Prematures

[Wigeon]

"Treatise on Ammunition, 1915." pages 250 and 251 discusses possible causes of premature in shells. A copy is available online  at the Internet Archive.  

During 1918 some 18 pounder Shrapnel ammunition prompted an alert when it was discovered that they had been issued with fuze sockets made of cast iron  rather than brass  and was liable to   blinds and prematures, due to failure of the socket. Source is WO 95-4002-6 War Diary Chief Ordnance Officer Ammunition South June 1918.

Examples of prematures and their probable cause taken from  WD for  Assistant Director Ordnance Service, Canadian Corps for the first half of 1918 

                                          4.5 inch How - bore obstruction.

                                          60 pounder- defective fuze spindle allowing detonator to function on gun firing.         

                                          9,45 inch Mortar - faulty bomb affected by the flash from the propelling charge.

                                          Stokes Mortar - overly sensitive detonator or detonator separating from fuze.

                                          Stokes Mortar - poor crimping allowing detonator to part company with the safety fuse component.

                                          No 28 Mk1  -   defective time fuze or too short a length.

 

 

 

 

                                              

[Matlock1418]

Wigeon,

Welcome to GWF

On 15/07/2020 at 03:53, Wigeon said:

"Treatise on Ammunition, 1915." pages 250 and 251 .... etc.

Thank you for your interesting input into my thread - something to look out.

The line that especially caught my eye in your post as rather different from the rest was: "4.5 inch How - bore obstruction" = ???

:-) M

[Wigeon]

Cringe time for my mistake regarding the  4 point 5 How incident, a less-hasty reading of the WD entry shows that while it was investigated as a premature, and  a gunner stated the shell exploded a foot outside the muzzle,  the  projectile did not go off near the gun and the investigating officer concluded the damage to the gun was caused by a gas build-up due to an obstruction. Apologies for that and sorry about the delay in replying, spending way too much time researching British anti-tank technology and tactics during the Great War

Regards,

Wigeon

[RaySearching]

Prematures

Dai Bach y Sowldiwr

is on hand to advise

[Matlock1418]

On 16/07/2020 at 10:18, Matlock1418 said:

The line that especially caught my eye in your post as rather different from the rest was: "4.5 inch How - bore obstruction" = ???

18 hours ago, Wigeon said:

regarding the  4 point 5 How incident, a less-hasty reading of the WD entry shows that while it was investigated as a premature, and  a gunner stated the shell exploded a foot outside the muzzle,  the  projectile did not go off near the gun and the investigating officer concluded the damage to the gun was caused by a gas build-up due to an obstruction.

Wigeon,

Thanks for your re-read and reply.

That clears that one up - actually an obstruction.

However could be also of interest as to the particular incident and the sort of consequence

Was the cause / type obstruction identified?

And what was the type damage / level of damage [and/or possible injury(ies)] caused?

Being a gunner was certainly not without its own equipment-specific risks.

Hope your other A-T research goes well.

:-) M

Edited 5 August , 2020 by Matlock1418

[Wigeon]

Hi,

A slightly edited version of the WD entry reads, 

COA  " investigated the premature which occurred on 6-4-18 in 4.5inch How.... No damage was done either to any of the gun crew or to the emplacement. He visited.... and saw the howitzer in question. The bore had split in two places at the muzzle, one split being about 11 inches long and the other about 3 and a half inches long.

           It was not thought that the damage to the piece was caused by faulty ammunition. Corps Ammunition Officer was informed.... and the Battery Commander stated in his report that the shell burst about one foot from the muzzle. If the shell had burst as stated the muzzle would most probably have been blown off and damage also done to the gun crew and emplacement. The cause was  probably some slight obstruction in the bore near the muzzle which impeded the flight of the shell, causing the accumulation of gases which split the bore."

          WD entry  is for 9th April and is available on the Library and Archives Canada website. 

          I do have figures for the premature rate compared to number of rounds fired for 18 pounder and 4.5 How for Spring 1917, if you are interested I can post them.

Regards,

Wigeon

[Matlock1418]

6 hours ago, Wigeon said:

I do have figures for the premature rate compared to number of rounds fired for 18 pounder and 4.5 How for Spring 1917, if you are interested I can post them.

Thanks and yes please.

:-) M

[Wigeon]

Hi,

the following is an extract from a letter sent by Haig to the Army Council dated 25 April 1917 which was copied to Southern COO(Ammunition) and enclosed in the latter's WD, National Archive ref WO 95/4000.

" The actual number of prematures in  the 18 pr gun during the period April 1st - April 15th was four, which worked out at one premature to every 317, 000 rounds of HE fired. During last December the proportion was one premature in every 19, 000 rounds. The corresponding figure for the 4.5 in Howitzer during the same period is 7 prematures, or one in 106, 000 rounds. In January last the proportion was one premature in 18, 000 rounds."

Wigeon.

 

[Matlock1418]

15 hours ago, Wigeon said:

" The actual number of prematures in  the 18 pr gun during the period April 1st - April 15th was four, which worked out at one premature to every 317, 000 rounds of HE fired.

During last December the proportion was one premature in every 19, 000 rounds.

 

The corresponding figure for the 4.5 in Howitzer during the same period is 7 prematures, or one in 106, 000 rounds.

In January last the proportion was one premature in 18, 000 rounds."

Many thanks.

I guess a very date-specific pair of examples, but very interesting and striking comparisons.

Looks like April 1917 was seemingly a lot better than 3-4 months previously, especially for the 18 pdr = c.1/16th the earlier

Good to see in hard figures.

I bet the were some rather pleased people in the RA.

Wonder if there is a wider period set(s) of figures to, for example, allow comparison with the hastily increased manufacturing required by the 1915 shell crisis and also even later when production might have settled down a bit.

I wonder if bad & better performance [i.e. quality] had any effect on manufacturers' received payments [i.e. penalties & bonuses]??

Interesting ...

:-) M

[Rod Burgess]

Apologies if I'm teaching my grandmother to suck eggs...

Artillery prematures

In order for a well-designed artillery impact or graze fuze to trigger, four actions are required and they must occur in the correct order.

• Set back

• Spin

• Creep forward

• Set forward

Set back occurs when the round is given a hefty thump from behind, at the instant the gun is fired and the shell starts to move forward. In the fuze, this causes a mass of some sort to ‘lag behind’ as a result of its inertia, which in turn allows another component of some sort to move under the influence of spin.

Spin occurs as the shell starts its way up the bore, gripped by the barrel’s rifling. Having been unlocked by the set back action, a mass ‘lags behind’ the spin of the shell and, in effect, turns relative to the fuze body. This unlocks the next action.

Once the shell has left the muzzle, it stops accelerating and starts decelerating under the influence of air resistance and gravity. This allows a spring of some sort to start to push the ‘setback’ mass forward again, albeit more slowly. Once the mass has crept forward it unlocks the final action.

Set forward is what happens when the shell strikes and rapidly decelerates. A mass within the fuze (often the same one as operated under set back and creep forward) jerks violently forward relative to the fuze body, under the influence of its own momentum, triggering the fuze to explode the shell.

In the case of shells intended to explode in the air rather than on contact, (shrapnel, smoke, star and possibly chemical), set forward will not occur and instead a time delay of some sort will trigger the fuze, provided the first three actions have occurred. The delay might be clockwork or possibly by the combustion of a powder train or match etc.

All of that mechanism has to be designed and constructed to incredibly fine tolerances to withstand the colossal forces encountered during the firing and flight of the shell. Clockwork time fuzes in particular are very intricate devices and, with a shell travelling at many hundreds of feet per second, have to be accurate to within milliseconds over the time of flight of the shell (ie from a few seconds to a minute or so for the very longest-range weapons).

A certain number of fuzes, especially time fuzes, so commonly used in WW1, will contain minute flaws in design or construction and will be prone to complete failure to set the shell off (a ‘dud’) or, possibly worse, premature operation causing an early explosion of the shell. Given the four actions described, you can see how artillery shell prematures can occur at different times in the shell’s short, but exciting journey from chamber to target. For example, if some internal fault has unlocked the ‘set forward’ action, there is a likelihood that the shell will explode as soon as it senses ‘creep forward’, ie shortly after leaving the muzzle – a so-called ‘muzzle premature’. Similarly, if poor manufacture or transport damage has released the latches on ‘spin’ and/or ‘set forward’, the shock of firing may trigger the fuze inside the barrel – a ‘bore premature’. Such failures are not unknown even with today’s improved materials and manufacturing techniques: in the rapidly-increased munitions industries of WW1, they were far more common. I have seen two barrels of the relatively modern M107 175 mm gun which were catastrophically damaged by muzzle prematures, one being peeled open like a banana for about the last metre of its length.

If a time fuze relied on a burning powder train, it was possible for the burn to be slower or faster than standard, causing early or late activation. With adjustable time fuzes, there is another possibility – incorrect setting of the fuze. When the time of flight for a given target on a given shoot had been determined, the fuze setting in seconds was calculated (or read of the gun’s fuze indicator). A tool called a fuze setting key was then placed over the fuze at the nose or base of the shell so that lugs on the tool engaged in holes in the fuze. The key was turned, winding the clockwork mechanism, the key removed and the shell loaded and fired. Any error in calculating or reading the required setting or in actually applying it to the fuze would result in explosion of the shell earlier or later than intended, or even not at all. Imagine doing that in the heat of combat, possibly in driving rain and or darkness.

I took part in a night firepower demonstration in about 1980 in which my battery of eight M109A1 155 mm guns were firing on a target which was to be illuminated by a single Abbot 105 mm on a different gun position. The computers handled the sums so that the Abbot’s illuminating round would go off, after the M109s had fired but sufficiently before their shells arrived, in just the right place for me, as observer, and a grandstand full of NATO brass-hats to see the results. The 105 mm round had a clockwork time fuze. The calculated time-setting was applied on the fuze key and the fuze was set by simply placing the key over the it and turning until a stop was reached, thus ensuring accurate repeatability. Unknown to us, an inexperienced, new and very chunky gunner was setting the fuzes. When I gave the order to fire no illuminating round appeared and Range Safety immediately called “Check Firing!” It seemed that the incredibly bright illuminator had gone off immediately after leaving the gun and everyone was staggering about temporarily blinded by the glare. Gunner Bloggs had turned the key the wrong way, using brute force to break the delicate fuze mechanism and unwittingly arming the shell to operate as soon as it was fired. A rapid enquiry failed to discover the problem so, after about 20 minutes, they tried again with the same result! Mis-setting a clockwork time fuze is an easy mistake to make but hat Abbot battery had to buy a lot of beer to keep us quiet.

[Matlock1418]

1 hour ago, Rod Burgess said:

Apologies

Absolutely no need for an apology.

Thank you for a really great, interesting and illuminating explanation on fuses and how a premature might arise - Just what was needed by me.

Very much appreciated.

:-) M

[MikB]

In APHEI rounds of my acquaintance, a light volute coil spring used spin to migrate outwards and clear the path for movement of the set-forward firing pin, and also gyroscopically to align a detonator ball's primer and quick-action fuze between pin and the septum covering the gaine. This ball was offset at an angle in the unfired round. There was a locking C-washer holding it in that attitiude until setback on firing released it.

 

The idea was that on impact the piercing point should crush and initiate an incendiary/oxidiser pellet fitted inside the aluminium ballistic cap, then pierce ballistic cap and armour and draw the resulting mass of burning thermite through the hole after it. Meanwhile the set-forward firing pin has hit the detonator cap on deceleration, initiated the ball's quick-action fuze to fire the nitrocellulose gaine and thus the RDX bursting charge.

 

This was an aircraft round of the 80s. I don't know if an explosion in environmental testing counts as a premature, but it was in translation of a lengthy report about one such that I came into contact with the design.  If such fuzing systems were in use in WW1, I'd expect they were most likely in naval AP rounds. 

Edited 18 August , 2020 by MikB

[Rod Burgess]

Those of you who are ex IGs or Ack IGs or Explosive Ordnance Technicians will have spotted that I greatly over-simplified my description to illustrate the basic principles of set back, spin, creep forward and set forward actions in fuzes. The designers of fuzes are clever people and have found numerous devious and ingenious ways to make safe and arm their devices but the basic ideas remain pretty much unchanged - until we get to clever stuff like proximity fuzes.

[Matlock1418]

1 hour ago, Rod Burgess said:

I greatly over-simplified my description to illustrate the basic principles of set back, spin, creep forward and set forward actions in fuzes.

And that's the way we, who are not ex IGs or Ack IGs [whatever those are??] or Explosive Ordnance Technicians, like it!

Some others' posts are are sometimes almost unintelligible - a different land/different language

Thanks for bearing us ordinary folks in mind Rod.

:-) M

[Rod Burgess]

Instructor in Gunnery and Assistant Instructor in Gunnery (the former are officers and the latter very experienced warrant officers) both of whom do a year-long course these days at the School of Artillery). I was neither of the above: I just knew which way to point it and which string to pull!

[Matlock1418]

 

18 minutes ago, Rod Burgess said:

I just knew which way to point it and which string to pull!

And at the date you mentioned before I knew a few who would be / were grateful that it went in the right direction and ended up in the right place for them.

Thanks.

;-) :-) M

Edited 18 August , 2020 by Matlock1418
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