The team then focused on an alternative approach—designing a cylindrical capacitor capable of reacting structural loads. To fabricate this concept, technicians would cover a dielectric-coated conductor with a conductive metal layer. Next, they would integrate the coaxial system into a hybridized, composite weave with carbon tow for additional reinforcement. Finally, they would lay up the material on a tool and inject it with resin to create an air vehicle’s primary load-bearing structure. Dielectric characteristics, which are the Achilles’ heel of any electrostructural system, govern capacitor performance. While the cylindrical capacitor approach enables precise control over dielectric layer quality, designing the energy’s ingress and egress paths into the structure is more complicated than simply using the parallel plate capacitor concept. Nevertheless, using the cylindrical capacitor composite allows engineers to integrate tens of thousands of feet of capacitor into an air vehicle’s structure.

To demonstrate the basic concept, AFRL researchers used a commercially available copper wire with Kapton™, an aromatic polyamide dielectric material available in numerous manufactured forms. Kapton material has voltage breakdown strengths of around 4500 V/mil and offers low electromagnetic losses. While designers did not intend for this material to form the basis of a transitionable solution, they did believe it capable of showing concept feasibility and establishing future research activity parameters. For the purposes of the initial feasibility investigation, technicians fabricated the first specimens with a conductive copper paint offering a uniform coat and good adhesion. The team conducted an initial capacitor test to examine performance using a Hewlett-Packard 4284A precision impedance meter. Results showed good capacitance, inductance, and resistance from 1 kHz to 1 MHz. The team considered these results significant, given the limited time spent optimizing the system.

The team then integrated the design into an experimental piece of carbon fabric material, evaluated its ability to maintain structural capacitance, and demonstrated the weaving process viability. This process required a significant amount of capacitive wire, since 1.0 sq ft of plain weave fabric requires 120 ft of cylindrical capacitor when used only in the fabric warp direction at 10 pics/in. The team created a small, taskspecific weaving machine to produce a structural capacitor fabric coupon measuring 8.0 in. wide by 14.0 in. long. Testing this first-generation specimen (see Figure 1) verified the capacitor concept’s preliminary feasibility. Researchers subsequently used this demonstrated performance in an integrated conceptual air vehicle fuselage design that ultimately generated 168.92 μF at high-voltage operation.