Laser starway to reduce energy requirements and focusing problems for laser pushed solar sails

Centauri Dreams covered the Charles Quarra Light Bridge (laser stairway) which was presented at the 2013 Starship Congress (48 pages).

Preprints of the paper is here

The ‘laser starway’ concept Charles Quarra present is a natural evolution of the work of both Robert Forward and Geoffrey Landis in extending the reach of beamed power into deep interstellar space, by taming the beam divergence that is ever present in all laser wavefronts. Beamed power gives us the possibility of leaving the source of energy at home, avoiding the exponential blowout of energy requirements imposed by the Tsiolkovsky rocket equation. But beamed propulsion is far from devoid of issues: The pointing accuracy, the huge laser sources and sails tens or hundreds of kilometers wide demand engineering capabilities that are still far from our current horizon.

Conceptually, the starway tries to push the idea of multiple lenses for beam refocusing, analyzed by Landis in the 1990s, in the direction of making them reusable: Can we take a string of lenses, deploy them between two stars and keep them operational for long periods?

The principal parameter that determines the available power for sail propulsion on the light bridge structure is the optical efficiency of the relays: As the optical efficiency of the relays stays below the critical value (which for a starway made of thousands of nodes will imply losses per node around 100 ppm) the efficiency of the laser thrust utilization grows rapidly.

Do we want one-off missions, demanding huge investments with limited overlap and resource sharing? Or do we want to build a long-term infrastructure that, once deployed, consistently reduces the complexity and cost of subsequent interstellar flights? Interestingly, this is not the first time we had to deal with this same exact dilemma: Roman engineers perfected the technology to build large bridges over rivers that divided Roman territories and provinces. This technology was key to keeping the Roman Empire connected across most of Europe for several hundred years. Building bridges was undoubtedly a daunting task relatively compared to the effort required to cross the river in floating boats, but once they were deployed, the long-term benefits in reduced travel time and risk vastly exceeded any potential building costs.

As promising as the concept sounds, there are a number of issues that need to be solved before we can start building them. First, we need to figure out exactly what it would take to deploy such a structure. A starting point for deployment analysis is the multiple-lensed beamed sail mission proposed by Geoffrey Landis, and the fact that this analysis exists goes a long way to proving that deployment is far from being impossible. Depending on the masses of the hundreds or thousands of relays, deployment might take somewhere between several decades to a few hundred years. But the grunt work of relay design is still ahead, and graphene has an enormous potential for reducing the relay mass, at least for the thermal management sections.

A interstellar network of this kind would not only enable interstellar transport, but would also make high-bandwidth communications much more feasible.

The refocusing element design of a starway will profit from certain key developments in adaptive optics and optical mirror mass reduction that are highly sought by the astronomical community.

Related laser propulsion work

Young Bae recently received a $500,000 NASA phase 2 NIAC grant for Propellant-less Spacecraft Formation-Flying and Maneuvering with Photonic Laser Thrusters. His work would recycle photons thousands of times by bouncing them between mirrors on the spaceship and the launching system. This would make the laser propulsion system thousands of times more energy efficient.

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