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XM777 Lightweight 155mm howitzer (LW155)

The XM777 Howitzer comprises two major assemblies: the Carriage and the Elevating Mass.

The Carriage comprises two subassemblies: the Body & the Saddle.

• The Body includes two forward stabilizers, and two split trails fitted with self-digging spades and dampers.
  • Mounted on either side of the body is a hydragas suspension unit fitted with stub axle and aluminum road wheel. A small hydraulic hand pump is installed at each wheel station to raise and lower the equipment into and out of action.
  • A centerpost on the body receives the saddle, and a rack segment, attached to the rear of the body enables the saddle to rotate through 400 mils either side when a pinion on the saddle-mounted traverse gearbox is engaged with this rack.
• The Saddle is fitted with yoke-mounted roll-screw and nut elevating gear actuated by handwheels on either side of the saddle.
  • The right-hand handwheel with simple clutch can be used to assist the left via toothed drive belts, pulleys and a cross-shaft. A two-piece extendable drive shaft imparts this manual drive from a handwheel gearbox via an intermediate gearbox mounted on the left-hand end of the roll-screw gearbox trunnion and into the elevating roll screw gearbox and planetary roll screw nut. This nut transmits the drive to the threads of a fixed roll screw terminating in a resilient gearbox fastened to the yoke, thus elevating or depressing the assembly according to handwheel rotation.
  • The howitzer's one-man-operated optical / mechanical sighting system is secured to the left-handed elevating gear. A two-man sighting system is employed on US artillery is provided for the JLW 155 Program.
  • A traverse gearbox unit is pivoted on mounting lugs attached to the back of the saddle. It comprises a handwheel-actuated input gearbox, an anti-backdrive unit, a torque limiting clutch and a lower gearbox, all combined.
  • The output pinion of the traverse gearbox meshes with the gear rack segment attached to the carriage body, providing 800 mils of traverse.
  • A toggle lever operates a mesh adjuster mounted on top of the saddle, either clamping the traverse gearbox into correct mech with the rack or disengaging the mesh for transit.

The Elevating Mass comprises an open-structured Cradle pivoted from the saddle and a 155mm Cannon mounted in the cradle.

• The Cradle consists of four large thick-walled tubes with lightweight perforated side frames, and linked together by yokes. The four cradle tubes perform a useful secondary role as nitrogen gas and hydraulic fluid pressure vessels for the recoil systems and the 'equilibrators' or balancing gear.
  • Recoil Cylinders are mounted inboard on either side of the cradle linked to an accumulator site on top of the cradle.
  • Equilibrators sited either side above rear of the cradle ease the handwheel effort when elevating or depressing the howitzer.
• The Cannon assembly is an M284 barrel, similar to that used on the American M109 A6 Paladin, fitted with M289 standard screw breech and M49 percussion firing mechanism.
  • Re-orientation has enabled the breech to open in the vertical plane. An automatic Primer Tube Feed Mechanism holding 10 primers is fitted to the breech.
  • A muzzle brake adopted from the M198 howitzer reduces blast over-pressure on the detachment and incorporates the towing eye assembly.

"Bolt-on, bolt-off" Pre-Planned Product Improvements (P3I) for the LW155 will provide this expeditionary weapon with advanced digital technology modules currently found only on heavy self-propelled weapons. These P3I systems fully support the Marine Corps Operational Maneuver From The Sea (OMFTS) and the US Army Task Force XXI objectives. P3I systems will include:

• A Mission Manager, digital fire control linking the weapon directly into the Advanced Field Artillery Tactical Data System (AFATDS);
• A Towed Weapon Inertial Navigation System (TWINS), providing inertial navigation during displacements and updated orientation to the Mission Manager during fire missions;
• An on-board Muzzle Velocimeter, providing the Mission Manager with automated updates for each round fired;
• A Direct Fire Sight, providing a first-round-hit capability by combining night vision, laser range finding, and computation of super elevation;
• A Power Supply System, providing reliable power through redundant access to a generator, the vehicle and a back-up battery mounted on the howitzer;
• A Powered Rammer to reduce crew fatigue and support higher rates of fire.

On 17 March 1997, a contract for a joint U.S. Marine Corps/U.S. Army Lightweight 155mm howitzer (LW155) was awarded to the Textron Marine & Land Systems Division of Cadillac Gage Textron, New Orleans, Louisiana by the U.S. Army Tank-automotive and Armaments Command, Armament Research, Development and Engineering Center, Picatinny Arsenal, New Jersey. Textron is teamed with Vickers Shipbuilding and Engineering Limited of the United Kingdom. The Contract award was the culmination of a nearly eleven month detailed "shoot-off" and analyses of commercially developed prototypes.

The LW 155 will replace the current M198 towed howitzer in both services with one that is nearly half the weight, yet performs equal to or better than the M198 in every respect. Prospective offers were required to deliver a prototype howitzer to Yuma Proving Ground in April 1996. A seven month competitive shoot-off was then conducted by the LW 155 joint Program Manager, which included technical testing at Yuma Proving Grounds and an operational assessment at 29 Palms and Camp Pendleton, CA.

In what LW 155 Joint Program Manager, Colonel Stephen Ward referred to as a "rigorous competition," the Textron-Vickers team edged out a team led by United Defense Limited Partnership of Minneapolis, Minnesota which teamed with Royal Ordinance of the United Kingdom. Colonel Ward stated that the "comprehensive shoot-off was America's first ever for choosing an artillery system" and expressed his appreciation to both competitors for their "professional conduct and arduous efforts" displayed throughout the competition.

The contract to Textron, worth approximately $40M, covers a three year Engineering, Manufacturing and Development(EMD) program in which eight prototypes will be built. During this phase, design changes will be made to reflect the shoot-off testing as well as improvements which come to light during the extensive testing which will occur during EMD.