Starship Sails Propelled by Cost-Optimized Directed Energy

TL;DR

This paper explores cost-optimized designs for microwave and laser propelled sails for interstellar travel, analyzing system costs, materials, and configurations to identify the most efficient propulsion methods.

Contribution

It introduces a cost-based framework for designing and comparing different beam-driven sail propulsion systems, including microwave, millimeter wave, and laser options.

Findings

Optimal sail size increases with lower mass and higher frequency.

System costs scale with kinetic energy and inversely with sail diameter and frequency.

Microwave, millimeter wave, and laser systems are currently of similar cost.

Abstract

Microwave propelled sails are a new class of spacecraft using photon acceleration. It is the only method of interstellar flight that has no physics issues. Laboratory demonstrations of basic features of beam-driven propulsion, flight, stability ('beam-riding'), and induced spin, have been completed in the last decade, primarily in the microwave. It offers much lower cost probes after a substantial investment in the launcher. Engineering issues are being addressed by other applications: fusion (microwave, millimeter and laser sources) and astronomy (large aperture antennas). There are many candidate sail materials: carbon nanotubes and microtrusses, graphene, beryllium, etc. For acceleration of a sail, what is the cost-optimum high power system? Here the cost is used to constrain design parameters to estimate system power, aperture and elements of capital and operating cost. From general relations for cost-optimal transmitter aperture and power, system cost scales with kinetic energy and inversely with sail diameter and frequency. So optimal sails will be larger, lower in mass and driven by higher frequency beams. Estimated costs include economies of scale. We present several starship point concepts. Systems based on microwave, millimeter wave and laser technologies are of equal cost at today's costs. The frequency advantage of lasers is cancelled by the high cost of both the laser and the radiating optic.