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global positioning, a laser range finder, and the ability to pinpoint and provide coordinates of hostile forces, whether they are on foot or in vehicles. LRAS3 is mounted on vehicles


such as Humvees and Strykers, but it can be used dismounted, as well. It enables Army Scouts to conduct 24-hour reconnaissance and surveil- lance missions while remaining outside the firing range of their adversary.


Challenging Considerations Te critical constraint the team


encountered with mechanical com- ponents used to create the LRAS3 involved design and development of the main sensor housing. Tis rect- angular housing (approximately 16 x 18 x 24 inches) needed to serve as an electronics housing, thermal manage- ment system, optical bench and system superstructure (Fig. 2). Lead time and system cost would


be driven by success or failure to achieve these functions. And, because this product was intended for light vehicles or dismounted applications, every bit of weight reduction would be valued by the final customer—the Scout. To meet the program objectives


for a robust yet cost effective system, Raytheon engineers performed a trade study comparing fabrication process alternatives including com- posites, weldment, metal assembly and casting. Te aluminum invest- ment casting process was selected because of its ability to provide complex configuration details that would reduce the piece count, shorten the design cycle, streamline assembly steps and ultimately lead to the low- est total cost for the system.


Design and Development Casting design and development


is not new to Raytheon. Te North Texas design team supporting Combat Systems programs has 40+ years of experience with complex casting appli- cations, including a team of mechani- cal producibility engineers who serve as subject matter experts in the design application of manufacturing processes and technologies. Te Raytheon Precision Manufacturing organiza-


Figure 1. The LRAS3 is used by U.S. Army Scouts.


tion provides stereolithography rapid prototyping, quick-turn prototype machining, and turnkey machining and finishing processes for production rate requirements. It is supported by an in-house supply chain organiza- tion. Each of these skills would prove vital throughout the development and fabrication phases of this project. Te overall size of the housing was


predicated on the subsystems, includ- ing electronics and optical elements. Once the ballpark size was estab- lished, it became important to define the required casting wall thickness. Although calculations showed thin walls would satisfy the anticipated structural loads throughout the hous- ing, the practical aspects of fluidity and


castability tilted the ultimate design toward a compromise that would be both lightweight and producible. As bosses, mounts, and other


functional features were modeled into the interior of the housing, wall sec- tions were relieved with cutouts and configurations to maintain a consistent wall thickness for castability and fur- ther reduce the weight. Tis created a significant challenge for the toolmaker and wax injection personnel (over 300 inserts were required), but ultimately was key to the success of this configu- ration. While the structural analysts were crunching their numbers based on estimates of internal component mass, the thermal analysts were devel- oping the approach required to heat sink the circuit cards and electronics. Teir solution resulted in a tall and dense forest of external fins, sufficient to provide required cooling through radiation to the environment (Fig. 3).


Determining Castability Once the general configuration


Figure 2. This is a concept drawing for the LRAS3 sensor housing.


parameters were set, it became pos- sible to evaluate the relative castability of these concepts. Stereolithography patterns were produced in several variations using the in-house capabil- ity of the Rapid Prototyping Lab at Raytheon Precision Manufacturing (Fig. 4). Tese patterns were invested and cast to evaluate feeding, fill, and aspects of metal fluidity.


July 2013 MODERN CASTING | 29


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