Large caliber artillery shells.

[tommywishbone]

When large caliber explosive shells are launched from, let's say, 105mm up to 16" naval artillery, why do the explosive shells not explode when the gun is fired? With the incredible force of the blast & the huge amount of fire/heat, it seems that the shells would simply blow-up when you pulled the "trigger". Perhaps it does happen sometimes, but it is so rare it does not get reported?

 

[CycloWizard]

Just a guess, but I'd say it's because the shells are designed so that only a frontal impact (on the curved part of the shell) can cause an explosion. The back part, where the explosion occurs, is probably very thick to prevent pressure transmission. I'm not sure what kind of explosives are used in these shells to say for certain what the ignition mechanism is though, so I could be wrong.

 

[sao123]

Originally posted by: tommywishbone
When large caliber explosive shells are launched from, let's say, 105mm up to 16" naval artillery, why do the explosive shells not explode when the gun is fired? With the incredible force of the blast & the huge amount of fire, it seems that the shells would simple blow-up when you pull the "trigger." Perhaps it does happen sometimes, but it is so rare it does not get reported?

because unless they are special explosive rounds...most artillary are just that.
Large armor piercing metal balls which are shot at high velocity. No different than the same shells which u shoot out of a normal firearm, just made out of a better metal.

If indeed they are explosive rounds they would operate much like grenades. They dont explode until the pin is pulled (Ie shot).

 

[dkozloski]

Artillary shells are filled with high explosive material such as TNT. This material requires a very intense detonation to initiate an explosion and is very safe. This is accomplished with the fuse. Part of the operation of the fuse is to prevent an unintended explosion near the gun crew so the fuse is rendered safe either on the basis of time or the number of rotations of the projectile imparted by the rifling. There are a multitude of designs meant for different purposes. Heavily constructed shells are used for armor piercing. Thinner shells with more explosive instead of steel casing are for bursting efffect. What is really remarkable is that during WWII fuses were developed that were working radars that exploded the shells when they approached aircraft. The Navy refers to them as VT or variable time. Other shells have timed fuses which are called a cut fuse. Probably a holdover from the days when the fuses were actually cut to different lengths to change the burn time. Timed fuses were in use nearly 200 years ago. If memory serves me correctly the actual acceleration rate of an artillary projectile is about 1200 G.s. The explosion of a round in the barrel is very rare and usually is the result of a dud that remains long enough in a hot barrel to "cook off."

 

[Alexstarfire]

They could always use a substance like C4 in it. You know, something that is stable until an electrical charge, or something of the sort, is introduced. Just a guess, but that's not likely.

 

[DrPizza]

Perhaps because the forces are lower during firing?
keeping it really simple,
change in momentum = impulse = F*t
when it strikes the ground, the time to deccelerate is much shorter (therefore the force is much greater) than when it is fired.
When it is fired, it accelerates over the length of the barrel from the expanding gasses. (Still, a very short amount of time, but more time than when it strikes the object)

Just for a rough idea, let's ignore air resistance and assume that it strikes the object with the same velocity as it has when it leaves the barrel. It goes from that velocity to 0 over a very short distance, say a few centimeters as the object deforms and the shell deforms somewhat during the impact. At the other end, they've got a nice long barrel, so it's accelerating along a distance of a couple meters, maybe 50 times longer than the distance it travels during impact, so the force at impact will be roughly 50 times greater.

Of course, I know nothing about these shells, so I may be wayyy wrong, but it's an idea

I haven't got a clue how they develop those bombs that penetrate 2 layers of concrete to hit deep bunkers.

Here's another example to make it very easy to envision:
Needed: 1 waterballoon, 1 waterballoon slingshot, 1 brick wall.
Fire the balloon... they usually don't break when being fired. Will break when it hits the wall.

2nd experiment: make a very simple rocket by putting fins on a 2-liter bottle, filling it up 1/2 way with water, and pressurizing it to about 100psi. (you would need one of those launchers that are easy to make or can be found at several science supply places). Place egg on top. Fire rocket. The egg will pretty much explode during the first small fraction of a second, almost as if it had been thrown downward and hit the bottle squarely. It can't handle that much acceleration. (IIRC, roughly 100 g's)

 

[Vee]

dkozloski already answered this question. Explosives which are sufficiently insensitive are used. The sensitivity of an explosive is a rather complex issue. To get some grip on it it's common to divide it into impact sensitivity, friction sensitivity, heat sensitivity and chock sensitivity. In order to endure high g an explosive need to have a good measure of impact insensitivity and a combination of integral structural strength and friction insensitivity. The problem is also dependant on the size and mass of the charge. The more sensitive explosive in the fuse manages high g by being very small.

C4 is not suitable for shells.

The first "modern military explosive" that was successful in quantity and possible to use in artillery shells was Lyddite. Lyddite was a mix of TNP, MNB and petroleum jelly. It is a slightly stronger explosive than TNT and was used in huge quantities in WW1. Before the advent of the TNP based explosives around 1900 the military was restricted to wet guncotton and gunpowder.

As drPizza deduced, the problem of getting the explosive to survive smashing through heavy naval armour plate and bunker concrete is more serious. In the early days at least four different solutions were developed.

The Germans took immediate advantage of the newly invented TNT and used Tritol (almost pure TNT with some DNT). The British evolved Lyddite into Shellite by adding 30% DNP. The Japanese simply padded their armourpiercing shells internally with a crash cushion for their Type 91 explosive (based on TNA). USA finally, saw the invention of "Explosive D" aka Dunnite. Explosive D is a salt. More precisely ammonium picrate, the ammonium salt of TNP.

AFAIK Explosive D is the most impact insensitive explosive ever to have been used in quantity. I'm also fairly certain Explosive D was used in the navy's armour piercing 16 inch shells to the very end. (as well as 14 and 8 inch).

But otherwise, since WW1 to recent days, TNT has mostly been the key desensitizing ingredient in many families of military explosives. I believe Hexotol, Cyclotol, Comp B, and Octol can be used in shells and be fired out of gun barrels. They all contain an amount of TNT. The TNT provides integral structural strength for the explosive and also works as a desensitizer.

Today some other solutions are being tried. DADNE and DNAN, serving the same purpose as TNT in formulations. Or, another type of explosive compound, the "PBX" - plastic bonded explosive. In these types of explosives, the mechanical strength and reduction in sensitivity is provided by some special plastic binder.

 

[dkozloski]

It's interesting to note that if the fuse fails to opertate, no matter what the shell hits, it will be a dud. As a general rule, shells meant for air burst or blast effect will have nose fuses. Armor piercing shells will have fuses in the base to protect them from damage until the shell penetrates as far as it is going to go before it detonates. Becoming familiar with all the configurations and designs of artillary shells is a career in itself.
Vee, the 16" shells I was schoooled on were loaded with TNT like most of the rest of the Navy's projectiles. The stuff would ooze out of stored shells, producing TNT exudate, and created a cleanup problem.
Another interesting sidenote. When an ammunition ship blew up in the Sacramento river near Port Chicago, California during WWll the blast sent artillary shells further than they would go if fired through the correct barrel. It seems to me that some were found clear down by Mt. View and Palo Alto nearly 20 years later. At least the EOD teams were sent down there to recover them.