MA answered some of these. I'll toss in some more 2 cents.
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Originally Posted by JonClarke
I think the cycler concept is a very ingenious use of some rather elegant orbital mechanics. As to its practicality, I have several issues.
1) The transit time for each leg is longer than the semi-Hohmann transfer orbit, thus the crew spends much longer time in the high radiation interplanetary environment and (potentially) in zero G.
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I'm not sure if this needs to be the case. My orbital mechanics is fuzzy, but the transit could be faster if the Earth-Mars pass was made near perihelion of an elongated orbit. Of course, this would screw up the 5 cycler system, or it could simply be an addition to it. So, maybe you could have some slower cyclers (currently envisioned: apoapsis near Mars, periapsis sunward of Earth) and some high speed cyclers (periapsis near Earth, apoapsis farther out than Mars). Time would tell if the investment would be profitable, kinda like flying Concorde instead of a jumbo-jet.
One of the key design philosophies of a cycler, so far as I know, is to be bigger so that the hazards are better dealt with. Thus, a cycler would have better radiation, thermal, and impact shielding and rotational gravity compared to a direct trajectory vehicle. Such design would make them bigger, but more safe, and not needing to change acceleration, which lends itself to making them bigger. I don't know if it was clear in the OP, but in my design they would essentially be rotating space habs/colonies that perform occasional transits. They wouldn't be megastructures, but they would be several times bigger than currently envisioned direct trajectory vehicles (such as variations on Mars for Less).
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2) The crew spends much longer times on on the martian surface. Combined with (1) this makes overal mission times very long.
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In addition to MA's idea of one-way, I think a set of Mars-Earth cyclers could make Earth-return as convenient as the outbound trip once craft and fuel are available for rendezvous with the passing cycler or one of the other cyclers.
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3) The cycler orbit is not a perfect match for Earth and Mars, up to five cyclers are therefore required to allow a transfer each launch window. Since each cycler is a substantial spacecraft this requires substantial investment in transit infrastructure rather than on the Mars surface, infrastrucure that furthermore is only used 2/5ths of the time. This renders the claims for less mass false.
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I thought there would be several cyclers and I admit it would be a substantial investment. I think that it would develop as Mars became a more common destination. I'm not positing this as part of a Mars development strategy, but as part of a Mars support strategy. It need not take money from Mars investment, unless we posit that space transit remains the province of government. It could become a commercial enterprise, albeit still getting a large percentage of their earnings from government contracts.
The claim for less mass was for per-transit inter-terminal vehicles. That it to say, a vehicle that would need to accelerate from earth, coast to mars, decelerate at mars, and repeat the process if it returns to earth would require more mass for the deceleration fuel and more acceleration fuel to get the deceleration fuel up to speed. Assuming the size and design of the transit craft and cycler are similar (for the sake of argument), this is an advantage for the cycler, which need not decelerate. However, it may be preferable to take that mass savings and turn it into a better vehicle; there's a range of possibilities.
On the other hand, we might have fewer cyclers and fewer windows and use those for large passenger loads. Important shipments of mass or critical personnel could use HTOs for a conjunction transit that is not serviced by a cycler. It might be useful to have colonists train on earth for an extended period anyway, so missing a conjunction might not make a difference in the overall colonization strategy.
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4) Launching to the cycler means a deep space rendezvous with very narrow launch windows and little margins. If a launch is delayed even minimally then I suspect that the window is missed.
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True. I would design the shuttle to be small and light and fast, with extra fuel to make any adjustments in order to extend the window of rendezvous. Also, it's not inconceivable that the cycler might retain it's engines for making adjustments in it's orbit periodically, and also use them for emergencies where it may need to decelerate a little to retrieve a crippled shuttle attempting to catch up to it (or it might send back a craft with the ability to take on passengers, push it to rendezvous, or decelerate it to an Earth abort trajectory). It's all delta-v and slowing down might be plausible if it has enough delta-v in the tanks to get back on schedule with an altered trajectory.
Or maybe there are some other methods of getting a shuttle to speed while still close enough to earth to be rescued. Maybe a magnetic launcher in orbit, or some sort of gravity slingshot or rotovator would make failure less likely. Of course, this is speculation and more advanced technology than I am positing as necessary for the cycler.
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5) The deep space rendezvous is very much a dock or die affair, if planet to cycler ferrys are minimalist affairs. To ensure the crew survives in the eent of a failed docking they have to be large enough to make the mission without the cycler, which makes the cycler redundant.
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See above. I think the shuttles or ferries would be minimalist affairs, but I suspect they would not launch from the planet but from orbit, perhaps from a space station or orbiting launcher. If they missed rendezvous, there might be something that could be done from the cycler. I would not design the shuttle to be survivable for a missed rendezvous. If I were to over-engineer it, I would make it faster, smaller and lighter and increase it's delta-v potential to reduce the chances of missing rendezvous.
It's a valid question, and the priorities at the time will drive transit systems design.
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6) Tranfering to the cycler orbit always requires more propellant than doing directly to Mars or Earth, sometimes much more.
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All things being equal, yes. The cycler system, as envisioned, would not be equal. The shuttles would be minimal in size, meaning the mass being accelerated to the cycler rendezvous would be less, requiring less fuel for the same delta-v.
If you assume that shuttles/ferries would be akin to cycler in size and design and that they would either rendezvous or make the journey on their own then it makes the cycler idea less practical. However, I wouldn't rule out the idea of rapid transit either. An HTO might be minimal cost in terms of delta-v and real money, but time is also money and the economics of space may make it desirous to get there sooner using constant boost or partial boost. If cheaper delta-v makes it plausible and practical to use constant boost or faster-than-free-fall transits then that would be the beginnings of the end of the cycler system, something I hope will happen, but do not expect until several decades or a century later.
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7) entry velocities at Earth and Mars are always higher than non cycler orbits.
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Yes, this would require more fuel to decelerate from. However, the advantage is that the craft being decelerated to planetary orbit is small, requiring less fuel for that delta-v. I'd expect that they would use the same shuttle to rendezvous with an orbiting station instead of landing directly to the surface. This would greatly reduce complexity, however, it might increase fuel requirements for either the shuttle or for the cycler if it is meant to refuel the shuttle. It will be an economic consideration.
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So in the end using a cycler means much larger spacecraft, more propellant and greater risk to carry out essentially the same mission that a non-cycler would use.
I am sure cycler orbits are good for something, just not for travelling to and from Mars.
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Yes and maybe. The craft would be much larger, larger than would be used for HTO or a powered trajectory between Earth and Mars. This would be their main advantage. The analogy would be taking a dinghy to an ocean liner for an ocean transit versus taking a yacht the entire distance.
The risks taken may be different but I'm not sure they are greater. I posit that a larger craft with more redundant systems would be more resilient for recovering from failures or damage and more durable with more protection in the space environment. A failure in an engine that prevents rendezvous could be bad, but so could an engine failure in a direct mission that causes the ship to undershoot or overshoot it's target trajectory.
I don't think the fuel argument has been made. A large cycler would require more fuel to get up to speed, but it would not need much afterwards. Shuttlecraft between cycler and orbit would require more delta-v but that would be a lower amount of fuel due to the lower mass of the shuttle. It would really come down to the size and type of craft envisioned in both scenarios (HTO, powered or cycler) but even if it is more fuel for the cycler system, the users may opt to exchange fuel savings for comfort.
The missions would not be essentially the same. Earth to Mars and Mars to Earth transit is only one part of the mission of a cycler in the OP. If a cycler both drops off and picks up passengers at Mars (something not posited in the Aldrin plan, but possible in mine), there is still a lot of space science that could be done aboard the cycler during the time between Earth drop-off and the next Earth pick-up.