Bofors gun fuze destruct delay

[BMG50]

As we know the 40mm AA Bofors gun projectile has two fuzes. One a impact at the front obviously. The other fuze at the base for self destruct. So if the projectile misses the target it explodes at a set time when the base fuze is timed out, so the projectile doesnt travel to the next town or village and disintegrates into a thousand pieces in the local vicinity avioding projectiles becoming live EOD when they land if they fail to go off. I was wondering if these base fuzes had different delay times, so if your target was at 3,000 feet your the base fuze would explode the projectile at this point but if your targets were say 6,000 feet or closer at 2,000 etc they would explode too soon or too late. Were there different delays or just one standard fuze that set off at a certain distance and acted as more of a deterrent to aircraft in the guns lethal range.

 

[Eggburt1969]

Firstly, what county's and what service's 40 mm Bofors high explosive (HE) projectiles (shells) are we taking about? The UK, America, etc...

For the UK, it has a fuze at the front, but the rear item is a tracer and igniter assembly (Tracer and Igniter Shell No. xx), it's not a fuze per sec.

Those listed in the UK's 1949 Anti-Aircraft Ammunition publication, and stated to being used with the 40 mm Q.F. HE shell, are as follows: Tracer and Igniter Shell No. 11; Tracer and Igniter Shell No. 12; and Tracer and Igniter Shell No. 14. Of these most had at either two, or up to three, Marks (Mk.) by 1949.

Going by the UK's aforementioned publication, the various tracer and igniter assemblies had a nominal 6.5 second functional delay. This delay will however vary due to pyrotechnic compositional variation and composition's pressing (force) within the assembly. The time delay will also vary due to atmospheric effects.

 

[BMG50]

I was talking about wartime UK. The base fuse also acted as tracer and burnt out to igniter to exploded the shell. I assume there were tolerances that affected the action of projectile but what was the spec of delay of the three, as you mention they were all at 6.5 sec delay, what distance on average did the projectile explode at give or take conditions. Looking at the delay of 6.5 seconds upon firing ignition of the tracer in the barrel and 6.5 sec distance of igniter in fuze exploding projectile looks like 5,000 to 6,000 feet, not sure.
I thought you could use same type ammo but with different bursting delays, if the aircraft being shot at was close or very far away.

 

[Darkman]

Early Mk 11 & 12 tracer-igniters burnt for 6.5 seconds, but Mk 14 was 12 seconds

 

[BMG50]

Interesting, so would a Bofors crew have two types of ammo ready for short or long range gunnery when the target presented itself or even mix, was this judged by the loader etc. Not sure how it worked. Of course the Proximity fuze was a game changer later on but, so i am told was to big to fit a 40mm Bofors projectile or even a L70. Correct me if i am wrong, trying to find out facts for a talk about gunnery using the Bofors gun, not sure if there was a weapons training pamphlet denoting practice and proceedure.

 

[Eggburt1969]

Early Mk 11 & 12 tracer-igniters burnt for 6.5 seconds, but Mk 14 was 12 seconds

Indeed, all Marks of the Tracer and Igniter Shell No. 11 and Tracer and Igniter Shell No. 12 in the 1949 UK book are listed as nominally 6.5 s.

The book only lists Marks 1 to 3 for the No. 11, and only Mk. 1 and Mk. 2 for the No. 12, so that's all I've got data on.

For some reason I misread the No. 14 details and for Marks 1 to 3 listed, which are listed in the book, the nominal time delay is 12 s.

Interesting, so would a Bofors crew have two types of ammo ready for short or long range gunnery when the target presented itself or even mix, was this judged by the loader etc. Not sure how it worked. Of course the Proximity fuze was a game changer later on but, so i am told was to big to fit a 40mm Bofors projectile or even a L70. Correct me if i am wrong, trying to find out facts for a talk about gunnery using the Bofors gun, not sure if there was a weapons training pamphlet denoting practice and proceedure.

No idea about having a mix of SD times, and hence ammo with No. 11/No. 12, and some No. 14 for longer ranges. The proximity fuzes during WW2 were pretty large, probably due to the old analogue electronics within them, and these components requiring quite large reserve batteries, this all took up space.

 

[tankbarrell]

As I understand it, the tracer igniter was simply intended to show trace and destroy the shell whilst still in the air to prevent damage to friendlies. It was not meant to detonate the shell in the hope of destroying the target.
The nose fuze was an impact fuze and that is how the target was supposed to be engaged.

 

[BMG50]

As I understand it, the tracer igniter was simply intended to show trace and destroy the shell whilst still in the air to prevent damage to friendlies. It was not meant to detonate the shell in the hope of destroying the target.
The nose fuze was an impact fuze and that is how the target was supposed to be engaged.

The thing about the nose fuze is, you have to be very very lucky to hit an aircraft target, an exploding shell in the vicinity of the aircraft would give you more luck of a strike plus the exploding rounds would be more of a deterrent to the enemy, you wouldn't want to encounter a flak burst. The trace and destroy was the main goal, flak being a bonus. Interesting to find out what the design concept was.

 

[Darkman]

The thing about the nose fuze is, you have to be very very lucky to hit an aircraft target, an exploding shell in the vicinity of the aircraft would give you more luck of a strike plus the exploding rounds would be more of a deterrent to the enemy, you wouldn't want to encounter a flak burst. The trace and destroy was the main goal, flak being a bonus. Interesting to find out what the design concept was.

Self destruct was a secondary aim. 40mm Bofors was impact only to destroy the target.

 

[RoyalPerthRegiment]

Loads were already well-said here. I just wish to add this diagram from British 1946 manual. The Self-destruct time was stenciled on to the shells as a red number + broken circle (in seconds). Therefore a Red-7-circles indicate No. 11 or 12 igniter fitted while Red-12-circle indicates No. 14 igniter fitted. I assume this allows troops to pick which type to use depending on planned engagement distance?

 

[BMG50]

Loads were already well-said here. I just wish to add this diagram from British 1946 manual. The Self-destruct time was stenciled on to the shells as a red number + broken circle (in seconds). Therefore a Red-7-circles indicate No. 11 or 12 igniter fitted while Red-12-circle indicates No. 14 igniter fitted. I assume this allows troops to pick which type to use depending on planned engagement distance?

View attachment 205512

That what i thought the burst all though self destruct was also a deterrent for the incoming enemy aircraft. To hit a moving target relying on the impact fuze was quite a challenge in its self that why flak shells had a better chance. I read that there was a very high percentage of misses actually hitting a moving aircraft.

 

[Bombdoc]

I don't think the tracer/igniter was capable of detonating the filling.. there was no detonator, only a thermal link into the main filling causing the fill to low order and split the shell.

The problem with having a detonating destructor would be that the ammunition would not be safe in storage. The Ordnance Board required the detonator to be separated from the filling by two separate mechanisms until the round armed. A fire in an ammunition store with tracer/igniters that detonated on tracer burnout would result in a shell detonating in storage, which is not acceptable, particularly on board ship.

 

[Eggburt1969]

I don't think the tracer/igniter was capable of detonating the filling.. there was no detonator, only a thermal link into the main filling causing the fill to low order and split the shell.

The problem with having a detonating destructor would be that the ammunition would not be safe in storage. The Ordnance Board required the detonator to be separated from the filling by two separate mechanisms until the round armed. A fire in an ammunition store with tracer/igniters that detonated on tracer burnout would result in a shell detonating in storage, which is not acceptable, particularly on board ship.

Yes, I quite agree. Hence the name 'Tracer and Igniter Shell No. xx', emphasis on the word 'Igniter', not detonator, and not a fuze. Though people keep saying it's a fuze here for some reason?

Though it's not meant to cause detonation of the fill, a deflagration to detonation transition (DDT) can occur. This is generally dependant on the geometry and type of fill, and the confinement of the fill.

The term 'low order' is generally not used any more in technical reference materials, as what causes full detonation, partial detonation, or just deflagration of the fill is more fully and more widely understood now.

 

[AMMOTECHXT]

I found a document online many years ago that gave a hit probability of one in 8,000 for percussion fuzed (i.e. impact / direct action) shell and about ten times less (1 in 800) for VT (proximity) fuzes. The point here is that the proximity fuze has to be within its detection distance of the target. I assume that radar controlled AA shell fitted with proximity fuzes likely increase the chance of a hit. And, as has been said, a tracer / igniter is not a fuze.

 

[BMG50]

I have seen bofors projectiles in documentary film clips fire at the target ie a towed target with the puffs of the exploding projectiles in the back ground. I thought the exploding projectile destructing would act as secondary deterrent, for it not to is a big danger to built up areas miles away, the very reason you should take to a shelter in a air raid.

 

[BMG50]

I don't think the tracer/igniter was capable of detonating the filling.. there was no detonator, only a thermal link into the main filling causing the fill to low order and split the shell.

The problem with having a detonating destructor would be that the ammunition would not be safe in storage. The Ordnance Board required the detonator to be separated from the filling by two separate mechanisms until the round armed. A fire in an ammunition store with tracer/igniters that detonated on tracer burnout would result in a shell detonating in storage, which is not acceptable, particularly on board ship.

Was there a CE pellet with the tracer section as the 2inch mortar doesn't use a detonator to set off the its explosive just a cap and CE pellet, the cap could be set of with heat rather than a strike. As for storage the 2inch mortar is stored with fuze separate fitted with a shipping plug and packed at depo ready for destination with fuze fitted in sealed box.
The same I assume with Bofors rounds stored with fuzes separate and fitted shipped ready for action. Fuzes stored separate. at another building or site. Bombs used by the RAF were stored at dumps collected and fitted with tails and fuzes before loading on aircraft.

 

[Bombdoc]

Was there a CE pellet with the tracer section as the 2inch mortar doesn't use a detonator to set off the its explosive just a cap and CE pellet, the cap could be set of with heat rather than a strike. As for storage the 2inch mortar is stored with fuze separate fitted with a shipping plug and packed at depo ready for destination with fuze fitted in sealed box.
The same I assume with Bofors rounds stored with fuzes separate and fitted shipped ready for action. Fuzes stored separate. at another building or site. Bombs used by the RAF were stored at dumps collected and fitted with tails and fuzes before loading on aircraft.

No.. The 2" Mortar HE was issued fitted with a 161 fuze at the factory. The 2" mortar was used by the lowest level of the Infantry and there is NO WAY these would have been issued plugged to the end user. Plugs would only ever have been used in production between filling and fuzing activities in the filling factory.

The 161 fuze has a shuttered stab detonator which is armed by setback. The fuze also has a stemmed CE exploder. I blew up the very last batch of 2"Mor HE bombs that the UK had in store (for SF use only) about 10 years ago. I have one of the cans on my desk as I type filled with pencils..




Bofors ammunition was also always issued fuzed and ready for use. Light AA is a quick reaction force and needs to get its ammunition quickly. There was an issue with certain 40/70 fuzes in the 1960s which were found to premature on hitting raindrops. As a consequence they were banned for training and the HE rounds were fitted with a PRF (Plug representing fuze) for practice use. This was done by the ammunition depots or the filling factories on an "as needed" basis. War reserve stock was stored fuzed. In practice, only field artillery ammunition was issued to the user plugged and had to be fuzed before firing. This was done for two reasons.. Large calibre shell (over 120mm) was typically stored and transported loose or on pallets. Due to the weight they were fitted with a ring plug to allow the shell to be slung for handling purposes. Fitted fuzes are vulnerable to side swipes and are only used where the shell is stored and moved in a box. Artillery also used to have a choice of impact or delay fuzes which they could fit to meet the operational need, hence the selected fuze was only fitted at the gun position. This has largely been replaced by multi role fuzes which do not need to be changed.

Aircraft bombs in storage are again typically kept plugged and bare, often just stacked in the open air. They are assembled on an "as needed" basis to fit the operation and aircraft type requirement by the bomb dump armourers prior to a mission. Tail units are often light and flimsy and if kept permanently on a bomb body would get damaged in storage. In WW2 aircraft bombs were stored loose and just rolled about by the Stigs (...of the Dump). These days they are fitted to a palletised bracket.

Assembling and fuzing bombs and shell is a potentially risky activity. The general rule is not to let the "end user" do this unless absolutely necessary. During WW2 some short cuts were taken with the manufacture of some components which gave them a short shelf life and which had to be replaced during the life of the ammunition, but this was generally done in ammunition depots. Wherever possible ammunition assembly should be done by trained specialists.