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We show that photophoretic aircraft would greatly benefit from athree-dimensional (3D) hollow geometry that pumps ambient air through sidewallsto create a high-speed jet. To identify optimal geometries, we developed atheoretical expression for the lift force based on both Stokes (low-Re) andmomentum (high-Re) theory and validated it using finite-element fluid-dynamicssimulations. We then systematically varied geometric parameters, includingKnudsen pump porosity, to minimize the operating altitude or maximize thepayload. Assuming that the large vehicles can be made from previouslydemonstrated nanocardboard material, the minimum altitude is 55 km while thepayload can reach 1 kilogram for 3D structures with 10-meter diameter at 80 kmaltitude. In all cases, the maximum areal density of the sidewalls cannotexceed a few grams per square meter, demonstrating the need for ultralightporous materials.

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