Aurora’s approach to prototype manufacturing is to seek the best trade-off between technical aspects such as weight, materials, design, fabrication and testing, Chu explains. For a prototype program in which a small number of units were to be made, limiting tooling costs became a critical factor since the tooling costs could not be amortized across quantities.
“Because standard tooling made of carbon or metal can be costly, we explored low cost tooling made of materials like foam or medium density fiberboard (MDF),” Chu says. “However, this tooling’s downside is that it’s limited to low cure temperatures, typically below 200 F. So we were forced to use materials that have low temperature cures and that do not require autoclaving. These materials typically have lower interlaminar strengths, but similar in-plane strengths to higher temperature, autoclaved materials. As a result, more material was needed in certain areas because of the materials low interlaminar strength. We also had to increase the number of parts slightly in order to have a design layout with loads paths appropriate to the materials,” he explains.
A combination of plain-weave carbon fabric prepregs and unidirectional prepregs were chosen for their properties, and the parts were manufactured by hand lay-up. “Assembly was done using the upper skin cure fixtures as a platform to reduce the amount of assembly tooling required,” says Chu.
Boeing gave Aurora full design authority for the wing and the two teams worked collaboratively throughout the project. This enabled a confident concurrent design and manufacture that significantly shortened the project schedule, Chu notes. “Our on-time and on-budget delivery of the wing is all the more impressive given that the wing was 12 percent under its weight allocation,” says Aurora CEO John Langford. “The design team performed extensive stress analysis to squeeze out each ounce where they could,” Chu adds. “For example, rather than using a uniform or constant minimum thickness layup, which would have been easier but heavier, the design team generated custom, optimized ply layups for each part. They also looked at the whole wing system and oriented the design so that more of the load was taken by the spar and less by the skins than in a typical wing design because of the weight savings.”