Tech Briefs

Using rapid prototyping technology to fabricate reusable projectiles for a nonclassical gun experimental application.

Rapid prototyping (RP) is the term most commonly used to describe additive manufacturing technologies. An additive manufacturing technology is any manufacturing process that fabricates a part by adding one layer of material at a time, one on top of the other, to produce detailed 3-D geometries directly from 3-D computer-aided design (CAD) models.

Development of radius of curvature (ROC) for blunt impactor.

The additive manufacturing process generally uses a computer-controlled deposition/curing process to create the individual layers, eventually culminating in a 3-D reproduction of an input CAD geometry. Some processes produce finished, fully cured parts, and others produce parts that must be cured as an additional process. This differs from conventional machining, which can be thought of as subtractive manufacturing. Conventional machining creates a part by cutting away material from a piece of solid stock material. Conventional machining can be combined with computer-aided manufacturing (CAM) software to produce highly complex geometries directly from CAD models.

There are advantages and disadvantages to each process that must be considered each time a designer wishes to take his/her design to the manufacturing stage. Even with the advancements in CAM software for conventional machining, the initial setup process requires a substantial amount of time and effort by the designer and machinist every time a part is manufactured.

Generally, RP technologies are relatively easy to set up and operate. There is less interaction required between the designer and the person operating the machine, which is typically the biggest time saver and error reducer when comparing the two manufacturing methods. For the purpose of this experiment, an RP manufacturing technology was chosen by the designer based on these principles.

The initial parameters of this experiment pointed toward RP technologies as a viable option. The experiment required a lightweight and robust material that could survive several blunt impacts before being discarded. An SLS technology was selected and the material chosen was a glass-filled polyamide material that had adequate impact resistance and durability. This selection was based on the previous experience of the US Army Research Laboratory’s (ARL’s) Guidance Technologies Branch (GTB) in the design and fabrication of sabots for nontraditional shaped projectile geometries used in smoothbore-gun-launched applications.

SLS technology uses a bed of powdered material that is introduced to a laser. The laser is controlled by a computer to sinter the particles of powdered material to form the aforementioned layers of material one on top of the other until the entire geometry emerges fully cured.

As part of their behind helmet blunt trauma (BHBT) research initiative, the Warfighter Survivability Branch (WSB) of ARL’s Survivability/Lethality Analysis Directorate (SLAD) was commissioned to design and build a projectile that could be used to record impact data between itself and a variety of target materials. The projectile needed to provide stable, repetitive flight for a set distance between a compressed air cannon, developed by SLAD in collaboration with the Weapons and Materials Research Directorate’s Flight Sciences Branch, and a target. Experimental results needed to be recorded with high-speed photography and by data collection onboard the projectile using a commercial-off-the-shelf (COTS) onboard recorder (OBR).

As part of the experiment, a specific frontal geometry was needed that could produce the correct amount of force on a desired impact area. The concept behind selecting the geometry was to launch an instrumented projectile that would simulate the impact caused by the deformed helmet after defeating a ballistic threat. A schematic for the design concept of the blunt-impact simulator is shown in the illustration.

Varying frontal geometries were developed to be evaluated during the first phase of the experiment. Of these geometries, two specific frontal radii of curvature (RoCs) were chosen for use in Phase 2 of the experiment. Phase 2 consisted of taking the selected frontal geometries and adapting them to a projectile that contained a COTS OBR and power supply with an external interface for data download and power recharging. Leveraging specific expertise in creating internal gun-hardened electronics for a variety of high-g applications, GTB developed an internal electronics package containing a COTS OBR that could be custom fit into the projectile geometry chosen from Phase 1 with a few modifications. The final product was a robust self-contained projectile that could be reused over multiple firing events, providing many valuable impact data points to the customer.

This work has been done by Douglas A. Petrick for the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) here under the Manufacturing & Prototyping category. ARL-0215