Experimental tests of beam-riding sail dynamics

Stability is a neglected issue in proposals to propel light sails by beamed power. Whether the beam comes from a laser or a microwave antenna, power falls with angle from the beam center. This drives a sail sideways under any lateral perturbation—“tumbling down the hill.”. While spin can help stabilize, the basic mechanics of pressures and sail averaging of them across its area remain unexplored in experiment, and only recently treated in theory. Here we report what is to our knowledge the first attempt to study beam‐riding dynamics in the laboratory, using a slightly overweighted pendulum. A sail attached to the pendulum bottom can be made unstable by adding weight to the top end. Stability and oscillation are possible if this is corrected by electrodynamic beam pressure on the sail, directed from below, torquing the pendulum. We present both data and analysis. Our major points are: Microwave powers of a few hundred W can hold a sail steady. This is made possible because of the gradient in beam power with sidewise angle. Our experiments agree with the Univ. New Mexico numerical studies which show similar stability conditions. At higher powers, the sail can be oscillated in angle. Time‐dependent feedback of beam power can manage a sail into stable motions across the beam. Theory shows this, but experiments are not yet done. Beam powers comparable to the strength of perturbing forces can plausibly achieve these effects in free sail flight.