An Cosantóir

An Cosantóir September 2019 www.dfmagazine.ie 24 | O R D N A N C E C O R P S A R T I C L E S E R I E S How your equipment works: Grenade Fragmentation BY LT ORDNANCE SCHOOL, DFTC T he Defence Forces uses the M72 hand grenade. It consists of approximately 60g of composition B explosives (A 60/40 mix of RDX/TNT), a plastic casing and a pre-notched coil surround- ing the explosives. The infantry section carries four grenades. Two are given to No. 1 and No. 2 riflemen (the assaulters). They are used as a semi-remote close-in attack of personnel in conventional opera- tions. The technical hazard classifications associated with the M72 grenade are blast and fragmentation. The fragmentation comes from the pre-notched coil surrounding the explosive fill. Upon initiation of the composition B main fill, the detonation wave travels omni-directionally from the initiation point. If we wer to alter the initiation point of the explosive fill, we would change the direction of travel of the fragments. Central initiation is where the main explosive fill is initiated from within its centre. Looking at the image below, the red dot indicates the initiation point of the explosive. The black arrows show the beam angle of the fragments. Central initiation gives a more favourable beam angle and allows the fragmentation grenade to better disperse its fragments. The M72 grenade follows this principle. Notice in the sectioned image of the grenade how the detonator is placed centrally in the grenade body. The geometry of the case of the grenade also effects the beam angle. Consider the shape of the M72 grenade. Egg shaped cases will yield a better fragment distribution. Many other grenades have similar shapes to yield better fragment distribution. More crude designs such as the Russian RG42 grenade have a cylindrical design. Whilst this may be easier to manufacture, the beam width will not be as favourable as the M72 grenade. This does present manufac- turing difficulties and thus drives the cost of manufacture of the M72 grenade up. Russian weapon and grenade designs tend to be cruder and cheaper to manufacture than western designs. Anti-personnel munitions are designed to fragment. Fragmenta- tion is achieved by one of three ways: Controlled fragmentation, Pre-formed fragmentation or Natural fragmentation. Natural frag- mentation would be the case of a munition fragmentating from the explosive composition initiating. Examples in service within the DF would be the 105mm high explosive artillery shell or the 81mm high explosive motor bomb. Pre-formed fragmentation are frag- ments that have been cast surrounding the explosive fill. Small ball bearings or cubes that are made before being placed surrounding the explosive fill are pre-formed fragments. The 40x46mm high explosive round fired from the M203 grenade launcher uses pre- formed fragments as does the RBS 70 bolide missile in service with the Artillery Corps. Controlled fragmentation is what the M72 hand grenade uses. The pre-notched metal coil has a built-in weakness in the material. The 'notch' is the weakness in the coil. The detona- tion of an explosive material is the propagation of a shock wave through the material. When the shock wave meets the notch on the coil of the grenade, the pressure increases because the shock wave is acting over a smaller area. Cracks begin to form in the coil due to the built-in weakness. The fragment is accelerated by the ex- plosive initiating and travels towards its target at high speed. A lethal fragment is one that can perforate a target. It is unlikely that a single fragment will kill the target. The threshold for the human body, with body armour protection is 10 J/mm2. Compara- tively, a tank requires 15000 J/mm2 for perforation! This shows that for an anti-personnel warhead to be effective, the fragment must have at least 10 J/mm2 to penetrate the target. This topic is one of many taught in detail during the Ammuni- tion Examiner Course and the Ammunition Systems Engineering block of an Ordnance Young Officers Course. A thorough under- standing of how warheads are designed and work allows the Ordnance Corps to fully support Óglaigh na hÉireann on-island and overseas. Figure 1: Fragment Beam Angle Figure 2: M72 Grenade Figure 3: Body Shape and Fragmentation Spread Figure 4: Pre-Notched Coil in M72 Grenade

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