DOI 10.2478/jok-2019-0100
Rafał BAZELA
Military Institute of Armament Technology (Wojskowy Instytut Techniczny Uzbrojenia)
DEVELOPMENT OF A SELF-DESTROYING FUSE FOR
ROCKET PROPELLED GRENADE MUNITIONS
Opracowanie zapalnika z samolikwidatorem do amunicji
granatników przeciwpancernych
Abstract: The paper presents a proposal for modernising the DCR-type self-destroying fuse
for rocket propelled grenade munitions. It features a description of the fuse’s structure concept, its design and operation by presenting its particular stages. The further part is a description of the structural materials used for constructing the modernised fuse. Then, a conducted analysis of laboratory and field testing of the developed fuse models was presented. The results of the field testing of the devised and developed modernised fuse technology demonstrator are shown. The high quality of the fuse’s operation was demonstrated. The research paper is concluded with a short summary and a list of the literature used.
Keywords: self-destroying fuse, rocket propelled grenade munitions, munition fuse modernisation
Streszczenie: W referacie przedstawiono propozycję modernizacji zapalnika z
samo-likwidatorem typu DCR do amunicji granatników przeciwpancernych. Opisano koncepcję konstrukcji tego zapalnika, jego budowę oraz zasadę działania, prezentując poszczególne fazy tej pracy. W dalszej części opisano zastosowane materiały konstrukcyjne do budowy zmodernizowanego zapalnika. Przedstawiono następnie przeprowadzoną analizę badań laboratoryjnych i poligonowych opracowanych modeli zapalników. Zaprezentowano rezultaty badań poligonowych zaprojektowanego i wykonanego demonstratora technologii zmodernizowanego zapalnika. Wykazano przy tym wysoką jakość działania tego zapalnika. Referat zakończono krótkim podsumowaniem oraz wykazem wykorzystanej literatury.
Słowa kluczowe: zapalnik z samolikwidatorem, amunicja do granatników przeciw-pancernych, modernizacja zapalników amunicji
1. Introduction
A correctly selected fuse is the basis for effective operation of rocket propelled grenade projectiles. For this reason, its execution should be based on the newest materials and technologies. On the other hand, its structure should ensure easy projectile arming, guarantee reliable projectile operation in any operating conditions, feature a self-destroying mechanism and meet specific defence standards. A very important element is also the ability to maintain combat capabilities after transport by various means (land, maritime, aerial) without distance limitations. In the case of equipping rocket propelled grenades with new fuses outside of the factory, the fuses should enable transport in any hermetically-sealed packaging used by their manufacturer. During designing and testing, it is important to pursue cost minimisation with maintenance of its tactical and technical parameters. Furthermore, when choosing the structural materials, it is necessary to remember about the need to achieve optimal weight and strength features.
2. Self-destroying fuse structure concept
The fuse’s concept assumes a mechanical structure with explosive train safety and a pyrotechnic self-destruct mechanism. The fuse protection system features a pressurised mechanical safety system that provides protection for the fuse in normal operating conditions, a friction and inertia safety system that protects against inertia forces when firing and a timed pyrotechnic self-destruct mechanism. The condition for arming the fuse is the execution of a sequential operating cycle, which includes the following:
–
fuse safety release by breaking the rigid safety catch as result of impact of gunpowder gases in rocket engines,–
permanent unlocking of the inertia safety system,–
DC pierced igniferous primer displacement into the “ready for piercing” position,–
igniferous primer piercing in the self-destruct system and ignition of thepyrotechnic self-destruct mechanism’s retarder mass.
The condition necessary for the fuse’s activation is its arming and the igniferous primer’s piercing in the “explosive train” or initiation of the flame detonating primer in the detonator by the retarder. The modernised fuse (DCR-2 type) consists of mechanical and pyrotechnic sets placed in the body - fig. 1.
The fuse’s body includes the following: pressurised safety system consisting of:
- diaphragm (15), - piston (14),
- rigid safety catch (13), - seal pads (7),
- retaining bolt (4), two-variant arming system:
- plunger bushing (5a) with and expanding ring (12), - ball (9a),
- shifter (10), - shifter springs (18),
• inertia system (...), consisting of the following: - plunger bushing (5b)
- rigid safety catch (9b) combined permanently with a bushing (5b) - shifter (10),
- shifter springs (18),
piercing mechanism, consisting of the following: - hammer with firing pin (6),
- spring (11),
self-destruct system, consisting of the following: - pierced igniferous primer placed in the shifter (10) - firing pin (16),
- self-destruct mechanism’s pyrotechnic retarder pressed into the shifter (10). The fuse body is combined with the detonator guard (1) using a cover (2).
Fig. 1. Modernised fuse concept, variant 1 – shifter (10) kept in factory position by the ball (9a): 1 – detonator guard, 2 – cover, 3 – ball blocking the movement of the hammer with firing pin in the unarmed safe fuse, 4 – retaining bolt, 5a – plunger bushing, 6 – hammer with firing pin, 7 – sealing pads, 9a – ball retaining the shifter in the unarmed safe position, 10 – shifter with the self-destruct mechanism’s pyrotechnic retardant and pierced igniferous primers, 11 – spring blocking the movement of the hammer with firing pin against buffer forces, 12 – expanding ring, 13 – rigid safety catch, 14 – piston, 15 – diaphragm, 16 – igniferous primer’s firing pin, 17 – retaining pin, 18 – shifter springs (J. Legieć)
The basic pyrotechnic system of the fuse is the “explosive train”, which consists of the following:
–
DC pierced igniferous primer placed in the shifter (10),–
flame detonating primer embedded inside the detonator placed in the guard (1). In a safe fuse, the “explosive train” is interrupted; the shifter (10) is placed in a shifted position that ensures the pierced igniferous primer’s withdrawal from the flame detonating primer. In this position, the shifter (10) obscures the channel inlet in the guard (2), through which the flame from the igniferous primer can arrive at the detonating primer.The modernised primer and its subassemblies and parts can be assembled with maintenance of the current safety principles applicable to fuse technology processes. The stages of assembly of the modernised fuse’s subassemblies are presented in fig. 3.
Fig. 2. Modernised fuse concept, variant 2 – shifter (10) kept in factory position by the rigid safety catch (9b): 1 – detonator guard, 2 – cover, 3 – ball blocking the movement of the hammer with firing pin in the unarmed safe fuse, 4 – retaining bolt, 5b – plunger bushing, 6 – hammer with firing pin, 7 – sealing pads, 9b – rigid safety catch retaining the shifter in the unarmed safe position, 10 – shifter with the self-destruct mechanism’s pyrotechnic retardant and pierced igniferous primers, 11 – spring blocking the movement of the hammer with firing pin against buffer forces, 13 – rigid safety catch, 14 – piston, 15 – diaphragm, 16 – igniferous primer’s firing pin, 17 – retaining pin, 18 – shifter springs (J. Legieć)
The key structural solution in the presented fuse concept is the arming system. The essence of the presented concept is the experimental determination of:
–
fitting and tolerances of the ball’s (9a) position in relation to the shifter (10),–
the notch outline in the shifter (10), in which the ball (9a) is placed,The selection criterion comes down to obtaining friction between the expanding ring (12) and the fuse body, with the value guaranteeing no movement of the plunger bushing (5a) within a time interval, from the time of the fuse’s safety release to the time of inertia caused by the rocket engine sequence.
The condition necessary for the correct operation of the fuse’s arming system is for the shifter (10) movement to take place after the ball’s (9a) falling out of the socket due to the total impact of springs (18) and the inertia caused by the rocket engine sequence.
The impact of springs (18) on the shifter (10) and consecutively: of the shifter (10) on the ball (9a) in the cocked fuse should be insufficient to arm the fuse. The ball in the arming system can be replaced with a rigid safety catch (9b) combined permanently with the plunger bushing (5b).
3. Fuse operation
In the time of ammunition exploitation until the time of firing, all fuse parts are in their factory positions – the fuse is safe and unarmed (fig. 3a). At the time of firing initiation (fig. 3b), the pressure of gases in the projectile’s rocket engine distorts the diaphragm (1).
The piston (2) breaks the safety catch (3) when moving into the inside of the fuse. The fuse’s safety is released. The friction between the expanding ring (5) and the fuse body maintains the plunger bushing (4) in its original position. The fuse remains unarmed.
Due to the inertia caused by the rocket engine sequence, the plunger bushing (4) moves towards the diaphragm (1) and overcomes the friction of the expanding ring (5) (fig. 3c).
The ball (6) unlocks the ability of the hammer with the firing pin (12) to move. The ball (7) stops blocking the shifter’s (9) movement. Due to the springs (8), the shifter (9) is moved to the “armed fuse” position. In the final stage of the shifter’s (9) movement, the pyrotechnic self-destruct mechanism retarder’s igniferous primer, placed in the shifter (9) is pierced by the firing pin (11). When the shifter (9) moves, the igniferous primer is placed in opposition to the firing pin’s head (12). The fuse is armed and ready for action: from the primer’s piercing or from the pyrotechnic self-destruct mechanism’s retarder. During the projectile’s flight to the target, the hammer with firing pin (12) (fig. 5d) is affected by buffer forces. The spring supporting the hammer protects the pierced igniferous primer against piercing due to the buffer forces.
At the time of the projectile’s impact with the target, the occurring inertia causes a shift in the hammer with firing pin (12) (fig. 5e) and piercing of the DC flame igniferous primer. The flame of the DC igniferous primer initiates the action of the TAP flame detonating primer and of the detonator (15).
If the projectile’s flight is longer than the action of the pyrotechnic retardant in the self-destruction system, the action of the TAP flame detonating primer in the detonator (15) (f) takes place based on the gas and flame impulse transmitted through the channel (16).
Fig. 3. Stages of assembly of the basic subassemblies of the modernised fuse: a) fuse body with sealing against gases from the projectile’s rocket propulsion system, b) assembly of mechanical subassemblies (the primer’s safety and piercing subassemblies), c) assembly of shifter with pyrotechnic elements (J. Legieć)
Fig. 4. Stages of fuse operation: a) unarmed safe fuse; b) fuse’s safety released due to the destruction (breaking) of the safety catch due to pressure p of the rocket engine gases; c) fuse’s arming as result of release of the lock preventing the hammer with firing pin movement, shift of the shifter and piercing of the self–destruction system’s igniferous primer due to inertia caused by the rocket engine sequence F and force of the shifter’s springs: 1 – diaphragm, 2 – piston, 3 – safety catch, 4 – plunger bushing, 5 – expanding ring, 6 – ball blocking the movement of the hammer with firing pin, 7 – ball blocking the shifter’s movement, 8 – shifter spring, 9 – shifter 10 – self–destruct mechanism retardant’s igniferous primer, 11 – immobile firing pin, 12 – hammer with firing pin (J. Legieć)
In the case of a missed firing or early impact of the projectile into the ground, the following are possible:
• fuse action due to the piercing of the DC igniferous primer at the time of impact or
• fuse action from the self-destruct mechanism at the end of the pyrotechnic retardant’s burning in the self-destruction mechanism
or
• projectile breakdown along with an explosion of the crushing material or
• partial projectile breakdown without an explosion of the crushing material. Such a case is reported to the field command as unexploded ordinance during firing.
Fig. 5. Stages of fuse operation: d) armed fuse with safety released in the flight path (affected by buffer forces N, the pyrotechnic retardant in the self-destruct system is burning; e) fuse action due to axial inertia caused by the projectile’s impact with the target or obstacle: 6 – ball blocking the movement of the hammer with firing pin, 7 – ball blocking the shifter’s movement, 9 – shifter, 12 – hammer with firing pin, 13 – spring of the hammer with firing pin, 14 – burnt part of the self-destruct mechanism’s pyrotechnic retardant, 15 – initiated detonator (J. Legieć)
Fig. 6. Stages of fuse operation: f) fuse action from the self-destruct system after the time of burning of the self-destruct mechanism’s pyrotechnic retardant: 12 – hammer with firing pin, 13 – spring of the hammer with firing pin, 14 – burnt part of the self-destruct mechanism’s pyrotechnic retardant, 15 – initiated detonator, 16 – channel transmitting the self–destruct mechanism retardant’s gas and flame impulse to the flame detonating primer in the detonator (J. Legieć)
The fuse’s structure was based completely on pyrotechnic parts, subassemblies, elements and auxiliary materials made from domestic raw materials. The selection of materials for the subassemblies and parts was justified by a strength analysis and an analysis of the structure’s mechanics functioning with consideration of the entire structure’s optimisation. The metal parts were made from aluminium, brass and steel alloys. In addition, the metal parts featured galvanic coatings or converse coatings providing protection against corrosion.
The fuse’s elements were made from metal, plastics, galvanic coatings, paints, varnishes, enamels, glues, sealing agents, greases and pyrotechnic masses that do not react with one another and do not cause corrosion of the fuse’s elements or hazardous chemical compounds.
4. Analysis of laboratory and field testing of fuse models
and their technology demonstrator
The laboratory testing featured checks of the fuse’s (non-action) safety in case of primer and gunpowder priming action, which confirmed the lack of ability of initiating both the primer and the gunpowder priming.
It also featured checks of the fuse’s safety in case of primer action in the unarmed fuse, which provided a negative result. Lack of primer insulation in the unarmed position from the detonator was ascertained. For this reason, the number of aluminium pads in the gunpowder priming subassembly was increased and the diameter of the opening in the plates was also increased, thereby compensating the presence of the firing pin in the opening, which is not present in a combat fuse. The aforementioned changes allowed for obtaining a positive result of the check.
The next test was to check the armed fuse’s action from the impact mechanism, which provided a positive result.
The armed fuse’s action from the self-destruct mechanism was also checked. The effect of this testing was the increase in the self-destruct mechanism igniferous primer’s piercing energy by replacing the shifter’s spring with a spring that would allow obtaining a several-fold higher force while maintaining the same bend arrow. In addition, a shifter lock was used in the position corresponding to the igniferous primer’s piercing.
The obtained positive results of laboratory testing allowed for the development of fuse models for field testing.
As part of the conducted field tests, the fuse body’s tightness against the ingress of gases from the projectile’s propulsion system was checked. The diameter of the openings under the retardant’s pyrotechnic path was also increased, which caused an increase in the quantity of the self-destruct mechanism pyrotechnic masses’ burning products, thereby improving the mechanism’s operation.
The testing allowed for the development of the self-destroying fuses’ technology demonstrators, which then underwent field testing.
The testing featured checks of the fuses’ safety during transport, safety in case of accidental safety release and arming, stability of pyrotechnic elements’ embedding in the shifter and the possibility of mechanical damage that could affect safety. After the testing, three fuses were disassembled and reviewed in terms of the following:
• changes in the mutual position of the fuse’s safety and arming parts, • pyrotechnic elements’ embedding in the shifter,
• occurrence of possible mechanical damage that could affect the fuse’s safety and/or action.
The results of all the aforementioned checks were positive. This allowed for checking the fuses’ safety in the case of falling from the height of 3 metres.
The testing was conducted on fuses with the detonator’s upwards and downwards positions. The testing methodology was based on placing a 2 kg grenade launcher on a steel
dummy, consisting of two parts, 1 kg each, and dropping the set from the height of 3 metres onto a horizontal cast-iron plate with the hardness of 100±10 HB. For fuses with the detonator in the downward position, the testing confirmed the fuses’ high quality and demonstrated no safety release or arming, intact embedding of pyrotechnic elements in the shifter and no mechanical damage that could affect the fuse’s safety and action.
As for the fuse with the detonator in the upward position, the self-destruct system action was ascertained, which was confirmed during a subsequent trial. After the fuse’s disassembly, it was ascertained that it became armed and its safety released due to the rigid safety catch’s breakage. An in-depth analysis of the structural documentation’s entries did not demonstrate deviations, however more restrictive conditions of the test featuring dropping was ascertained for modernised fuses when compared to those without modernisation. Therefore, the subsequent testing featured the conditions projected for non-modernised fuses and demonstrated very positive results.
5. Summary
It is therefore possible to state that the modernised fuses (DCR-2) met the technical requirements imposed during fuse safety testing when dropped.
Furthermore, it is necessary to state that the technology demonstrators of the modernised fuses (DCR-2) are resistant to adverse transport conditions and are safe after falling from 3 metres.
The technology demonstrators’ field testing provided not fully satisfactory results, especially in terms of the self-destruct system’s correct operation and meeting the requirements concerning the top and bottom arming limit.
The results of the field-testing point to the need of modifying the current structural version to ensure the correct and reliable operation of the self-destruct system and to the need of conducting repeated field testing for the modified fuses in grenades (applies to PG-76).
The need to modify the DCR-2 fuses is justified by the fact that the military storage facilities include substantial numbers of the RPG-76 grenade launchers, the exploitation of which is suspended for safety reasons (no self-destruct system).
It is advisable to bring the DCR-2 fuse’s structure to the implementation form and to use the fuse to arm the new warhead version for the RPG-76 grenade launcher.
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