Laser Ion propelled interstellar generation ship using this century technology

Dana Andrews looks at a minimum transit speed of 2 percent of the speed of light (fifty years per light year) for an interstellar generation ship. This makes for transit times in the neighborhood of hundreds of years, depending on destination. A key requirement is the ability to decelerate and rendezvous with the destination planet using a magnetic sail (magsail) that can be built using high-temperature superconductors. Andrews also assumes 20-30 years of research and development before construction actually begins.

He assumes that a space-based infrastructure sufficient to begin active asteroid mining will be in place.
He cites a thirty-five year timeframe for getting these needed systems operational.
Some of the R&D could presumably be underway even as this infrastructure is being built, but we also have to take into account what we know about the destination.

Credit Dana Andrews and H/T Centauri Dreams

Andrews develops an interesting take on using space-based lasers, working in combination with four-grid ion thrusters using hydrogen propellant and optimized for a boost period of thirty-two days. The specific impulse is 316,000 seconds. The lasers are mounted on a small asteroid (one or two kilometers) and take advantage of a 500-meter Fresnel lens that directs their beam to a laser reflector on the spacecraft. Let me take this directly out of the paper:

An actively steered 500 m diameter Fresnel lens (risk scale 3) directs the beam to the laser reflector on the Generation Ship Spacecraft, where it is focused onto hydrogen-cooled solar panels (risk factor 1), using a light-pressure supported ¼ wave Silicon Carbide reflector (risk factor 2.5). The light conversion panels operate at thousands of volts at multiple suns (risk factor 2) to allow direct-drive of four-grid Ion thrusters using hydrogen propellant and optimized for short life (30 days) and very low weight (risk factor 3). The four-grid thrusters provide a Specific Impulse (Isp) of 316,000 seconds operating at 50,000 volts using hydrogen. The triple point liquid hydrogen propellant is stored in the habitat torus during boost (the crew rides in the landing pods for the duration of the 32 day boost period), so there is no mass for propellant tanks. After acceleration the crew warms up the insulated torus, fills it with air, and moves in.

I think with spiderfab that very large multi-kilometer space based optics could be produced sooner. I would also consider molecular nanotechnology in the 40 year timeframe.

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