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In 1985 Buzz Aldrin AKA “Dr. Rendezvous” proposed using a pair of space station habitats in solar cycler orbits as “Up and Down Escalators” for crewed Earth-Mars transfers. https://en.wikipedia.org/wiki/Mars_cycler

The primary advantage is to reduce transit time and propellant consumption by leaving the most massive components in solar orbit. These two advantages are connected since required radiation shielding mass is proportional to transit time. None of the active shielding methods proposed to date appear to offer a weight savings over bulk material shielding. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.733.9425&rep=rep1&type=pdf .

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Cycler space stations (“Castles”) uses were outlined in What uses would the Aldrin-Cycler have?

In particular, radiation shielding mass, habitat and solar panels can be left in orbit instead of being repeatedly relaunched.

The big downside appears to be the delta V at each end of the transfer, but this is only being applied to the small mass of cargo and its "taxi", not the entire “castle”. Propellant consumption is proportional to the mass being "delta-ed".

The cycler concept seems to attract little attention compared with discrete spacecraft using Hohmann transfer orbits.

Question: Why does the Buzz Aldrin Mars Cycler not play a larger role in planning for Mars exploration?

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    $\begingroup$ I wonder if there is any financial downside to building two large space stations in deep space that must be maintained for decades. $\endgroup$
    – Organic Marble
    Commented Nov 25, 2021 at 20:33
  • $\begingroup$ The answer would depend on the total cost and the best alternative. If the best alternative is building all those space station facilities into every interplanetary ship and launching it twice each round trip, the numbers might look good. The Devil, as always, is in the details. $\endgroup$
    – Woody
    Commented Nov 25, 2021 at 21:30
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    $\begingroup$ Sure, if the Funding Entity is willing to go all in. Big Plans that involve a lot of up front spending are tough sells. Look up the Integrated Program Plan sometime. en.wikipedia.org/wiki/Space_Transportation_System $\endgroup$
    – Organic Marble
    Commented Nov 25, 2021 at 21:34
  • $\begingroup$ You make a practical point. The Cycler is more along the lines of Elon's, "Building ~1,000 Starships to create a self-sustaining city on Mars' is SpaceX's Mission" type hand-waving. $\endgroup$
    – Woody
    Commented Nov 25, 2021 at 22:27
  • $\begingroup$ Why do cyclers not play a larger role in planning for Mars exploration? For about the same reason that Aircraft Carriers did not feature very largely in the Wright Brothers' plans for their Flier. $\endgroup$
    – CuteKItty_pleaseStopBArking
    Commented Nov 26, 2021 at 4:54

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A cycler makes sense where there is expected to be a small but steady shuffle of humans back and forth between Earth and Mars on a fixed timeline. Notably boarding and leaving a cycler involves more DV than a Hoheman transfer would take so only makes sense where 'buy once' costs boosting life support and radiation shielding add up through re-use.

It becomes less useful if:

  • life support technology is not closed and needs regular topping up (see DV penalty)
  • The bulk of the mission mass is actually needed at Mars (landers, habs etc)
  • Abort options are desirable (parking vehicle at Mars may be helpful)
  • Some/all of the crew intend to stay on Mars
  • Your business model is making rockets designed to refuel on Mars (SpaceX)
  • Aim is to minimise cost of a bootprint on Mars

There have been a number of different Mars proposals over the years and most of them have probably looked at cyclers, but have run it against the needs list and found it was not a value add, most likely on the cost two re-usable cyclers over one (probably simpler) single purpose transit vehicle. Note that humans, especially humans with elections to win are not necessarily good long term planners.

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