Karl Schroeder delves into the science behind the novel and noteworthy space-travel tech he's invented for his forthcoming book, Permanence. Permanence literally floored me — I picked up the manuscript, started reading it on my way out the door, and found myself, hours later, sitting cross-legged on the floor by the door, still reading. The science behind Karl's stories is wonderful.
The Schroeder cycler is initially accelerated from the Earth or a colony star to some percentage of lightspeed. The velocity would be determined by how long the cycler can survive the battering of hard radiation at speed, and also by the turning radius required by its course. Once in motion, we leave the cycler in motion. It uses a combination of Lorentz Force turning and gravitational slingshot (if feasible) to alter its trajectory so that it passes by a number of stars in succession, finally returning to Earth to begin the cycle again. Even at half lightspeed, a typical cycler might take a century or more to make such a grand circuit of local interstellar space; however, if new cyclers are being launched every few years, there could eventually be more than enough of them to supply all the traffic that the solar system can afford to send out.
