Japanese ww2 'adiabatic' fuzes for 20mm shells

[msxyz]

I recently read here http://www.ammunitionpages.com/download/130/japanese about a series of fuzes, employed by the Japanese, for their 20mm autocannon rounds, which didn't use the usual striker but relied solely on a column of hot air to fire up the lead azide capsule.

Does anybody have more info or actual pictures of these fuzes?

It seems to me quite a dangerous and not very efficient fuze. Dangerous because there is no safety mechanism. The starting capsule is practically exposed on the nose of the shell at the end of a very short passage. Not very efficient because the 'air piston' striking the capsule solely depends on the shell hitting head on something that is hard enough to flatten the head of the fuze squishing the air in it. Its only advantage to me seems that is cheaper to produce as it doesn't need a lot of precision machined parts. Was it a 'last ditch' design employed in the last months of war?

 

[A.G. Williams]

I recently read here http://www.ammunitionpages.com/download/130/japanese about a series of fuzes, employed by the Japanese, for their 20mm autocannon rounds, which didn't use the usual striker but relied solely on a column of hot air to fire up the lead azide capsule.

Does anybody have more info or actual pictures of these fuzes?

It seems to me quite a dangerous and not very efficient fuze. Dangerous because there is no safety mechanism. The starting capsule is practically exposed on the nose of the shell at the end of a very short passage. Not very efficient because the 'air piston' striking the capsule solely depends on the shell hitting head on something that is hard enough to flatten the head of the fuze squishing the air in it. Its only advantage to me seems that is cheaper to produce as it doesn't need a lot of precision machined parts. Was it a 'last ditch' design employed in the last months of war?

The RAF used such "air column" fuzes as well, both during WW2 in the 20mm Hispano and postwar in early 30mm ADENs. The RN also used the Hispano fuzes in 20mm Oerlikon ammunition. The 20mm air-column fuzes were No. 253, 254 and 258; these replaced the original No. 252 family of strikered fuzes.

Simplicity to aid rapid production in wartime was certainly a factor, but I don't know how the performance compared. I think that the air-column fuze designs may have been easier to adjust for the delay-time required, by changing the shape of the air column.

 

[msxyz]

Interesting. I didn't know this design was used extensively also by the British. Still, I'd like to know more about its safety and efficiency. A fuze design like this also doesn't take into account that every shot that misses its target is going to fall back on earth somewhere. Many contemporary German fuzes even had self destruct mechanism despite being so small! Thanks for the info. I'll try to search if there are more info about this similar British designs.

 

[A.G. Williams]

I don't think that the air column fuzes were particularly dangerous - to act, the air column had to be compressed extremely rapidly; in other words, the shell needed to hit at very high velocity to function, it wouldn't go off just from being dropped onto a concrete floor. Somewhat worse than this, I suspect, were the Japanese fuseless HE rounds which just had a thin metal nose filled with PETN...

You are right that self-destruct mechanisms were useful in certain situations but this didn't have to be achieved by the fuze. The simplest form of SD was to fit a tracer in the base, linked to the HE by a narrow channel. The shell exploded as the tracer burned out.

 

[msxyz]

I found a low res image of a No.254 British fuze:

The air pocket in front of the primer capsule is visible. Behind it, the usual threaded cylinder acting as anvil and containing a small booster charge. This fuze is indeed very simple to manufacture and it's probably made of a handful of parts. Compared to some intricate German designs with dual (stetback+centrifugal) safety system and mechanical timed self destruction, this must really be much cheaper and easier to manufacture!

...still not fully convinced about its efficiency though I suspect a fuze like this can punch through the aircraft skin with ease and even if the front cap that closes the channel is made to rupture and collapse (acting like a syringe piston) I suspect that the shell has to hit a strut or some other structural part of the plane before exploding. On a side note, I remember an interview with an Italian aviator who used to fly on SM.79s (the 'hunchback' bomber that caused so much trouble in the Mediterranean theater to Allied forces) boasting that their planes were not affected by British cannon fire as the shells would simply go through them without exploding. The SM.79 had a steel tubular structure covered with waterproof fabric.

 

[A.G. Williams]

On checking, the Soviet 20mm ShVAK also made at least some use of air column fuzes.

 

[Greyman]

Interestingly (and probably unsurprising) it was the French that figured out the Hispano fuze functioned with the entire striker mechanism omitted. The British 253 strikerless fuze was developed based on these 1939 trials.

The main reason for the switch was the over-sensitivity of the original French/No.252 fuze.

The US Mk.26 fuze -- which was a copy of the British No.253 -- could "withstand a 40 foot drop on to armor plate without detonation", would not detonate on water impact, and would function on 3mm steel.

The sensitivity was increased slightly in the No.253 Mk.II due to the Mk.I not detonating at air fighting ranges of over about 300-350 yards. I could be wrong on that figure, going off memory.

 

[Greyman]

Found a relevant bit:

With the No.253 Mk.I fuze, the shell will not detonate on the skin of an aircraft, but will detonate on structure, including self-sealing tanks; a 4" to 8" hole being blasted in the substance against which the shell detonates up to ranges of 400 yards. It will blast a hole in 7mm of plate if it detonates thereon. This fuze gives its best performance at ranges from 0 - 250 yards.

- Gun Sub Committee​

 

[OldParts]

Greyman, would you share a reference? I am always looking for more technical information on weapons and engines. Thanks!

 

[elbmc1969]

Interestingly (and probably unsurprising) it was the French that figured out the Hispano fuze functioned with the entire striker mechanism omitted. The British 253 strikerless fuze was developed based on these 1939 trials.

The main reason for the switch was the over-sensitivity of the original French/No.252 fuze.

The sensitivity of the French fuse was intentional: they were worried about cannon shells passing straight through fabric-skinned bombers without effect. Unfortunately, they ended up a bit behind the times when the Germans showed up with all-metal aircraft and all of the cannon shells scored surface detonations ...

 

[A.G. Williams]

The sensitivity of the French fuse was intentional: they were worried about cannon shells passing straight through fabric-skinned bombers without effect. Unfortunately, they ended up a bit behind the times when the Germans showed up with all-metal aircraft and all of the cannon shells scored surface detonations ...

The RAF were aware of this problem, which made the original HE shell with a strikered fuze relatively ineffective. Ultimately, this was solved with the air-column fuze in the HEI shell (the incendiary element added to its destructiveness) but in the short term, the solution was to fire "ball" (target practice) rounds which had inert projectiles. The destructive effect of 129-gram 20mm high-velocity projectiles was quite impressive even without any explosive filling.

 

[Greyman]

Greyman, would you share a reference? I am always looking for more technical information on weapons and engines. Thanks!

Sorry, most of my material is in my big mess of a stash that's been accumulated over the years. Most of it not readily available for uploading / online viewing. That said the portion about the US fuze is from "U.S. Navy Projectiles and Fuzes" and I see a copy is online here: U.S. Explosive Ordnance (link is straight to the Mk.26 fuze page).

The sensitivity of the French fuse was intentional: they were worried about cannon shells passing straight through fabric-skinned bombers without effect.

Indeed -- I have a 1937 British report on the Hotchkiss 37-mm and 25-mm AA guns, and it was explicitly pointed out that each would function successfully on fabric and that a penetration depth of less than 1-mm would occur before detonation.

 

[OldParts]

I found a reference web site called BulletPicker. If you search for PD, No. 253 Mk 1 - 3 then you should arrive at: PD, No. 253 Mk 1 - 3 and a diagram for the fuze and a diagram of the US Cartridge, 20mm HEI, Mk 1 which is stated to be almost the same as the British cartridge (no sure what is meant by the comparison). There are also references to several very old ordnance manuals covering shells up to 40mm. I also found a manual printed around 1948 that included the results of tests to determine the probability of defeating aircraft fuel tanks including both petrol and JP-3 with incendiary shells at: https://apps.dtic.mil/dtic/tr/fulltext/u2/a800109.pdf Very complicated subject but the observational instrumentation available was very limited compared to what I am sure is available today.

The tests included self-sealing tanks. BulletPicker has links to many, many ordnance manuals. It is for those interested in history.

 

[A.G. Williams]

I don't normally pay much attention to fuzes, but I've just checked my collection and discovered a 40mm Bofors L/60 round with what appears to be an air-column fuze - at least, the appearance is the same as a fuze of this type for a 30mm Aden LV.

 

[msxyz]

Interesting. Can you please post a picture of it?

 

[bdefen]

Great new book about the Allied development of the anti-aircraft proximity fuse:

12 Seconds of Silence — Jamie Holmes (jamieholmesbooks.com)

 

[A.G. Williams]

Interesting. Can you please post a picture of it?

Two pictures of the 30mm Aden LV (30x 86B) with a Type 914 air compression fuze. The very similar Bofors to follow....

 

[msxyz]

Much appreciated, thanks

 

[A.G. Williams]

Bofors 40mm L/60 with what appears to be an air compression fuze:

 

[A.G. Williams]

Bofors 40mm L/60 with what appears to be an air compression fuze:

View attachment 602587

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I have posted these photos on an ammunition forum with lots of fuze experts, who tell me that it is a standard strikered No. 250 or 251 fuze with the components removed and a hole drilled through it. The Aden is genuine, though.