AA Shells Detonation
 

I was looking at some data re WW II bomb damage to a coastal UK town that was not the target for heavy Luftwaffe air raids. I wondered how many of the bomb damaged buildings were actually struck by AA shells that failed to detonate in the air and exploded on impact when they returned to earth ?

Just how reliable were the WW II timed fuzes fitted to heavy anti aircraft shells and if the shell failed to explode in the air, would it explode on impact with the ground ? Ground damage from exploding 3.7", 4.5" or 5.25" shells were unlikely to be reported ,as such, in the local press at that time.

PTTF AA fuses
 

If the fuses on the AA shell were PTTF ( powder train time fuses) which were adjustable for the mount of time till detonation these I believe were later replaced by proximity fuses.
If the powder train which was in a serpentine grove machined into two sections of the fuse body was interrupted there would not be a detonation at the preset altitude and the shell would continue on a ballistic arc till the shell returned to impact the ground where it may or may not explode.
Fuse types are super quick or impact, impact delay, PTTF, proximity or others that I don't know about.
I have a background in military explosives and years as a commercial pyrotechnician.

Aerial bombs are a lot bigger than AA Shells. German bombs HE bombs sizes were 50kg, 250kg (most common type), 500kg, 1000kg and 1800kg. A 3.7 in AA shell was only 28 lbs. (12.7kg), most of which was in the steel.
What type of damage are you talking about? Were there any AA guns positions in the area you’re concerned about? Being a coastal town even if it is not targeted that does not rule out bomb damage. Damaged German bombers would often dump their bombs before flying back over the channel.

John The points you raise are well taken and there is no reason to question the reported bomb damage to buidings that were destroyed during an air raid. I am referring to buildings that sustained damage that left much of the structure still standing. Some published photos show damaged buidings that were not severely damaged, but never a word that it was caused by an AA shell that failed to detonate in the air. The UK town I had in mind had several AA gunsites located quite close to residential housing. It has been reported in one or two articles that the German papers never referred to property damage caused by their flak shells, but military observers knew that this was the case. My question relates to data that must be in the literature, recording the probable percentage of HAA shells that failed to detonate before impacting the ground.( It is too long ago to recall the figure quoted by an IG staff instructor at a AA gunnery course I attended.)

Why be concerned about shells with defective fuzes? Functioning fuzes present just as much of a concern and were the main reason for Air Raid warnings for the local populace.

Depending on the Mark of 3.7" gun, the rate of fire was between 10 to 25 rounds per minute. With each gun of a four gun Battery firing 20 rounds a minute of 28lb shell, the Battery is putting 1 TON of steel up in the air each minute. What goes up must come down. Typical building damage following an Air Raid included broken tile and slate rooves, broken windows, and damage to vehicles and livestock in the open. And that was without the Enemy dropping a single bomb. :eek:

I have some shrapnel pieces of 3.7" shell here, and even after 60 years the edges are razor sharp. If these were to fall on a building from 20,000ft, I'm sure they would shatter brickwork.

I interviewed a transport Sgt many years ago, who told me about watching a dogfight way, way above them. They were in North Africa, and behind the lines. All very interesting until spurts of sand started getting kicked up, and one of his blokes was hit on the helmet with a shell casing. After that it was 'dive for cover under the trucks' time. As even the spent brass could cause minor damage, I expect razor sharp lumps of hot steel would be quite lethal/damaging.

Mike C

The above post was prompted by my questioning(in general ) the reliability of WW II bomb damage reports that are in the archives of many UK municipalities. These data are on record for the town and the surrounding rural area :- Air Raids 85
HE Bombs dropped 567
Incenderary Bombs 12454
Land Mines 13
Other Types 212

Damage to Buildings
Destroyed 350
Major damage 900
Light-Mod damage 14000

It is my understanding that these data were compiled from the local ARP and Police reports recorded at the time. If the figure for the incenderary bombs is correct,there is little recorded evidence that they did much damage to the town itself. It also seems resonable to conclude that much of the light to moderate damage to buildings was due to AA shell shrapnel. as pointed out by Tony Smith.