Here's the wikipedia stub and the Popular Mechanics article for the Buz Aldrin version.


The basic idea, if you're not familiar with it, is to have a large craft continuously moving at high velocity between the orbits of Earth and Mars without stopping. The orbit would be designed to make a pass by the planets at regular intervals. The premise is that it reduces the amount of material needed to be accelerated for each transit by limiting it to a small passenger craft that travels to or from the planet to the cycler and vice-versa.

I like the idea from that article, but I would want to alter and expand it. The current idea is near future with dainty craft, and wants to use aerobraking to slow a second class of cycler (Mars-Earth transit cycler) into orbit, board, and then boost for a slower, unrotating trip back to Earth. I wonder if it would be better to forgoe that second stage variation and just "waste" the fuel to accelerate up to a full-size/first-class cycler. This would be plausible once there is a fuel infrastructure on or near Mars.

The Aldrin idea would want to leave the cycler uninhabited and shut down as it waits for an Earth-Mars transit. However, I suspect it could be a useful platform for various astronomy activities, such as solar observing and as relays during conjunction (when one planet is eclipsed by the sun). The cycler-as-a-colony concept would mean it's is rather self-sufficient with recycling technologies, using solar energy, and only needing occasional inputs of stock materials (food, etc) after startup.

The basic reasoning is that an interpanetary craft has the same needs as a space colony (protection, arti-grav, recycling), except for thrust, and that once thrust is generated, the only difference is slowing it down into orbit around a planet or letting it sail past. If we want to send people, but not the entire vessel, to the planet, then you only need to decelerate a shuttlecraft or lander (to the surface or an orbiting station, perhaps another rotating colony). I think this would be a plausible step between dainty missions like Mars-For-Less and the desired but speculative big and fast interplanetary ships with lots of delta-v at their disposal. Since we don't know if or when we'll have those types of big and fast craft, or when they will become competitive once we have them, I think it's fair to examine this idea as valid starting in a couple decades and operative for several decades to a century or more using current technology.

There are limitations to the idea of a Mars cycler. It would only really be useful for perishable items like passengers and some small cargo. It would probably still be preferable (cheaper) to ship large amounts of bulk or large items via a slow boat. Each cycler has limited windows of use, making it desirable to have multiple cyclers running routes in order to have a semi-regular schedule. (I admit I don't have a complete understanding of the orbital mechanics to schedule one myself, but I've read there should be several, due to opposition occuring at different point in the orbit each time. Opposition would be the shortest transit, other trajectories going closer to the sun might also be plausible.)

Feel free to criticize or expand the concept.

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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.

2) The crew spends much longer times on on the martian surface. Combined with (1) this makes overal mission times very long.

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.

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.

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.

6) Tranfering to the cycler orbit always requires more propellant than doing directly to Mars or Earth, sometimes much more.

7) entry velocities at Earth and Mars are always higher than non cycler orbits.

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.

Jon

Only if the missions are always return-to-Earth missions. For missions where the passengers (colonists) stay on Mars, that is not an issue.

Another good point. Perhaps having larger but fewer missions would be the solution, that is, sending 40-50 people instead of 7-8.

Since the launch to the cycler would be made from orbit, I think that rendezvous would not be a big problem.

That is correct. But given enough maneuvering fuel, that should not be a critical problem.

AFAIK, it would require almost exactly the same amount of fuel. That is because the shuttle would be accelerating to the exact same velocity as it would if it were going solo.

I don’t understand why that would be the case. If the transit velocity is the same, and the orbital path is the same, the entry/reentry velocities should be the same.

Initially, it would be difficult if not impossible to justify cyclers. However, eventually, using large cyclers, possibly asteroids, the advantage would be in providing luxury accommodations for passengers on the long voyages between planets. Although the initial energy requirements to get the cycler into the proper orbit would be very large, the payoff would be in being able to provide perhaps 10-20 times the amount of living space for passengers as well as freshly grown food, fancy entertainment, and larger passenger loads. It would be something like the difference between a 30 foot yacht and a 600 foot cruise ship.

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Except it will still take more mass and propellant to do it via a cycler than directly.

I agree, I can see a possible role here.

Possibly, if the window if several orbital periods long. I haven't been able to find any calculation of the window in any of the stuff I have read.

Given enough propellant is the rub. If it takes more propellant to go via the cycler than directly, what is the point?

From memory transfering to a cycler orbit requires several km/s more dV. it also varies significantly from encounter to encounter

Again, here is the rub. The orbits are not the same. You have to match orbits with the cycler first. The orbit the cycler takes is close to but not exactly the same as the transfer orbit. In fact you don't want the cycler in an orbit too close to the one taken by a transfre spacecaft because it will pass to close to earth and Mars and be peturbed. This can be corrected, but requires propellants. Al of which have to be ferried to the cycler.

Again, I agree, there may be a role here. However but the time we need to ferry large numbers of people we may also have more powerful rockets that can make the trip in 4 months with larger payloads. I have no idea which is more probable.

cheers

Jon

MA answered some of these. I'll toss in some more 2 cents.

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).

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.

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.

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.

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.

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.

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.

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.

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Hello, Ara Pacis, JonClarke et al,
The Cycler Transit System is an interesting concept that could be utilised to further advantage. However an even more advantageous approach is not to go to Mars, but to enable International Crews to commute to Mars.
Wilson Aerospace Systems originated SOMTS in the Late 80`s when it was realised that Mars Exploration Staging would benefit immensly had an additional Planet been located midway between Earth and Mars. So, why not create our own "mini planets" so to speak. The concept was further developed in 2001 / 2002 specifying and submitted to NASA. (No comment there however ?),
A feasibility study was imlemented proposing graduated staging of a Mars Expedition where Astronauts would first travel one third of the way to Mars, and then return to Earth. The purpose of this excercise is deep space hardware and software proving, crew response to extended deep space isolation etc. The primary objective of course is to deliver an International Space Station into a Solar Orbit. A production line manufacturing International Space station would utilise economies of scale to
gradually populate the Earth Mars spatial segment with these Solar orbiting Mars Transit Stations. Each SOMTS would be accompanied by SRB type Rockets to provide commuting Astronauts with return to Earth transportation.

Suggested first level Solar Insertion Orbits are. Earths Orbit of 93.000.000 miles, first level Plus 22,000,000 second level plus 10,000,000
mars Orbit of plus 15,000,000 miles. Approximate Solar Orbit durations are Mars 686 days SOMTS OSO (Outer solar Orbit) 572 days) SOMPS ISO (Inner Solar Orbit) Earth Solar orbit 365 Days, Each ISS would be accompanied by
de-orbit burn capable SRB`s. This is a very expensive permanent Transit System which enhances and maximises crew surviveability. Orbital dynamics may enable a modest reduction in ISS unit requirements wher ISS units may populate a specific sector. Kevin Wilson.

Do you have a link to a more detailed explanation?

Jon

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]

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.

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.

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.

Regardless of who pays and how, if cyclers take more infrastructure and more propellant.

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

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.

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?

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.

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.


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.

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.

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.

Again, I don’t see why a cycler would have more redundant and resilient systems than a transit vehicle.

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


What space science could a cycler do that would not be done equally well by a transit craft?

Cheers

Jon

Large scale off Earth colonies are likely still decades in our future, but it will be even longer, unless we work out and test some of the details this decade. Colonies that cycle about the inner solar system would be more interesting than a colony in permanent Earth orbit, and would perhaps attract more of our best people to live there. I agree, the close approaches do change the cycler orbit; very close, changes the orbit radically, such that the original cycler idea mostly disappears.
We need at least two cyclers, unless there is some sort of back up system, perhaps 500 million tons with a full load of cargo. Each needs at least two shuttle craft, so we have a backup system. With 60 tons of reaction mass, the cyclers can fine tune their orbits, perhaps doing a slingshot maneuver around a planet or large moon ten times per century with rarer close approaches with little slingshot. I'd like to see a century of essential supplies on each cycler so Earth can be repopulated if all surface humans die in a disaster.
Fast single trip ships can have very high re-entry speeds, so that may not be an argument against cyclers. Neil

Is the premise that as one craft arrives, it transfers it's kinetic energy to the orbiting craft such that they swap velocities? What would such a setup look like?

I can imagine one such possibility, but I need some to check and see if my intuition matches the realities of physics...

In my setup, you would have a two craft of equal mass. The craft orbiting Mars would be orbiting in such a way that the plane of it's orbit was perpendicular to the approaching velocity vector of the craft arriving from Earth. A tether of sufficient length is paid out such that it's endpoint is at the same orbital altitude as the orbiting craft, and lies in the orbital plane. It's length is sufficient not to over-G either craft in the upcoming maneuver.

The arriving craft hooks into the end of the tether, and they do a 180-degree swap before the post-orbiting craft, now departing craft, let's go, leaving the arriving craft in the same orbit as the departing craft.

The focus isn't to send the craft returning to Earth on the exact vector. Rather, it's to ensure the arriving craft is in a proper orbit.

Once there, the arriving craft reels in the tether, takes care of business, and awaits the return of the next craft so that it, too, can be flung back in the general direction of Earth.

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There is no reason why non-cycler craft haave to be to be single mission.

Approach velocities for Earth and Mars can be extremely high with cyclers, of the order of 12 km/s at Mars and 11 km/s at Earth over and above minimum energy transfer approach speeds. Include these and martian escape velocity means that total approach velocities for Mars and Earth would be of the order of 19 and 22 km/s in the worst case scenario. This is the price of using the intrinscially inefficient orbits required by cyclers. http://www.troymcconaghy.com/storage/AIAA_2002-4420.pdf

Jon