M109

M108
Light Self-Propelled 105mm Howitzer
NSN 2350-00-440-8810

This version had the T-196 development designation. It was designed at the same time as the M109. The weapon mounts a 105mm M103 howitzer with a small fume extractor but no muzzle brake. It was only produced in 1962 and 1963. The M108 is in service in Brazil, Spain, Taiwan and Turkey.

M109
Self-Propelled 105mm Howitzer
NSN 2350-00-440-8811

The original (A0) production version of the M109 developed from the T-196, introduced in 1963. Fitted with a short (23-caliber) 105mm M126 howitzer with muzzle brake and a distinctive, large, "lazy D" shaped fume extractor. Maximum M126 projectile range of 15,914 yd (14,600 m). The vehicle carried 28 rounds.

M109A1
Self-Propelled 155mm Howitzer
NSN 2350-00-485-9662

The M109A1 mounts the longer M185 155mm gun and incorporates elevation, traversing and suspension improvements. It can fire a high-explosive round 19,800 yd (18,100 m). The M109A1 weighs 53,070 lb (24,070 kg) fully loaded and is 29ft 8-in (9.04-m) long including the barrel. The first M109A1 conversion kits were produced in 1972. The first converted from standard vehicles became operational in 1973.

M109A2
Self-Propelled 155mm Howitzer
NSN 2350-01-031-0586

The M109A2 is a new production weapon which incorporated 27 mid-life improvements to the M109A1. The M109A2 variant howitzer featured many system improvements, including a redesigned rammer, improved recoil mechanism, engine operation warning devices, re-designed hatch and door latches, improved hydraulic system and larger turret bustle carrying 36 155-mm projectiles. Production began in 1978. About 850 were produced for the US Army and Army National Guard. The M109A2 is still in current production under a technology transfer agreement between the USA and Samsung in South Korea, the K55 and K55A1. Since this collaboration, Samsung Techwin, or now Hanwha Techwin, has developed its own Howitzer, the K9. The K9 utilizes many parts that are similar or identical in design to the M109.

M109A3
Self-Propelled 155mm Howitzer
NSN 2350-01-031-8851

The M109A3 is a depot converted M109A1 incorporating 27 mid-life improvements. The M109A3 had an improved M178 cannon mounting, boresight alignment driver and selected RAM and safety kits, which included a fuel system air purge, driver's instrument panel, bustle/rack, propellant stowage, torsion bar, counter-recoil buffer, and upper recoil cylinder. Introduced in 1980, most of the global M109 fleet were retrofitted to the M109A3 designation standard. The performance of the M109A3 is identical to the M109A2.

M109A4
Self-Propelled 155mm Howitzer
NSN 2350-01-277-5770

The M109A4 designation was used for the 737 US Army Reserve Howitzers re-fitted under the NBC and reliability, availability and maintainability (RAM) program completed in 1993. The system upgrades included in this specfic re-fit included:

• 180A electrical charging system
• Vane-axial fan assembly increasing cooling capacity and reliability
• Twin hydraulic filters to replace the single power-pack filter
• External power receptacle
• Crew compartment sub-floor drains for torsion-bar pockets
• Traverse mechanism upgrade of internal gearing and clutch and addition of hydraulic tubing and external clutch valve
• Rewire air cleaner switch to shut off when vehicle in neutral
• Add starter-circuit protection relay to prevent over-cranking
• Move slave-start receptacle to driver compartment
• Add protective covers over sensors mounted on engine hood

M109A5
Self-Propelled 155mm Howitzer
NSN 2350-01-281-1719

The M109A5 was an upgrade of the M109A3 to the M109A4 standard with additional features including the Reserve Component/Modified Armament System (RC/MAS) that, with the installation of the M284 cannon assembly and M182 gun mount upgraded the weapons to the A6 variant standard. US Fleet upgrades were completed in 1994 and exports began in 1998. The M109A5 fields upgraded components providing greater durability. The powertrain was upgraded with a higher output (440HP), low heat rejection (LHR) engine and an improved transmission (Allison ATD-XTG 411-4A) for increased mobility. A4 features such as fire suppression, an over-cranking protection device to prevent starter burnout, a 180A alternator that extends battery life, the elevation/equilibrium cylinder and turret traverse clutch assembly are re-designed to reduce failures, protective covers for the engine electrical sensors to prevent accidental damage, relocated and easily accessible filters improve hydraulic power pack filtration, easier to replace track pads and external road wheel lubrication points simplify maintenance. The suspension system was strengthened and an external NATO power receptacle was added to allow the M109A5 to be electrically powered by the M992 Field Artillery Ammunition Support Vehicle (FAASV).

The M109A5 variant is the the most advanced "factory" variant of the original M109 production series in use by non-USA Armed Forces.

M109A5+
Self-Propelled 155mm Howitzer
NSN 2350-01-281-1719

The M109A5+ is the designation given to field upgraded M109A5 variants. The M109A5+ retains the same National Stock Number (NSN) as the M109A5. Many manufacturers have fielded system improvements since the M109A5 which are designed to be incorporated into the exported M109A5 vehicles providing improved mobility, communications, battlefield integration and fire control.

M109A5+ Field upgrades can include:

• Fire Control Upgrade
• Increased firing range of up to 36 km
• Burst Firing Capability (3 Rounds in 15 Seconds)
• Engine Upgrades and Improvement
• Transmission Upgrades and Improvement
• Suspension Upgrades
• Track Upgrades
• Communications Upgrades
• Battlefield Data Management
• Armor Augmentation
• Driver and Commander Assistance Systems
• Electrical System Upgrades (MIL-STD-1245A and better)

M109A6 Paladin
Self-Propelled 155mm Howitzer
NSN 2350-01-305-0028

The M109A5 Howitzer was the subject of several US Army programs for improvement and sustainability ending in the merger of the Howitzer Improvement Program (HIP) and Howitzer Extended Life Program (HELP) into the M109A6 Paladin variant standard in 1989. The major improvements that define the A6 Paladin variant are:

• Upgraded gun mount that accepts a 39 and 58 caliber 155mm cannon
• Secure SINCGARS radios
• Paladin Digital Fire-Control System (PDFCS)
• Onboard diagnostic systems
• Increased ammunition load
• Additional armor: 5083 aluminum with composite spall liners and supplemental armor
• Upgraded engine and transmission

M109A7 Paladin Integrated Management PIM
Self-Propelled 155mm Howitzer
NSN 2350-01-577-0830

The latest M109 variant designed for the US Army is the M109A7, formerly known as the M109A6 Paladin Integrated Management (PIM) during development. The US Army has procured a fleet of 580 sets of M109A7 howitzers and M992A3 ammunition support vehicles with M992A3 being a factory rebuild of the existing M992A2 vehicles. The first M109A6 and M992A2 vehicles were disassembled and reassembled to M109A7 and M992A3 standard as part of low-rate initial production beginning in summer 2014. Production deliveries were scheduled through 2022.

The Paladin Integrated Management builds on the A6’s advances, but with so many changes it was almost a new-build program. The BAE/Army partnership re-used the turret structure and the main 155/39mm gun. Additional range and accuracy depends on using new projectiles like the rocket-boosted & GPS-guided M982 Excalibur, or ATK’s non-boosted PGK screw-in guidance system. Both are explicitly contemplated in the Paladin PIM’s loading systems. Maximum rate of fire also remains unchanged from the M109A6, as cannon tube structure and temperature remain the limiting factor for sustained rates of fire. The Paladin Digital Fire Control System is somewhere between old and new and continuously software upgraded. The system has continued to receive upgrades and is being produced by BAE and Northrop Grumman. GPS is currently provided via older PLGR systems with data sent to the Dynamic Reference Unit – Hybrid (DRU-H inertial navigator), but the obsolescence of electronic components within this box signals that DRU-H and PLGR are on the future replacement list.

Chassis

Previous M109 upgrades hadn’t altered the M109’s 1950s configuration. The new chassis were fabricated & assembled with components from the M2/M3 Bradley IFV (engine, transmission, final drives), in order to create more commonality across the Heavy Brigade Combat teams. THe new chassis adds to the overall weight by less than 5%, but the combined effects of the new chassis and more robust drive components give Paladin PIM the ability to operate at higher weights than the M109A6 GVW maximum of about 39 tons/ 35.4 tonnes. The higher weight rating can support the weight of the T2 add-on armor and underbelly armor add-on kits.

All-Electric

The M109A7 PIM incorporates technologies from the Future Combat Systems 155mm NLOS-C (Non-Line-of-Sight Cannon), including modern electric gun drive systems to replace the current 1960s-era hydraulically-operated elevation and azimuth drives. The removal of the hydraulic systems saves the crew a tremendous amount of maintenance and they retain manual backups for gun laying just in case. The shift to an electric turret included a major redesign of the vehicle’s power system converting the 600 hp engine’s work into up to 70 kW of 600VDC/28VDC for use by various on-board systems. The power system’s modularity means that if any one of the motors inside fails it can be replaced in the field within less than 15 minutes using the same single part type. In soldiering terms that means the howitzer crew can handle the problem themselves and continue the mission instead of withdrawing for repairs.