The Interstellar Cycler concept was envisioned by science fiction author Karl Schroeder in his novels Halo and Permanence.
No matter what technique you use, it takes an enormous amount of energy to accelerate a craft up to near-light speed. So much so, in fact, that it may not be worth an equal amount of energy expenditure to simply decelerate it again at its destination. The interstellar Cycler envisions a vessel that never slow down, but uses various techniques to effect turns around a local neighborhood of stellar objects to inscribe a rough circle. A cycler may visit anywhere from a handful to many dozens of star systems, and interstellar objects such as brown dwarves, before returning to its point of origin and completing a circle.
The inhabited systems it passes accelerates smaller cargo vessels to catch up to it and decelerate cargoes the cycler drops off. This way, no one star system has to foot the entire cost of building and launching the interstellar vessel; they only have to absorb the cost of accelerating and decelerating much smaller cargo vessels. The cycler also takes some of the cargo to slowly build itself up. In Schroederís novel, the original cycler started out as an unmanned magsail probe; after many centuries, it grew into a full-scale, fully-inhabited fleet of city-sized structures.
In Schroederís scheme, a magnetic sail (see the Light Sails article) is accelerated by an array of powerful particle beams out of its home solar system. It then deploys large loops of highly charged superconductive wires. These interact with the interstellar medium and galactic magnetic fields, using the Lorentz Force (the force exerted on charged particles in a fixed magnetic field) to very, very gradually turn the vessel. This is combined with other techniques, such as long-range assists from the home systemís particle beams, gravity slingshots, and on-board reactive engines such as ion drives, to slowly turn the vessel and align it with its next destination. After it reaches a certain level of velocity (about 6% lightspeed), it could perhaps use magnetic field scoops to gather hydrogen from the interstellar medium to power a bussard ramjet-like system that can allow it to turn much more effectively.
Its a matter of some debate whether affecting large, gradual turns at significant fractions of lightspeed is actually more energy conservative that simply slowing the vessel down at its destination. Obviously, the shallower the angle of the turn, the less energy would be needed, and the more significant masses it can pass to use for gravity assists would also be useful. Thus, interstellar cyclers would probably work best when used on routes heavily-populated with star systems and interstellar objects such as brown dwarves.
As a cycler passes a system, that systemís inhabitants use lightsail or magsail craft to accelerate cargo to meet with the cycler, then use the same system to decelerate cargo left off by the cycler. Th particle beams used to accelerate the cargo may also be used to help accelerate and/or turn the cycler as it passes through. The cycler ship is slowly built up as it passes from system to system and cycle to cycle, drawing upon resources sent to it by civilizations along its route.
As with interplanetary cyclers, it is very probable that multiple interstellar cycler craft will be used along any one route, staggered anywhere from a few years to a few months apart. In this way, an interstellar trading economy can remain viable even at otherwise prohibiting STL speeds, as new goods would be arriving regularly at frequent intervals. Schroeder proposed that the original cycler on a route can "reproduce" after it accumulates enough resource donations from its member civilizations. The crew of the cycler slowly builds up another cycler vehicle en route, and when complete the new vessel slows down and drops behind its "parent" ship before it accelerates again a few months or a few years later. The process is repeated as many times as necessary to create a fleet of cyclers for any one route.
Permanence and Halo by Karl Schroeder
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