PROJECTILES, 155-MILLIMETER: HERA, M549 AND M549A1
M549: Std AMCTC 8753, dtd 1971. M549A1: Std.
Fragmentation and blast effect against personnel and materiel. Also extends the range and improves effectiveness of 155mm M109 and M109A1/A2/A3 self propelled and M114A2 and M198 Towed Howitzers.
These projectiles consist of two major components, a warhead filled with 16 pounds of Composition B high explosive (M549) or 15 pounds of TNT high explosive (M549A1), and a solid propellant rocket motor. These components are threaded together so that the outer steel shells of both form a stremlined ogive. A supplementary charge is installed in the deep cavity of the nose. A rotating hand encircles the assembled projectile near the base. A rocket cap is threaded into the base. The cap is removed prior to firing to allow ignition of the rocket motor for extended range. The rocket motor body contains seven pounds of solid rocket propellant arranged in two segmented grains. Each of the three segments of the forward grain contains an ignition pellet. The motor nozzle is recessed in the center of the boat tail rocket motor base of the projectile, and thrust is along the longitudinal axis.
The M549/M549A1 projectiles have a lifting plug designed to protect the projectile fuze area against accidental damage. The new plug has an oversized (3-3/4 in.) flange. If this protective lifting plug is broken at the neck area, the threaded portion of the plug will remain in the projectile and the projectile cannot be fuzed. No attempt should be made to extract any portion of a broken plug from a projectile; the projectile is not to be used and should be returned to supply point.
The projectile M549/M549A1 also has a new type of grommet designed especially to fit the configuration of this projectile. It is of polycarbonate composition.
When the weapon is fired, the rotating band engages the barrel rifling to impart spin to the projectile for stability in flight. The obturator and rotating band form a seal to prevent leakage of gas pressure past the projectile. Rapidly expanding gases from the burning propellant charge propel the projectile through the barrel with the velocity necessary to reach the target. Extended range is obtained through rocket assist, the rocket cap is removed prior to weapon chambering exposing the pyrotechnic delay assembly in the base of the rocket motor. When the projectile is fired, the propellant gases ignite the delay which burns for approximately 7 seconds and then sets off the rocket igniter to initiate the rocket motor propellant. The rocket motor burns for approximately three seconds. This additional thrust augments the velocity and consequently, the range of the projectile. If a PD or ET is used, the fuze detonates the supplementary charge and the supplementary charge detonates the warhead filler either on impact or at the preset time.
Difference Between Models:
Model M549 is filled with Composition B; Model M549A1 is filled with TNT.
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