal is the sort of job where you just wing it with stuff that turns up every month or two. What knowledge could they lose in three years which changed how they decided to proceed?

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Re: I don't understand explosives. That bomb in Exeter.

Post by Fishnut » Thu Mar 04, 2021 2:24 pm

Martin Y wrote:
Thu Mar 04, 2021 2:16 pm
Fishnut wrote:
Thu Mar 04, 2021 11:36 am
I can't remember where I saw it so take this with a grain of salt but I'm sure one article said they don't see the Exeter type of bomb much and the last one was a couple of years ago.
One of those news reports quotes a bomb disposal person saying they hadn't dealt with one for 3 years. Now I don't want to get all Dunning Kruger about an expert's field but 3 years doesnt sound all that long. And I doubt bomb disposal is the sort of job where you just wing it with stuff that turns up every month or two. What knowledge could they lose in three years which changed how they decided to proceed?
My point wasn't that they'd be out of practice but that we don't hear see this size of bomb being found very often so have no idea what a controlled explosion of that type normally looks like.
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Re: I don't understand explosives. That bomb in Exeter.

Post by Martin Y » Thu Mar 04, 2021 3:33 pm

Yes, that's fair, I have no idea what size of explosion they could expect with the options they had, given its location and condition, but I do suspect that having large pieces of metal which they had brought in hurled onto nearby buildings wasn't intended. Of course there's probably a lot more to it than I have any notion of. It wasn't until I read the reports that I considered that they were not only trying to deflect the blast up rather than out (with a couple of hundred tons of sand) but they also had to consider how to minimise the shock transmitted through the ground which could have damaged buildings and utilities in less obvious ways.

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Re: I don't understand explosives. That bomb in Exeter.

Post by FlammableFlower » Fri Mar 05, 2021 3:11 pm

The last one that I know of was the one in Bath 2016 - but in that case it was half the size and they deemed it possible to move it, so the following day after it had been discovered it was transferred to a quarry near Shepton Mallet where the controlled explosion took place so very much out of sight and it probably didn't matter too much if it caused any damage to the surroundings! Whilst there are lots of comments about unexploded ordnance being found on quite a regular basis they don't say the size.

I can't imagine being the poor sod having to drive it there... I wouldn't want to go over any bumps to fast by the looks of it.

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Re: I don't understand explosives. That bomb in Exeter.

Post by Boustrophedon » Fri Mar 05, 2021 9:24 pm

I saw a documentary about the German fire brigade's approach to unexploded ordnance; They lobbed RAF bombs it in the back of a flatbed and drove it of to a quarry for blowing up. It would seem that British explosives are more stable.

A lot of explosives like picric acid and Lead azide can react on contact with metals to form much more sensitive compounds, particularly when moist. I don't know if the Germans were using picric acid as an explosive in WW2, but that could also account for the reluctance to move the Exeter bomb.
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Re: I don't understand explosives. That bomb in Exeter.

Post by Boustrophedon » Mon Mar 29, 2021 3:12 pm

Tom Scott (has someone else) explains.
https://www.youtube.com/watch?v=OOWcTV2 ... l=TomScott
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Re: I don't understand explosives. That bomb in Exeter.

Post by jaap » Mon Mar 29, 2021 3:25 pm

Boustrophedon wrote:
Mon Mar 29, 2021 3:12 pm
Tom Scott (has someone else) explains.
https://www.youtube.com/watch?v=OOWcTV2 ... l=TomScott
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