eli5 why do fuses for explosives stay lit underwater?


I’ve seen videos of people throwing firecrackers and dynamite and things like that into water and they still explode, but how do the fuses stay lit?

In: 128

The fuse doesn’t draw oxygen from the atmosphere to burn. Instead it contains an oxidiser. So it can burn under water.

There are a few kinds of fuse, it’s not fair to say *all* fuses are good underwater.

But the gist is that burning requires 2 ingredients, a fuel and something called an “oxidizer”, which is usually just oxygen gas from the air itself. So something like a piece of wood, the wood is the fuel and the oxygen from air is the oxidizer. Put wood in the water and it goes out because you removed the oxygen gas.

Certain materials, like gunpowder contain both the fuel and the oxidizer in one, they just need heat to start burning. You can fire a bullet underwater or in outer space, no air required.

So if you make a fuse literally out of gunpowder (yes, they do have these) it can burn underwater if you make it right (there are other problems with burning underwater like cooling the flame).

Safety fuses, invented in the early 1800s, consist of a core of gunpowder wrapped in some sort of waterproof material, originally something called jute rope, but later replaced with other more reliable materials. Gunpowder contains it’s own oxidizer, so it will burn underwater so long as it stays dry.

Fire requires three things to burn, often called the *fire triangle*. You’ll often hear this phrased as “air, fuel, and heat”. Remove any one of the three, and the fire goes out.

The thing is, “air” is a bit of an oversimpilification. What fire really needs is an *oxidizer*: something to fuel the chemical process called *oxidation* characteristic of burning. As the name implies, most of the time the oxidizer is oxygen, and most of the time that comes from the air.

But the oxygen doesn’t *have to* come from the air. As long as a suitable oxidizer is available (and there’s enough fuel and heat to start or sustain the reaction), other substances can be used. Most common fuses actually infuse the cord with an oxidizer to make it more reliable: the fuse burns in a controlled way and at a predictable rate, and outside factors like moisture won’t affect it. This is considered an important safety feature: you can control how long the fuse will burn by measuring out a specific length, and it’s very hard to make it burn any faster or slower.

The fuse needs oxygen to burn. If you make the fuse with some ingredient that can provide that oxygen (like rust, which is iron and oxygen), you can make a fuse that doesn’t need to rely on oxygen from the surrounding air to burn, and it can burn underwater.

You also want to make sure that the thing that contains the explosive is waterproof too, such as a plastic tube or paper dipped in wax, so that the explosive material will explode. Water-logged dynamite does not go boom, for example.