Three-dimensional photophoretic aircraft made from ultralight porous materials can carry kilogram-scale payloads in the mesosphere

We show theoretically that photophoretic aircraft would greatly benefit from a three-dimensional 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 Reynolds number) and momentum (high Reynolds number) 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 large vehicles can be made from nanocardboard material, as previously demonstrated at smaller scales, the minimum altitude such vehicles can levitate at is approximately 55 km, while the payload can reach approximately 1 kg at an altitude of 80 km for vehicles with a 10 m diameter. 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.