Advanced Composites Design for the WASP Project

In addition to working frequently with plastics and reinforced polymer compounds, the staff of Metis Design is also experienced in engineering with advanced composite materials. One application where advanced composite materials are necessary is with unmanned aerial vehicles (UAV's). UAV's must have a small profile to avoid detection, while still having the ability to travel long distances. Therefore the structure of the vehicle must be very strong yet very light.

The Wide Area Surveillance Projectile, or WASP, project was a DARPA seedling initiative led by Draper Laboratory to design a low cost UAV that would be capable of being launched from an artillery shell in order to provide down-range loitering surveillance. As consultants on this program, Metis Design collaborated with Draper to provide systems engineering support, alternate concepts for operation and composites engineering. This composite work focused on the design, analysis and manufacture of WASP's structural components including the fuselage and wings, which had to withstand launch forces of up to 15,000 g's.

Phase I of the project, demonstrated that aluminum components were too heavy for the application. During Phase II, carbon fiber reinforced polymer composites were introduced as the principal structural material. The use of composite materials added significant complexity to the design, and several analyses were necessary to prove that the material could perform successfully. MDC worked on the design and analysis of the composite laminates for the fuselage, shroud, wings and attachments.

For the fuselage, candidate laminates were examined using a classical laminated plate theory (CLPT) code written in Matlab. Next, I-DEAS was used to generate a finite element mesh for each component. Then static and dynamic analyses were performed on the mesh using ABAQUS. For the static case, the stresses determined by the ABAQUS analysis were input to an equation solver to determine if the design would fail. The dominant failure mode was predicted by comparing the results of the dynamic and static models.

In order to establish that the predicted failure loads were accurate for given laminates, compression test experiments using a servo-hydraulic testing machine were performed. Test laminates were manufactured at MIT to the specifications determined during the analysis phase. The quantitative comparison of prediction and experiment was reasonable, though not sufficiently accurate to permit complete detailed design without testing. The dominant failure mode for the structure, determined in the testing, was generally consistent with that determined by the analysis, however. This result showed that compression tests could be used to prove the capability of a structure. This testing, which could be performed at MIT, reduced the amount of expensive air-gun testing the project needed to perform.

AS4/3501-6 graphite epoxy pre-preg was used for the fuselage. Such a material was too stiff for the wing and tail sections, so a wet lay-up manufacturing method was used at those locations instead. The airfoil design employed a lightweight top and bottom laminate, with a foam-filled center. Like the fuselage, the aerodynamic structures were extensively tested to establish structural integrity. These structures underwent an additional bending test due to concern about the cantilever design of the wings.

One of the most challenging design problems in WASP was the design of the wing attachments. The wings needed to fold in order to fit in the artillery shell. In the interest of time, standard spring-loaded stainless steel cabinet hinges were bonded between the two wing sections using a micro-fiber thickened epoxy mixture. Holes were drilled through the hinges to allow the flowing epoxy to form a rivet-like connection between the laminate and the metal. The hinge was also covered by a small piece of woven carbon fabric to help prevent delamination of the bond. The wings, tail fins and rudder were all connected to main structure using CNC-machined aluminum root pieces. Additionally, a thin steel shroud was designed to protect the stowed device during deployment. A finite element analysis was performed to solve for the stresses experienced by the shroud during loading. The determined stresses agreed with earlier calculations, confirming that the shroud would protect the WASP vehicle with a large margin of safety.

The design of WASP resulted in a patent, which was awarded the Best Patent of the Year by Draper Laboratories for 2002.

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