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Reticulated Foam Materials for the Evaporative Cooling of Hypersonic Vehicles & for Control of Secondary Electron Emission in Space Electric Propulsion

  • Author(s): Dickstein, Dylan Abraham
  • Advisor(s): Ghoniem, Nasr M
  • et al.
Abstract

An experimental- and simulation-based approach is used to investigate the fundamentals and applications of aerospace ``metamaterials''. This dissertation trisects the subject and delves deeply into the following topics: secondary electron yield suppression, controlling transpirant in hypersonic leading edges, and mass loss processes via arc jet testing. All three segments use computer simulations to produce data whereas experiments are incorporated for only the former and latter. In the first topic, secondary electron yield (SEY) is measured in solid copper and copper foam through direct measurements using a high vacuum electron gun chamber at The Aerospace Corporation in El Segundo, CA. These results are compared with ray-tracing Monte Carlo simulations. It is found that the 4.6\% volume fraction copper foam has approximately a 20\% reduction in yield compared with its solid counterpart. Furthermore, SEY is determined to be inversely proportion to volume fraction. The second focus is investigated by modeling a 5$^\circ$ half-angle leading edge wedge with a semicircular nose tip of 1 mm radius of curvature. The complex method of optimization is used to maximize the cooling effectiveness in a porous leading edge in Mach 6 conditions. We found that by focusing transpirant on regions with the highest incident heat flux, the wedge inherits a 6\% increase in cooling effectiveness compared with the isotropic permeability case. In this manner, foam volume fraction is inversely proportional to incident heat flux. The last segment is explored by performing arc jet experiments on lanthanum hexaboride (LaB$_6$) discs, corroborating results with computer simulations of the setup, and involving such characterization tools as microscopy, spectroscopy, and X-ray diffraction. It was measured that an incident heat flux of 19.5 MW/m$^2$ causes a LaB$_6$ surface to recess at 0.11 mm/s. Moreover, the material was determined to have a nearly constant surface recession rate for energy fluence magnitudes exceeding about 400 MJ/m$^2$.

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