Graphite-aluminum composites in advanced space radiators

摘要

The objective of DWA Composite Specialties, Inc., in this second phase SBIR program was to fabricate a full-scale space radiator built around an experimental folding heat pipe supplied by Thermocore, Inc., of Lancaster, PA. The radiator framework and radiating panels had a mass of 1.5 kg/m (1 pound/foot) of radiator length, utilizing 16 square meters of tapered graphite-aluminum panel. This effort grew out of a successful Phase 1 SBIR program investigating the application of high thermal conductivity metal matrix composite materials to space radiators. Results of analysis conducted i1 the Phase 1 effort indicated that thermal transfer per unit mass of radiator could be improved by approximately 17 percent by substituting a high thermal conductivity graphite-aluminum metal matrix composite (MMC) for conventional aluminum in radiating surfaces. DWA fabricated the internal support structure around the heat pipe and assembled a series of graphite-aluminum panels into a lenticular section radiator of monocoque construction. The lenticular section was about 32 mm (1.25 inch) deep in the center, tapering to about 2 mm (.06 inch) at the edges. The active radiator was approximately l3.5 m (44 feet) by 0.6 m (2 feet). MMC surfaces were applied to both sides of the heat pipe. The thermal transfer rate per unit mass was optimized by fabricating the MMC radiator skins themselves with a tapered cross section, varying in thickness from 0.63 mm (.025 inch) at the center to 0.20 mm (.008 inch) at the outer edges. An analysis determined the taper required to maintain a constant thermal flux density through the radiator from the root to the outer edge. The analysis indicated that a 30 percent weight savings could be achieved without sacrificing thermal transfer rate. The graphite-aluminum panels were fabricated by consolidating a set of plies of uniform thickness and varying widths approximating the profile specified. The minimum ply thickness available through DWA's manufacturing process is approximately 0.13 mm (.005 inch). The panels therefore consisted of 5 layers of graphite-aluminum stacked to form the required profile. All the graphite in the panels was aligned with the fibers perpendicular to the heat pipe. This orientation of fibers resulted in the maximum thermal conductivity away from the heat pipe. In addition to the construction of the radiator, a series of thermal transfer performance and thermal conductivity measurements were made. DWA constructed a small thermo-vacuum chamber with a liquid nitrogen-cooled cold wall. Six specimens were tested: copper, aluminum, tapered unidirectional graphite-aluminum, constant thickness unidirectional graphite-aluminum (in both orietations), 0-90 cross-plied graphite-aluminum, and /- 45 cross-plied graphite-aluminum. The thermal conductivity of the same materials was measured. For the anisotropic composite materials, the conductivity was measured in three orthogonal directions.