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I will give an example with the moon.

If some spacecraft is put either in orbit around both earth and moon or in highly eccentric orbit that intersects with the moon orbit, some other spacecraft can dock to it and "ride" it to the moon.

A problem will be docking with so much faster craft on different orbit and also slowing down of this craft over time, but it can gain speed from gravitational slingshots from the moon, or it can be reboosted with cheap fuel from the moon for example?

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This sort of spacecraft is known as a "cycler".

You hit on the problem with it: you have to match its trajectory/velocity exactly in order to dock with it, so if you can reach the cycler, you could already reach the cycler's destination.

There's no slowing down of the cycler for the same reason. In principle, if you connected to the cycler with a very elastic tether, you could borrow velocity from it, but that's completely impractical.

The potential advantage of the cycler is that it can provide space and non-consumable amenities for the journey, thus saving mass on the shuttle that rides the cycler. That's not a huge win for translunar flight, where it's only a 3-5 day trip each way, but for e.g. routine flights between Earth and Mars, it's more attractive. The mothership Hermes in "The Martian" is acting as an Earth-Mars cycler, though that wasn't its original mission design.

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  • $\begingroup$ I wonder how this is necessary and why won't someone think of some mechanical system for capturing spacecraft on different orbit. Also, when I see docking I always wonder why they don't fire some hooks or mechanical arms at the final few meters, but this is a different question... $\endgroup$
    – Nikolai Frolov
    Commented Jan 23, 2018 at 19:31
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    $\begingroup$ I don't think capturing another craft with multiple km/s speed difference is going to work well and not kill the passengers. $\endgroup$
    – zeta-band
    Commented Jan 23, 2018 at 19:49
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    $\begingroup$ @Nigel - I guess Hermes wasn't designed to be a cycler from the beginning, but it's acting as a cycler within the story, as it goes from Mars to Earth, back to Mars, and back to Earth without entering parking orbits. Edited to make that clearer. $\endgroup$
    – Russell Borogove
    Commented Jan 23, 2018 at 20:55
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    $\begingroup$ @patstew I'd love to see some back of the envelope math for a rotating cycler-hook that can manage, say, 1km/s capture velocity. Earth-moon or Earth-Mars velocity is > 3km/s; saving less than 1km/s seems hardly worth the effort. $\endgroup$
    – Russell Borogove
    Commented Jan 23, 2018 at 20:58
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    $\begingroup$ @Nigel The Common Berthing System used on ISS does use a mechanical arm to complete the last few meters of docking, but it starts from zero relative velocity. Harpooning an arriving spacecraft would be inherently unsafe. Any proposed method of capturing a spacecraft on a different orbit has to cancel the speed difference somehow (lots and lots of bungee cords?), and any safe method of doing so will far outweigh the straightforward option of carrying enough propellant to match velocities. $\endgroup$
    – Russell Borogove
    Commented Jan 23, 2018 at 21:06
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The problem is that when you rendezvous with such cycler (and possibly dock) you are already on the same orbit and you had to spend the fuel to get you to the Moon. So you mostly do not need the second craft for anything.

For the Moon trip that does not make much sense. On the other hand cyclers are often suggested for Mars trips: when you put one big cycler in Earth-Mars loop, you can then possibly use much smaller crafts at each planet to move people and supplies (needing less propellant hopefully) but you can enjoy the big ship on the long trip.

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  • $\begingroup$ It may be worth mentioning that a cycler does not save energy overall: All the mass that's going to Mars or wherever had at some time to be accelerated to match the proper trajectory. It does save energy for specific flights though because all the supplies could be accelerate beforehand (and put on the cycler) at an arbitrary, potentially more convenient schedule. In a way, a cycler is like the powerbank you carry for your phone on long trips ;-). $\endgroup$
    – Peter - Reinstate Monica
    Commented Jan 24, 2018 at 13:04
  • $\begingroup$ Well, it might save the energy which would otherwise be needed to accelerate a much bigger interplanetary ship multiple times. But how much of it would be used by putting the cycler itself in place and by maintenance is another thing. $\endgroup$
    – jkavalik
    Commented Jan 24, 2018 at 13:09
  • $\begingroup$ True. let the big mass cycle. $\endgroup$
    – Peter - Reinstate Monica
    Commented Jan 24, 2018 at 13:32
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Yes. This concept is known as a Lunar Cycler. Especially interesting is the Backflip Lunar Cycler, which meets the Moon once a month, and the Earth twice a month. A search for 'lunar cycler' will bring up several papers.

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    $\begingroup$ Just did some research and found the Wikipedia article. Got the idea from Skott Manley doing something similar in Kerbal Space Program though. $\endgroup$
    – Nikolai Frolov
    Commented Jan 23, 2018 at 19:21
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    $\begingroup$ @Nigel: Since you mentioned KSP, you might want to try doing a docking maneuver or two in KSP without first precisely matching orbits. You'll find out why it's not that practical. :P (If you want to actually try making a cycler in KSP that does something useful, I'd suggest installing a life support and/or colonization mod of some kind. Without one, there's not much point to a cycler, when Kerbals are happy to just sit for years in their equivalent of a Mercury capsule.) $\endgroup$
    – Ilmari Karonen
    Commented Jan 23, 2018 at 23:54
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    $\begingroup$ If you do try this in KSP, please throw me a link to the resulting video. I like explosions :-P (If they don't phase right through each other, that is) $\endgroup$
    – John Dvorak
    Commented Jan 24, 2018 at 6:22
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Besides cyclers, a reason to have dedicated vehicles to go from orbit to orbit is to be able to use the best vehicle design for each leg of the trip. One can imagine a winged shuttle bringing people up from earth to an orbiter. Then the orbiter goes from low earth orbit to lunar orbit. There people transfer to a lunar lander to reach the lunar colony on the surface. The orbiter never lands, and the lunar lander never goes further than lunar orbit. The orbiter could have low thrust electric propulsion and heavy shielding. The lunar lander would have legs and higher thrust engines. Also fuel and equipment is not carried unnecessarily through a leg of the trip it is not needed for. The orbiter need not carry the lander or fuel for it.

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