Main categories of WW1 fuzes
The use of detonators with modern HE shells
Inertia mechanisms and percussion pins
Safety and arming mechanisms
Springs and pins : basic arming systems
Stirrups and ramps : inertia arming system
Pyrotechnic arming systems
Centrifugal force arming systems
Percussion fuzes systems
Percussion fuzes systems without delay
Delayed percussion fuzes systems
Super-quick fuzes systems
Time fuze systems
Tubular time fuzes
Revolving discs time fuzes
Clockwork time fuzes systems
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This much important mission gave to this often small part of the ammunition one of the most important roles for the correct operation of all the giants, guns and projectiles, that are spoken about in the other pages of this website.
The fuzes that can still be found nowadays in the ploughings, embedded in earth or chalk and half destroyed, are remnants of precision mechanics and pyrotechnics systems... For a good understanding of the explanations and diagrams of the section 'WW1 fuzes', one might want to take some time to familiarize himself with some technical concepts of WW1 fuzes mechanics and pyrotechnics, one of the impressive examples of the creativity at the service of crime and horror...
Several types of fuzes were in use in 1914-1918. They can be roughly categorized on the basis of the timing needed for triggering the shell explosive charge :
'Percussion fuzes' were triggered by the impact shock at the arrival on the target, causing the explosion of the shell main charge.
'Delayed percussion fuzes' were percussion fuzes whose triggering at the impact was delayed by some fractions of a second, causing the explosion of the shell main charge after some penetration in the ground or target.
'Super quick (or instant effect) percussion fuzes' were percussion fuzes whose triggering at the impact was made as quick as possible before any penetration in the ground or target.
'Time fuzes' were triggered at a precise point of the projectile trajectory, by the ways of a pyrotechnic or clockwork time countdown system.
'Double effect fuzes' also named 'Time and percussion fuzes' combined the time fuze and percussion fuze behaviors, allowing to chose one or the other, and to ensure explosion at the impact if it happened before the end of the countdown or if this latter was defective. More generally, double effect fuzes could be the name of percussion fuzes with selectable optional delay.
'Multiple effects fuzes' were made by combining more than two different behaviors, including for instance in the same fuze a short delay, a long delay and a time system. The use of 'universal shells' at the beginning of the war gave way to very complex multiple effects fuzes.
These parts of the artillery ammunitions, designed to trigger the burst at a precise part of the shell trajectory or at impact despite severe conditions before the shell was fired (humidity, shocks, corrosion, ...) and during its shooting (huge accelerations and decelerations, high heat, rapid spin, violent shocks) were high precision mechanisms.
Of course, a fuze was also supposed to present all the safety guarantees for the gun crews : it had to be designed in order to sustain uncareful handling during transportation, uncontrolled storage conditions, and the violent acceleration of the shooting start in the tube without causing premature explosions, likely to destroy guns and crews... This additionnal specification added to the fuzes design complexity.
It is interesting to note that the trend of the pre-war was to develop more and more sophisticated fuzes, while the war experience, new types of shells and artillery techniques, and war economics induced at the contrary the use or more and more simple fuzes.
The embarked explosive charge of the fuze could be strong enough to trigger the explosion of the main charge of the projectile when filled with gun powder, but with the very powerful modern and more stable chemicals for high explosive shells (for example molten TNT or mélinite main charges), the fuze charge was only igniting a 'detonator' (also called 'intermediate charge' or 'primer') that had sufficient explosive energy to cause the shell detonation.
In these cases, French technology most often used to add a small separate primer at the base of the fuzes, and a bigger intermediate detonator inserted into the shell head.
Germans army engineers more often designed 'Detonators-fuzes' where the fuze body could be assembled (or was factory preassembled), with a large primer in a single part (often equipped with safety systems) before its mounting on the shell body.
|French 24/31 Typical french high explosive shell pyrotechnic line : from the left to the right, the fuze itself, the adaptator screw ring, the small primer usually screwed on the fuze tail, and the bigger intermediate detonator often mounted into the shell body
|German Dopp Z 96 fuze assembled on a HE shell, showing the primer attached to the tail before mounting on the shell top gaine
The mecanics science principle of inertia was widely used in the WW1 fuzes design.
This pnenomenon is the same than the one that projects forward non immobilized objects in a car braking suddenly, or backwards when the same car accelerates violently, and is modelized by the famous Newton formula 'F = M x a'. It was equally applicable to any free moving object inside a fuze body attached to a shell violently accelerating in the gun tube at the departure, or decelerating even more violently when hitting its target.
Associated with a pyrotechnic igniter that needed the penetration of a hard pin (named 'percussion pin', see an example at left) into a cap filled with a flammable solid (most of the time mercury fulminate, named 'percussion cap'), it allowed a pyrotrchnic activation under the effect of an acceleration or a shock.
|The way the fuzes inertia mecanisms worked was most often comparable to the one exposed in the scheme at right :
|This same principle was applied both for :
|Time pellet system of a French 30/55 time and percussion fuze. On the picture, from the left to the right : the percussion pin, the safety spring, and the detonator cap.
In this last case, the apparatus was turned upside down so that the mobile graze pellet was moving backwards.
Fuzes were (and still are...) dangerous pieces of equipment, designed to create a flame, an explosion or even a detonation on a shock or when ignited. Safety devices were necessary to ensure they would not act this way elsewhere than in enemy territory, just when they were wanted to do so. Springs and pins : the basic arming devices Stirrups and ramps : inertia arming systems : At impact on the target, the newly composed graze pellet, fastened together by the staple mechanism, was projected forwards against the safety spring with enough energy to compress it entirely. The detonator cap hitted the percussion pin, and the resulting flame was propagated by a channel machined in the base of the fuze to the main charge. Pyrotechnic arming system At impact on the target, the inertia energy was be sufficient to project the graze pellet against the percussion pin while compressing the safety spring, and ignite the main charge through a base communication hole. Centrifugal force arming system The centrifugal force effects were used in numerous fuzes or detonators safety devices, as shown in the following examples :
Uncareful handling during transportation, accidental falls during manipulation, surrounding enemy shelling shockwaves were only examples of so many things that could frequently happen to the fuzes in an usual war environment before their use. Military engineers had to invent safety devices that would inert the fuzes before they were assembled to the shell and shot by the gun.
But another danger existed within the gun itself at the very first instants of the fuze active life : the brutal acceleration of the departure could also cause premature triggering of the fuze and shell explosions still in the barrel, often destroying both the material and its servants... This was another reason for designing safety and arming techniques that would avoid such catastrophies.
Removing the safeties of a fuze is called 'arming'. This could be done manually while the fuze was still attainable, but had to be done automatically from the time it disappeared into the gun breech hole. The corresponding devices are many, and the following list is just intended to give an idea of their varieties, using pins, springs, centrifugal force or pyrotechnics.
The basic safety device of a percussion system was the 'safety spring', or 'creep spring', that was inserted between the percussion pin and the detonator cap. This spring could be replaced by, or complemented with an deformable cap protecting the pin, but that would be flattened easily by a sufficient pellet pressure.
These deformable items had to be strong enough to resist to accidental falls of the fuze, but weak enough to be completely compressed when needed at departure or on the shock of arrival. Needless to say that the balance between these two diverging goals was difficult and could eitehr lead to friendly losses, or dud shells...
One simple way to secure completely the fuze was to insert a safety pin (or twin safety pins) into a hole transversing it, that would block the mobile items whose movement was necessary for the triggering process, such as the graze pellet.
This was a real secure system that was used in numerous German or British fuzes. The drawback was the fact it had to be removed manually before the shell was inserted into the gun barrel, and therefore providing no safety from premature explosions in the bore.
Some percussion fuzes used that principle with a safety pin that would be sheared by internal moving pieces only by the impact on target energy.
French fuzes often included an auto-arming device, designed to remove a mechanical safety under the action of the shock of departure. This additional safety gave more guarantees than the basic safety spring that was then just kept for providing the in-flight safety, and allowed to avoid safety pins that could be dangerous if accidently removed in the heat of the moment, and were detrimental to the rate of fire since it added an additional operation from the firing crew.
These inertia auto-arming systems all had the same goal of keeping the graze pellet and the percussion pin out of reach of each other before the shot.
French most famous ones were named 'Robin' system and 'Peuch' systems.
Stirrup ('Robin') system
One of these classical French auto-arming systems was the stirrup spring type, appeared in the XIXth century with the 'Budin' (1875) percussion system and widely used during WW1 with the 'Robin' percussion system.
A mobile starter-bearer (or 'detonator cap pellet') was maintained against the back wall of the fuze by an arming spring that was also pressing on an intermediate tubular mobile inertia block, itself kept separated from the front wall and the percussion pin by the classical safety spring.
The inertia block was equipped with a stirrup spring ('staple' being the litteral translation from French). There was no way for the percussion pin to meet the detonator cap in this configuration and the fuze was safe.
At the shot departure : under the shock, the intermediate tubular mobile block compressed the arming spring and comes around the mobile starter-bearer. The force was sufficient for the staple mechanism to fasten itself on the notches practised in the starter-bearer : the arming spring was from now on definitely compressed, and a classical graze pellet was reconstituted that way, the safety spring only was now preventing the starter to reach the percussion pin during the flight.
The Robin system has been widely used in French fuzes. One classical example is be the 24/31 Mle 1899/08 fuze
The wartime scheme at far left shows that fuze, using a Robin arming system for the percussion system in the tail and another kind of stirrup system in the top head mechanism ('Lejay' type) for an additional safety. The picture at left shows the remains of such a fuze that can be easily comparred with the scheme.
This mechanism was also used in other fuzes, including the time and percussion fuze 30/38 Mod 1884 and its numerous followers.
Below, a dismantled Budin fuze, ancestor of the Robin system, showing the percussion pin, the safety spring, and the starter-bearer equipped with a staple mechanism that had to be compressed by the departure shock energy in order to enter inside the inertia block.
Helicoïdal ramps ('Peuch') system
Another classical French auto-arming system was the helicoïdal ramps type invented by M. Peuch. In this system first implemented in 1914 in the fuze 24/31 Mle 1914, and later in the mine-throwers fuze 24/31 Mod 1916, the accidental meeting of the percussion pin and the detonator cap was prevented by an intermediate cylinder that could shorten itself thanks to an ingenieus system of helicoïdal ramps.
An alternative to the French inertia arming system was introduced by the German military engineers with the pyrotechnic safety. These devices were based on a blocking stem and gunpowder grain system.
This system was widely used in German percussion fuzes as well as in time fuzes. Its advantage was that it allowed to arm the fuze only after the full combustion of the compressed gunpowder grain, that is somewhere on the shell trajectory after leaving the gun barrel. On the other hand, combustion gasses needed some more machining in the fuze for the fumes vents and the necessary igniting system, adding to manufacturing complexity.
In these kind of devices, the classical graze pellet / safety spring / percussion pin system was blocked by a safety stem resting on a compact compressed gunpowder column that prevented any movement.
The only way to arm the percussion system was to burn this compressed gunpowder grain. This was initiated at the shell departure either by :
At the shot departure, the shell rude departure acceleration in the gun barrel triggered the concutor mechanism. The resulting flame went through a communication channel to the compacted gunpowder grain that began to burn.
During flight, when the combustion of this powder column was over (this took some time and therefore prevented the presence of an already armed shell in the gun tube, potential source of prematures), the arming device blocking stem was released and the graze pellet was free to move.
Wartime scheme of the KZ14 German percussion fuze, using the pyrotechnic safety system. One can see the system housed on the top of the fuze in two cylinders, one with the concutor, the other one with the compressed gunpowder grain and the stem, as well as the fume vent.
Section of the same fuze, showing the same mechanisms damaged by the explosion.
A completely different pyrotechnic safety system was integrated inside the medium and large minenwerfer fuze ZsumMW. In these devices, the combustion of compressed gunpowder grains ignited at shot departure was needed to allow the action of springs opening metallic jaws that were blocking the movements of the main percussion system at rest.
Another classical way of arming fuzes after the departure of the gun was to take the opportunity of the spin movement of the shell when fired with a rifle bore tube. This rapid movement gave way to very high centrifugal forces in the shell and fuze.
That effect was mainly used by German fuzes and some British or French ones. One of the specificities of such a phenomenon was that the centrifugal force remains applied to the shell during the whole trajectory, while the inertia needed for stirrup mechanisms was only available during the acceleration par of the trajectory, that is inside the gun barrel.
The centrifugal effect gave way to various systems, and was also used for detonator safety systems.
In the centrifugal force based safety systems, the moving pieces were blocked at rest by side stops resting on springs. The force needed to arm the system was the one created by the side stops under the action of the centrifugal force to compress the springs. The pehnomenon coul be computed easily on the basis of the side stops mass, the spring resistance, the shell spin and the fuze geometry.
At the shot departure, the shock had no influence on these systems, but the rotational movement (spin) of the shell given by the gun inner grooves applied quickly a strong centrifugal force (the shell spins at to several thousands of rotations per minute around its axis) to all the elements of the projectile.
Under the effect of this force, the side stops were pushed back towards the walls, compressing their springs, and released the movements of the graze pellet.
From this moment and during the flight, the fuze was armed and only the safety spring of the main percussion system prevented the graze pellet to enter in contact with the percussion pin.
When hitting a target, nothing prevented the impact energy to project the graze pellet against the percussion pin while compressing the safety spring, triggering the shell explosion.
Traditional centrifugal safety system (with a single side stop) of the percussion system of the German HZ14 Fliehb. fuze
Centrifugal detonator safety of the German KZ11Gr fuze, with a sliding block interrupring the pyrotechnic line between the fuze and the detonator, moved away by the action of cenrifugal force created by the shell spin, and blocked at rest and while in the gun barrel by a pyrotechnic safety.
Centrifugal detonator safety of a HZ16 German fuze, evolution of the precedent KZ11. This safety was generalized to many German fuzes post 1916
Springs and pins : the basic arming devices
Stirrups and ramps : inertia arming systems :
At impact on the target, the newly composed graze pellet, fastened together by the staple mechanism, was projected forwards against the safety spring with enough energy to compress it entirely. The detonator cap hitted the percussion pin, and the resulting flame was propagated by a channel machined in the base of the fuze to the main charge.
Pyrotechnic arming system
At impact on the target, the inertia energy was be sufficient to project the graze pellet against the percussion pin while compressing the safety spring, and ignite the main charge through a base communication hole.
Centrifugal force arming system
The centrifugal force effects were used in numerous fuzes or detonators safety devices, as shown in the following examples :
Percussion fuzes were designed to trigger the burst of the shell when hitting the ground, an obstacle or the aimed target. Originally dedicated to siege guns and projectiles, they became more and more used during the war with all guns including the fieldguns, with the progressive replacement of the classical shrapnell shell for anti-personnal missions by HE shells, inspired by the trench war experience.
Depending on the type of target to be destroyed, it could be necessary to fine tune the precise moment of the explosion in relation with the moment the shell body itself would impact the target.
Yet at the beginning of the war, HE shells fuzes could be equipped with delay systems that would trigger the shell burst some hundredth or tenths of a second after the impact, allowing the shell to perforate a protective coating of concrete, wood or shielding before exploding. During the war, this kind of behavior proved very useful against trenches, deep dug-outs, tanks, or even to create large craters, or to let gaz shells liberate their poison slowly from the ground.
The war experience also induced the fighting armies from each side to feel the need for a percussion fuze that would act so quickly that it would trigger the explosion of the shell before the warhead really entered into the ground or the target. This is how superquick fuzes were invented and mounted on HE shells for anti-personnal, barbed wires destruction or gaz spreading missions.
Non delayed percussion fuzes
|At the beginning of the war, 'percussion fuzes' (also named 'direct action fuzes' mainly equipped the siege and naval guns. They were based on the percussion system described above, with a mobile graze pellet and a percussion pin (and hence sometimes named 'graze-action fuzes').
One of the classical concerns of such fuzes was their safety, since their users wanted them to burst the shell when hitting the target, but not when accidently hitting the ground due to a mishandling. Different arming systems were found and used (see above). Such basic fuzes could malfunction (not exploding) when hitting a ground too soft, or slowed down in water.
|Theoretical scheme of a German pure direct action fuze based on inertia system.
Delayed percussion fuzes
|Classical direct action fuzes could prove inefficient when hitting a steel shield or a reinforced concrete surface. In both cases, the shell would burst before it could penetrate into the hard surface, and the shield would be left almost intact.
Delaying the fuze action for less then a tenth of a second would let the shell enough time to pierce the shielding and explode behind it or into it, with much more damages. This result could be obtained by inserting a delay (a tiny compressed gunpowder grain) in the classical pyrotechnic line of a percussion fuze, between the graze pellet inertia system and the detonator.
This kind of fuze was also found pretty useful with some kinds of gaz shells when the desired effect was that the projectiles digs himself into the ground and slowly liberates its poison for a zone interdiction, against fortresses with perforation shells (with specific fuzes inserted into the shell base to avoid their premature destruction on the concrete), or against tanks.
|Zoom on the pyrotechnic line of a French 24/31 Mle 94/08 percussion fuze including a small 0.05 sec delay just before the detonator.
|Whereas French and British fuzes were most of the time originally built with or withour delay, or could be quickly adapted by inserting a delay into the tail, the German engineers wanted to create polyvalent fuzes with selectable delays.
This behavior was obtained either by the use of :
|Wartime scheme of a theoretical selectable separate pecussion systems
|Wartime scheme of the BdZ10 base fuze delay selection system, based on a single percussion system whose flame could be directed to different channels delayed or not, by the means of an external selector screw.
Operation of the selectable separate percussion systems type :
|Position 'No Delay' ('o.V.') : at the shot departure, the arming percussion system flame is directed to the compressed gun powder grain that blocks the movements of the stem maintaining the no delay percussion system and burns it. At the impact, the 'no delay' percussion system works while the delayed one is blocked, and its flame is immediately transferred to the detonator via a communication channel passing under the delay.
|Position 'With Delay' ('m.V.') : at the shot departure, the arming percussion system is now directed to the compressed gun powder grain that blocks the movements of the stem maintaining the delayed percussion system and burns it. At the impact, the delayed percussion system works while the non delayed one is blocked, and its flame is transferred to the detonator via a communication channel including a delay.
|This kind of selectable delay systems gave way to very complex material, as it can be seen on this German percussion fuze with optional delay GrZ04. The left picture shows the inner scheme with the two separate percussion systems, while the right one is a tail view of the same fuze showing the complex inner organization with the holes of the two percussion systems and different safety systems.
This complexity was a real handicap in wartime economy, and the new fuzes designed during war were much simplified.
Super-quick percussion fuzes
|The need for a new kind of fuze came early during war, for all fighting nations : the applications seemed numerous for a system that would detonate the shell before it penetrates into the ground.
This is how the idea came of a very simple system based on a solid rod pointing forward on the top of the fuze and pushed back when hitting a target. This pure rigid mechanical effect proved quicker than the inertia / spring based systems, and the rod top position far ahead from the shell was another factor inducing a burst before the shell enters into the earth. Moreover, these systems could be made much more sensitive that the inertia ones and trigger when hitting 'weak' targets, provided specific safety devices were added...
The typical use of these ammunitions were mainly :
|The basics of such a 'push-back fuze' (more often called 'superquick fuze' or 'intantaneous fuze' can be seen on the very simple French mine-thrower fuze on the left, and schematized on the right.
In this example, the only safety mechanism is a safety pin that was sheared by the rod when hiiting the target. It is intersting to notice on the left tha a variant existed for this superquick fuze with an additional delay !!! One must remember that in the case of trench artillery the use of sensitive superquick fuzes was dictated by the nature of the grounds and low projectile speeds and not by the need to maximize surface effects.
In the other example top left (French I.A. fuze), the long rod has been replaced by a short one, the whole percussion mechanism and pyrotechnics being located on the top of the long fuze, and linked to the shell body via an explosive wire.
|At the shot departure, the half-ring chocks were ejected by the centrifugal force of the spinning shell, or were removed manually in the case of the trench mortars.
During flight, the percussion rod was prevented to slide in the cylinder despite the wind pressure, thanks to the mild steel safety pin.
When hitting the target, the head of the striker hitted the target at full speed, the rod was drove back in the tube and the mild steel pin was sheared by the shock, letting the percussion pin encounter the starter, triggering the detonation.
Obviously, this operation needed that the shell attaining the obstacle with a good perpendicularly compared to the axis of the stem. This was generally the case for the indirect curved shots of the trench artillery with fin tailed projectiles, for howitzers projectiles shot at high angles, or for the field guns direct flat shots against standing obstacles.
|Varitaions of the French superquick fuzes
|Theoretical scheme of a typical German superquick fuze, that usually kept an ogival shape, sometimes elongated.
|1917 version of the German Fieldgun superquick fuze 'EKZ17'
Shrapnel shells were in 1914 the main ammunition in use by the fieldguns of all the fighting nations. Just as with the older fragmentation shells, the efficacity of their anti-personnal effects was dependant of the position they bursted just in front of their target, in order to sprinkle steel fragments and lead balls on it. Since there is no impact of the shell with the target to give the signal for the burst, the fuze had to be a precise mechanism integrating a count down.
The role of the 'time fuzes' devices was to trigger the explosion at the end of a given lapse of projectile flight time (generally from some seconds to almost a minute), corresponding to the distance needed, given the knowledge of the projectile speed.
Pure time fuzes were generally limited to the use of Anti-Aircraft artillery, where one wants the shell to burst around the flying target but where it is important that an unexploded shell does not explode when landing back on friendly land. Most of the time, these systems were associated with a percussion mechanism, making these fuzes 'Time an Percussion' fuzes.
Three main types of time fuzes, were in use during WW1. Two of them were based on the slow and regular combustion of 'pulverine' (compressed gunpowder) track with an approximate speed of 1 cm a second, and the third one appeare lately and was based on clockwork mechanics :
Tubular time fuzes
|This kind of time fuze was exclusively used by the French artillery, for instance with their famous fuzes '22/31 mod. 1897' (whose wartime scheme is shown at left, and a surviving example at right), or the '30/55 mod.1889'.
The following explanation is made for the first model, that was equiped of an additional classic percussion fuze device in its tail. This particular fuze thus was a 'double-effect' (or 'time and percussion') one.
|In this fuze, a stretched lead tube containing the compressed gunpowder is rolled up in a spiral shape along a hollow conical barrel. In the inner housing of this barrel is lodged an time pellet (also called 'concutor') inertia system designed to ignite the time system track at the shot departure, as well as a compressed gun powder disc.
The lower part of the spiral tube was connected to a gunpowder room, placed under the barrel, just above a traditional percussion fuze device. The barrel and the fuzing tube were covered with a 'soft metal hat', engraved with a spiral groove just over the path of the fuzing tube, in which seconds graduated marks were stamped.
Operation of the tubular time fuzes :
|For a time-triggered behavior, before the shot the artillery man punched the hat groove graduation at a precise place using a specific equipment called 'fuse-borer' (in French 'débouchoir'), according to the aimed time of flight before explosion, given by the battery officer. This operation made a communication between the compressed gunpowder of the fuzing tube and the inner room of the barrel.
The shell shock of departure in the gun activated the concutor (or time pellet), that started the combustion of the compressed powder disc. The flame of this disc was communicated to the compressed gunpowder of the fuzing tube just at the punched place, and the two tube sections began to burn in two opposed directions, at the speed of 1 cm a second. When the combustion of the lower section reached the lower gunpowder room, the explosion of this latter passed through the percussion apparatus to ignite the burst charge.
|In the case of the double effects fuzes, the explosion could also be caused at the impact time if that one occured before the end of the flight time selected by the punching of the time groove, or if the fuze cap had not been punched and therefore a pure percussion behavior selected. In this case, the explosion was commanded by the lower traditional percussion fuze system.
This system, called 'French Time System', was derivated into several fuses models until WW2.
|Fusing spiral groove of a French 25/38 Mle 1880. The lead inner barrel has been pierced at the place of a graduated cap hole
|Family picture of several French fuzes derivated from the French Time System, including some pre-WW2 fuzes.
|Fusing spiral groove of a 30/55 Mle 1884 French fuze
Revolving discs time fuzes
|Apart from the French Army, all of the other nations used the revolving discs time fuze systems rather than the tubular time systems.
Almost as quick to set up than the tubular time fuses (whose sometimes fragile mechanism was one of the contributing conditions to the impressive shooting rate of the 75 mm gun), they were however somehow easier to manufacture but on the other hand required more metal.
Again, most such fuzes were equipped with an additional percussion system that gave them a selectable percussion fuze behavior that could be selected intentionally, or would act if the time system failed or did not finish its countdown before hitting the ground. In this case, these kind of fuzes were called 'double-effect' or 'time and percussion' fuzes.
In some rare cases such as with anti-aircraft use, or with some minenwerfers (see at left), pure time fuzes were designed as well.
|This time, a line of compressed gunpowder dust is located in circular channels dug in the base of two superimposed metallic discs, one being mobile (by rotational movement) and the other fixed. Both these channels are interrupted on a small segment of their circumference. The gunpowder line of the higher disc (fixed) is connected at one end to the concutor placed in the axis of the fuse, via a connection channel.
The gunpowder line of the mobile lower disc is connected at one end the burst charge of the detonator located at the base of the fuse axis. A communication channel, machined in the mobile disc makes it possible to connect the higher and lower gunpowder lines at a selected place, just by rotation of the mobile lower disc
Operation of the revolving discs time fuzes :
|The departure shock of the shell caused the operation of the time pellet ('concutor') system, igniting the gunpowder line in the communication channel. The flame progressed at an approximate speed of 1 cm a second, successively in the upper fixed disc gunpowder channel track until the moment it reached the mobile communication channel whose position was set up by the disc rotation, then the lower disc gunpowder line.
The combustion of this one lead the flame to the burst charge whose firing finally commanded the detonator explosion or the one of the shrapnel shell rear charge.
The setup of these fuzes operation was always made by a preliminary rotation of the graduated mobile disc, placing the wanted duration of flight (in seconds or in hectometers) in front of an index engraved on the fixed disc. Specific devices existed to fix the disc to the set rotation before shooting, so that it did not turn during flight under the effect of the shell spin.
|These fuzes generally were 'double effect' type, pure 'percussion behavior' could be selected pointing the index on the (international !) Roman Cross symbol.
In this configuration, the gunpowder tracks of the discs did not communicate, and the combustion stopped when the higher track was burned.
Only the traditional percussion system integrated in the fuze could then trigger the shell explosion at the impact time.
|When the index was positioned on the '0' mark (sometimes at the place of an evacuation vent for the fumes), the two communication channels were aligned, so that the detonator ignition was quasi instantaneous, corresponding to the needs of a grape-shot firing at point blank.
|All other positioning of the index induced a time counting behavior, whose duration was shown by the engraved figured of the lower disc, pointed by the index, and corresponding to the combustion time of the sum of the gunpowder lines.
|Increasing the angle allowed to increase the flight duration time
|The maximum duration was reachable by the time needed to burn the sum of the circumferences of the gunpowder lines of the two discs.
The maximum set-up time could be increased for long range guns in specific fuzes by using a slower burning composition for the gunpowder track, or by increasing the number of discs.
The combustion of all these gunpowder tracks and elements generated fumes, so that multiple windows were machined in those fuzes in order to let the gasses escape.
|Superb section made by Alain Dubois of a British n°80 time and percussion fuze. See the communicating channels between the discs
|Revolving discs German HZ05 time and percussion fuze : the destroyed model shows the sliding grooves of the disappeared mobile disc
|Opened British n°80 time and percussion fuze, nice view on the central housing where the time pellet was located.
Clockwork time fuses
|A completely mechanic design also existed in France and Germany with clockwork based fuzes.
These kind of devices offered the advantages of giving long countdown times without influence on the fuze dimensions (where conventional pyrotechnics woul have needed to use long tracks, hence big fuzes). Moreover the mechanic apparatus were not sensible to humidity and atmospheric pressure, two factors that could change the burning properties of the gunpowder tracks of the conventional time fuzes.
These fuzes were often dedicated to the Anti-Aircraft artillery, but were already sometimes in use with the long range German guns as early as 1914. They probably were quite expensive to produce (see the schematics of the German Dopp Z08 at left...), and therefore were quite rare.