The M549 is a High-Explosive Rocket Assisted (HERA) 155mm howitzer round developed for use by the US Military.
Development
The projectile was developed to provide extended range for standard and developmental howitzers. The projectile has two distinctive preassembled components—the high explosive warhead and the rocket motor. The warhead is fabricated from high fragmentation steel for increased effectiveness and contains a bulk-filled explosive. Currently there are two models.
Variations
The M549 contains 16 pounds of Composition B and is restricted from use with the new top Zone 8S M203 Propelling Charge. To assure compliance with safety requirements in newer weapon systems, which are capable of using the M203 Propelling Charge, a conversion to TNT fill in lieu of Composition B was introduced in September 1977 with Type Classification of the M549A1. The M549A1 contains about 15 pounds of TNT.
The 155 mm M549/M549A1 HERA is a separate-loading munition with a two-part forged shell body. The forward HE-filled shell body is constructed from high-fragmentation steel (HF1) and is of a low-drag aerodynamic profile. The rocket-motor body makes up the rear of the shell, this constructed from 4340 steel. The motor body weighs 13.5 kg (approximately 3.175 kg of which is propellant), is 266.7 mm long and is encircled by a welded overlay copper driving band and obturator band, both protected during storage and transit by a polycarbonate composition grommet. At the front of the projectile, the fuze cavity is protected during transport by an energy-absorbing lifting plug, which protects the fuze area from damage during storage, transit and handling. Before loading, the lifting plug is replaced by a fuze, and the protector cap over the rocket-motor nozzle is removed; the 155 mm M549/M549A1 HERA is not intended to be fired in the ‘rocket-off’ mode.
On-target effects are enhanced by the shell’s HF-1 steel body.The 155 mm M549 HERA forward shell body is filled with a nominal 7.26 kg of Composition B. The M549A1 HERA filler is 6.8 kg of cast TNT.At the instant of firing, the propellant gases ignite a pyrotechnic delay train in the rocket motor. The delay burns for approximately seven seconds. By the end of this seven seconds, the end of the delay train reaches the rocket’s primary igniter and subsequently lights the motor’s two propellant grains via an igniter pellet.