Payload Carrying Capabilities of Ultralight Lightsail Spacecraft

Lightsails harvest the radiation pressure of incident light for the propulsion of spacecraft, eliminating the need to carry and expel reaction mass, and in principle allowing acceleration to relativistic velocities.¹ Recent efforts have sought to develop technologies for ultra-lightweight lightsail space probes that could be launched using an earth-based high-power-intensity laser, allowing execution of interstellar flyby missions on a time scale of a few decades.² This mission concept calls for a spacecraft comprising a ~1 g, ~10 m² lightsail, carrying a payload of similar mass, including all sensors, communications systems, and power sources.³ Prior work has identified materials and technologies suitable for construction of lightsails and payload components within these extremely limited mass budgets.⁴ However, the question remains as to whether such large-area ultrathin lightsail membranes could support the attachment of payloads without tearing or destabilizing during acceleration, owing to the changes in mass distribution and the concentration of inertial forces at the points of attachment.<br/><br/>Here, we study the effects of various payload integration schemes on the behavior of interstellar lightsail spacecraft during acceleration, to determine the limits on payload mass and distribution imposed by the strength and stability of the lightsail. Specifically, we look at lightsails made from silicon nitride (SiN_x), with either specific shape curvature or flat photonic metagrating patterned surfaces, which have been previously shown to achieve beam-riding stability in the absence of an attached payload.⁵ We start with static structural analysis to determine the limits on localized mass loading for the membranes, concluding that for the nominal acceleration target of ~10⁵ m/s², the membranes can support center-mass payloads on the order of 100 g. However, we show that localized reinforcement of the membrane, by increasing its thickness, can increase this limit several-fold with modest mass penalty to the sail. Furthermore, distributing the payload mass between multiple components spaced across the sail can further reduce the peak stress. <br/><br/>Then, a time-domain mesh-based simulation tool, specifically developed for lightsail modelling, is employed to study the behavior of unconstrained flexible membranes with integrated payload mass(es) during acceleration. This allows us to evaluate not only the mechanical integrity of the lightsail by observing the dynamic stress distribution, but also the effects of payload integration on the trajectory and ultimately the beam-riding stability of the integrated sailcraft. We have observed that modest (~100 g) center-located payloads can be carried by the previously reported self-stabilizing lightsail designs, with little reduction in stability or strength margins, and are working to further optimize designs to determine the limiting payload mass that can be carried in center-located and distributed-mass configurations. Results of the simulations will be presented, including acceleration trajectories, stress and temperature distributions, and animations of shape perturbations. <br/><br/>1. Lubin, P. <i>JBIS </i><b>2016,</b> 69, 40-72.<br/>2. Breakthrough Starshot Initiative. https://breakthroughinitiatives.org/initiative/3<br/>3. Parkin, K. L. G, arXiv:1805.01306<br/>4. Atwater, H. A.; Davoyan, A. R.; Ilic, O.; Jariwala, D.; Sherrott, M. C.; Went, C. M.; Whitney, W. S.; Wong, J. <i>Nat Mater </i><b>2018,</b> 17, (10), 861-867.<br/>5. Gao, R.; Kelzenberg, M. D.; Atwater, H. A. <i>In Preparation</i> (<b>2022</b>)