Shells and triggers

[Nathan Greenfield]

On the 24 April 1915, at 2nd Ypres, Major V. Odlum watched as a shell landed amongst a group of soldiers that included Odlum's brother. According to Odlum's report his brother "simply ceased to exist."

I would like to know the sequence by which a shell actually explodes. Obviously, the trigger in the nose has to be depressed. But what happens next. Torpedoes, for example, explode because a circuit is completed that sends a current down a cellonoid that starts the main explosion. I know there is no electric source in shells, so it has to be physical/chemical.

Thanks in advance.

Nathan

[Max Poilu]

This is with reference to percussion fuzed shells and grenades (as opposed to time/percussion). These fuzes are very simple indeed, much of the 'innards' is concerned with making sure the fuze is safe to handle and load and only armed once the ordnance is launched.

In most cases there is a movable piece of relatively substantial mass held away from the percussion cap or detonator by a creep spring or other safety mechanism. This is enough to make it stable in flight but as soon as the shell's velocity is checked by the ground or another object the inertia causes the striker to be pushed back initiating the detonator (then sometimes a small first charge) nd in turn the main charge. Simple really!

[Guest]

High Explosive Shell

In a high explosive shell, the main filling is high explosive, which causes the shell to fragment into splinters. The effects of the fragmentation are far more lethal, over a wider area, than the blast from the filling.

In high explosive, there is a detonation in which a shock wave disrupts the molecular structure of the explosive, unlike low explosive (gunpowder, cordite, etc.) where the explosion is causedf by rapid combustion.

If we take TNT as an example of a main filling, it is powerful but not very sensitive. This means that if you fired a TNT shell without a fuze, it would not explode at the other end. Similarly, you can burn a block of TNT without it exploding (usually that is - still best to take cover).

There is a measurement called the "figure of insentitivity" (F of I) and on this scale, high explosive fillings have a figure of 100 or over. TNT is about 120.

High explosives used as detonators, however, are very sensitive (F of I 20 or less). In modern artillery fuzes, detonators usually consist of lead azide, but I am not sure that this was so common in the Great War and there are a number of others.

You can cause detonators to detonate by percussion or ignition. Electric detonators in effect cause heat and detonation is the result of ignition. In standard artillery fuzes, detonation is caused by percussion, where a striker is driven into the detonator. This can be achieved in various ways, using either deceleration forces, or the hammer effect of the nose of the fuze striking a hard surface (but problems may arise in muddy conditions).

Much of the internal mechanism of fuzes is either to prevent accidental or premature detonation, or to prevent it detonating the main filling if it happens. This is achieved by shutter mechanisms, etc. which are operated by the forces acting on the fuze following firing and during the early part of its flight.

Between the small detonator and the main filling, there is what is sometimes called an exploder, gaine, or booster. This may be part of the fuze and/or a separate component. This is somewhere midway between the high sensitivity of the detonator and the low sensitivity of the main filling. In effect, it boosts the power of the detonating wave to a level sufficient to detonate the filling.

Combustion time fuzes, were generally unsuitable for HE, so airbursts we not common at the time, although they are far deadlier. During WWII, the Germans in particular made good use of mechanical time fuzes and the allies developed proximity fuzes, originally for AAA, but used by field artillery in small quantities from late 1944.

Shrapnel

Shrapnel shell made use of combustion time fuzes and a low explosive charge (gunpowder) and did not replicate the blast effect or fragmentation of HE.

[Nathan Greenfield]

Thanks.

Since Odlum mentions only his brother as being killed, it seems reasonable to assume that the shell was high explosive rather than shrapnel.

Does anyone the temperature of the blast bubble?

Cheers,

Nathan

[Max Poilu]

Shrapnel shells are designed to explode in the air above the target not at ground level. Plenty of stories from the Great War of HE shells landing amongst a group of men - some men being blown to kingdom come while others remained unscathed.

If you have a search for 'shrapnel' or 'shell' there is a wealth of info from previous discussions that will be of interest.

Detonators used in the Great War utilised fulminate of mercury. Again a search will bring lots of info.

[Guest]

Does anyone the temperature of the blast bubble?

<{POST_SNAPBACK}>

I don't know the answer. Probably bloomin' hot, but cooling rapidly. The fragments certainly are extremely hot and it is a very bad idea to pick them up before they have had time to cool.