How does it work, especially the old ammo (WWII and before)?
If you are referring to artillery ammunition then the self destruct function was typically incorporated into the fuze or tracer element.
Examples, greatly simplified:
For Time/Impact fuzes there was typically an adjustable time ring, containing a train of fine gunpowder, integral to the fuze, with markings on the ring that typically corresponded to seconds. The time ring was set by a fuze setter (typically somewhere between a few seconds to 10’s of seconds, depending upon the fuze design) before the projectile was loaded into the gun. Upon firing the gun the powder in the time train was ignited and after burning the set number of seconds the flame from the burning time train had traveled to an opening inside the fuze that allowed it to ignite a booster charge which in turn ignited the main charge in the projectile. If the projectile hit the target before the set time, then an impact element in the fuze ignited the booster charge.
Some projectiles had a base tracer element that contained a passage to the main charge in the projectile, when the tracer completed its burn the main charge was ignited.
Late in WW2 there was the development of the Variable Time (VT) or Proximity Fuze which via internal electronics sent out a radio wave signal, when the projectile neared a target or the ground, a detector in the fuze recognized return signal radio waves bouncing off the target or the ground and ignited the projectile or the fuze may have had a built in time limit so when that time was reached without encountering a target the fuze ignited the charge in the projectile.
Most USN large caliber projectiles were marked on the back to identify the self-destruct feature. Here’s a 3"/50 with a Mk 58 VTF fuze that is Non Self Destruct (NSD). And a 5"/54 with a Mk 59 VTF and the same feature.
I was hoping you would chime in, nice pics!
Before 1945 SD features were were used only by few countries, usually basing on tracers and some German late war designs had mechanical SD fuzes which remained state of the art untill today.
Fuzes containing electronics are a different story then as SD features there are easy to incorporate.
Also the question should specify what type of ammunition is meant. As there are somewhat different systems used in different caliber ranges and other (non bore launched) ammunition.
Firstly, thanks to all answerers !!!
Secondly, I meant large calibre ammo, mostly AA, since it would come back to earth and explode next to its point of origin if projectile had no SD feature and missed their target.
Yes, AA ammo is typically SD. The Luftwaffe also introduced it for home-defence fighter ammo in WW2 to avoid killing their own civilians on the ground.
It is also common to find SD fuzes used with 40mm grenade rounds and similar, to avoid dangerous projectiles being left on the battlefield if the impact fuze doesn’t work.
Isn’t self-destruct a little misleading describing AA ammunition?
Of course, AA ammunition usually had a self-destruct feature to avoid duds lying around; a safety feature.
But my understanding is that AA fuzes like the German S/30 (a clockwork fuze) had the primary purpose to detonate exactly at the height of the aircraft. So the time fuze functionality was to detonate the projectile as close to the target as possible. Allied VT fuzes were the culmination of this concept. Didn’t self destruction only come as a sort of by-product?
By the way, I recall the Mark III pyrotechnic time fuze of the U.S. 3" AA Mark IX projectile being called Scovill fuze. (Not my field of expertise, but I know of it because it served as a basis for the today forgotten G3 drag function.)
I can’t speak for the modern VT fuzes, but the early models, at the end of WW2 through the KW did not have SD features. This was mostly because the design of the fuzes had not progressed to the point where auxilliary devices such as tracers, AuxDet fuzes, etc would not interfere with the operation of the VT fuze itself. Unexploded projectiles falling back to earth or water were one of the hazards of AA.
Mechanical Time (MT) fuzes are by their very nature all SD mechanisms. Others, such as percussion or base detonating fuzes could easily be made SD with the use of a tracer. 40mm fuzes are typical of them.
The US 37mm and 40mm AA rounds had, as bdgreen stated, a burn through tracer which ignited a small wafer of black powder in the base. I always wondered if this was enough to detonate the explosive main charge, usually tetryl. The initials of SD makes us always think “self destruct”, but for these rounds in particular, the term in the publications is “shell destroying”. You may think that the terms would be synonymous, but I learned different after finding several of the 37mm fired AA rounds that looked like they had been subject to a low order detonation, i.e., just split open and the main charge and fuzing gone. Now I knew what the black powder wafer was intended to do. It had enough pressure to open the projectile like a piece of pop-corn but, in most instances, not give it a full detonation. Cheers, Bruce.
The few publications that I have on VT fuses use the term self destruct. A minor point.
Self Destruct or Shell Destroying projectiles can be a two-edged sword. Is it better to have one big piece of steel falling from the sky, or hundreds of small pieces? I think I’d take my chances with one big one. After all, what are the odds?
One big piece of steel that blows up on impact, scattering fragments at high velocity…
I am reminded of an incident a few decades ago when a Royal Navy practice shoot went wrong and a 4.5" shell ended up in someone’s back garden. The RN spokesman reassured the shocked householder that it was nothing to worry about as the shell was an inert practice type. Yup, 55 lb of steel landing at supersonic velocity, nothing at all to worry about… :-D
We had a scary moment in Korea when we were on auxiliary line in just fox holes. We were told they would be firing overhead with 155’s. About half way through the fire mission some puffy clouds drifted by and the proximity fuses started going off. We were peppered by shrapnel cutting several tree limbs off and puncturing a BMG water jacket but luckily nobody was hit, Took about 15 minutes to get the firing stopped. Ever see Marines climb completely inside their helmets?
In my reply for the 37mm and 40mm I stated that for those [u]particular[u] rounds, the acronym “SD” was for “Shell Destroying”. Later fuzes, such as those Ray is speaking about, clearly have in the pubs a “Self Destruct” feature. Wasn’t trying to argue, just trying to point out that, sometimes, the acronyms we use can have two different meanings. Cheers, Bruce.