M777 Howitzer Cannon - A Titanium Design Study

Topics Covered

Background

The Howitzer M777 Cannon

Improvements in The M777 Howitzer over the M198

Use of Titanium Enables the M777’s Weight to be Kept to A Minimum

Design Criteria for the Howitzer M777

Ti6Al4V, The Material of Choice

Ti6Al4V Justifies Its Choice

Design Challenges for the M777 Howitzer

Smaller System Footprint

Centre of Gravity

Manufacturing the M777 Howitzer

Streamlining the Production Process

Reducing Labour Requirements

Reduction in Raw Materials and Cycle Times

Performance Improvements of The M777 Howitzer

Successful Firing Trials

Background

Through the unprecedented media coverage of the war in Iraq we are all aware of the cost to military and civilian lives if firepower is not in the right position at the critical time. Accurate and mobile ground artillery plays an important role in modern, fast paced warfare, and a new ultra-lightweight field howitzer developed by BAE Systems RO Defence will be a huge asset to military forces when it is deployed in about a year’s time.

The Howitzer M777 Cannon

Howitzers are self-propelled or towable cannon artillery that can fire at both high and low angle trajectories. The new M777 155mm lightweight Howitzer will deliver a host of significant improvements compared to its predecessor the M198, which is being phased out of service.

Improvements in The M777 Howitzer over the M198

Improvements achieved in the M777 include a 42% reduction in basic system weight, a 25% reduction in size and retention of the 30km firing range using NATO-standard ammunition.

Use of Titanium Enables the M777’s Weight to be Kept to A Minimum

The M777s new capabilities and performance standards have been set by a revolutionary new design that enlists a material form unprecedented in land-based artillery - titanium Ti6Al4V alloy investment castings. The use of this material has enabled BAE Systems to attain the system weight goal established by the US Department of Defence (DOD), which was seeking a new design that would contribute to its military transformation effort.

Design Criteria for the Howitzer M777

Among the many systems under evaluation, the DOD wanted a new artillery piece that could be rapidly deployed in response to threats around the globe. With a production weight of 3.7kg, the M777 fits the bill and can be transported by helicopter, transporter aircraft or ship, and towed by a 4x4 vehicle weighing 2.5 tonnes or more.

Ti6Al4V, The Material of Choice

Bearing in mind the system requirements, candidate materials for the howitzer's major structural components were limited. So with many objectives linked to the transportability of the new howitzer, particularly by aircraft, it is no surprise that the designers turned to Ti6Al4V alloy, a long-standing choice among military aircraft designers for its unbeatable combination of light weight and high strength.

Ti6Al4V Justifies Its Choice

The idea of using this alloy for a howitzer’s structural components was met with scepticism by some. But titanium investment castings demonstrated their ability to meet tough criteria for weight reduction and overall ruggedness during numerous tests in the developmental phase, as well as in subsequent operational assessments. In addition, the material met the durability objectives set by the joint US Army and US Marine Corps. For example, the M777 achieved considerably more than a 900 ‘Mean rounds between system abort’ level of performance, exceeding the objective by a reassuring margin during tests. Moreover, it successfully completed a 20 year corrosion test, which called for the test gun to fire dozens of rounds after the corrosion testing had been completed.

Design Challenges for the M777 Howitzer

The M777 howitzer’s designers faced a number of challenges in meeting the tough weight, transportability and on-ground mobility objectives set for the new system. The M198, itself only a few decades old, represented a major improvement over its predecessor. Making another round of order-of-magnitude improvements taxed the limits of known production technologies for land-based weapons systems.

Smaller System Footprint

When exploring their options for the new artillery, designers envisioned a significantly smaller system footprint, and analysis showed that the desired levels of lightness and compactness could only be achieved by design changes that incorporated new and innovative dual function structures and pressure vessels.

Centre of Gravity

The design of the new system resulted in a non-traditional artillery configuration, which placed the centre of gravity (CG) in a ‘static-out-of-balance’ configuration. The CG of the recoiling mass is located in front of the howitzer's trunnions when the gun is not firing, an approach that allowed engineers to design the trunnions much closer to the ground than was possible with previous generations of weapons.

This design strategy served to position the weapon's overall CG as low as possible. Coupled with a low trunnion position, this counteracts the right hand torque generated by the weapon on firing. The recoiling mass remains within the system structure keeping the system highly stable.

Manufacturing the M777 Howitzer

Converting fabrications to castings does more than help deliver superior performance in the field, it also gives BAE Systems superior performance in its production process. These benefits include simplifying, streamlining, standardising and containing the cost of the manufacturing process.

Streamlining the Production Process

One significant achievement is the consolidation of 973 detailed parts into 196 single-piece castings - an 80% improvement. A part-count reduction of this magnitude helps streamline the production process by reducing or eliminating a host of manufacturing and administrative burdens that inevitably accompany alternative processes. Administrative record-keeping tasks in purchasing, supplier management, accounts payable, shipping, receiving, material handling, inventory management, inspection and more were all significantly streamlined or, in some cases, permanently eliminated as a result of part reduction.

Reducing Labour Requirements

Labour requirements were also reduced, since fabricated structural assemblies for the M777 would have required 2,458 welds, whereas the cast components need only 483 welds another 80% improvement. Moreover, the length of the welds was cut by 77%.

Reduction in Raw Materials and Cycle Times

Further savings accompanying conversion to the casting process include reduced raw material input requirements and much shorter manufacturing cycle times - typically 25-50% less than the cycle time for the fabrication of respective assemblies. These added savings were made possible by the tighter control of process variables that now characterises the investment casting process. Today, the parts that emerge from the process are completely interchangeable. This is a knock on effect of continuous improvements in the precision, reliability and repeatability of the wax injection, shell-making, vacuum melting, casting and heat treating phases of the investment casting processes, which are all automated.

Reductions in cycle time are also achieved thanks to vertical integration within Howmet, and the expanding role the company is playing in managing finish-to-print tasks. By way of example, the majority of material and technical inputs into the casting production process were accomplished at the company's investment casting campus in Whitehall, Michigan. This single location is home to Howmet’s research centre, which used SLA pattern making in the engineering and manufacturing development stage of the project to make test articles and prototypes rapidly and relatively inexpensively.

Whitehall is also home to Howmet’s titanium alloy production and speciality material production facilities. The Specialty Materials operation produces the waxes used in the pattern making process, and is adjacent to the Ti-Alloy and Ti-Cast operations, which provide the raw material and foundry resources for the majority of the Howmet castings used in the M777 howitzer. The castings for the M777 are all hot isostatic pressed in Howmet’s facility, co-located with the Ti-Cast operation.

Howmet's Temperaft operation in Cleveland, Ohio, was called on to make the tooling for the howitzer’s castings. Tempcraft also designs, builds and supplies the microprocessor controlled wax injection equipment used to create the patterns required for the investment casting lost wax process.

Performance Improvements of The M777 Howitzer

Performance capabilities in a range of crucial combat support areas have also been greatly improved, such as a 25% increase in the ability to destroy enemy vehicles in close support combat, a 70% improvement in survivability, and a 500% increase in counter fire exchange ratios. The lightness and compactness of the new system makes it extremely nimble, and it can be set up in three minutes, move out in two minutes, and conduct firing missions from as many as four different locations within one hour.

Successful Firing Trials

Initial firing trials of the M777 met with success last summer and a US Army/US Marine Corp operational test and evaluation phase is scheduled for August 2004. The new lightweight howitzer looks set for its role as the US Army’s new generation of medium force weapon, with the British and Italian Armies expected to place orders soon.

 

Source: Materials World, Vol. 12, No. 6, pp. 22-24, 2004.

 

For more information on this source please visit The Institute of Materials, Minerals and Mining.

 

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