  Breakthrough Starshot is an initiative to prove ultra-fast light-driven
nanocrafts, and lay the foundations for a first launch to Alpha Centauri within
the next generation. Along the way, the project could generate important
supplementary benefits to solar system exploration. A number of hard
engineering challenges remain to be solved before these missions can become a
reality.
  A system model has been formulated as part of the Starshot systems
engineering work. This paper presents the model and describes how it computes
cost-optimal point designs. Three point designs are computed: A 0.2 c mission
to Alpha Centauri, a 0.01 c solar system precursor mission, and a ground-based
test facility based on a vacuum tunnel. All assume that the photon pressure
from a 1.06 {\mu}m wavelength beam accelerates a circular dielectric sail. The
0.2 c point design assumes \$0.01/W lasers, \$500/m$^2$ optics, and \$50/kWh
energy storage to achieve \$8.0B capital cost for the ground-based beam
director. In contrast, the energy needed to accelerate each sail costs \$6M.
Beam director capital cost is minimized by a 4.1 m diameter sail that is
accelerated for 9 min. The 0.01 c point design assumes \$1/W lasers,
\$10k/m$^2$ optics, and \$100/kWh energy storage to achieve \$517M capital cost
for the beam director and \$8k energy cost to accelerate each 19 cm diameter
sail. The ground-based test facility assumes \$100/W lasers, \$1M/m$^2$ optics,
\$500/kWh energy storage, and \$10k/m vacuum tunnel. To reach 20 km/s, fast
enough to escape the solar system from Earth, takes 0.4 km of vacuum tunnel, 22
kW of lasers, and a 0.6 m diameter telescope, all of which costs \$5M.
  The system model predicts that, ultimately, Starshot can scale to propel
probes faster than 0.9 c.
