An impact fuse detonates an explosive charge when it hits the ground or a target object . The detonators are used in almost all types of military ordnance : high-explosive bombs , torpedoes , land and sea mines , rocket weapons ( guided missiles ), hand grenades and the grenades fired from tubular weapons ( explosive or mine projectiles ).
Types of ignition
Impact detonators could be divided into different categories.
According to the type of release:
- The direct-acting percussion fuse has an ignition needle that is pushed directly into the initial explosive when the target material hits it, thus triggering the detonation. This simple principle is used in many head-impact fuses, an example of which is the British No 1 Mk2 of the First World War.
- In the case of indirect impact fuze (also inertial fuze), the delay of the ordnance upon impact causes a striking mass to move further forward due to its inertia and hit the detonator (which is braked together with the ordnance). This principle is used with all rear impact fuses, examples are bottom fuses of armor piercing grenades, squeeze head grenades or rear fuses of high explosive bombs.
- By pushing in the ignition needle, highly sensitive percussion fuses have a combination of direct and indirect impact ignition by advancing the detonator in the hammer. Since the ignition needle and detonator move towards each other on impact, such detonators respond more quickly. All large artillery fuses (from 7.5 cm caliber) of the German Army in World War II were highly sensitive impact fuses.
The above modes of operation require the impact of the weapon with the direction of movement parallel to the longitudinal axis of the weapon. This is usually ensured by the external ballistic properties of the ordnance (stabilization of the flight path through rotation of projectiles, stabilization or control of the flight path through tail units in bombs and missiles). However, these detonators are not reliably triggered at non-optimal angles of impact ( ricochets ), and duds can occur.
- All-round impact fuses usually have the design features of highly sensitive impact fuses (movable ignition needle and movable detonator carrier); In addition, they have another ring-shaped mass piece, which, due to its conical shape, drives the ignition needle and the detonator carrier together in the event of impacts across the trajectory, so that the function is also triggered in the event of lateral impacts. Examples of all-round appealing German artillery detonators are the light infantry grenade detonator 23 (l.Igr.Z. 23) and the heavy infantry grenade detonator 23 (see figure 23).
According to local arrangement:
- Head detonator : Located on the head of the ammunition, it is the first part that hits the target and detonates the explosive charge. Very often used in many types of ammunition e.g. B. aerial bombs, grenades, rockets, guided missiles.
- Bottom detonator : Located in the bottom part of the ammunition and is therefore protected when it hits the target. In many types of ammunition e.g. B. aerial bombs, grenades, rockets, guided missiles are used.
- Transverse fuze : Flush sunk on one side. With aerial bombs, guided missiles
- Point Initiated, Base Detonated (PIBD) : The components of the detonator are located in both the top and bottom sections. The initiating fuse is located in the head part, the detonator, which detonates the explosive charge, in the bottom part. A very fast response time and triggering in the bottom part is necessary, especially with shaped charge projectiles , so that the shaped charge jet can develop. There are two basic types of PIBD detonators, pyrotechnic and electric. The pyrotechnic PIBD detonators work according to the so-called "spit back" principle (German: "spit back"). The detonator in the head part initiates a small detonator charge. This can be designed as a small shaped charge. The jet of fire from the first detonator goes through the empty space of the shaped charge and hits the actual detonator behind the shaped charge. The earlier shrapnel projectiles worked on a similar principle . There are two variants of the electrical PIBD detonators. In the first variant, a piezoelectric crystal generates an electrical pulse when it hits the target . This pulse is transmitted to the electrical detonator via an electrical line or other conductive parts. A second variant of the electrical PIBD igniter is a non-closed circuit with a voltage source (e.g. battery) and in the head part with two electrical contact surfaces and a gap between them. The impact pushes the electrical contacts together so that they touch each other and thus complete the circuit. The closed circuit then ignites the detonator.
Impact detonators are often combined with delay elements: the explosive device should not detonate immediately on impact, but rather penetrate the target object a little first in order to take effect. This is necessary, for example, with armor-piercing weapons (detonation only after penetration of the armor), but is also required for impact fuses of the artillery in order to combat different targets. In this case, detonators are often used in which the function can be set with delay (mV) or without delay (o. V.) before firing. The German artillery fuze impact fuze 23 modified with 2 delays (AZ 23 umg. With 2 V.) could even be set between o. V., 0.2 seconds and 0.8 seconds delay. The delay was usually achieved by a pyrotechnic fuel.
As early as the Second World War, in addition to mechanical impact fuses, electric bomb fuses were also used on the German side. In these, the detonator was ignited by electrical current, which was generated either during the impact by the movement of a ring magnet through a coil by means of induction (sensitive impact fuse eAZ (66) for fragmentation bombs SD 10 A) or when the bomb was dropped in a capacitor was stored and could flow to the detonator on impact by closing electrical contacts (for example AZ (25), AZ (28), AZ (35)). Nowadays, electric detonators are used, for example, in shaped charge weapons, since they have only minimal (for closing the electrical contact) or no mechanical processes at all (when igniting via piezo detonators ) and thus only the smallest delays. Even in weapon systems that are mainly triggered by sensors (proximity detonators), an electric impact detonator is often available.
Safety requirements for detonators
The general safety requirements for all detonators also apply to the impact detonators:
A long shelf life (at least ten years) is generally required for military ammunition. During this time, neither the safety nor the functional properties may be impaired. This relates, for example, to corrosion, the reaction of the different materials in the igniter with one another or the penetration of moisture, dirt, etc. into the igniter.
Handling and transport safety
The fuse elements of the detonator must effectively prevent unintentional unlocking and, in particular, unintentional triggering during the expected loads during handling and transport. The requirements for military detonators are significantly higher than, for example, in the civilian sector, since, in particular in the event of an emergency due to hectic rush, enemy fire, etc., proper handling is not always guaranteed.
The purpose of interrupting the ignition chain is to prevent the detonation from being transmitted to the subsequent ignition elements ( boosting or transfer charge) in the event of an unintentional ignition of the detonator (e.g. through fire, bombardment, etc.) . This is achieved, among other things, by the fact that the detonator is housed in a movable carrier that is only brought "into functional line" when the safety device is released. Nowadays, two independent forces have to cancel the interruption of the ignition chain for this purpose (with artillery detonators usually acceleration and rotation forces ). The requirement for detonator safety was not generally implemented until after the Second World War, until then the detonator in the so-called priming charge (which consisted of detonator, booster and transfer charge and was inserted into the explosive directly under the detonator) was always in use in German artillery fuses the functional line.
In the case of bomb detonators, the safety of the detonator against unintentional triggering in collisions with birds , but also in collisions with other aircraft up to a crash, must also be guaranteed during the approach to the target until the bomb is dropped . This also results in the possibility of emergency dropping, that is to say dropping the bombs in an emergency situation without unlocking them and detonating on impact.
Front pipe protection , mask or fall protection
After firing (with barrel weapons) or dropping (with dropping ammunition), especially with impact fuses, the complete release of the fuse should be delayed at least until the projectile or the bomb has moved far enough away from the gun or the aircraft that a detonation occurs this is no longer endangered. This is to prevent, for example, when firing from camouflaged cover, the detonator can be triggered immediately in front of the barrel muzzle by camouflage material or a randomly flying bird and thus endanger your own gun operation.
Railway safety, rain safety
With artillery fuses, especially with impact fuses of small-caliber automatic cannons, the design must avoid triggering by small objects in the trajectory (insects, raindrops, hailstones). This is mainly necessary for percussion fuses, which only have a cover membrane on the fuse tip in front of the ignition needle. Damaged launch tiles can lead to early cracks .
Release and sharpening
Every percussion fuse must be unlocked before it can be used. In the past, the unlocking was also the sharpening, with today's explosive devices this is separated. The unlocking (pre-sharpening), for example by pulling a split pin, enables the sharpening. The explosive device is only sharp after the sharpening event has occurred - in the case of a grenade, for example, this is the acceleration when it is fired or, in addition, the centrifugal force of its spin.
Sample data from a grenade
- Pre-sharpening: pulling off the spacer
- Sharpening: acceleration of more than 100 m / s²
- Front tube safety device: Sharpening takes place with a short delay so that the shell does not explode in the immediate vicinity of your own gun
- Service delay: none
- Disarming: 1 second after impact
- Resharpening possible: no
Sample data from a free-fall bomb
- Pre-sharpening: pulling the locking pin
- Sharpening: notching at a height of more than 1500 m
- Impact delay: 0.7 seconds (corresponds to three floors of fall height)
- Defusing: 3 seconds after impact (2.3 s after intended detonation)
- Resharpening possible: no
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