How do underwater explosives work?

I'm just curious after reading the post about the 14000 tons of explosives that is supposed to lie on the bottom of the sea just off the Thames estuary....

www.abovetopsecret.com...

Despite being in the forces for many years, my job never required the use of explosives, apart from a few times in S. Armagh but that was it. No underwater stuff though.

I just wondered how underwater explosives differ from the normal ones? You'd think the water would cancel a detonation out, or ruin the explosive wouldn't you? I'd have thought something that old (as in the post above) would be so perished and clogged up that it would be no good to anyone. Then again we always have the beach cleared near here when the old WW bombs wash ashore.

I know there are people here with a great deal of explosive experience, so i thought i'd ask.

CX.

Most modern day explosives used for military applications come in a plastic wrapper that will protect it from water for quite a long time.

The explosives themselves are very waxy so will hold together in water more than long enough for them to be set and detonated.

Explosives in bombs are encased in thick steel which takes a long time for water to rust through. Also the explosives in bombs are poured in while in a liquid state, so there is no oxygen trapped inside the case to work at deteriorating the compound.

Old civil war era stuff like cannon balls are still fully functional even after 100+ years in the ocean, because they use black powder. Black powder lasts for ever. Even if it gets wet, once it dries out the black powder is still good actually it is more sensitive once it dries out.

reply to post by watchitburn


Thanks for that, interesting stuff.

Guess i've watched too many films where they've made a bomb safe by sticking it in a bucket of water.

CX.

I thnk that the thing about putting stuff in a bucket of water is more applicable to things like fireworks. You'd be mainly dousing the wick or potential spark on the inside with something cold and potentially blocking access to the oxygen in the air.

Many explosives are mixed so as to already contain an oxidizer in the chemical mixture. Consider how some explosives can even go off in space. As in the example of gunpowder, all three chemicals act together to provide the fuel as well as the oxygen source. It's just a matter of getting something hot enough near to it to cause it to ignite.

if its in one mass like a ship wreak nothing is going to cancel out 14000 tons of explosive.( Liberty ship SS Richard Montgomery)
en.wikipedia.org...
www.ukdiving.co.uk...
www.ssrichardmontgomery.com...

it would make one very large hole in the water.

By the way liberty ships had a maximum load of about 8000 tons so there could not have been 14000 tons of explosives on the ship or it would have needed wheels.

I believe it's like underwater welding in that a bubble of air is created at the point of detonation, to give the charge some oxygen for fuel. After that point the explosive charge does its work.

also one thing that should be noted is that water is an incompressible fluid.... This means it transmits explosions EXTRAORDINARILY WELL. A good example of this is the krakatoa shaped charge efp. ON land it can penetrate a one inch steel plate at 25 yards while under the sea it's designed to breach the hull of a warship!

The reason for this difference in potential is due to the nature of explosions. Basically an explosion is a very rapid and violent outgassing of material. Now remember when I pointed out that water is an incompressible fluid? When you create a blast wave underwater it VIOLENTLY displaces the water transmitting a much higher percentage of the force of the explosive outgassing into everything around it. Compare this to air where you have to be much closer to be effected by an explosion of much higher magnitudes versus in the water.

For more info check out howstuffworks' page on how underwater explosions work.
science.howstuffworks.com...

reply to post by CX


Underwater high explosives are the same as those used above water. High explosives do not need air to explode because the oxidizer is part of the explosive. The way they work is that a shockwave is generated as part of the explosive train with a booster explosive. A common booster is PETN and it initiates the main charge which is usually a less sensitive explosive. RDX and HMX are the usual main charges in modern military explosives and are of a class called nitramines.
What happens when an underwater explosive goes off is that hot gas expands rapidly in what is called an adiabatic expansion...that just means too fast for heat transfer. At a certain point, the surface area is big enough for the surrounding water to cool the gas and it contracts rapidly. This compression reheats the gas and it expands again but not so much as the initial explosion. This oscillation can continue for a few cycles as the gas rises to the surface and eventually bursts through. See WW2 movies of depth charges; the big splash is the gas making it to the surface after all the damage is done.
The shock wave damages vessels by rapid motion and pressure effects. A torpedo under a surface ship has the properties of bending the hull back and forth during the oscillation. See examples on youtube of the Mk 48 ADCAP breaking modern vessels in half. For the comprehensive thermohydrodynamics of an underwater explosion see "The Science of High Explosives' by M.A. Cook.

You should also look up Dambusters, theres quite a few good docies about the raids. Basically some goofy engineer came up with the extraudinary idea to blow up German dam walls, a mission as epic as the Doolittle raids I suppose.

What he did, and this was genius, is attach a very heavy bomb in the shape of a large whiskey barrel to the bottom of a bomber. Before the release the bombs were already spinning at a high rpm. The bomb had to be released at the exact time, speed, altitude and distance. When the bomb his the water it would skip across the surface untill it reached the dam wall, sink a few metres untill a depth charge made it go boom. The mission was a huge success.


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