3D photophoretic aircraft made from ultralight porous materials can carry kg-scale payloads in the mesosphere

We show that photophoretic aircraft would greatly benefit from a three-dimensional (3D) hollow geometry that pumps ambient air through sidewalls to create a high-speed jet. To identify optimal geometries, we developed a theoretical expression for the lift force based on both Stokes (low-Re) and momentum (high-Re) theory and validated it using finite-element fluid-dynamics simulations. We then systematically varied geometric parameters, including Knudsen pump porosity, to minimize the operating altitude or maximize the payload. Assuming that the large vehicles can be made from previously demonstrated nanocardboard material, the minimum altitude is 55 km while the payload can reach 1 kilogram for 3D structures with 10 meter diameter at 80 km altitude. In all cases, the maximum areal density of the sidewalls cannot exceed a few grams per square meter, demonstrating the need for ultralight porous materials.