Lightweight Howitzer casts a lean shadow

Nov. 20, 2003
Investment cast titanium helps drop cannon weight by 42%.
The major structures of the M777 Lightweight 155-mm Howitzer are made from investment cast 6Al4V titanium. Investment casting drops the cannon's weight to 9,200 lb, 42% less than its M198 Howitzer predecessor.

It's axiomatic that the lighter the equipment, the more rapidly it can deploy. This is especially true of armament systems such as the Lightweight 155-mm Towed Howitzer. It needed to shed close to half its weight to meet new rapid deployment criteria a joint U.S. Army and Marine Corps team devised.

Moreover it had to lose weight without sacrificing any of the 30-km range of the gun it was intended to replace. A business that is now part of U.K.-based BAE Systems -- RO Defense got the nod for its design.

The prototype, now called the M777 Lightweight 155-mm Howitzer, weighs 9,200 lb (42% less than its M198 Howitzer predecessor) and fires NATO-standard 30-km-range ammunition. Another version, Model M777E1, incorporates a 500-lb digital fire-control system boosting its weight to slightly more than 9,700 lb.

The M777 took advantage of lightweight, high-strength aerospace materials. It's also the first time investment-cast titanium has served in a land-based armament system. The gun employs 28 investment-cast titanium parts, most of which come from Howmet Castings, an Alcoa business, Whitehall, Mich. Besides being significantly lighter, the M777 is 25% smaller and more compact than its predecessor. This smaller "hoof print" came from design changes characterized by dual-function structures and pressure vessels, and by placing the system center of gravity "out of balance."

The M777's configuration is described as "static-out-of-balance." The CG of the recoiling mass sits in front of the Howitzer's trunnions (i.e., pivot points) when at rest. This design lets the trunnions be close to the ground, keeping the weapon's overall CG as low as possible. The low CG counteracts the right-hand torque the weapon generates when it fires. The recoiling mass remains within the system structure during firing and keeps the relatively compact, lightweight system stable.

Three primary advantages of investment casting provided the impetus to move away from hogouts and fabricated assemblies: leaner manufacturing, reduced cycle time, and lower costs. For example, conversion to castings dropped part count by 57% and reduced weldments by more than two thirds. It also slashed manufacturing cycle time and cost significantly. Typically, the casting processes takes just 25 to 50% of the time needed to hogout or fabricate an assembly. Casting also uses less raw material, less labor requirements and shop-floor space, and results in fewer part numbers to track.

Part-count reduction simplifies assembly, welding, and inspection, while streamlining and simplifying administration. The ability of the investment-casting process to produce large, complex, asymmetrical, near-net shapes expands design freedom and can vastly simplify production, assembly, and finishing operations. Cast-in features, for example, such as datum pads help accelerate setup and machining operations.

The major structures of the new Howitzer are made from 6Al4V titanium. Primary selection criteria included lightweight, high strength, and elastic properties. Originally designed as titanium fabricated assemblies, the investment castings improve operational ruggedness, parts commonality, and interchangeability. In addition, the M777 got significantly higher ratings for survivability, partly because it can be emplaced in 3 min and displaced in two, thus facilitating "shoot-and-scoot" tactics.

The M777 also successfully completed a 20-yr corrosion test, with test guns firing dozens of rounds.

Investment casting capabilities and benefits

Parts count reduction: Reduces assembly
Large shapes: Reduce welding/inspection
Complex shapes: Enhance design freedom
Near-net shapes: Trim raw material requirements
Asymmetrical shapes: Promotes simplification
Cast-in features: Significantly cut machining
As-cast surfaces: Streamlines finishing operations

PART COUNT (CASTING VERSUS FABRICATION)

Titanium
Structure
Fabrication Casting
Cradle 324 172
Body 215 166
Saddle 116 9
Stabilizers 70 2
Spades 120 60
Trails 98 8
Elevating yoke 19 1
Buffer yoke 11 1
Total 973 419

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