US Army test fires 3D printed grenade, grenade launcher
This demonstration shows that additive manufacturing (commonly known as 3-D printing) has a potential future in weapon prototype development, which could allow engineers to provide munitions to Soldiers more quickly, the US Army said earlier this month.
The printed grenade launcher, named RAMBO (Rapid Additively Manufactured Ballistics Ordnance), was the culmination of six months of collaborative effort by the U.S. Army Research, Development and Engineering Command (RDECOM), the U.S. Army Manufacturing Technology (ManTech) Program and America Makes, the national accelerator for additive manufacturing and 3-D printing.
Every component in the M203A1 grenade launcher, except springs and fasteners, was produced using additive manufacturing (AM) techniques and processes. The barrel and receiver were fabricated in aluminium using a direct metal laser sintering (DMLS) process. This process uses high-powered precision lasers to heat particles of powder below their melting point, essentially welding the fine metal powder layer by layer until a finished object is formed. Other components, like the trigger and firing pin, were printed in 4340 alloy steel, which matches the material of the traditional production parts.
The purpose of this project was to demonstrate the utility of AM for the design and production of armament systems. A 40 mm grenade launcher (M203A1) and munitions (M781) were selected as candidate systems.
To be able to additively manufacture a one-off working testable prototype of something as complex as an armament system would radically accelerate the speed and efficiency with which modifications and fixes are delivered to the warfighter, the US Army said. Researchers would be able to manufacture multiple variations of a design during a single printing build in a matter of hours or days instead of waiting months for a prototype.
Depending on a part’s complexity, there can be numerous steps involved before it is ready for use. For instance, in the case of RAMBO, the printed aluminium receiver and barrel required some machining and tumbling. After printing, the components were cut from the build plate, and then support material was removed from the receiver.
The barrel was printed vertically with the rifling. After it was removed from the build plate, two tangs were broken off and the barrel was tumbled in an abrasive rock bath to polish the surface. The receiver required more post-process machining to meet the tighter dimensional requirements. Once post-processing was complete, the barrel and receiver underwent Type III hard-coat anodizing, a coating process that’s also used for conventionally manufactured components of the M203A1. Anodizing creates an extremely hard, abrasion-resistant outer layer on the exposed surface of the aluminium.
The barrel and receiver took about 70 hours to print and required around five hours of post-process machining. The cost for powdered metals varies but is in the realm of $100 a pound. This may sound like a lot of time and expensive material costs, but given that the machine prints unmanned and there is no scrap material, the time and cost savings that can be gained through AM are staggering. The tooling and set-up needed to make such intricate parts through conventional methods would take months and tens of thousands of dollars, and would require a machinist who has the esoteric machining expertise to manufacture things like the rifling on the barrel.
Beyond AM fabrication of the weapon system, ManTech also requested that a munition be printed. Two RDECOM research and development centres, the U.S. Army Edgewood Chemical and Biological Centre (ECBC) and the U.S. Army Research Laboratory (ARL), participated in this phase of the project to demonstrate RDECOM’s cross-organizational capabilities and teaming. An integrated product team selected the M781 40 mm training round because it is simple and does not involve any energetics—explosives, propellants and pyrotechnics are still awaiting approval for use in 3D printing.
The M781 consists of four main parts: the windshield, the projectile body, the cartridge case and a .38-caliber cartridge case. The windshield and cartridge case are traditionally made by injection molding glass-filled nylon. ARL and ECBC used selective laser sintering and other AM processes to print glass-filled nylon cartridge cases and windshields for the rounds.
The .38-caliber cartridge case was the only component of the M781 that was not printed. The .38-caliber cartridge case was purchased and pressed into the additively manufactured cartridge case. Research and development is underway at ARDEC to print energetics and propellants.
In current production, the M781 projectile body is made of zinc. Zinc is used because it’s easy to mass-produce through die-casting, it’s a dense material and it’s relatively soft. The hardness of the projectile body is critical, because the rifling of the barrel has to cut into the softer obturating ring of the projectile body. The rifling imparts spin on the round as it travels down the barrel, which improves the round’s aerodynamic stability and accuracy once it exits the barrel. Currently, 3D printing of zinc is not feasible within the Army. Part of the beauty of AM is that changes can be made quickly and there is no need for retooling.
ARDEC researchers used modeling and simulation throughout the project to verify whether the printed materials would have sufficient structural integrity to function properly. Live-fire testing was used to further validate the designs and fabrication. The printed grenade launcher and printed training rounds were live-fire tested for the first time on Oct. 12, 2016, at the Armament Technology Facility at Picatinny Arsenal, New Jersey.
Testing included live firing at indoor ranges and outdoor test facilities. The system was remotely fired for safety reasons, and the tests were filmed on high-speed video. The testing included 15 test shots with no signs of degradation. All the printed rounds were successfully fired, and the printed launcher performed as expected. There was no wear from the barrel, all the systems held together and the rounds met muzzle velocities within 5 percent of a production M781 fired from a production-grade grenade launcher. The variation in velocities were a result of the cartridge case cracking, and the issue was quickly rectified with a slight design change and additional 3-D printing. This demonstrates a major advantage using AM, since the design was modified and quickly fabricated without the need for new tooling and manufacturing modifications that conventional production would require. More in-depth analysis of material properties and certification is underway. The RAMBO system and associated components and rounds are undergoing further testing to evaluate reliability, survivability, failure rates and mechanisms.
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