Shell Fuse Design

[Rockdoc]

I've recently purchased Part X of The Official History of The Ministry of Munitions from the Naval & Military Press and, surprisingly, it's too bad a read. I was expecting something far drier. Inevitably, I suppose, it assumes a certain level of prior knowledge when discussing technical aspects and that has given me trouble in understanding some of the discussion about fuse designs and limitations. I understand that fuses were time-delay adjustable within limits but, as the main part of the timing 'system' was the burning of a stick of fuel (is that a gaine?) I'm not sure how this adjustment was made. Can anyone point me to any illustrations of fuses or a layman's description?

The second puzzle comes on page 22 of Part VI, where there is a discussion on the problems encountered when using standard fuses for anti-aircraft gunnery. It isn't difficult to see how the dynamic air pressure affected the rate of burning and delayed the explosions of the shell but, in the penultimate paragraph, it says that unexpected problems were encountered as the muzzle velocity was increased with the introduction of the 3in 20cwt gun. In the final paragraph it says that this was traced to the rotational speed of the shell and was countered by changing the pitch of the rifling to reduce it. Why would the rotational speed affect burning when the fuse was on the rotational axis and was small in diameter?

Keith

[MikB]

In the final paragraph it says that this was traced to the rotational speed of the shell and was countered by changing the pitch of the rifling to reduce it. Why would the rotational speed affect burning when the fuse was on the rotational axis and was small in diameter?

Keith

And even if it did, wouldn't it be vastly cheaper and simpler to change the fuse material, or its calibration, or even just revise the setting instructions, rather than rebuild the whole gun and remachine the rifling? Seems crazy on the face of it.

Regards,

MikB

[Rockdoc]

What's remarkable throughout the discussions about shells and, particularly, fuses is the degree to which operational concepts were empirically based rather than on scientific principles. To check a fuse setting for a gun used conventionally you set one, fire it and observe the result. If it went off too soon you tweak it one way and if it went off too late you tweaked it in the other direction. When anti-aircraft guns were introduced they pretty much had to determine how the fuses worked in practice before they could start work on the problems.

To answer your point, they were desperate to increase the muzzle velocity of AA guns. Let's say a plane was a 12,500 feet and directly overhead. It would take slightly over 5 seconds to arrive if the muzzle velocity was 2,500 ft/sec, by which time who knows what the pilot may have done. Firing at a target that was approaching or leaving the area would only increase the flight time of the shell so getting the shell there quicker was paramount. By the end of the War, planes were flying at 26,000 feet and the constant development in aircraft meant a similar need for development in counter-measures.

Keith

[nigelfe]

I'm still waiting for the 4 volumes I ordered, but having read (and copied) some sections of the original draft in Kew a few decades ago.

In WW1 all UK time fuzes (eg as used with shrapnel, star, incendiary, etc) were igniferous (sometimes called combustion) using special quality gunpowder. A gaine was only required when an HE fill had to be detonated (ie gunpowder bursting charges as used with shrapnel were ignited more easily). Air density (resistance) varies with altitude and atmospheric conditions and affects the time of flight, which must be allowed for in the fuze length. It's usually said that igniferous fuze burning rate is affected by atmospheric conditions, but I not come across a physical explanation. Its possible that this is confusion with the effect on the shell's time of flight, if the burning rate of gunpowder is affected by atmosphere then it will be documented somewhere in the (19th cent?) scientific literature.

Unfortunately I've never come across any references to (never mind explaining why) the speed of rotation affected burning speed. My first thought is that there might be an Ordnance Board proceeding on the matter.

[Rockdoc]

That section of the book says that they tried to simulate shell flight under laboratory conditions, which sounds decidedly hairy, but were only able to use muzzle velocities of a few hundred feet per second. The results under field conditions held until a certain threshold was reached, after which problems occurred. I asked a friend with experience in industrial chemistry for his thoughts and he was surprised, offering only that the rotational speed may force the flame-front to to the outside of the fuel and become annular.

There's no doubt that they understood that dynamic pressure of the air affected the fuse but the number of changes they made to the design of the shell for all natures, especially the radius of the head which they increased considerably, strongly suggests to me that the Royal Arsenal was working on the principle "if it works, don't fix it." The book states that operational practice was based on the empirical principle of suck it and see. The observation of shell fall and effectiveness was badly affected, for example, by a change in the charge that produced no smoke. They had to introduce a smoke-generating element so that the spotters could determine the effectiveness of the impact.

The Germans appear to have been much more scientific in their approach and there are examples of German shells and shell fragments being returned to Woolwich for examination.

Keith

[Pete1052]

If I recall correctly the proximity fuze of World War II was the invention of British scientists. In the U.S. the type used by field artillery is called the VT fuze, for variable time.

Edit: The British did some conceptual experiments on the proximity fuze, but it was a U.S. Navy development program which made the first ones that worked. Click here for information on the fuze.

[truthergw]

The science behind the fuze burning may have been expressed as a set of partial differential equations, many physical models are. Positions of planets, times of tides and so on. These sets of equations do not have a closed form, that is they do not have an exact answer. The ' solution' is an approximation to the desired accuracy. Before the advent of computers, these sets of equations were very difficult to approximate. The range tables for Artillery were produced in the same way as tide tables. Analogue computers which consisted of sets of gears with sliding transmission etc. Operating them and deriving tables of data was as much an art as a science. The first electronic computers were also analogue but soon gave way to digital. and were used for range tables in USA. What I am saying is, fire it and see what happens, was probably as scientific as it could be at the time. Analysis of German fuzes and shells would be mechanical and metallurgical.

[TonyE]

Rockdoc - the powder train in the fuse that was ignited on firing and which could be adjusted for time of flight was not a "stick" of powder. In its simplest form there were two circular trains of powder, one fixed and one in an adjustable ring in the fuze. This ring could be turned so that a hole in the adjustable ring passed the flame from one train to the other at different points depending on the position of the ring. Thus the time from ignition of the powder train on firing to the point where it detonated could be adjusted.

I hope that explanation makes some kind of sense, but I suggest you have a look at one of the official "Treatise on Ammunition" (1915 would be best as it is available in reprint) for diagrams of Time and Percussion fuzes. Hoggs and Thurston "British Artillery WWI" also has some explanations.

As the burning of the powder train is annular, I can see that the speed of rotation could affect the rate of burn but would not like to explain it scientifically. I believe that another change that was made to No.80 fuzes to modify them for AA work was the removal of the percussion element and its replacement by a wood block, but I don't know the Mark number (if it had one).

Regards

TonyE

[George Armstrong Custer]

The purely practical problem of setting the fuze for each shell was greater in wartime than anticipated before it. Fuze setting was done by moving a graduated ring on the fuze, but the graduations were small and difficult to see in the darkness or heavy rain. These were precisely the conditions frequently encountered on the Western Front. [........]

High explosive shells in themselves were very useful but in combination with one of the key British technological developments of the war, they became supremely effective. This was the No 106 instantaneous fuze:

"The usual fuse contained a slight delay and the shell penetrated a foot or two into the ground before exploding. For wire-cutting, this entailed two disadvantages: first, a crater which impeded the progress of attacking troops; and second, the slight upward trajectory of the shell splinters which caused most of them to miss the wire. The only technique used for wire-cutting by artillery was therefore shrapnel, bursting low at short range, which was rarely possible. But with the introduction of the instantaneous fuze, No 106, wire-cutting with high-explosive shells became the most effective method." - Second Lieutenant Frank Parish, 53rd Battery, 2nd Brigade, Royal Field Artillery, 6th Division. *

* From Cambrai 1917: The Myth of the First Great Tank Battle, by Bryn Hammond, (2008), p. 29.

Such little nuggets of first hand technical knowledge quoted in this book are invaluable, and alone seem to make the book almost pay for itself.

[Rockdoc]

.....What I am saying is, fire it and see what happens, was probably as scientific as it could be at the time. Analysis of German fuzes and shells would be mechanical and metallurgical.

The book does not criticise the use of empirical formulae and practices for conventional gunnery but states that these long-standing concepts failed altogether when applied to AA gunnery. Elsewhere I have read that the practice, at least in the early days, of AA Sections was for the guns to fire alternately, with the spotting for the first gun's shell being used to help set up the gun and fuze for its next shot and so on. Empiricism was clearly alive and well but it would appear to have been woefully inadequate when it came to AA work. That's no criticism of the RGA because there wasn't a single British AA gun anywhere when war broke out so everyone would have been in the dark.

Although metallurgical examinations would certainly have been done and German fuzes were stripped - especially their clockwork fuzes which were in action long before the British managed to copy it successfully - one of the aspects that greatly concerned the RA was the increased ranges the Germans were extracting from existing artillery. They could not keep up and our gun emplacements were being pounded without any means to retaliate. It was the examination of the design of German shells that helped drive the development of British shells with noses having greater and greater radii of curvature. I haven't found a number for the radii at the start of the War but they couldn't have been much more than two calibres because increases were made to three and four calibres early on. Some shell designs reached eight-calibre radii towards the end of the War. All this work was dine in order to increase the effective range by even a few hundred yards. That's a very significant change in design and manufacture.

Tony, your explanation of the way a fuse worked helps my understanding enormously. GAC, the development of fuzes is featured strongly in the section of the book I'm currently reading. What fascinated me is how a fuze that was satisfactory when tested by Woolwich proved disastrous when put into the field. The No 100 was a case in point, being rapidly superseded. Even so, it wasn't until the 106 you mention that the things became really reliable.

Keith

[centurion]

All forms of gunpowder are mixtures not compounds and physical factors such as the degree to which they are compressed can effect the rate at which they burn. Because of this I can see that air pressure (rather than the amount of oxygen in the air as the powder effectively supplies its own oxygen) could be an important factor. Similarly the centripetal force generated by rapid rotation could also affect the physical properties of the powder in the fuse.

[Rockdoc]

I was particularly confused over the rotational effects because, until I read TonyE's post, I did not know that the timing element was a powder-filled spiral whose effective length could be varied. It would seem likely that the flame-front moving to an edge as the result of centripetal forces could cause serious disruption to the overall rate of burning. It would also seem likely that the powder in the spiral would have to be poured in rather than being a pressed, solid mass so there would be a greater chance for the components to separate.

Keith

[nigelfe]

Typically in a UK igniferous time fuze in 1914 the special quality gunpowder was actually in a ring not a spiral, this channel had a U shaped cross section and I think it was filled by pressing powder into the U. A lower ring led to the powder channel connected to the exploder.

Fuze setting was not ToF (UK didn't adopt this until well into WW2), it was an arbitrary scale, with setting lengths given for each tabulated range in the Range Table.

With surface to surface fire the observer could determine the corrector by ranging, basically this was a correction for non-standard conditions. Obviously finding the corrector for AA fire was next to impossible.

At some point Fuze Indicators started to appear, these had the range/fuze length relationship for the type of gun (and charge), and offsets for the corrector setting as well as corrections for MV. Remember that the usual UK procedure was to order the range to the gun, not the elevation angle, so each gun had its own fuze indicator to provide its fuze setting. By WW2 these were usually called 'Fuze Bar Indicator' (FBI) because of their shape, in pictures you usually see them secured in brackets across the top of the shield.

The UK shells at the start of WW1 were 1.5 or 2 crh (tangent type crh, secant type was adopted in the 1930s). The improved ballistic shape had a signicant effect on range (there are numbers for 18-pr on my web site's ammo page).

[Rockdoc]

In the notes my Grandfather made about the duties of the detachment, he says that the No 2 uses the "fuze dial" and that he adjusts the scale up and down. I suspect this may have been what you called a fuze indicator. In case it makes things clearer, the duties he listed for the 11 men are:

At the order “Action” the numbers will take up their positions & work as follows:-

1 on the ground in a convenient position to command and supervise his Detachment & where he can see the signals of the Battery Commander. He orders “Go On” (commence firing) & “Stop”. He passes all orders & acknowledges same by saluting. He is responsible that his layers are on the target ordered & not in some other aeroplane.

2 places himself on the right of the gun at the fuze dial. He adjusts the vertical scale “Up” & “Down”. He should occasionally glance at the order board to check his fuze.

3 places himself on the left of the gun at the elevating wheel. He lays for elevation (usually by means of the telescope) & reports “On Target”.

4 on the right of the gun, adjusts the lateral deflection as ordered. He should be able to set deflections blindly (e.g. one turn clockwise equals on degree right, etc). He should occasionally glance at the order board to check his deflections. He should stand well clear to avoid crowding the other numbers

5 is on the left of the gun at the traversing wheel. He lays for line (usually over open sights). If he uses the telescope he must keep his eye away from the eyepiece to avoid shock of recoil. On first picking up a target he will, when using open sights, order 3 to elevate or depress in order to bring the target into the field of view of No 3's telescope.

6 on the right of the gun, opens and closes the breech. He will keep clear of the recoil & must not keep the breech in the most fully open the position when the gun is being loaded but should allow the catch retaining breech mechanism open in the hole bored for it on the right side of the carrier; otherwise the extractor will not allow the new cartridge to go home.

7 places himself on the left of the gun in rear of 5. He loads and fires. When loading he will push the round home with his closed fist until it is engaged with the catch retaining cartridge. After gun fire has been ordered he will fire as soon as loaded & continuing loading & firing until “Stop” is ordered. He will fire by pushing the lever forward. He must be careful to keep clear of recoil. He receives ammunition from 8 & at a change of fuze will call “Fuze In” loud enough for the No 2 to hear.

8 supplies ammunition to 7 back of the hand up and fuze to the right. Ammunition may, however, be thrown up to 7 – palm of the hand upwards & fuze to the right. On a change of fuze he will give back the round in his hand to 9 or 10 receiving a correctly fuzed round in its place.

9 or 10 place themselves at the most convenient box for the supply of ammunition, changing their position as required. They set and alter fuzes, handing to 8 rounds set at the fuze shown on the order board.

11 attends to the tightening of the jacks and wheel scotches in action & will assist the wagon drivers in the supply of ammunition from the wagon when necessary.

Keith

[johnreed]

The problem with the Fuzes was due to the powder train in use at the time. As the gun was fired an igniting detonator in the fuze struck a fixed needle, the resulting flash lit the end of the powder train, which then began to burn at a speed determined by the composition and density with which it was packed. But it soon became apparent that something was upsetting the reliability of the Fuze. The Anti-Aircraft Experimental Section built a vacuum chamber and installed a spinning table inside so that the fuze could be operated in conditions of spin and rarefied atmosphere exactly like those to be met in the air when fired. These experiments showed that the burning powder was often extinguished due to the drop in air pressure at great heights. Another defect found was that the centifugal force of the spinning shell, caused the burning area of the powder to detach itself from the rest of the powder train and this caused the failure to pass on the ignition. By reformulating the powder and changing the internal arrangements in the fuzes, they were made more reliable, but when the war ended 20,000 feet seemed to be the greatest height that the combustion fuze could be relied upon to function with any degree of accuracy.

John

[Rockdoc]

Superb, John! Clear as crystal. Thank you. Are the reports of the experiments available anywhere? Kew, perhaps? As a test engineer of long standing I'd enjoy reading them.

Keith

[johnreed]

Keith

I cannot give you a definate document held at Kew but a search of MUN 4/ might give you a list of any documents you might be interested in. One document that I have come accross is MUN 4/3259 Organisation: Formation of Anti-Aircraft Equipment Advisory Committee.

John