Role and main types of fuses
Inertia arming systems
Centrifugal force arming systems
Stem and powder grain arming systems
Time fuse mechanisms
Tubular time fuses
Revolving discs time fuses
Optional delay fuses
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This crucial role made this small part one of the most important elements for the operation of all the giants, guns and projectiles, that we spoke about in the previous pages.
It is therefore something like precision mechanisms elements that one can still find nowadays in the ploughings, embedded in the earth or chalk... For a good understanding of the explanations and diagrams of my section ' Some fuses', you may take some time to read hereafter some technical concepts of WW1 pyrotechnics, or the ingeniousness with the service of the horror...
Three main types of fuses existed in 1914-1918 :
The 'percussion fuses' functioned by the hit shock on the arrival on the objective, causing the immediate explosion of the main charge (fuses without delay or instantaneous fuses), or after a very short time (fuses with delay).
The 'time fuses' ignited in a very precise point of the projectile trajectory, thanks to a countdown mechanism.
The 'double effect fuses' or 'Time and percussion fuses' combined the two preceding types of operation. There were also 'triple effects' or even 'quadruple effects' fuses (with or without delay).
These apparatus, designed to function with precision in the short flight time and in difficult conditions of environment and acceleration, were real precision mechanisms.
The proper charge of the fuse could be strong enough to provoke the explosion of the main charge of the projectile, but in certain cases (for example with molten TNT or mélinite main charges), the fuse charge was only igniting a detonator (intermediate charge) which, only, had sufficient explosive energy to cause the principal detonation. In this case, French artillery used to screw a specific separate detonator at the base their traditional fuses. On the opposite, the Germans designed 'Detonators- fuses', in a single part.
Of course, a fuse was also to present all the safety guarantees : to sustain not necessary careful handling during transport, uncontrolled storage conditions, and the violent acceleration of the shooting start in the tube without causing premature explosions, likely to destroy guns and crews...
The percussion fuses functioned with the effects of the inertia: a small sliding inertia block, maintained against the back wall of its housing by a safety spring, carried a mercury fulminate starter. At the time of the impact against the target, the inertia block was violently projected forwards by a sufficient inertia force compressing the spring, putting in contact the starter and a percussion pin fixed on the front wall of the housing, igniting the starter flaming.
The same principle was applied for the igniting mechanisms of the time fuses, called in French 'concuteurs', that we will find in some further section. In this case, the ignition having to occur at the start of the shell in the gun, the device was rotated by180°, and it is backwards that the moving part could slide.
Of course in the practice, it was necessary to block the movements of the inertia block during handling, for obvious reasons of safety. The safety spring could not provide sufficient guarantee against shocks, therefore another safety device had to be used for the perfect immobilization of the inertia block. This device was called the 'arming device', and 'armed the fuse at the very last moment. That could of course be a safety pin, but in most of the cases the arming was done at the beginning of the gun shot by the action of inertia, able to compress a safety spring, or by the centrifugal force of the shell spinning under the action of the grooved tubes, either by combustion of a compacted powder hold releasing the movements of a stem.
Percussion fuses with inertia arming system :
We will meet again this basic principle, simple and effective, in many fuses of this period : The starter-bearer is maintained against the back wall of the fuse by an arming spring which is pressing on a mobile inertia block. This mobile runner, maintained between this latter arming spring and the safety spring separates the starter from the percussion pin, fixed on the front side of the fuse housing. The inertia block is equipped with a staple mechanism. The charge will be ignited only if the starter and the percussion pin are violently put in contact.
At the beginning of the shot in the gun tube, the mobile block compresses thanks to inertia the arming spring. Under this effect, the staples mechanism down and comes is fastened on the notches practisedin the starter-bearer : the spring will remain compressed henceforth ! The flight of the shell is carried out in this configuration, only the safety spring is now preventing the starter to touch the percussion pin....
At the impact on the target, the now unique piece composed with the starter-bearer and inertia block, fastened together by the staple mechanism, is projected forwards, always thanks to inertia. This time, it is the safety spring that is violently compressed. The starter comes to run up against the percussion pin, and ignites. The explosion is propagated by a channel machined in the base of the fuse.
Percussion fuses with centrifugal force arming system
There was a first alternative exists for the arming of the fuses : this time, the starter-bearer is maintained isolated from the percussion-pin by side stops resting on springs.
The starting shock does not cause anything, but the rotational movement (spin) of the shell given by the gun inner grooves applies an important 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 are pushed back towards the walls, compressing their springs, and releasing the movements of the starter-bearer, still maintained against the rear side of the fuse housing by the safety spring. But now, nothing prevents any more the force of the impact to violently project, just like in the preceding case, the starter-bearer against the percussion pin while compressing the safety spring, igniting the final explosion...
Percussion fuses with stem and powder grain arming system
The next alternative arming system was introduced by the German military engineers. Like in the two preceding cases, the starter-bearer is still maintained apart from the percussion pin by the mean of a safety spring that just waits to be compressed by the shock of arrival on target. But what is new, is that the arming mechanism is now pyrotechnic : a stem, resting on a compact compressed powder column prevents this movement. An classic percussion ignition system, that ha sbeeb studied some lines before, called concutor (in French 'concuteur') , is ready to ignite this 'gunpowder grain'.
The shell rude departure acceleration causes the projection of the percussion pin of the concutor on the starter, compressing the safety spring of this device. This violent meeting ignites, via a communication channel, the compacted gunpowder grain. When the combustion of this powder column is finished (not instantaneous and thus preventing the presence of an already armed shell in the gun tube, potential source of accidents), the inertia block stopping stem is free to move.
Once again, at the moment of the impact on the target, the inertia will be sufficient to project the starter-bearer against the percussion spin while compressing the safety spring.
It must be precised that, since the combustion of the concutor gunpowder created fumes, a window was machined in those fuses in order to let the gaz escape.
The use of grape-shot, balls or shrapnels shells, implied that the projectile should explode 'spontaneously' near its target, in order to sprinkle steel glares and lead balls on it. In this situation, there is no insurance that an impact of the shell with the objective occurs, and thus no certain shock to give the signal for the detonation.
Therefore, the role of the 'time fuses' devices is to start the explosion at the end of a certain lapse of projectile flight time (generally several seconds), representing the distance covered so far as one knows the speed of the projectile.
Two main types of burning fuses, or time fuses, existed at the beginning of the twentieth century, both based on the slow and regular combustion of 'pulverine' (compressed gunpowder) with an approximate speed of 1 cm a second :
Tubular time fuses
This principle was exclusively used by the French, with their famous fuses named 22/31 mod. 1897, and 30/55 mod.1889. The following explanation is made for the first model, that had in addition to the time-dependant system, a classic percussion fuse device. This particular fuse thus was a 'double-effect' one (fusing and percussion)
In this fuse, a stretched lead tube containing the compressed gunpowder dust is rolled up in spiral along a hollow conical barrel. In the inner housing of this barrel is lodged an inertia mechanism intended to ignite the time system at the shooting departure, called 'concutor', as well as a compressed gun powder disc.
The lower part of the spiral tube is connected to a gunpowder room, placed under the barrel, just above a traditional percussion fuse device. The barrel and the fuzing tube are covered with a 'soft metal hat', engraved with a spiral groove just over the path of the fuzing tube, in which seconds graduated marks have been written.
Before the shooting, the artillery men 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. This operation makes a communication between the compressed gunpowder of the fuzing tube and the inner room of the barrel.
The shock of the departure of the shell in the gun actuates by inertial effect the concutor, who starts the combustion of the compressed powder disc. The flame of this disc lights the compressed gunpowder of the fuzing tube just at the punched place, and the two tube sections begin to burn in two opposed directions, at the speed of 1 cm a second. When the combustion of the lower section reaches the lower gunpowder room, the explosion of this latter passes through the percussion apparatus to light the burst charge.
In the case of the double effects fuses, the explosion can also be caused at the impact time at the condition that this one occurs at the end of flight time lower than the one selected by the punching of the time fuse, or if the time fuse had not been punched. In this case, the explosion was commanded by the lower traditional percussion fuse system.
This system, called 'French Time System', was derivated into several fuses models until WW2.
Revolving discs time fuses
Apart from the French Army, all of the belligerents used the revolving discs time fuse systems. Slightly slower to regulate than the tubular time fuses (whose fine but sometimes fragile mechanism was one of the necessary conditions to the impressive shooting rate of the 75 mm gun), they were however easier to manufacture.
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.
The shock of the departure of the shell causes the operation of the concutor. This latter fires the gunpowder line of the channel of communication. The flame progresses at the approximate speed of 1 cm a second, successively in the upper fixed disc gunpowder channel line until the moment when it meets the mobile communication channel, which is ignited it ignite the lower disc gunpowder line. The regular combustion of this one leads to the burst charge whose firing finally commands the detonator explosion.
The programming of these fuses operation was always made by a preliminary rotation of the graduated mobile disc, placing the wanted duration of flight in front of an index engraved on the fixed disc.
These fuses were generally of 'double effect' type, pure 'percussion behavior' could be selected pointing the index on the (international !) Roman Cross symbol. In this configuration, the gunpowder lines of the two discs do not communicate, and the combustion stops when the higher line is burned. A traditional percussion device only ignites the explosion at the impact time.
When the index was positioned on the indication ' 0 ' (sometimes at the place of an evacuation vent for the fumes), the two communication channels were aligned, and the final 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. The maximum duration was reachable by the time needed to burn the sum of the circumferences of the gunpowder lines of the two discs.
To be exhaustive, let us quickly mention that purely mechanical time fuse devices, very close to the clock industry mecanisms, were developed mainly for certain anti-aircraft shells.
According to the objective to be reached, it could be necessary to finely regulate the precise moment of the explosion compared to the moment of the impact. When it was preferable to let the shell penetrate under ground to cause large craters, or to perforate a protective coating of concrete, wood or shielding before exploding, one could want a delay of some hundredths of seconds. On the other hand, it was important in certain occasions to maximize the surface effects (anti-personnel, steel shielding destruction ...), and therefore it was then necessary that the explosion release takes place a very short moment before the warhead really touch the objective.
Fuses with optional delay
A very short time delay could be obtained between the actuating of the percussion mechanism and the detonation of the main charge by addition of a 'delay', made of a 'compressed gun powder grain', in the pyrotechnic route. In most of this time projectiles, this delay could be added to the detonator at the time of the shell preparation.
In other cases (and particularly for the German ammunition for which the fuse was equipped with a fixed the detonator), it was possible to select an behavior with or without delay. This device, described below, imposed the existence of two distinct percussion systems, selected by an programming disc or screw.
Position 'No Delay' : at the departure of the shot, the concutor ignites the gun powder grain of the arming mechanism, releasing the movements of the 'No Delay' percussion mechanism stem. At the impact, this latter functioned and immediately fired the detonator via a communication channel passing under the delay.
Position 'With Delay' : at the departure of the shot, the concutor ignites the gun powder grain of the arming mechanism, releasing the movements of the 'With Delay' percussion mechanism stem At the impact, this latter functioned and ignited the compressed gunpowder delay via a communication channel passing above this latter. The explosion could take place only after complete burning of this delay, that is to say some hundredths of seconds later than in the first case.
Instantaneous fuses or pushed-back stem fuses
In certain cases, the few hundredths of seconds that separated the explosion from the hit of the shell on the objective were enough to let the projectile cross the target and explode too far behind, or slightly dive in the earth, making a nice hole but making too small damages the surface material.
In this case, it was possible to use a 'instantaneous fuse', to maximize the surface effects : the shock detection mechanism precedes the shell at the arrival with a few centimeters, and provokes the explosion on the surface, by the intermediary of a stem. Half-ring chocks and a mild steel pin prevented the mechanism from functioning at rest.
At the departure of the shot, the half-ring chocks are ejected by the centrifugal force of the spinning shell, or are removed manually in the case of the trench mortars. The stem still cannot slide in the cylinder, blocked by the safety mild steel pin. The shell follows its flying trajectory, and when the head of the striker hits the target at full speed, the mild steel pin is sheared, the stem is drove back in the tube, and the percussion pin encounters the starter, igniting the detonation.
Obviously, this operation needed that the shell comes to run up against the obstacle with an acceptable perpendicularly compared to the axis of the stem. This was generally the case for the indirect curved shots of the trench artillery, or for the field guns direct flat shots against standing obstacles.