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Originally Posted by Ara Pacis
MA answered some of these. I'll toss in some more 2 cents.
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Sorry AP, I missed this, until the thread rose to the surface again today.
[b]JC 1) The crew spends much longer times on on the martian surface. Combined with (1) this makes overal mission times very long[b]
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Originally Posted by AP
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.
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That's an interesting idea, I can't see why it should not work.
But to match with the "faster" cycler orbit will take more propellant than with a slower one. Since the dV requirements are already against even a slow cycler, this solves one problem at the expense of making another one worse.
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Originally Posted by AP
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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Regardless of the nature of the orbit, a spacecraft travelling between planets is going to be large enough to deal with the hazards adequately. So there is no reason to suppose that a cycler will have better protection against any of the hazards than an equivalent non-cycler spacecraft
JC 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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Originally Posted by AP
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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Five cyclers is enough to provide two-way transport, as I recall.
JC 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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Originally Posted by AP
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.
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Regardless of who pays and how, if cyclers take more infrastructure and more propellant.
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Originally Posted by AP
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.
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Let's try some rough calculations from Earth to Mars orbit.
LET
The mass of the transit craft and the cycler be the same ~ 50 tonnes
The mass of the ferry spacecraft be 20 tonnes.
The mass ratio to place the transit craft into Earth-Mars transit orbit be equal ~ 1.5
The mass ratio to place the transit craft into Mars-Earth transit orbit be ~ 1
The mass ratios to place the ferry spacecraft into cycler orbit from Earth to be ~3 and from Mars to be ~2 (worst case numbers)
The Earth orbit and Mars orbit insertions to be 100% aerocapture.
There be local production of propellant and other consumables.
The mission cycles be of the same length
All spacecraft are reusable. This will require five cyclers and four ferry craft OR two transit craft.
THUS
To establish the intrastructure the transit craft approach requires 100 tonnes and the cycler approach 250 tonnes (not including support facilities) = that is the cyler requires 190 tonnes of spacecraft more.
Propellant to establish the cycler system is 750 tonnes
Propellant per mission cycle is 100 tonnes for the cycler and 125 tonnes for the transit craft.
THEREFORE the cycler saves 25 tonnes of mass per mission cycle. It would take 30 cycles to make up the propellant cost of establishing the cycler constellation.
It would take a further 8 cycles to make up for the mass cost of establishing the cycler constellation, BUT if propellant costs in orbit are 25% of spacecraft costs THEN it would take 32 cycles.
CONCLUSION - It would take 62 cycles (~135 years) for the cycler to pay its way. By that stage the cycler would well and truly need replacement
Can we build large deep space craft with operating lives of this period?
Another factor often forgotten is cargo. This would be sent one way and would not benefit in any way from going via cycler. This would be 50-100 tonnes of cargo and 75-140 tonnes of propellant
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Originally Posted by AP
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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It would make a big difference to the plans of the people already there when they have to wait another 2.2 years for the arrival of those human resources!
JC 4) 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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Originally Posted by AP
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.
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There are still two. problems. How small are the ferry craft? They have to be able to carry the passengers and crew, consumables for the journey (1.2 tonnes per person), spares and consumabls for the cycler, plus of course the structure, engines and tankage to support all this. They are not that small.
Secondly fast means more propellant, and it goes up expontentially, as I recall.
Now these are solvable problems, but is the solution worth the cost?
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Originally Posted by AP
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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If you need cycler specific infrastucture it just adds to the cost burden/
JC 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 event 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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Originally Posted by AP
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.
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So much depends on the extreme reliability of the deep space rendezvous.
JC 6) Tranfering to the cycler orbit always requires more propellant than doing directly to Mars or Earth, sometimes much more.
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Originally Posted by AP
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.
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See above. It is an unresolved question of how small the ferry craft have to be. They have to carry the crew, passengers, personal cargo, all consumables for the trip, supply items for the refurbishment of the cycler. They have to maintain the people on board for several weeks at a time.
JC 7) entry velocities at Earth and Mars are always higher than non cycler orbits.
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Originally Posted by AP
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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See above!
JC 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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Originally Posted by AP
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.
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I don't see why the cycler would be any larger in terms of crew habitat facilities than a cycler. Both would be optimised for the time the crew spend on board, which are of the same order in both cases.
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Originally Posted by AP
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.
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Again, I don’t see why a cycler would have more redundant and resilient systems than a transit vehicle.
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Originally Posted by AP
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.
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It depends on the type of cyclers and how close they pass to Earth and Mars. The closer they pass to a planet, the more they will be perturbed away from the cycler orbit and the more propellant they will need to get back onto it
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Originally Posted by AP
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.
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What space science could a cycler do that would not be done equally well by a transit craft?
Cheers
Jon