A Discussion of a Mars Cycler Transit System

[Ara Pacis]

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.

[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.

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

[MentalAvenger]

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

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

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.

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

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.

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

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.

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

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

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.

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

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.

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.

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.

[JonClarke]

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

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

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

I agree, I can see a possible role here.

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

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.

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

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

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.

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

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.

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.

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.

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

[Ara Pacis]

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

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.

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).

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

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.

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.

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.

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.

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.

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.

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.

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

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.

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

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.

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.

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.

[Kevin Wilson]

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.

[JonClarke]

Hello, Ara Pacis, JonClarke et al,
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.

Do you have a link to a more detailed explanation?

Jon

[JonClarke]

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

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]

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.

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.

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).

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.

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.

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.

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.

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

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.

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

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.

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.

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.

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?

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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

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.

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

Cheers

Jon

[neilzero]

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

[mugaliens]

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.

[JonClarke]

Fast single trip ships can have very high re-entry speeds, so that may not be an argument against cyclers.

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

[mugaliens]

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

So? Just use either longer tethers to keep the G's low, or immersed technologies to allow the high G's. I don't know what the optimization point might be for such a maneuver, but my gut answer is somewhere around 17 G's, which can be done without chloroflourocarbins.

[JonClarke]

So? Just use either longer tethers to keep the G's low, or immersed technologies to allow the high G's. I don't know what the optimization point might be for such a maneuver, but my gut answer is somewhere around 17 G's, which can be done without chloroflourocarbins.

The theory of tether propulsion is fine. The practice is going to be very difficult and complex. The solutions they offer may be more complex and risky than those offered by others approaches. Other than dynamic elegance what advanatages does it really have?

When do you envisage such a system being used? For initial missions or for the establishment of settlements and after?

Some issues that come to mind.

1. Design and construction of complex tethers that can reel and unreel repeatedly.

5. 2. How will differences in orbital planes and orbital be met with the tether propulsion system?

3. How much propellant is still required by the ferry craft?

4. How will the tethered system adjust its attitude and orbit?

5. What is the mass of the the cycler system?

6. What abort options are there if the tether does not work or rendezvous fails?

Note that 17 G for periods of more than a fraction of a second is not going to be acceptable under any circumstances.

Jon

[mugaliens]

Me: "my gut answer is somewhere around 17 G's, which can be done without chloroflourocarbins."

Note that 17 G for periods of more than a fraction of a second is not going to be acceptable under any circumstances.

Jon

Sorry. I meant with chloroflourocarbons.

Do you understand the concept? Complete immersion in oxygenated, breathable chloroflourocarbons of the same density as the human body?

You'd barely feel 17 Gs. You wouldn't feel it at all except for the fact your bones are more dense than the rest of your tissues.

[JonClarke]

Do you understand the concept? Complete immersion in oxygenated, breathable chloroflourocarbons of the same density as the human body?

You'd barely feel 17 Gs. You wouldn't feel it at all except for the fact your bones are more dense than the rest of your tissues.

I am quite familiar with the concept. Immersion in liquid to counter acceleration is as old as Tsiolovosky. He used suggested water. As I understand it, chlorofluorocarbons are only an advantage if you want to do liquid breathing.

There are several problems. Liquid breathing is still very experimental and risky. You don't want anything to go wrong. Even simple liquid immersion requires bulk tanks and a lot of water. Probably a tonne of water per person, perhaps 2 plus the tank and associated plumbing. This has to be mounted in the spacecraft firmly enough to deal with acceletation. Ever tried bracing 17 tonnes, even briefly?

cheers

Jon

[mugaliens]

There are several problems. Liquid breathing is still very experimental and risky. You don't want anything to go wrong. Even simple liquid immersion requires bulk tanks and a lot of water.

No! A simple comformal cradle molded to one's body is sufficient for the water approach.

Probably a tonne of water per person, perhaps 2 plus the tank and associated plumbing.

What plumbing? Static water. And try 40 lbs per person, not 2,000 lbs (unless you really want to immerse them in an entire room...). If you're worried about controls, picture this - waterproofed fingertip controls.

This has to be mounted in the spacecraft firmly enough to deal with acceletation. Ever tried bracing 17 tonnes, even briefly?

You mean aside from the internal bracing linking the Saturn V's engines with it's 4,500 ton infrastructure?

<smirk>

And 40 lbs of water in a conformal carbon-fiber shell weighs perhaps 60 lbs, in addition to the 200 lb human, so call it, max, 300 lbs.

Not 17 "tonnes."

Where's your engineering!

Ok. I can see you don't believe me.

Take a polystyreme cooler. Yes, the one's that fly apart at the slightest breeze and have no tensile strength whatsoever.

Please recall that neither does concrete, which is why for any compressive load that's more columnar than a pyramid, we use steel-reinforced concrete.

I can put my 200 lb frame on a 4 oz piece of polystyrene and it won't break. That's an 800 to 1 advantage, and it's unreinforced.

If I wrap it in a carbon-fiber shell which conforms to my foot and puts perhaps a 50% prestress on the material, it can easily withstand 1,000 lbs, which brings the advantage to 4,000 in a 1-G environment.

Thus, for a 17 G application, the advantage would only be 235, or, for a 50% margin of error, just 156.

Thus, for a 200 lb body, it would require that the entire shell, polystyrene and carbon fiber wrap, need not exceed 1.28 lbs.

Material engineering is an interesting science, folks. If you can't follow/picture the physics involved, respond and I'll do my best to paint it for you.

Try this on for size: I can stand on an empty cola can. It doesn't collapse. I weigh 200 lbs. The can weighs, what? (someone with a scale weigh the can). 1 oz? I don't know, but please respond. If it's 1 oz, that's a 3,200 advantage. Is this bringing things home for those of you out there who're still in the brick and morter phase?

That's the advantage, the limits with which we're working, here, in this day and age. They just don't compare to the old days.

While vacationing in Hawaii, I once hefted an 8 ft board built using wood. Only someone who was less than 5 ft could ride it because it was so dense (didn't displace much of it's weight in water). I next hefted a same-displace fiberglass, foam core board. Less than half the weight, and now we're upwards of a near 200-pound rider. The third board was a combination of pre-stressed carbon fiber, foam, spruce (of all things), and kevlar. It weighed in at half the fiberglass board. The kevlar portions were laid in with polyester resin, while the carbon with epoxy resin.

25% the weight, and the same displacement. In just 45 years.

Then, the grand finale - a surfboard that was designed by some leading aeronautical engineers. If you could buy it on the market it would have cost more than $100 grand. But it weighed in at less than 30% of the previous board.

That's a 12-fold increase in weight reduction vs cargo-carrying capacity in just 45 years.

Folks, there are some very smart people out there who're doing bang-up jobs on distributing loads while minimizing the supporting densities required to do so under varying conditions.

I honestly wouldn't be surprised if a modern-day engineered skyscraper like the Empire States Building weighed less than 25% of the original but was 30% stronger.

If it weren't, the newest skyscrapers on the market would never be possible.

Nor would acceleration couches built using 17 tons of old-technology design vs 100 lbs of new-technology design.

If you can't make the mental leap, I'll design the dang thing myself, and be it's first test subject for a full ten minutes, provided you post $100 plus all expenses (and I can do it for less than $20,000 additional expenses for a proof model...

[JonClarke]

JC There are several problems. Liquid breathing is still very experimental and risky. You don't want anything to go wrong. Even simple liquid immersion requires bulk tanks and a lot of water.

No! A simple comformal cradle molded to one's body is sufficient for the water approach.

That an interesting idea. It would save some water mass for sure. I assumed a tank capable of full immersion you would about 1 cubic m of water, and thus a tonne.

JC Probably a tonne of water per person, perhaps 2 plus the tank and associated plumbing.

What plumbing? Static water. And try 40 lbs per person, not 2,000 lbs (unless you really want to immerse them in an entire room...). If you're worried about controls, picture this - waterproofed fingertip controls.

You have to be able to pump the water in and out for maintenance purposes. If you are doing to immerse the human body in it the water needs to be filtered and sterilised. Water is too massive and valuable a commodity in space travel to leave idle in an acceleration couch. When not in use it should be pumped elsewhere. All of which requires plumbing.

JC This has to be mounted in the spacecraft firmly enough to deal with acceletation. Ever tried bracing 17 tonnes, even briefly?

You mean aside from the internal bracing linking the Saturn V's engines with it's 4,500 ton infrastructure?

I expressed myself poorly, apologies. I was trying to capture the point that normally spacecraft interiors don’t need to be braced for 17Gs.

Most of the time the spacecraft interior does not have to be braced for 17 Gs. So most of the time this is dead mass as opposed to useful mass. It would be better to see ways of eliminating the need for such mass.

The only reason for the 17Gs seems to be so that you can use tether propulsion. Since the advantages of tether propulsion in this case are not clear, and were only raised to try and overcome the problems with cyclers, of which the advantages are also not clear. If the cycler isn’t use, you avoid all these issues.

And 40 lbs of water in a conformal carbon-fiber shell weighs perhaps 60 lbs, in addition to the 200 lb human, so call it, max, 300 lbs.

Not 17 "tonnes."

300 “pounds” at 17 Gs is 1.275 tonnes in real units. Plus the support structures and plumbing.

Ok. I can see you don't believe me.

It is not a question of believing or disbelieving you, it is a question of trying to work out your reasoning.

Take a polystyreme cooler. Yes, the one's that fly apart at the slightest breeze and have no tensile strength whatsoever.

Please recall that neither does concrete, which is why for any compressive load that's more columnar than a pyramid, we use steel-reinforced concrete.

I can put my 200 lb frame on a 4 oz piece of polystyrene and it won't break. That's an 800 to 1 advantage, and it's unreinforced.

If I wrap it in a carbon-fiber shell which conforms to my foot and puts perhaps a 50% prestress on the material, it can easily withstand 1,000 lbs, which brings the advantage to 4,000 in a 1-G environment.

Thus, for a 17 G application, the advantage would only be 235, or, for a 50% margin of error, just 156.

Thus, for a 200 lb body, it would require that the entire shell, polystyrene and carbon fiber wrap, need not exceed 1.28 lbs.

What relevance does polystyrene, concrete, and carbon fibre has to the penalties of accelerating at 17 G?

I can stand on an empty cola can. It doesn't collapse. I weigh 200 lbs. The can weighs, what? (someone with a scale weigh the can). 1 oz? I don't know, but please respond. If it's 1 oz, that's a 3,200 advantage. Is this bringing things home for those of you out there who're still in the brick and morter phase?

That's the advantage, the limits with which we're working, here, in this day and age. They just don't compare to the old days.

While vacationing in Hawaii, I once hefted an 8 ft board built using wood. Only someone who was less than 5 ft could ride it because it was so dense (didn't displace much of it's weight in water). I next hefted a same-displace fiberglass, foam core board. Less than half the weight, and now we're upwards of a near 200-pound rider. The third board was a combination of pre-stressed carbon fiber, foam, spruce (of all things), and kevlar. It weighed in at half the fiberglass board. The kevlar portions were laid in with polyester resin, while the carbon with epoxy resin.

25% the weight, and the same displacement. In just 45 years.

Then, the grand finale - a surfboard that was designed by some leading aeronautical engineers. If you could buy it on the market it would have cost more than $100 grand. But it weighed in at less than 30% of the previous board.

That's a 12-fold increase in weight reduction vs cargo-carrying capacity in just 45 years.

Folks, there are some very smart people out there who're doing bang-up jobs on distributing loads while minimizing the supporting densities required to do so under varying conditions.

I honestly wouldn't be surprised if a modern-day engineered skyscraper like the Empire States Building weighed less than 25% of the original but was 30% stronger.

If it weren't, the newest skyscrapers on the market would never be possible.

Nor would acceleration couches built using 17 tons of old-technology design vs 100 lbs of new-technology design.

What’s the relevance of surf boards and skyscrapers to designing for 17Gs?

To me this all seems an ad hoc way of trying to save the cycler concept which, for all its mathematical elegance, does not do anything useful, at least in terms of getting people to and from Mars.

cheers

Jon

[gpk7]

One of the nice reasons to use a cycler is that you can look for a rock that's already nearly in a cycler orbit, then use it for shielding.

The idea is to find an asteroid that's about 10 meters in diameter and convert it to rubble. This can provide a layer about 2-meters thick around a reasonably sized crew compartment, which is sufficient to do a reasonably good job of stopping cosmic rays. Essentially, you form the rubble into a cylinder, then dock your spacecraft inside. When you get near Mars, you leave the shield, then rendezvous with Mars. The rubble shield continues along its orbit, ready for future use.

The big advantage is that you now don't have to boost the heaviest part of your spacecraft into a Earth-Mars transfer orbit: it's already in one. That means you may be able to afford to eventually build up a relatively large collection of cyclers, enough so that there will always be one ready, whenever you want to travel.

There are lots of such rocks. Thousands of near-Earth asteroids are known with sizes above 1 km, and from cratering studies, it is known that there are far more small rocks than large one.

More detail can be found at http://kochanski.org/blog/?cat=29.

[mugaliens]

That an interesting idea. It would save some water mass for sure.

I think we can skip the plumbing, too. A simple comformal gel-pack, with sealed, flowing gel in which you're "immersed" while under acceleration would do fine. The only mass that's required is the thin layer between the human contour and the carbon-fiber conformal pack, plus a slight additional amount to flow up and over most of the top layer to provide true, 360-degree encompassing support, like if you've ever laid down in a half-filled waterbed. This will most closely approximate total submersion.

[quote]What relevance does polystyrene, concrete, and carbon fibre has to the penalties of accelerating at 17 G?

Lightweight compressive resistance, heavyweight compressive resistance, and tension resistance

What’s the relevance of surf boards and skyscrapers to designing for 17Gs?

Highly complex structures maximized for strength to weight ratios under dynamic loads.

To me this all seems an ad hoc way of trying to save the cycler concept which, for all its mathematical elegance, does not do anything useful, at least in terms of getting people to and from Mars.

cheers

Jon

Well, that may be true!

[Ara Pacis]

Wow, I started this almost a year ago and people brought it back to life and I missed it. I'll have to go back and re-read the posts.

The simple fact is that cyclers work if you have a big enough system. If it's a half-hearted occasional transit, then might as well do it with point-to-point rockets.

[mugaliens]

Wow, I started this almost a year ago and people brought it back to life and I missed it.

It's alive... It's alive!

The simple fact is that cyclers work if you have a big enough system. If it's a half-hearted occasional transit, then might as well do it with point-to-point rockets.

Any insight on what the payload/frequency thresholds would be?

[Ara Pacis]

Any insight on what the payload/frequency thresholds would be?

I downloaded/read a study/pdf here comparing cycler orbits to direct visits (so-called stop-over cyclers) and it seemed to conclude that, all things being equal, direct visit was just as economical or more so for several trajectories using standard estimates for vehicle mass based on current mission profiles. There were some comparisons that favored cyclers.

Cyclers and stop-over cyclers have been compared in terms of ΔV, propellant mass, flight time and revisit time. In terms of ΔV only, it seems that stop-overs are generally more advantageous even because they do not require a docking manoeuvre on hyperbolic orbit. On the other hand when the mass budget is calculated, 6S8 and 6S9 cyclers become competitive. However these cyclers require a total of 12 vehicles to take full advantage of all launch opportunities. Aldrin’s cycler (which offers the lowest number of needed vehicles) is generally too expensive, nevertheless its optimised version presents an appreciable gain in ΔV especially for the taxi. This suggests that an optimised version of the 2L3 cycler which requires only 4 vehicles) could become competitive compared to stop-overs. The revisit time and transit time are dominated by the geometry of the two planets and by its periodicity and does not seem to be dependent on the particular transportation system.

Howeover, one big caveat for any such comparison is that the design parameters probably at least an order of magnitude smaller than my proposed infrastructure scale project. The PDF linked above assumes 70 tons for the spacecraft and 15.5 tons for the taxis. I'm thinking of smaller intra-orbital taxis (5-10 metric tons loaded) and larger cyclers of thousands of tons. I'm not thinking of small vehicles like Mars Direct but vehicles similar in size (if not mass) to aircraft carriers. Despite what the concluding sentence suggests, the greater the mass difference and specific delta-v requirements between cycler and taxi, the less favorably a direct stop-over visit with a vehicle of similar total mass will compare.

They would also do more than simply service an outbound drop-off at Mars and then, later, an inbound pick-up at Mars. It might also rendezvous with solar-orbiting stations in the asteroid belt and perform some refining and manufacturing work on materials in transit as well as performing astronomical work as for interferometry and lab experiments and special observations and continuing life sciences work.

I'm thinking that the cyclers could have one or more large rotating sections for long duration habitation. There might also be a long tail sticking out the rear where an electromagnetic launcher can be used to decelerate the taxi while accelerating the cycler as a mass-driver propulsor for orbital maneuver if necessary (or the velocity could be shed later if necessary). The taxi could also use onboard motors to adjust velocity still further. Depending on velocity, the taxi could rendezvous with circular orbiting stations or, perhaps more likely, rendezvous with a high speed mini-cycler in a highly elliptical orbit close to escape velocity. This mini-cycler could convey the taxi, pax and cargo to one, of possibly several, stations in high circular orbit at its apoapsis. Note: the idea of extra stations and mini-cyclers may be superfluous depending on future delta-v capabilities and on traffic.

[mugaliens]

Equipment which is much more massive can easily change the direction of a craft's velocity, but it's magnitude is more elusive...

...or is it?

Let's look at this from the perspective of a kid on a swing. Twist the swing up, then let loose. Tighten up, spin faster! Loosen up, spin slower.

In a tethered system, velocity changes are only a changed tether-length away, and what energy is expended tightening up a teather is easily recovered letting it out, later, same as a hybrid's speeding up and using electro-braking to slow down.

So, with simple planning and an electric motor/generator, the tethered system itself is fully capable of imparting both changes in magnitude, as well as direction, when it comes to velocity.

[Ara Pacis]

Equipment which is much more massive can easily change the direction of a craft's velocity, but it's magnitude is more elusive...

...or is it?

Let's look at this from the perspective of a kid on a swing. Twist the swing up, then let loose. Tighten up, spin faster! Loosen up, spin slower.

In a tethered system, velocity changes are only a changed tether-length away, and what energy is expended tightening up a teather is easily recovered letting it out, later, same as a hybrid's speeding up and using electro-braking to slow down.

So, with simple planning and an electric motor/generator, the tethered system itself is fully capable of imparting both changes in magnitude, as well as direction, when it comes to velocity.

Did you post this in the wrong thread?

[mugaliens]

No. Search this thread for "tether." It's one way of economizing the physics of a cylical system, akin to regenerative braking for hybrids. Expending reaction mass for both acceleration and braking we must do for one-shot deals, but doing it for a cyclical system is no better than wasting all that energy as heat in the brakes.

[Ara Pacis]

Oh, you didn't quote any referant and the last I saw someone talking about tethers was with IsaacKuo in teabinge's thread.

So, you think a tether, like a rotovator, could rendezvous with an interplanetary cycler on it's high-speed pass in it's hyperbolic orbit in order to exchange mass, such as interplanetary taxis?

[JonClarke]

One of the nice reasons to use a cycler is that you can look for a rock that's already nearly in a cycler orbit, then use it for shielding.

The idea is to find an asteroid that's about 10 meters in diameter and convert it to rubble. This can provide a layer about 2-meters thick around a reasonably sized crew compartment, which is sufficient to do a reasonably good job of stopping cosmic rays. Essentially, you form the rubble into a cylinder, then dock your spacecraft inside. When you get near Mars, you leave the shield, then rendezvous with Mars. The rubble shield continues along its orbit, ready for future use.

The big advantage is that you now don't have to boost the heaviest part of your spacecraft into a Earth-Mars transfer orbit: it's already in one. That means you may be able to afford to eventually build up a relatively large collection of cyclers, enough so that there will always be one ready, whenever you want to travel.

There are lots of such rocks. Thousands of near-Earth asteroids are known with sizes above 1 km, and from cratering studies, it is known that there are far more small rocks than large one.

More detail can be found at http://kochanski.org/blog/?cat=29.

The problem is that no such asteroid is known. They may not exist, or, if they do be in such far from ideal orbits that the amount of propellant to get to and from that orbit is prohibitive.

Jon

[mugaliens]

The problem is that no such asteroid is known.

???

Have we looked? There are several thousand possible candidates to choose from... And if we don't find an exact match, move two or three smaller ones and lash them together.

[cjameshuff]

The problem is that no such asteroid is known. They may not exist, or, if they do be in such far from ideal orbits that the amount of propellant to get to and from that orbit is prohibitive.

They exist, many in this size range have been seen hitting the atmosphere and a few have spotted on close passes. (one was just discovered...2009 BD)

The idea that they are rare is a bit absurd...what could possibly prevent them from forming or so selectively destroy them? There's likely far, far more of them than the larger asteroids...millions, if not billions. The odds of none being in useful orbits are extremely remote. The difficulty is in spotting them.

It's not a necessity, though. You could add radiation/micrometeorite shielding incrementally. Haul cargo and cycler equipment/shielding panels on the initial trips until it's well enough equipped and protected to handle passengers, continue expanding capacity and capabilities from there. The initial cycler could be little more than a bus with power, communications, propulsion, and a sling of some sort.

Even if you require that manned transfer vehicles be capable of delivering their passengers to the destination alive in the event of a missed rendezvous, there can be clear benefits to rendezvous with the cycler...aside from comfort and health beyond simple survival, there would be greater safety margins and more options in case of trouble with access to cycler-based supplies and equipment. Also, the requirement of keeping the occupants alive doesn't necessarily mean mission success...an abort could very well mean jettisoning mission equipment in order to achieve the needed delta-v.

In reality, however, I suspect a missed rendezvous would mean something's gone wrong enough that chances for survival are extremely slim. No matter how capable the transfer vehicle is when working properly, something has caused it to not perform as expected. In the case of a cycler rendezvous, there's at least the potential of sending a rescue craft from the cycler, which is likely much closer to the transfer vehicle in time and delta-v than any other location in the solar system.

[JonClarke]

They exist, many in this size range have been seen hitting the atmosphere and a few have spotted on close passes. (one was just discovered...2009 BD)

The idea that they are rare is a bit absurd...what could possibly prevent them from forming or so selectively destroy them? There's likely far, far more of them than the larger asteroids...millions, if not billions. The odds of none being in useful orbits are extremely remote. The difficulty is in spotting them.

Yes, I known such bodies exist. I have been to their impact sites. But a Mars cycler has is a very precise orbit, one that is constantly being peturbed by close encounters with Earth and Mars. The chances of an asteroid in such an orbit is doing to be astronomuically small IMHO.

It's not a necessity, though. You could add radiation/micrometeorite shielding incrementally. Haul cargo and cycler equipment/shielding panels on the initial trips until it's well enough equipped and protected to handle passengers, continue expanding capacity and capabilities from there. The initial cycler could be little more than a bus with power, communications, propulsion, and a sling of some sort.

Nothing about cyclers is neccessary. They are the hard way to do things. IMHO.

Even if you require that manned transfer vehicles be capable of delivering their passengers to the destination alive in the event of a missed rendezvous, there can be clear benefits to rendezvous with the cycler...aside from comfort and health beyond simple survival, there would be greater safety margins and more options in case of trouble with access to cycler-based supplies and equipment. Also, the requirement of keeping the occupants alive doesn't necessarily mean mission success...an abort could very well mean jettisoning mission equipment in order to achieve the needed delta-v.

How does any ofn this add up asd an advantage over sending people and cargo directly to Mars.

In reality, however, I suspect a missed rendezvous would mean something's gone wrong enough that chances for survival are extremely slim. No matter how capable the transfer vehicle is when working properly, something has caused it to not perform as expected. In the case of a cycler rendezvous, there's at least the potential of sending a rescue craft from the cycler, which is likely much closer to the transfer vehicle in time and delta-v than any other location in the solar system.

Rescue craft from the cycler? How big do you want to make this thing? If you don't have a cycler you don't need any rescue craft.

Cyclers are still a slower, more expensive in propellant*, and require more in space infrastructure and operations than any other way of getting to Mars. What's the point?

If you want to go to Mars, send stuff to Mars, keep the space infrastructure needed to go there to a safe minimum.

*Even if you you tethers for momentum transfer it is still more efficient to go directly to Mars. And tethers are just a concept.

Jon