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The most commonly proposed plan to build a colony on Mars (notably the plan SpaceX is actively building) involves building a ship, launching it to space then have it (or part of it) land on Mars, then it stays there (part in orbit part on mars) until it's time to leave and flies back.

What I'm wondering is that what if instead there will be a spaceship between the two planets that never stops? each time it nears one it uses gravity assists to slingshot to the other planet, all cargo (astronauts, fuel, etc) is loaded via shuttle rendezvous in the time of the slingshot, same for disembarking.

While this have pros & cons the one I want to focus on is fuel economy, on one hand it saves the need to decelerate and accelerate to and out of orbit each time (and potentially get a gravity assist boost to help out each time too) but on the other the course will likely need more corrections as the ship never stops to wait to just the right time to begin the interplanetary part of the journey.

Will overall this save fuel? is it possible to roughly know how much (in percentage)?

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    $\begingroup$ Not really an empty shell, life support, the bridge, the engines, storage space, crew quarters, mess hall, etc, are all parts that stays in motion so never has to decelerate and accelerate, the only mass you truly need to accelerate and decelerate is the mass you truly want to land on the other planet, the rest never stops $\endgroup$
    – cypher
    Commented Aug 10 at 22:16
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    $\begingroup$ Yes, all right, but in the end you need to describe the system and figure out the mass of the stuff which keeps moving compared to the mass which needs to accelerate and decelerate. In the end, a rocket is made of 95% propellant and 5% dry mass; sure the payload mass launched from Earth is smaller, but you need to calculate the orbit of the shell to figure out the speed to which the payload needs to accelerate when launched from Earth, and then how much fuel is needed. $\endgroup$
    – AlexP
    Commented Aug 10 at 22:36
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    $\begingroup$ @AlexP that's the question, if I knew the answer to how much fuel is needed per kg I wouldn't have asked it now would I? $\endgroup$
    – cypher
    Commented Aug 10 at 22:44
  • $\begingroup$ If you supply the orbit of the ever moving shell then I'm sure that there are people who can tell you how much fuel is needed. But we need to know the orbit of the shell. $\endgroup$
    – AlexP
    Commented Aug 10 at 22:48
  • $\begingroup$ It would be interesting to see what kind of orbit can be modelled if we suppose the ship can count on encountering a certain size payload each time it passes Earth or Mars, captures that payload using a "magnetic launcher" type of induction system that extends out a long distance from it, and fires it into orbit around the next planet it passes. Ideally recycling much of the induced current to release one load as it takes up another. Without seeing a simulation I have no idea if you could make it work though. $\endgroup$
    – Mike Serfas
    Commented Aug 11 at 1:36

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You are describing a "cycler". It does indeed allow for some savings due to the cycler's mass only needing minor trajectory adjustments.

However, the spacecraft meeting up with the cycler has to match its trajectory. If it didn't dock with the cycler, it would just continue on to Mars on its own. So, as far as just delivering payload to Mars is concerned, there is nothing to be gained...in fact, the cycler must use a much higher-energy trajectory that takes more propellant to reach, so it is much less efficient for cargo.

The only advantage is that your ship can make use of the cycler's facilities while docked. The cycler can have more space, more radiation shielding, more extensive recycling capabilities than a ship could carry, hydroponics for growing fresh produce, etc. All this is more a matter of comfort rather than efficiency, though. Especially since you will need many cyclers, each of which may take several orbits to come back into alignment for another trip. That's a lot of infrastructure to have drifting through interplanetary space without anyone using or maintaining it.

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    $\begingroup$ Should have known that it was already an idea that was thought of, and the wiki link also provide a numric answer to the question In the case of the Aldrin cycler, use of a gravity assist reduces the fuel requirement by about 24.3 metric tons (26.8 short tons), or 15 percent, 15% is a whole lot of fuel saving $\endgroup$
    – cypher
    Commented Aug 11 at 9:02
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    $\begingroup$ @cypher: The 15% mentioned here is for originally launching the cycler onto its orbit, it is NOT about the cost of the taxi doing a planet -> cycler trip, or cycler -> planet trip. You should in fact expect to burn more fuel than on a direct trip. On the other hand, the taxi can be smaller, and have minimal recycling facilities (for either water or oxygen), which may reduce its weight, and therefore lead to better fuel efficiency. Even better, though, would be to find a way to fling those lightweight taxis at the cycler with external acceleration. $\endgroup$
    – Matthieu M.
    Commented Aug 11 at 13:41
  • $\begingroup$ @cypher as Matthieu M. says, that's a one-time savings specifically for putting the cycler in its orbit after launch. That involves putting the cycler into solar orbit and waiting for it to swing past Earth to do a flyby, which would take an additional three years...it's not a maneuver you'd use to send passengers to the cycler. Even you had such a "shortcut", you could use it to send lone spacecraft as well with no need for a cycler...anything capable of reaching the cycler can also reach the cycler's destination on its own, it just can't make use of the cycler's services during the trip. $\endgroup$
    – Christopher James Huff
    Commented Aug 11 at 13:57
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    $\begingroup$ Why would anything need to be in interstellar space? $\endgroup$
    – Harrychink
    Commented Aug 12 at 4:10
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Earth and Mars move on their orbits, optimal launch window happens about every 26 months. A ship constantly moving between them would have to go very close to Sun rather often.

Mars at it's farthest point is about 6-7 times farther than at it closest. Freedom to chose launch date is extremely valuable.

A ship constantly moving between Earth and Mars seems to be rather impractical for this reason alone.

Colonization likely would require much more than a single ship and mass of stuff moved to Mars will be many times greater than that of stuff that needs to be returned for decades if not centuries.

Most of the mass that this ship would have to accept would be the fuel necessary to decelerate the payload.

You save some fuel because you don't have to accelerate "life support, the bridge, the engines, storage space, crew quarters, mess hall, etc" but most of this stuff has to be present on your shuttles that have to achieve full travel speed - e.g engines. So, you don't save much on them. Worse, after the shuttle finished loading/unloading it has to lose the speed to return to Earth/Mars.

Any savings seems to be marginal even ignoring difficulties with the orbit.

Repairing this "permanent" ship would be a separate headache.

It's rather difficult to make this idea attractive. Too many problems.

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Frame Challenge: Not one spaceship but many i.e. Mars Cyclers

https://en.wikipedia.org/wiki/Mars_cycler

The basic idea is that there are series of orbits that have been calculated to allow a space ship to 'cylce' past the Earth and Mars regularly using zero fuel once in that orbit. Fuel is required to enter and leave these orbit's but in theory a second space ship can be used to meet an incoming vessel, dock with it and transfer personnel and equipment before separating for a return trip back to it's point of origin while the 'Cycler' continues on its way.

The orbits are however slow so a trip in one will take months for a one way Earth to Mars journey (and the same coming back). But the thing is you can place as many Cyclers in a specific orbit as you want so passengers could for example boost off Mars to meet an incoming Cycler for a return to Earth and if for some reason they are delayed? They can just reschedule and catch next months Cycler. Basically there could be a chain of these ships running a constant zero fuel 'bus run' between Earth and Mrs with a regular timetable for vessels scheduled to arrive at and depart from both planets (Note: the Cyclers would have to expend some small amount of fuel from time to time on minor orbital adjustments and as I said at the beginning you do have to expend fuel to get them into these orbits to begin with.

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    $\begingroup$ The URL was wrong. I fixed it but couldn’t get the formatting right. $\endgroup$
    – SRM
    Commented Aug 11 at 2:16

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