Cyclers To Mercury

[Moonguy]

How would crews get to Mercury? Is there any way we can beat the high delta-V penalties for classic style ballistic flights? Yes, there is: Cyclers.

Mercurys orbital period is 87.97 days. Earths is 365.25 days, or 4.15 Mercury years. A cycler deployed to a 351.5 day orbit, with Earths orbit as the aphelion and Mercurys orbit as the perihelion, will encounter Mercury every time it (the cycler) reaches perihelion.

Crews departing Earth would still have to generate a high delta-V. In fact, cycler missions would require a delta-V around 9.5 km/sec. while some standard (Hohmann) transfers can be done for 6-7 km/sec. The difference is that a cycler mission puts most of the payload mass required for the 176 day transfer time on the cycler. This drastically lowers the payload mass injected into the transfer. In a classic Hohmann transfer scheme, a manned payload would be injected with an upper stage able to effect the Mercury rendezvous and orbit insertion. For a 10 ton payload and a stage using a J2-X engine, a mass ratio of at least 4.1 would be needed for even the most favorable MOI delta-V, which is about 6.3 km/sec. The resulting stage would be about 60 tons. Pushing this into a transfer orbit from Earth would require a stage which also has a 4.0 mass ratio and hence masses over 100 tons by itself.

The cycler reduces the requirement to launching the crew in their Earth-return capsule. This could be an Orion, A DragonRider derivative or something similar. If it had the same 10-ton mass as the first example, it would not need to be boosted with a second stage. Instead, propellant for the maneuvers at Mercury would be derived from water stored on the cycler. Food and living accommodations would also be on the cycler as well. A 15 ton cycler could easily store enough food and consumables for several mission cycles. These supplies would be replenished to the cycler periodically by the same solar sail that deploys the cycler to the 351 orbit. The water used for propellant would also be supplied using solar sails. Initially this would be from Earth, but it would eventually come from Mercury.

With an orbit of 351 days, the cycler would encounter Earth every third orbit of the cycler. This is due to the synodic period of Earth and Mercury is 115.9 days. Multiplied by three yields 347.7 days. There is just over a 4 days discrepancy between an exact encounter., However, launch windows to Mercury are open for about 20 days, so it could be assumed a delta-V penalty would be incurred to make up for the 4-day difference.

The cycler mission requires much less propellant be available in Earth orbit. Only one stage is used. This is refueled at the cycler for the MOI burns and again for the return to the cycler for the return trip. If the Earth-entry interface velocity is too great for the return modules heat shield, the crewed stage could refuel a third time to execute a burn into Earth orbit. This architecture enables a crew to launch to Mercury every 347 days. Twice as frequently as flights to Mars.

 

 

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Edited November 16, 2013 by Moonguy

[Enthalpy]

Do I understand properly that the cycler is an accommodation and life support for astronauts, and that the cycler stays permanently on the Earth-Mercury-Earth transfer orbit while only the occupants are brought to and from it at the end stations?

 

Interesting provided someone wants to transfer many crews. You may also want to double-check the launch opportunities, since Mercury's orbit is very eccentric.

 

Hohmann transfers, in a useable form, are available on the web in the thesis of

Jarret Mathwig

On Properties of the Hohmann Transfer

just beware that his transfer times are wrong for very eccentric transfers.

 

When estimating a necessary delta-V, one has to use the Oberth effect.

 

I feel the J2-X is oversized for 60 tons. Two RL-10 should suffice; they provide roll control and are more efficient because they push less through more exit area. Take RL10-A or -B according to the available stage diameter. Presently hydrogen isn't stored for days, but this will be done soon.

 

You may also consider my Solar thermal engine. It works more easily nearer to the Sun and offers isp=1267s. That helps. Though, each 4.4m concentrator pushes only 2.4N near Earth, so a chemical engine is better to first leave the planet.

 

Don't worry about the heat shield. Galileo entered Jupiter's atmosphere at 39km/s. But do worry about the deceleration: returning from Moon was already hard enough at 11km/s, because Earth's curvature limits the time available to brake. The proper solution (to my opinion the only one) is a body that provides downlift to keep the vehicle in the atmosphere. At 8km/s around Earth, centrifugal force is 1g, so a crew accepting 9-1g (plus the deceleration by the strong drag) could arrive with 24km/s.

 

You should definitely consider a Venus slingshot. Unmanned probes take many gravity assists from Earth and Venus then Mercury, since delays aren't so important for them, but a single Venus flyby takes no extra time and must save much fuel. Advantageously, the inner planets offer many launch opportunities. Strong alternative to the cycler, and it combines with the Solar thermal engine.

 

Some scenarios for manned missions limit the unit mass to be launched and bring redundancy very important to my eyes. Typically, you can preset in Mercury orbit the descent-living-ascent module, the return module (or the module or propellant needed to reach your cycler), both by separate launches. Once these are in place, you can send the crew. More launches by a smaller rocket, that is, perhaps one which already exists. Then you can have backups in Mercury orbit, for the descent-living-ascent module and for the return module; their cost adds only once for all missions. They can follow a slow economic route through successive flybys or use a Solar sail.

[Moonguy]

Do I understand properly that the cycler is an accommodation and life support for astronauts, and that the cycler stays permanently on the Earth-Mercury-Earth transfer orbit while only the occupants are brought to and from it at the end stations?

 

Interesting provided someone wants to transfer many crews. You may also want to double-check the launch opportunities, since Mercury's orbit is very eccentric.

 

Hohmann transfers, in a useable form, are available on the web in the thesis of

Jarret Mathwig

On Properties of the Hohmann Transfer

just beware that his transfer times are wrong for very eccentric transfers.

 

When estimating a necessary delta-V, one has to use the Oberth effect.

 

I feel the J2-X is oversized for 60 tons. Two RL-10 should suffice; they provide roll control and are more efficient because they push less through more exit area. Take RL10-A or -B according to the available stage diameter. Presently hydrogen isn't stored for days, but this will be done soon.

 

You may also consider my Solar thermal engine. It works more easily nearer to the Sun and offers isp=1267s. That helps. Though, each 4.4m concentrator pushes only 2.4N near Earth, so a chemical engine is better to first leave the planet.

 

Don't worry about the heat shield. Galileo entered Jupiter's atmosphere at 39km/s. But do worry about the deceleration: returning from Moon was already hard enough at 11km/s, because Earth's curvature limits the time available to brake. The proper solution (to my opinion the only one) is a body that provides downlift to keep the vehicle in the atmosphere. At 8km/s around Earth, centrifugal force is 1g, so a crew accepting 9-1g (plus the deceleration by the strong drag) could arrive with 24km/s.

 

You should definitely consider a Venus slingshot. Unmanned probes take many gravity assists from Earth and Venus then Mercury, since delays aren't so important for them, but a single Venus flyby takes no extra time and must save much fuel. Advantageously, the inner planets offer many launch opportunities. Strong alternative to the cycler, and it combines with the Solar thermal engine.

 

Some scenarios for manned missions limit the unit mass to be launched and bring redundancy very important to my eyes. Typically, you can preset in Mercury orbit the descent-living-ascent module, the return module (or the module or propellant needed to reach your cycler), both by separate launches. Once these are in place, you can send the crew. More launches by a smaller rocket, that is, perhaps one which already exists. Then you can have backups in Mercury orbit, for the descent-living-ascent module and for the return module; their cost adds only once for all missions. They can follow a slow economic route through successive flybys or use a Solar sail.

Yes, that is a good basic description. Add the function of storing propellant water and producing LO2 and LH2 for Mercury Orbit Insertion (MOI) and the Earth Return burn and you have it.

Mercury’s orbit has an eccentricity of .2056. The Cycler would also have an eccentric orbit. The precession of Mercury’s nodes complicates things long term. Part of the answer to these issues is to keep the sail that delivers the Cycler to its operational orbit attached to the Cycler to perform corrections over long periods of time. Alternately, a high-impulse system could be designed into the Cycler for the occasional adjustments required, similar to mid-course correction maneuvers.

The J2-X was baselined here for standard Hohmann transfer using the larger payload required for the traditional approach. It would take everything the SLS Block II has to get even a small manned payload to Mercury orbit. In the Cycler concept, the manned crew vehicle and its single-stage system only needs something like the RL-10-B-2. In any case, I really do not want to use SLS components for anything if I can help it. Not at $2 Billion a pop.

I am considering your STE. Given the flux potential available at Mercury, it would be a very economical boosting system for manned flights back to Earth. I also like its simplicity and supportability. Another point about an STE: it can be a bi-modal system to generate substantial electrical power – possibly in a powersat application.

Not a healthy situation at all. The Cycler resolves this problem be enabling a returning crew vehicle to fuel up for an insertion burn into Earth orbit instead of an all atmospheric return. Or, alternately, some combination of the two. The point is the propellant is on board the Cycler to allow this. Other scenarios, providing any kind of high-impulse return to Earth is a logistical nightmare. We have enough of those already.

I would be happy to utilize Venus Gravity Assists, if only they were more frequent and did not require the crew to be en-route so much longer. Venus and Earth line up for transfer every 17 months. So this option would only be available for maybe a third of the potential flight opportunities. The Cycler routinely gets crews to Mercury’s orbit in 176 days. Flight times for the VGA flights to Mercury range between 145 days and 305 days, with most being well over the 176 days for the Cycler.

You correctly define the function of the Mercury Orbiting Transfer Station, or MOTS. This unit is also delivered to a 1000 km circular, near-polar orbit over Mercury by a solar sail. As in the case of the Cycler, the sail remains attached to the MOTS to serve as a sunshade and to provide station-keeping propulsion. The MOTS carries two manned landers with it. It also carries two unmanned landers that deliver key equipment to the surface. Like the Cycler, the MOTS carries a mass of water to provide propellant for the first few landings. Eventually, water from Mercury itself is used to replenish the MOTS on a continuous basis.

Both the MOTS and the Cycler(s) are developed from propellant tanks salvaged commercial launchers. Any tanks would do as they are only containing water at any given time. For my claculations, I use a LO2 taken from expended single-stage orbiters. These tanks are 4 m in diameter, 8.4 m long with a volume of 88 cubic meters and a dry mass of just over a metric ton each. There are six on the MOTS (though some are empty when it is deployed to Mercury orbit) and on each of the Cyclers. All together, a fuluy loaded Cycler would suppoirt ten one-way missions or five round trip missions.

Edited November 18, 2013 by Moonguy

[Enthalpy]

O yes, the semimajor axis of both Mercury's orbit and the transfer orbit keep their orientation, so the eccentricity isn't a worry. My mistake. Good news.

[...] I am considering your STE. [...] I also like its simplicity and supportability.

As long as one describes how an idea shall work, it looks simple.

When you develop and test it, discover what needs additions and complements, it often gets less simple...

My old engineer feeling sees it useful and feasible.

 

Another point about an STE: it can be a bi-modal system to generate substantial electrical power – possibly in a powersat application.

The concentrators can be the same as for my generator:

http://saposjoint.net/Forum/viewtopic.php?f=66&t=2051

either the same design or the same parts switching the functions.

[...] storing propellant water and producing LO2 and LH2 for Mercury Orbit Insertion (MOI) and the Earth Return [...]

You plan to find the water at Mercury's pole, is that it? Otherwise, I'd rather transport oxygen and hydrogen ready to use than water. Long storage of oxygen will be made soon, actively cooled hence even near the Sun, and hydrogen is feasible as well.

 

That would also be a reason for hydrogen at the descent and ascent module(s). If not found locally, I'd prefer a dense fuel, because its small tank and the insulation can be sturdy, while I fear Mercury stones flying against the insulation.

 

Are reasonable and reliable estimates available about how much water, how concentrated, that is how useable, is on Mercury? Because, if it's a little bit of snow deep within much soil, in concentration unknown until someone checks in situ, it demands a robot to verify first, and the extraction plant is probably heavier than the propellant...

 

Oxygen is available for sure in the sand or rocks. Solar heat can separate it, maybe through a zinc cycle. If hydrogen is hard to produce locally, you could consider:

- Bring the hydrogen, produce the oxygen locally. At 7:1 or 8:1 ratio, you do save mass.

- Find lithium locally? In our Moon not, but in Mercury? Together with locally found aluminium, it burns in oxygen, and pushes.

- Bring a safe fuel instead of hydrogen. Phytane for instance: liquid from -100°C to +300°C. RP-1 performance.

[...] As in the case of the Cycler, the sail remains attached to the MOTS to serve as a sunshade and to provide station-keeping propulsion.[...]

I do like Solar sails and believe all agencies should put much bigger efforts in them. Ulysses for instance would have been earlier on a better orbit than with chemical propulsion and a Jupiter assist.

 

Sails tested up to now have like 10m diameter and their manoeuvre isn't fully convincing. Unless they progress quickly (keep an eye on Jaxa's Ikaros), a manned mission would rely instead on ionic propulsion, which is proven by many geosynchronous satellites.

Venus and Earth line up for [direct] transfer every 17 months. The Cycler routinely gets crews to Mercury’s orbit in 176 days.

What are the constraints due to radiations on humans? On Earth-Mars-Earth, Nasa seeks a 90 day trip because of radiations.

 

I believe to remember - but may be horribly wrong - that the radiations not easily stopped by the craft are cosmic rays, which are attenuated by the Solar wind or its magnetosphere, so radiations would be less critical at Mercury than Mars. Is that correct? And is a long stay on Mercury possible within a normally shielded lander, or does it demand to bury a station?

 

If a short stay is mandatory, it will leave only the direct shot without Venus' gravity assist! That would impose non-chemical propulsion.

 

What are these 17 months? It looks like a synodial period, but a round trip is shorter. Both Mercury and the transfer rotate quicker than Earth around the Sun, so one needs to wait on Mercury less than a local year, doesn't he?

Edited November 19, 2013 by Enthalpy

[Moonguy]

O yes, the semimajor axis of both Mercury's orbit and the transfer orbit keep their orientation, so the eccentricity isn't a worry. My mistake. Good news.

 

As long as one describes how an idea shall work, it looks simple.

When you develop and test it, discover what needs additions and complements, it often gets less simple...

My old engineer feeling sees it useful and feasible.

 

The concentrators can be the same as for my generator:

http://saposjoint.net/Forum/viewtopic.php?f=66&t=2051

either the same design or the same parts switching the functions.

 

You plan to find the water at Mercury's pole, is that it? Otherwise, I'd rather transport oxygen and hydrogen ready to use than water. Long storage of oxygen will be made soon, actively cooled hence even near the Sun, and hydrogen is feasible as well.

 

That would also be a reason for hydrogen at the descent and ascent module(s). If not found locally, I'd prefer a dense fuel, because its small tank and the insulation can be sturdy, while I fear Mercury stones flying against the insulation.

 

Are reasonable and reliable estimates available about how much water, how concentrated, that is how useable, is on Mercury? Because, if it's a little bit of snow deep within much soil, in concentration unknown until someone checks in situ, it demands a robot to verify first, and the extraction plant is probably heavier than the propellant...

 

Oxygen is available for sure in the sand or rocks. Solar heat can separate it, maybe through a zinc cycle. If hydrogen is hard to produce locally, you could consider:

- Bring the hydrogen, produce the oxygen locally. At 7:1 or 8:1 ratio, you do save mass.

- Find lithium locally? In our Moon not, but in Mercury? Together with locally found aluminium, it burns in oxygen, and pushes.

- Bring a safe fuel instead of hydrogen. Phytane for instance: liquid from -100°C to +300°C. RP-1 performance.

 

I do like Solar sails and believe all agencies should put much bigger efforts in them. Ulysses for instance would have been earlier on a better orbit than with chemical propulsion and a Jupiter assist.

 

Sails tested up to now have like 10m diameter and their manoeuvre isn't fully convincing. Unless they progress quickly (keep an eye on Jaxa's Ikaros), a manned mission would rely instead on ionic propulsion, which is proven by many geosynchronous satellites.

 

What are the constraints due to radiations on humans? On Earth-Mars-Earth, Nasa seeks a 90 day trip because of radiations.

 

I believe to remember - but may be horribly wrong - that the radiations not easily stopped by the craft are cosmic rays, which are attenuated by the Solar wind or its magnetosphere, so radiations would be less critical at Mercury than Mars. Is that correct? And is a long stay on Mercury possible within a normally shielded lander, or does it demand to bury a station?

 

If a short stay is mandatory, it will leave only the direct shot without Venus' gravity assist! That would impose non-chemical propulsion.

 

What are these 17 months? It looks like a synodial period, but a round trip is shorter. Both Mercury and the transfer rotate quicker than Earth around the Sun, so one needs to wait on Mercury less than a local year, doesn't he?

Thanks for the clarification regarding the orbits.

As for the plan being ‘. . . based on finding water on Mercury’ Later, yes. Initially, though, the MOTS would have enough water on board to support several flights of manned or unmanned landers to Mercury’s surface and back. For the Cycler’s water supply, water would be obtained from Mercury and transported by sail to the Cyclers. These deliveries would be faster because of the 8-fold increase in photon flux at Mercury allowing a greater rate of acceleration for a given payload mass.

For those who have not heard, the MESSENGER team has confirmed the presence of massive amounts of water ice at both poles on Mercury – and the presence of a mysterious ‘hydrocarbon’ material, likely comet-derived. The total amount of water ice is at least equal to that on the Moon, likely greater. It’s still under study. . .

I am desperately hoping they go for a sail-propelled Mercury landing mission next. The ice has been characterized as solid ice with dust mixed in. ‘Snow’ is not quite the same thing. Figures of hundreds of billions of tons have dominated these findings. I will get updated on that before I say more.

I do not contemplate the use of solar sails for manned flights. They are the most economical way to go for cargo, but they take too long for crews. At these distances from the Sun we want to seriously decrease the amount of time the crews are in transit. There is an exception: If a crew module limited to five tons mass were carried by a large sail (1,000,000 m2 or more) it might be delivered to Mercury after several months, but with zero propellant used foir departure or MOI. I have not yet worked that out and want to give it quality attention. I’m open to comments on it, though. . .

On the Cyclers, crews are protected by 4 meters depth of water all around them or masses of equipment where there is no water mass. Also, the interior of the habitat is insulated with radiation absorbing materials such as hydrogenated polyethylene. As for NASA’s 90-day ‘requirement’, every study NASA has ever done was predicated on providing sufficient radiation protection within the payload mass limit of the transportation system used. I suspect the 90-day figure was politically motivated to justify development of fast nuclear powered systems on the ground of safety. I don’t buy it.

You are essentially correct about the Sun’s magfield protecting Mercury from cosmic rays. Radiation from the Sun itself is another matter. When the first crew arrives they will have a turnkey habitat waiting for them to inhabit while they build the first permanent base facilities. They will land just before sunset, set up some lights, and have over 88-days of protected EVA time to do the construction of the base. Mercury itself protects them from solar radiations during the night.

The 17 months (~520 days) refers to Earth’s synodic period with Venus. Nominal mission stays on Mercury are multiples of Mercury’s year if the Cycler is used. With the Cycler, there is no variation in the transfer times (176 days each way) between Earth and Mercury. However, since the Cycler relieves the propellant mass issue by such a great amount, missions are no longer needing to be defined by delta-V. The Cycler enables much longer stay times at Mercury as long as they come in increments of 88 days. Generally, a basic mission plan would have the crew on Mercury for two Mercury years, or 176 days.

[Enthalpy]

90 days trip to Mars, necessary or not? I didn't check by myself, from existing radiation data. Checking it would be feasible, but I confess I havn't done it.

 

Are these 90 days motivated by proponents of nuclear propulsion? Not necessarily! Esa and Nasa had studies made on magnetic shields for the crew. I read the study for Esa, and it was full of mistakes, which I told at NasaSpaceFlight. Other papers were just as wrong, as they ignored the true properties and limits of superconductors, and the danger of superconducting coils (=boom). No idea how good or bad the study for Nasa was, but I suppose more people have noticed how poor the design proposal for Esa was.

 

From that story, I rather imagine that the radiation threat is very real, that Esa and Nasa had hoped to answer it with a magnetic shield, an as critics told the designs were unrealistic and the principle out of reach, Nasa decided to reduce the trip time to 90 days, and meanwhile has defined an other mission (bring an asteroid near Earth, send astronauts to visit it then).

 

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About Solar sails: the biggest now are around D=10m, and the lightest are around 6µm thin (though ideas, and hand-sized demonstrators, exist for thinner materials). For comparison, a space blanket is around 25µm thin. 6µm polyimide weigh 9g/m², beams to support the film easily double the mass. We're at 19t/km² already, without any deployment mechanism nor payload.

 

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Agencies consider building a thick station by stereolithography fed with regolith. This needs material with adequate granularity. Esa has a nice demonstrator meant for our Moon, using lasers powered by electricity. At Mercury (and our Moon?), perhaps light concentrators suffice to melt the sand; if not (on Mars and if needed elsewhere) the efficiency of lasers pumped by Sunlight would outperform electricity. Such lasers exist with >10% efficiency at a Japanese university.

 

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Land on Mercury just before sunset... You mention a polar Mercury orbit. Does this constraint the orbital plane of the Cycler? If yes, is this constraint compatible with Earth's position? It may (or not) be necessary to switch to an equatorial orbit. Or land at the poles, then at any time: light is still strong, but at least land is cold - put sunshades as soon as possible.

 

Occupants stay for two Mercury years, so they will get full Sunlight. How does the temperature keep bearable? Station buried deep enough? And at what daytime temperature shall the machine operate to build the station? Sunshade are one thing, but Mercury's soil will be hot, and no paint can protect against ambiant IR radiation and at the same time radiate heat away.

 

I really thought you were targeting a pole. Make an igloo there: much easier than a stone house.

[Moonguy]

I am interested in the 90-day Mars mission issue but I need to look into their reasoning before I comment further on it. Might be a good subject for a later thread here. . .

As far back as the mid-1970’s JPL researchers were confidently talking about 2μ/m2 sail material in time to build a probe to Haley’s Comet in the mid-80’s. That would be 2000 kg/km2 of sail material. Doubling that with the booms and mast brings that to 4000 kg. The sails used for the Mercury project are 820 m per side square sails with CFRP masts and booms. Total load for the sail is 3 gm/m2 (~2017 kg) for the sail material alone. Assume a similar mass for the spacecraft control systems, docking unit etc. and related structure and you get 4034 kg.

I do know that the radiation threat is very real and will not be resolved by even the fastest trip times we can do for either Mars or Mercury. Shielding of some sort will be needed. Prior to looking into the Cycler concept for Mercury, I had the idea of inserting the crew module into an upper stage tank so that a couple meters of LH2 protected the crew during the coast phase of the mission. Clever, I guess, but not practical. The Cycler protects the crew with four meters of plain ol' water.

 

The MOTS is in a near-polar orbit to minimize heating. The rendezvous between the Cycler and Mercury takes place after an in-plane transfer, so the crew vehicle will need to make a plane-change maneuver to go into the same orbit as the MOTS. The Cycler continues on it's in-plane orbit. There is a penalty for the plane change, but it does not seem ruinous. Alternately, the first base could be positioned nearer the equator with a subsidiary operation at the pole limited to producing water and regional exploration. The MOTS would have to be in a very elliptical orbit where the apoherm is several thousand kilometers above the daytime hemisphere. This would lessen heating, but still allow access to an equatorial base.

Once the crew lands, they have 88 days of darkness where the surface temps will be cold enough to make you forget what warm is. After sunrise, the surface takes up to six weeks to heat up to the boiling point of water. Ionizing radiation after sunrise concerns me, but the crew could have a total of 18 weeks to build and bury a shelter before it gets too hot. By 'shelter' I include facilities for protecting spacecraft and equipment form surface conditions as well.

[Enthalpy]

How does a polar orbit reduce heating of the Mercury Orbiting Transfer Station better than an equatorial does? I'd agree that Mercury's soil heats the station, and the soil is colder at higher latitudes than in the morning and evening, but my fear is that over mercury's year, the polar orbit will get exposed to Sunlight all the time, instead of benefitting Mercury's shade.

 

Maybe you don't need the Mercury orbiter to change its orbital plane. The cycle can be almost ecliptic, and pass over or under Mercury, easing a polar insertion for the Mercury orbiter. A few 1000km North or South are little, over an Earth-Mercury transfer orbit.

 

To produce accurately thin polymer films for the Solar sail, I suggest a process and machine there

http://www.scienceforums.net/topic/78265-solar-sails-bits-and-pieces/

under the fourth =======.

Sail sizes we could test on Earth i a soccer/rugby stadium are 1.7hm² and 3.2hm² - only for constructions with radial booms, and based just one size... I still ignore how to make the booms, later maybe. Hectometer sizes are already far-fetched, and I'd consider them the biggest step from the present situation. Unless, of course, better ideas pop up.

 

I'd prefer any shelter to pre-exist the arrival of a crew, because many things will inevitably go wrong. But equipment needs protection only against the heat; it survives UV and ionizing rays for long, with proper design already known and tested.

 

Did you consider a large white film on the soil around the base, instead of burying everything? If it suffices, I feel it more exalting than sending equipment and astronauts to a cave, which isn't so much better on an other planet than on Earth. Easier to transport and install as well. At 0.38AU, easy e=0.90 and a=0.18 give you 300K. Optical Solar Reflectors for spacecraft outperform that.

[Moonguy]

For the MOTS, we want an orbit that is exposed to the Sun full time.

The MOTS is put in a near-polar orbit because the pole is where the base is to be. At least within 5 degrees of the pole. The polar orbit of the MOTS allows its photovoltaics to be exposed to the Sun 24/7 while the majority of the MOTS’ surface area (ie, radiators) is facing out to black space. Also, With a polar orbit that is at a very slight angle to the terminator, the MOTS can be flying over al-dark terrain for half the orbit, reducing reflected heat load even further. In a polar orbit, the orbit can be circular.

If an equatorial orbit is used, the MOTS will have to be in a very elliptical orbit with the apoherm well above the daylit side to minimize reflected heating. A major headache with an equatorial orbit like this is the line ups for descent to the surface base are complicated by the need to begin descent at the point of periherm, but the base site is not always positioned right for the most efficient descent path.

 

Designing the Cycler’s orbit to pass above or below Mercury would seem to be the way to resolve the plane change problem. The only issue would then be if the Crew Vehicle has to do a plane change between two polar orbits. Bizarre to think about, but functionally the same problem as going from an equatorial to a polar orbit. My guess is this would be a minimal propulsive maneuver because we would inject into a polar orbit over the terminator. Since the MOTS is kept orbiting over the terminator (or nearly so) the difference might only be two or three degrees. Very much better than an 87-degree plane change from the equator!

Regarding sail testing. . . I’m not sure any test done on Earth’s surface can really be conclusive. Even if a ‘large’ (hectare or better) sail could be tested in a vacuum chamber, the effects of gravity would still leave questions. Since sails are designed to operate under microgravity conditions, it makes sense to test them under such conditions. IMHO, that was IKAROS was all about and it proved the concept in its basic terms.

Why not just test a full-sized sail, in space? Even large sails are light enough to be injected directly into interplanetary transfer trajectories. That is an ideal test because the real issue is handling a sail over interplanetary distances. The launcher would only need to be able to get the payload to a high Earth orbit with a perigee well above Earth’s atmosphere. If the test works, we have an operational sail that can bring payloads to Mercury on a repetitive basis.

Since Mercury is so much like the Moon, I originally considered base designs that were much like the LESA program for the Moon. Basically big tin cans connected together over several years to make a complete base. Self-landing modular shelters are ideal for early, short-term habitats. The problem is they have no real future because you get diminishing returns by deploying module after module designed cookie-cutter style. At some point you want larger volume or more specialized layouts not accommodated by modules confined to the dimensions of an Earth-launch vehicle payload shroud.

There is more than one reason for burying the base. Ionizing radiation is just the biggest one. Deliberately burying to ten meters allows the shielding dead-load to equal the pressurization load of a full atmosphere in the habitat. The pressure structure (a fabric in the current plan) would have almost no differential stress across its surface and so could be very simple in construction. I have been looking into the properties and formulae for Beta-Cloth that could be made from Mercury’s own resources. If that works out the possibilities for structures on Mercury are staggering!

[Enthalpy]

I didn't fully grasp the Mot's polar orbit. Is its plane to rotate over the Mecury year, so that the Mot's permanently circulates near dawn and dusk? Rotating an orbital plane uses to require unaffordable delta-V.

[Moonguy]

I didn't fully grasp the Mot's polar orbit. Is its plane to rotate over the Mecury year, so that the Mot's permanently circulates near dawn and dusk? Rotating an orbital plane uses to require unaffordable delta-V.

The MOTS' orbit is Sun-synchronous. Mercury rotates beneath the orbit so that over the course of 56 days - Mercury's period of rotation - the MOTS will view Mercury's entire surface. The sail that delivers MOTS to this orbit remains attached to perform station-keeping maneuvers.

[Enthalpy]

To my understanding, sun-synchronous orbits use Earth's flattening, combined with the orbit's well-adapted inclination, to let the orbital plane precess by one turn per year.

 

Mercury orbiting faster and having zero flattening

http://en.wikipedia.org/wiki/Mercury_(planet)

do yo have reasons to believe a sun-sychronous orbit is feasible at all?

 

As well, it demands a precise knowledge of the non-spherical terms of the planet's gravity field. It would be worth checking if these are known for Mercury, reached by very few missions. I suppose Mercury has a tetrapolar field (as two separated masses in its orbital plane), not a flattened one.

 

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On its Hohmann orbit, the cycler would have 7.5km/s velocity relative to Earth near it (above Earth's gravity field), and 9.6km/s relative to Mercury. While the cycler permits to preset the necessary fuels amounts on board in several launches from Earth, and as well the heavy life support necessary during the interplanetary transfers, each delta-V (to and from the cycler) is still big.

 

Could you detail more what propellants, in what amount, and coming from where, are used? Especially from the cycler to Mercury orbit and surface, and from Mercury surface and orbit to the cycler?

[Moonguy]

One of the objectives of the MESSENGER mission's to characterize mercury's interior make-up due to mass distribution irregularities. The Bepi-Colombo mission will also have an interest in this. All I can say for now is that the real situation is 'To Be Determined'. My guess at this point is Mercury's interior, while definitely fractured and differentiated, is actually fairly evenly mass distributed. I say this because Mercury has a molten outer core that would tend to distribute mass (in a molten state') evenly. This core is very large compared to the planet's volume. I'm not discounting mascon effects over formations like the Caloris basin, but they might not be as pronounced as lunar mascons. More data is needed. . .

 

The 'Sun-synchronous' orbit of the MOTS derives from the initial orbit being established over the terminator, then maintained in that orientation by the solar sail and the occasional higher-impulse maneuvers. The latter are done when the MOTS is lowest in mass due to propellant consumption.

 

As you noted the velocity requirements for Cycler-based Earth departures and Mercury arrival are higher than the lowest possible case for standard Hohmann missions. The Crew Vehicle's propellant tankage must be designed for highest propellant mass required for any phase of the mission. that would be the 9.5 km/sec. you noted. That is a scary number for payloads of 20 tons, which is about what interplanetary crew modules (usually Mars bound) tend to be. A crew module for Cycler missions could reasonably be a third or a fourth of that. This is where propellant mass is saved.

 

Eventually, propellant water comes from both Mercury and the Moon. Initially, actual propellants are brought up from the Moon to an orbit around the Earth-Moon L2 point. The propellants are loaded directly onto the Crew Vehicle. There is no 'depot' required because the delivery vehicles are sized to provide the full propellant loads in one or two flights of the lunar-based tankers (that is, near simultaneous flights of two tankers). A Mars mission, by comparison, might need three or four tankers per mission, making a depot more of a necessity. The departure maneuver is timed for less than 48 hours after propellant loading.

 

I do have calculations for vehicle mass ratios, but, alas, they are based on the assumed use of the SLS Block II large upper stage and its J2-X engines. For a 9.5 km/sec. delta-V at Mercury, the mass ratio would have to be 8.1. Not a happy number, but not hideous for a vehicle starting with full tanks in an Earth orbit only 140 m/sec. to C3=0. With full tanks, the vehicle would have a 2.0 km/sec. delta-V margin to effect an Abort-To-Earth maneuver if needed. . .assuming the problem causing the abort did not disable the engines altogether. So I have homework to do to pin down the engine and propellant requirements for the Cycler option.

Edited November 27, 2013 by Moonguy

[Enthalpy]

But my guess is that mass distribution at Mercury is very uneven, making a strong quadrupole field in the orbital plane. Like two separate masses, which are aligned with Sun at perihelion and perpendicular to Sun's direction at aphelion. This would fit Mercury's day length, which lets it alternate synchronously the face it shows to Sun from one perihelion to the next.

 

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Speed on a low Mercury orbit is 3005m/s, so active means that rotate the orbital plane by 360° (in 88 days to be Sun-synchronous) must waste 18.9km/s, whoop. For a Solar sail:

- The necessary acceleration near the poles, well over 5mm/s², is a bit too much even at 0.38AU

- A dawn-dusk orbit would need a sunwards acceleration at one pole, not achieveable from a Solar sail

- It needs to manoeuvre twice an orbit, or rather, the sail spins permanently. Strong design constraint on the vessel.

And for ionic propulsion, both 5mm/s² and 18.9km/s are a lot. Imagine you eject plasma at 100km/s for a reasonable mass expense: perfect efficiency and 5t would demand 1.25MW, oops.

 

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My unreasoned gut feeling is that the scenario relies on many technologies yet unproven, and knowledge about celestial bodies still not firm, especially whether any ice can be exploited on our Moon... Anyway, once you have propellant at the Earth-Moon L2 point, the cost of reaching the Mercury transfer is less than 7.5km/s, because the vessel would dive near Earth to accelerate there.

[Enthalpy]

"My guess" existed long before:

http://books.google.de/books?id=xPqEeB-SRvUC&pg=PA189&lpg=PA189&dq=mercury+planet+%22mass+distribution%22&source=bl&ots=_ve4fLyIGj&sig=JpF7d9ec8Kg68cfP6osC1pUims8&hl=en&sa=X&ei=2-ScUtKIA4b9ywO534GwDg&ved=0CB0Q6AEwADgK#v=onepage&q=mercury%20planet%20%22mass%20distribution%22&f=false

and the geoid anomaly, fig B there

http://www-geodyn.mit.edu/zubersite/pdfs/Smith_335Science2012.pdf

shows excess mass at two antipodes.

 

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Moonguy, I hope you don't get discouraged by the difficulty of the Sun-synchronous orbit!

 

On such a mission, I'd try to invest a bit more on the thermal design of all spacecraft. They can be made more rugged, enabling them to provide comfortable temperatures under any circumstance. This would (1) make the mission safer (2) ease a lot the rest of the design, especially the choice of the orbits.

 

For a craft hosting several people, you'll need an active thermal design anyway, because of the size. Then you can super-insulate the complete hull (it takes materials similar to multilayer insulation, but for high temperature, for instance an equivalent made of metal or ceramic), and circulate a controlled throughput of gas to a heat dump designed to be cold even if facing the Sun. Either orient the dump to turn its back to Mercury, or have several faces and flow the gas to the currently cold one.

 

Cold despite facing the Sun isn't that hard. Optical Solar Reflectors are essentially mirrors made of silica (very transparent to Sunlight) covered at the rear face with a reflective metal. They achieve figures like a=0.03 (when clean) and e>0.85: at minimum 0.307 AU or 14500W/m², they would stay at 308K. Tweaking the angles a bit, like a roof shape facing the Sun, would make them cold : 259K with 2*30° apex angle; a pyramide would improve further, an umbrella as well.

[Moonguy]

I'm not at all discouraged by anything you have contributed to date. I knew going into the study that this would be a complex undertaking. There are 1001 details I have yet to think about, much less resolve. That is why I brought the matter to these forums. Alas, this is the only forum where I have gotten any kind of credible, useful feedback. Pretty sad when you consider some the minds that dwell here. . .

 

On the up side, I have yet to see anything that has 'SHOWSTOPPER' stamped on it.

 

I only wish I had more time to work through these issues. . .

[Enthalpy]

The rugged thermal design can be simpler than I thought.The radiating element, which can be the complete spacecraft or not, is divided in sectors isolated from another, each seeing only a part of the Universe. Then some sector(s) will see neither Mercury nor the Sun, however close to Mercury you are. Circulate the cooling fluid only to the cold sector(s).

 

If nicely white, the sectors will have at most ~700K, so many metals (with durable white coating) and ceramics survive it without limitation.

[Moonguy]

On the discussion concerning the orbit for MOTS being in a terminator orbit, I may have had a useful re-think. The MOTS only really needs to be in a terminator orbit to receive incoming Crew Vehicles or launch Crew Vehicles into the proper Trans-Earth trajectory. Any other time the MOYS can be reached from the polar base no matter what the MOTS' orbital orientation to the Sun is. I should think that reduces propulsive requirements for orbital maintenance.

 

The Oberth effect for Earth departures also has another important advantage. The flight time from L2 to Earth encounter would be more than long enough to determine if all the systems on the spacecraft had performed properly during the injection burn. If not, there would likely be time to effect a direct entry abort.

[Enthalpy]

Oberth works if you push near Earth, when speed obtained from terrestrial attraction is maximum, not at L2.

 

Leaving untouched the orbit of the Mercury orbiter between arrival and departure would be the economic option ... But you don't have that much choice! Because the departure date depends on the relative positions of Mercury and Earth, and the time between possible arrival and possible departure is rarely a multiple of a Mercury half-year.

 

Put in different words, a Polar Mercury orbit permits to leave at 0° or 180° direction from the arrival direction - all observed from a fixed reference frame. Alas, this is rarely the proper direction to join Earth. Or do you have this piece of luck? You mentioned a simple period ratio about the cycler.

 

An equatorial orbit at Mercury permits to leave at any date.

[Moonguy]

Oberth works if you push near Earth, when speed obtained from terrestrial attraction is maximum, not at L2.

 

Leaving untouched the orbit of the Mercury orbiter between arrival and departure would be the economic option ... But you don't have that much choice! Because the departure date depends on the relative positions of Mercury and Earth, and the time between possible arrival and possible departure is rarely a multiple of a Mercury half-year.

 

Put in different words, a Polar Mercury orbit permits to leave at 0° or 180° direction from the arrival direction - all observed from a fixed reference frame. Alas, this is rarely the proper direction to join Earth. Or do you have this piece of luck? You mentioned a simple period ratio about the cycler.

 

An equatorial orbit at Mercury permits to leave at any date.

It was my understanding that to encounter Earth from L2 required a Delta-V of about 1.45 km/sec. If so, a propulsion system designed for 9.5 km/sec. total Delta-V would still have 8.0 km/sec. potential Delta-V at Earth encounter.

 

Stay times at Mercury are multiples of Mercury's orbital period. The Mercury-Earth transfer opportunities (ie, launch windows) come at 115.9 day intervals. The Cycler's orbital period is 351 days. This is almost exactly three (actually 3.05) synodic periods, which defines the launch windows. Keep in mind that launch windows from Mercury are open for approximately 20 days, so I'm thinking the '.05' noted here is not a problem even though it is not a perfect, match-up mathematically. Mercury's orbital period is 87.97 days. Multiplying this by 4 gives 351.88 days.

 

The orbital characteristics of the MOTS defines the times when arriving and departing spacecraft can do their thing. Recall the idea of designing the spacecraft to handle Delta-V up to 9.5 km/sec.? Mercury Orbit Insertions can be made as low as 6.1 km/sec. Every calendar year has at least one, often two, flight opportunities where the MOI is we4ll below 8.0 km/sec. For the latter example, that would leave a 1.5 km/sec. Delta-V 'surplus' available to effect a slight plane change if the MOTS was a tad off in alignment with the incoming/outgoing trajectories. Since the 0-180 degree meridians are perpendicular to the Sun-Mercury line every other perihelion (ie. every 176 days) arrivals and departures can occur every third synodic period, or 347.7 days. If I am correct (sound of nails being bitten in background. . .) the departure flights would have to be off-optimum by about 5 days, but still within the 20-day launch window limit.

 

Functionally, I have always preferred an equatorial orbit for the MOTS and an equatorial location for the surface Base. The problem is all the water for propellant is at the poles. If it is there at all. the logistical issues for supporting an aggressive launch schedule are disquieting. . .

Edited December 5, 2013 by Moonguy

[Enthalpy]

OK, if the cycler's period is 4 Mercury orbital periods exactly, it works. Then the Polar orbital plane around Mercury will be parallel to Mercury's orbital speed around the Sun after 4.000 Mercury orbital periods. As the Hohmann transfer arrives and departs parallel to the Mercury orbit, the Polar orbit permit to arrive and leave properly.

 

I wonder about the necessary accuracy between the periods of Mercury, Earth and the cycler. Maybe 10 or 20 days can be compensated once, at some cost, but what happens over time? I understand you want to re-use the cycler for many missions over years, so the cycler must remain synchronous with both, and accurately I would say. Does this allow a cycler orbit easily used to join Earth? Can the smaller vessel between the cycler and Earth spend a time short enough, with acceptable fuel expense, despite the orbit mismatch? (You might try to spread the orbit mismatch among Mercury and Earth).

 

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The Oberth effect is more favourable than this. It just tells that if your craft must have 11.0km/s speed to escape Earth from a low altitude (where your craft has already 7.8km/s if it's in orbit), and you want 7.5km/s above Earth's gravity well, then you only need 11²+7.5²=13.3² km/s near Earth. Plunging from L2, your craft would already have some 10.5km/s (to be checked) so it need just 13.3-10.5=2.8km/s added at perigee.

 

It tells that fuel at L2 is more efficient than on low-Earth-orbit (provided it has not been brought from Earth...), because from Leo, the craft would require 13.3-7.8=5.5km/s. That's a factor of 2 in the mass. Is it worth mining the Moon? Err, only if the heavy mining machine sent there is lighter than the produced fuel!

 

Note to all students: I nearly got thrown out of my engineering school for being so bad for physics, especially in orbital mechanics, so don't lose hope

 

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I'd like to get relieved from a doubt:

Mercury orbits the Sun in 88 days

Earth does it in 365 days

So the cycler takes 211 days, because its semiaxis is the arithmetic mean, and semiaxis cubed give periods squared.

 

Additionally: the cycler's apside direction is fixed, so it needs Earth and Mercury to be timely at its aphelion and perihelion. To my understanding, the synodial period is useable only if one can choose freely each time the orientation of the transfer orbit. How many orbits of Mercury, the cycler and Earth happen to synchronize? I imagine 4:1 aren't the whole story, it would need something like 16:7:4, which isn't very frequent. And how accurate must this be, if cumulating over time? Or did I miss something?

[Moonguy]

At least you had orbital mechanics! I learned mine on the street. . .and it probably shows.

The ideal rendezvous is one where the Cycler and Mercury are at exactly the same point on Mercury's orbit at exactly the same time. Practically speaking, this never really happens. Both the Cycler and the Crew Vehicle need to have some margin in their ability to self-modify their orbits. the trick is to not have to modify it by very much. What I gather you are saying is that the Cycler will tend to rendezvous with Mercury on successive orbits, but at progressively greater distances. This may very well be the case, but I'm thinking it will need a computer analysis to say for sure and by how much.

 

There is a very interesting article in the current issue Astronomy Magazine, by Robert Zimmerman, that brings into question the availability of water-ice on the Moon. It does not make a conclusion about it either way, but it really gives you a reason to have a good think before planning anything.

[Enthalpy]

My course was not useable. Learnt meanwhile.

 

What I say about the cycler's apside line is that is does not follow the Earth-Mercury synodial period - unless there's some pice of luck I have not seen.

 

The synodial period of 116 days corresponds to a transfer orbit that has every time a different orientation. Earth moves by 114° meanwhile, Mercury by 360°+114°, to offer a new Hohmann transfer opportunity that has turned by 114° - but the cycler stays on its orbit.

 

If you wait 3 periods, the Hohmann transfer moves by 343°, too different from 360°. After 22 periods, you get 7*360° minus 3°, probably too much.

 

Do I get properly that this is a fundamental issue? You might try (nothing simple) to use Mercury and Earth flybys to rotate the cycler's orbit, if any feasible. Or, very hypothetical, let a huge Solar sail rotate the cycler's orbit - but then, a cycler loses its advantage over a one-time transfer vehicle.

 

In case the cycler can't be kept, you may consider a descent-ascent module and a return module preset on Mercury orbit. Gravitation assistance can put them there after years. Then, improved propulsion (Solar thermal for instance) preferably with one Venus flyby may bring a crew there and back. No local resource necessary

[Enthalpy]

This limit to cyclers is well-known, as is clear from Wiki's cyclers to Mars for instance. Their operation is, as a consequence, less favourable than one trip per synodial period.

 

The best operation imagined is indeed to choose their period so that both planets permit a transfer whose apside line moves little. As this can only be approximative, well-adjusted planet flybys shall give the cycler the residual correction.

 

Since the period needed for the cycler does not match a Hohmann transfer between both planets, its orbit around Sun is to pass by one planet but overshoot it. Of course, the relative speed increases a lot. And less frequent opportunities mean that a short stay on the distant planet requires an other cycler to come back.

[Moonguy]

As noted earlier, this idea (Cyclers) is complex enough to warrant a computer-based analysis of the orbital configurations over a long period of time.. I have contacted a number of sources and will see who raises there hand first... I think it fair to table further discussion on Cyclers for a week or two that info is in hand. I'll address the matter at that time.

 

When the project started, the first concept for crew transport was 'Mercury Direct' where the crew left Earth on a combined Earth Return/Crew Habitat/Mercury Lander Module, but with no MOI stage. The crew just landed on Mercury directly from a flyby orbit. A neat concept that did not require Cyclers, the MOTS, the Mercury Orbit Insertion stage or orbital refueling anywhere except for the Departure from Earth. Still, it has issues of its own and is a very high-risk approach. I'm ok with the risk level, but that is a difficult hurdle to get most folks over, so I have not pushed it.

 

Manned spaceflight is always about making trade-offs between conflicting requirements. Nature never gives you everything you want or need. I'm not particularly concerned about what method is used. All I'm concerned with is choosing an approach that is sustainable in terms of cost and complexity. So, moving on. . .