Description
This cartridge is a high explosive, obstacle reduction, tactical service round with tracer. The M908 is used in the 120mm, smooth bore M256 cannon.
The Cartridge; 120mm, HE-OR-T, M908 is a high explosive, obstacle reduction, tactical service round with tracer. The M908 is used in the 120mm, M256 smooth bore tank and is a fin stabilized round with a discarding sabot. The baseline design contains a propulsion system consisting of a combustible cartridge case, case adaptor, nineteen-perforated hexagonal JA-2 propellant, a propellant containment device (cloth bag), and an M129 primer assembled to a metal case base and seal assembly (all are currently used on the M829A1). The projectile consists of a subcaliber projectile and three piece aluminum sabot. The subcaliber projectile combines a fuzing system and a chemical energy warhead (Composition A3 Type II). The three segment sabot is secured to the warhead body by a nylon obturator and a steel retaining ring. The fuzing system includes: M774 base element, flexible communication circuit, and frontal impact switch assembly (FISA). The conical nose of the projectile consists of the FISA coupled to the warhead body and a steel nose coupled to the FISA, which closes upon impact with a ground target. A flexible electrical cable carries the signal of FISA closure to the M774 base element. An inertial switch in the M774 base element act as a backup to the FISA. Either switch will delay the function of the warhead beyond the point of impact with the target. This delayed functioning is ideal for the reduction of concrete ground targets.
The M774 base element is a dual environment safe and arm (S&A) device. The M774 receives an electric firing pulse from the FISA which then triggers the base element electronics to fire the M69 electric detonator. The M69 detonator is contained in the rotor which provides a physical separation of the M69 detonator from the fuze electronics until the subprojectile has traveled a safe distance downrange. The first safety feature of the mechanical S&A lock consists of three leafs and a spring, originated so as to release the rotor upon forward acceleration. The second safety feature is a drag weight which senses the decelerating force/drag of the projectile as it leaves the muzzle. As the drag weight senses drag, it moves out of the way of the rotor allowing the rotor to rotate to the armed position as designed.
Once the M774 base element is armed and receives an electrical firing pulse to trigger the detonator, the detonator, lead, booster, and warhead explosives initiate in sequence destroying the target. The explosive train located in front of the base element consists of the lead cup, booster, and Comp A3 explosive. The warhead explosive is contained in the body with a shaped copper liner, in front of the booster. The liner provides the penetration capability for the system.
An aluminum fin assembly with tracer is attached to the aft end of the subprojectile by way of an aluminum fin adapter. The fin has beveled leading edges and T-tabs on the outside diameter to increase the effective fin area. Spin, which is inducted by a twist in the fin blade, provides the subcaliber projectile with greater in-flight stability and accuracy.
Functioning
The operational characteristics of the M908 cartridges is basically the same as that which is utilized for all HEAT-T tank ammunition. After the chambering the cartridge, a voltage is applied to the primer. As current flows through the primer, the igniter charge is initiated which in turn initiates the benite strands. The burning benite, which is evenly distributed within the primer body, initiates the propellant charge. The expanding gases generated by the burning propellant expel the projectile into the gun barrel leaving only the metal case base and primer body behind. During the propellant burn, the tracer element in the fin assembly ignited which provides the projectile with tracking visibility. The silicone rubber seal and obturator band at the base of the projectile prevent blow-by of propelling gas during travel in the barrel. The obturating band and retaining ring also function to maintain projectile in bore entering and integrity.
Upon muzzle exit, the air resistance against the front of the sabot breaks the retaining ring and when the obturating band around the sabot breaks, the sabot falls away in three pieces leaving only the subprojectile to travel to the target. The fin assembly with six equally spaced fins, imparts spin to the subprojectile, thereby stabilizing its flight aerodynamics.
The accelerating of the projectile in the gun tube allows the release mechanism to release the rotor from the first safe position. As the projectile travels downbore the acceleration forces decrease until the rotor can overcome the forces and start its rotation to the armed position. The inbore acceleration of the fuze allows the setback voltage generator to charge up the firing capacitor. As the projectile leave the gun muzzle, the drag weight senses the increased drag forces and moves out of the rotor’s way, allowing it to arm.
Upon direct impact with a target, a firing signal is sent to the M774 base element. The base element’s firing capacitor provides the necessary current to initiate the M609 detonator, which initiates the lead, booster, and warhead explosives in sequence.
Hazardous Components
Filler - Composition A3
Propellant - JA-2
Detonator - M69
Primer - M129
Physical Data
Length, overall - 38.74 in, 984 mm
See Also
Nothing else to see.
Source(s)
