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A cycler is a craft that repeatedly passes between two or more planets. The concept is that a large spacecraft with lots of amenities could be used as a residence for long interplanetary trips, smaller shuttlecraft would do the high-delta-V transporting of the crew between the destination planets and the cycler as it flys by. I've seen it discussed a lot in KSP, but here are some articles on the concept. Quick introduction: Mind-numbingly complicated but thorough: I'm not sure of the practicality of cyclers over fast high-energy missions, but the search for cycler paths fascinates me a great deal. Earth-Mars cyclers are the most studied. Almost all papers I've seen make 3 simplifying assumptions about the motions of the planets to ease the search, as follows: 1) The orbits of Earth and Mars are circular and coplanar. 2) Mars' orbital period is 1.875 times Earth's. 3) Mars makes no change to the cycler's path, only Earth does. As we shall see, these assumptions cause problems when it comes to actually flying these paths. Some of the cyclers are described are ballistic, that is, after the cycler is placed in its path no further dV needs to be expended other than small course corrections. Others require substantial manuevers to keep them on track, for instance the Aldrin cycler requires a 230m/s or so correction every synodic period. Kerbalnot posted his execution of the Aldrin cycler in KSP here, check it out. A key element to cycler trajectories is that the positions of the planets and the ship repeat after a set time, this time has to be a multiple of the Synodic period of the two planets. In the case of the ideal Earth-Mars this is 2.143 years, for the real planets it is 779.95 days or 2.135 years, for Kerbin and Duna it is 227.38 Earth days or 909.5 Kerbin days. This means that if your ship leaves Kerbin on a given day, and then comes back to it 454.76 Earth days later with the same energy and moving in the right direction it can repeat the Kerbin-Duna-Kerbin path indefinitely. Note the requirement about the energy! It is easy to find a path that goes K-D-K, and even one that does it in 455 days, but it will come back with much more energy that it left with because of that flyby of Duna! If you don't use a Duna flyby or a deep-space maneuver then it is tricky to get back to Kerbin when it is that extra .135 orbits further in its path, so the big challenge in cycler searches is moving the apsis of your Solar orbit forward. In the last few years a very good cycler orbit has been discovered for the Earth-Mars run, it is called an "S1L1-Ballistic cycler". Here's the paper by the original discoverers. Here's a video of the path: Check out the low Earth and Mars intercept speeds, most other cyclers fly by much faster, especially ones that repeat after only two synodic periods. This cycler works even better with Kerbin-Duna because Kerbin's orbit really is circular, so running it in stock KSP is a great way to study it. Also Duna's SOI has a limit, so you can fly fairly close to Duna without it affecting your path unlike in the real world. So I first figured out the S1L1 path without the flyby of Duna, I wanted to see if Kerbin system could emulate the flyby angles and rotation of the apsis necessary without the complication of flying by Duna. Here it is. Note I'm using Earth time, I'll translate to Kerbin time below. I'm using a great little mod called Slingshotter, it is helpful for planning this sort of thing. Success! My ship returned to Kerbin 456.4 days after leaving it with only 9m/s less V-infinity. It only used 11m/s for course corrections after the initial 1255m/s to get started. Note that one cycler would only do the Kerbin-Duna run once every 2 synodic periods, and only in one direction. You would need another one timed to encounter Duna on the way in for the return trip, and another in-out pair to cover the other synodic periods. These limitations are common to all cyclers, I've seen paths that would require 14 cycler ships in order to be able to get a ride inwards and outwards every synodic period! So in Earth days if you initially leave Kerbin at 0 you must fly by it again on day 301.24 and day 454.76. In Kerbin time this is day 1205 (2Y 353D) and day 1819 (4Y 115D). Now I ran the path again, but this time with a flyby of Duna. Ouch! The flyby of Duna added a lot of energy. I think this can be compensated for by flying by alternating sides of Duna, or one could stay outside it's SOI at all times. That leads to spending several days in the shuttle vehicle and a couple hundred m/s of dV to get from the far-away cycler to the planet and back though. The bigger problem is that Duna's orbit is not circular and so it will not be in the same place relative to Kerbin every 454.8 days! This means you must change the cycler orbit constantly back and forth to pass close to Duna each time. This could be done using flybys of Duna, but I don't have a tool to calculate how to set this up yet. I need something that handles multi-orbit, symmetric double flybys. In Kerbin time this cycler started Y1 d67.68, flew by Duna Y1 d104.3, 1st Kerbin flyby Y2 d2.8, 2nd Kerbin flyby Y2 d158.4. In Kerbin time this is start Y1 d267.72, Duna Y1 d414.2, 1st K Y4 d190.2, 2nd K Y5 d386.6. Has anyone else tried this?