Cyclers To Mercury

[Enthalpy]

Cyclers are just less straightforward than I thought. Their proponents are well aware of the limits and have already suggested workarounds for Mars - but not really simple with several vessels, sometimes a hybrid operation with aerobraking and slingshot; always with regular orbit adjustments and gravity assists. Having added all this, their interest becomes less obvious.

 

Maybe your contacts could also tell how much a transfer to Mercury costs, in the nearly-vanilla scenarion, where only Venus provides gravity assist, only once per leg? Much easier to tell than a cycler, such a scenario permits a quick transfer and a short stay, and must cope with preset descent-ascent module and return module, using accessible propulsion technology.

 

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Back to ionizing rays. I thought the difficult ones were cosmic rays which are hard to shield, but scenarios to Mars worry about Solar storms, suggesting that Solar rays are the main danger. Hence I wonder about a mission closer to the Sun than Mars is.

 

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Here a hint to a thermal design near Sun and Mercury.

to suggest that a spacecraft can maintain a decent temperature in any circumstance, because some directions can be protected against both Mercury's heat and against Sunlight.

 

Beware this is just a sketch, and thermal design is tricky. Here I don't detail the temperature of the tips of the shaders, but they do radiate towards the cooling zone. Though, these questions are known in spacecraft thermal engineering. Maybe a mobile sunshade is necessary, hopefully not.

 

My conviction is that a convincing solution, working in all circumstances near Mercury, must be feasible - and that I'd prefer to rely on a better thermal design than on a special orbit, for it offers better alternatives if something goes wrong. Same argument in favour of preset modules, which can be redundant.

 

Marc Schaefer, aka Enthalpy

[Moonguy]

I agree about Cyclers being 'less straightforward'. What is at issue, aside from crew safety, is the on-going cost of using Cyclers as opposed to other alternatives. A system of Cyclers designed to provide at least one flight opportunity per year seems like it would be hideously expensive. In fact, if we stay away from using the SLS units (at $2 Billion/per launch) it is possible to develop a Cycler system well below the costs quoted for Mars cyclers. That is a particular question I am developing as it is the whole center of the issue: 'What is it going to cost?' Once I have a good picture of just know complex the Cyclers are, I can be more detailed about that.

 

Thermal management for the MOTS is not solely the strategy of flying over the terminator to minimize heating. It has thermal rejection built into both the propellant manufacturing system and the crew habitat. Your idea of using radially-mounted radiators (or are they inert thermal 'vanes'?) is very practical.

[Moonguy]

Many apologies for being so long delayed in getting back to this thrill-a-minute subject!!

I did get a couple of 'analysis' about cyclers back from some associates. One was incomprehensible to me. the other basically concluded with 'deploy a lot of cyclers and it might work.' Not particularly useful. . .

 

For some historical reference here, I originally worked out a scheme once suggested by Samuel Herrick, the originator of NASA's early navigation systems. The idea was to do away with going into orbit at all and just do a direct landing from a vehicle on a flyby trajectory. The manned lander would have to be pretty beefy, but it was still lighter than a full up MOI stage + lander. The landing velocity would be several times that for a lunar landing, but the payload was small, only about two tons. The big problem, at the time, seemed to be nowhere for the crew to go if the lander could not land.

 

I hope we can discuss this more in the future. . .

[Enthalpy]

One other transport candidate to Mercury is my Solar thermal engine

http://www.scienceforums.net/topic/76627-solar-thermal-rocket/

which gets a decent mass fraction there within few months, instead of many years of Venus and Earth flybys as is done presently with chemical engines. Though, I found difficult to bring samples back from Mercury with direct flights there and back; it would better use preset hardware.

 

The big improvement should be a single Venus flyby combined with the Solar thermal engine. One single slingshot adds little constraints on the flight opportunities and must enable the two-way trip, as not much is missing.

 

One paper about slingshot, with ready-to-use formulas on pages 8 and 9, thank you so much:

http://maths.dur.ac.uk/~dma0rcj/Psling/sling.pdf

 

I first advance other topics, then must get familiar with the slingshot, so if I come back with it for Mercury, it won't be immediately.

[Moonguy]

With upwards of 1250 sec. Isp, I too am interested in Solar-thermal for sending crews to Mercury. Given that it uses Liquid Hydrogen, I conceptualized a stage with a crew cabin integrated into the stage tankage so that the LH2 provides the radiation shielding (most of it anyway) without adding mass for shielding. The STR is only really need for the MOI phase. Even 10 km/sec MOI only require mass ratio around 2.7.

[Enthalpy]

I've put there an illustration of the slow (hence unmanned) transfer of heavy equipment (return vessel, descent-ascent module...) to Mars; it should look similar for Mercury, except for a potential Venus flyby:

http://www.scienceforums.net/topic/83289-manned-mars-mission/#entry809990

Figures and spreadsheets are in the message before.

 

One interrogation I have is about the radiation dosis suffered by a crew during a Mercury mission (besides the uneasy idea of living in an oven like Mercury). If - unsure! - the worst radiations emanate from our Sun and decrease with distance as R^-2, but the transfer and residence times decrease as R^-1.5, then the cumulated radiations would be worse over a Mercury mission than a Mars mission.

 

I certainly agree that a shorter mission has other advantages: for the crew, the operation personnel, the public.

[Moonguy]

Radiation dangers are about how much exposure you allow the crew to have. The mass of the crew module for the inter-planet transfer phases is critical in this respect. Right now, the concept of choice is for a laminate radiation shield and use of both potable and waste water (in separate units!) to absorb radiation. It is easy to design shielding mass for any given material or even a series of layered material. The hard part is putting it all together in a mass that can be launched for well below the national debt in cost.

 

The most dangerous exposure time is the period when the crew begins its descent to the surface to rendezvous with the pre-positioned surface elements waiting for them. If the crew descends over the night side, they are blocked from the Sun's heat and most deadly radiations but the spot on the surface where they are to rendezvous would be just a few days (less than a week) away from sunrise. Not enough time to deploy the first base (~ 1 day) AND cover it with enough regolith to secure against radiation AND deploy energy units, So the crew descends over the day side where they are assaulted by the heat from Mercury's surface as well as the Sun. Putting the base near one of the poles and having the crew enter Mercury orbit over the poles eases the problems somewhat.

 

What I do not know at this point for a certainty is what thickness of regolith is needed to reach the NASA baselines for exposure limits. For that reason, I just assume a depth of ten meters regolith all around. That is a lot of regolith to move around. . .

[Enthalpy]

What are the reasons for a crew to stay on Mercury during daytime? Fuel cells can provide electricity, and waiting for a good position of Earth and possibly Venus can be done elsewhere - orbiting Mercury when leaving for Earth isn't even useful with my solar thermal engine which wastes the Oberth effect.

 

Nighttime has obvious advantages: fighting cold is easy (-50°C in Antarctica means a good coat) but heat would be very difficult, and the planet shields against Solar ionizing radiation. This leaves 80 terrestrial days on the surface.

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

So a mission that lands where night begins, makes a few pictures with a flag, grabs some stones, and scrambles before the day arrives, looks safer and easier. Or isn't it?

 

10m regolith, I hope it's less. Our terrestrial atmosphere is very efficient against even cosmic rays and weighs "only" 10t/m², or about 3m sand - astronauts staying 80 days don't need the same protection as for 80 years on Earth. You had also considered landing at a pole: why not make a shield of water ice? It's easier to process and the lighter O+H produce less Bremsstrahlung than Fe+Al+Si+O.

[Moonguy]

The entire purpose of the project is to establish a permanent base on Mercury to make Mercury useful. The economics strongly favor a base over a brief visit. So the first crew has the challenge of setting up a base that can house an engineering crew and a science/exploration crew. This includes a lab where hundreds of kilograms of rock samples can be examined instead of the minimal amount that could be sent back on a return flight. The construction work can be done at night under lights with the goal of having the base covered (as needed) as soon before sunrise as possible.

 

As a thought experiment, a 10 meter thick shield covering a base that had a 25 m x 25 m footprint (625 m2) would keep a bulldozer with a 2 m3 bucket busy for about 260 hours, assuming an average of 5 minutes to transport/dump each bucket load. That is just over ten days. Obviously using two bulldozers would cut that down to less than a week. . .

 

On Mercury, the average density of regolith poured to make a shield mass 10 meters thick would have an average density of 1.9 metric tons /m3. A 10-meter shield would therefore weigh 19 x .38 or 7.22 metric tons per square meter of surface on the base. This corresponds to roughly 30 kg/in2 or 13.5 lb/in2. More could be added to just exactly equal one full atmospheric pressure inside the base, if it were inflatable. Essentially zero pressure differential increases safety margins.

During the daytime, the crew would remain in the base and maintain the base's agricultural system. The 88-day daytime is long enough to see some crop plants all the way through their growth cycle to harvest. Chances are the crew would have the garden up and running long before sunrise, so the second sunset would see the crew with a full harvest in place, all of the critical systems set up and perhaps hundreds of rock & soil samples examined before the next crew arrives.

Edited June 11, 2014 by Moonguy

[Enthalpy]

As alternatives to a bulldozer, you may consider a conveyor belt (faster, but is it lighter?), or a machine that throws the regolith through vacuum on the base to be shielded. And I still hope 10m aren't necessary, but have no excellent reason for it.

 

Is Mercury's soil texture known? To my knowledge, only orbiters have been near Mercury, and just a few ones. If a crew depends on regolith, this must be assessed before.

 

Bringing a few 100kg samples back must not be very difficult. A one-craft mission with the solar engine can bring 300kg from several asteroids. By presetting the return module around Mercury, we must achieve the same from a robot. My personal interrogation is rather if a semi-autonomous robot can choose and obtain the samples as wisely as a geologist would.

 

As far as I know, no human has ever lived from food grown away from Earth. Biosphere 2 achieved on Earth to grow food but not to remove the carbon dioxide from the air. My fears:

- we still ignore how many and how varied organisms are necessary to sustain food growth over seasons

- does the soil regenerate in a small closed environment?

so an experiment in real size should first be conducted at a place where the crew can come back quickly, say in Earth orbit.

Anyway, once air and most water are recycled, humans don't need so much supply; it can be brought by cargo. One could even produce simple food, like glycerine, from carbon dioxide and water - and Mercury's poles have water, they say.

[Moonguy]

All of the life-support and agricultural technology issues are already being worked on in the interest of lunar and Mars exploration. The assumption here is that Mercury simply draws on technologies developed with experience from those two venues. The same goes for mining and mineral processing technologies. This is one of the reasons why the Mercury project is so cost effective. There is very little 'Mercury specific' technology needed, at least not in the early stages.

 

Transportation is probably the biggest exception. Solar thermal is my choice for sending crews to Mercury, but has not been considered by NASA for lunar or Mars operations. They still think in terms of the J2-X, lunar water mining and (gulp!) L2 depot/station for departure. Goes a long way top explain why NASA can't sell their program to the public. . .

[Enthalpy]

Pity, I thought you also wanted to put humans on Mercury before Mars since the mission is shorter - but this is a project very different from what you consider, for sure.

 

Solar thermal engine: the idea takes time to percolate. Esa had made a design a few years ago, and it had some uneasy points, especially a transparent window for the concentrated light - I didn't check if they had catchers as good as mines for heat leaks. Then, operation at low chamber pressure, which dissociates some hydrogen, improves the isp from 800s to 1200+, which helps also. My concentrators+chambers look feasible. And I've described some pretty-cute missions which are more sexy than just Leo-to-Gso.

 

So possibly my design and missions are more appealing than what Esa had. But it takes time for mission planners to realize what gets possible and what stays not, and re-think how to organize a mission. My first thoughts were in 2010 and I still learn how to use the toy, so someone who heard about the engine through indirect ways legitimately needs time.

 

Manned Mars yes, I design a mission script presently. Mercury probably. For the Moon, the solar thermal engine may preset heavy equipment in orbit before people arrive there quickly, that is by chemical engines.

 

J2-X: it's good stuff. It has the Isp of its thrust and nozzle area, so if one needs the MN, fine. For a bit less thrust, I wish four or six RL-10 chambers had a common turbopump and set of actuators. And for sure, performance results from adopting good propellants (pumped oxygen+hydrogen, or pressure-fed oxygen+kerosene, instead of tetroxide+hydrazine) more than individual engine performance. The ballon tanks I describe elsewhere enable it, even without active cooling for the duration of a Moon mission.

[Moonguy]

Virtually everything you are saying is why I am confident about the 'transportation problem' (to Mercury) being solved. I particularly agree with you about the need for new ideas to 'percolate'. I grew up in a space advocacy that torpedoed itself with incessant arguing over 'where to go first' or who's idea for how to go was better. . . It made the entire movement a joke. Sadly, I have to admit I engaged in many of these arguments. I like to think I have evolved to where I see the value in 'synergy'. In the present case, this means using Mercury to boost astronomy and planetary exploration efforts and human settlement of Mars and other venues. A Mars settlement effort would provide a needy customer for things Mercury can provide economically. Less clear to me is what a Mars settlement would do to pay for the goods it got from Mercury. Its an interesting question, but I have not had time to work on it.

Edited June 13, 2014 by Moonguy

[Moonguy]

The 'Cycler' concept has dropped off the radar. For now, the idea is to focus on the first few ('Alpha') missions based on non-recovered systems.

A departure stage is sent to L1. This unit carries a payload of 15 tons. This includes a solar sail (3880) kg, a self-deploying habitat (NOT a 'tin can'), and a docking module. The stage is equipped with water electrolysis units (as in the case of the Cycler), liquefaction and propellant transfer equipment. At L1, the propulsion stage's tanks are filled with up to 100 tons of water. The solar sail and habitat module are deployed and the cluster departs from Earth on a transfer intercept course to Mercury's orbit. This course is elliptical and actually crosses Earth's orbit twice.

 

Much later (1 year+?), a second launch delivers a crewed capsule to the same L1 location where it is refueled with cryogens. As the cluster vehicle approaches, the crewed capsule launches into the same trajectory, where it eventually conducts rendezvous and docks with the cluster. The crew remains on board the cluster for the balance of the transfer, which takes about 78 days. Prior to Mercury encounter, water from the cluster's supply is processed into cryogens sufficient for either an orbit injection at Mercury (with subsequent rendezvous with the afore-mentioned MOTS) or it can be outfitted for a direct descent to Mercury's surface. Such a mission could be supported by existing Atlas, Delta or Falcon systems. the upper stage elements would be derived from the Advanced Centaur systems.

[Enthalpy]

Nice to see you again!

 

Could you detail where the water and cryogens come from, to fill the departure stage and capsule at L1? Sent from Earth, or mined from the Moon and sent from there? And why electrolyse water instead of sending the separate propellants?

 

Do I understand that the departure stage goes to the transfer orbit using its sail, the capsule with a chemical engine, and after joining both the sail brakes both at Mercury? Did you consider a single vessel that departs Earth chemically and brakes by sail at Mercury? Earth departure takes "only" asymptotic 7533m/s (circular, coplanar Hohmann...), or 5537m/s from Leo, still reasonable - not optimum, but safer as it avoids docking far from Earth.

 

Did you consider a slingshot at Venus? Several ones permit presently to put 1t at Mercury starting an Atlas or Delta from Earth. I still haven't tried to estimate the benefit, but a single slingshot at Venus must be key to a manned mission.

 

Delta-V and mass estimates at each stage would be nice, since this is a hard nut for Mercury. Braking duration by sail also - I understand this is a prospective scenario for which the sail does not need to exist today, and Mercury is a good place for a sail.

 

Is the Mercury habitat brought with the crew, preset in orbit, preset on the surface, buit there from local materials...? Shall the crew stay during daytime?

 

And: how does the crew come back? Accelerate by sail, slingshot at Venus, aerobrake at Earth? Not necessarily very heavy, but the harware must be at Mercury. Is it the same vessel as for the first leg?

[Moonguy]

1) All water/cryogens comes from the Moon. There is no Depot involved. All water/propellant transfers are direct to vehicle.

2) 'Departure Stage' is a misnomer. The stage carrying a solar sail only delivers a payload of 15 tons to the L1 point. The 15 ton payload includes propellant production equipment, a habitat module and the solar sail. At L1, the tanks of the delivery stage are filled with water, the solar sail deploys and the entire assembly departs into a heliocentric orbit with a one-year period. This orbit intersects the orbit of Mercury at one of the nodal crossings.

3) The sail and its payload - now called a Tanker - may have to make one or more orbits before the crewed vehicle is ready for launch. Preserving cryogens for that long is difficult.

4) The Crewed Vehicle launches 'some time later' as Mercury is positioned for encounter. The Crewed Vehicle launches into the same exact orbit as the Tanker when the Tanker encounters Earth.

Assuming a successful rendezvous and docking, the crew occupies the Tanker Habitat for the cruise phase of the flight - approximately 78 days.

5) Prior to Mercury encounter, the crew processes water into cryogens and fills the tanks of the Crewed Vehicle's propulsion system. At encounter, the Crewed Vehicle performs the MOI and rendezvous with the MOTS.

6) To return, the Crewed Vehicle performs the same maneuvers as at launch from Earth. The Tanker used for return would not necessarily be the one they arrived on. The Tankers are single-launch items and regarded as expendable. their total mass when filed with water is 185 tons. This demands very minimum mass for the Crewed Vehicle injection mass as all of the maneuvers are budgeted up to 10 km/sec.

[Enthalpy]

Mmmh, some points are clearer. I didn't get the full picture, as the scenario seems sophisticated.

 

2) Has L1 an important advantage over a high Moon orbit? The gravitation energy in Earth's potential is nearly the same, it's cheaper to reach from Moon's surface which should almost compensate the departure from there, and we reach a Moon orbit faster.

 

2 also) The one-year orbit can be synchronized with Earth, but how often is Mercury at a suitable location? If I remember properly, Mercury's orbit doesn't resonate exactly with Earth's one, and approximations get quickly unmanageable at the scale of the Solar system.

 

I'm carefully more pleased with the one-year orbit, which should keep compatible with Mercury's orbit inclination and with Mercury's eccentricity. Raising the apohelion to the asteroid belt is the easier part with a Solar sail. But how long does it take to lower the perihelion to Mercury? Solar sails use to navigate quicker when spiralling on almost-circular orbits.

 

3) I have zero worry about storing cryogens. This technology is necessary for any progress in space travel, is currently being developed, and will be available - sooner than big solar sails. I describe balloon tanks of welded steel for instance there

http://www.scienceforums.net/topic/60359-extruded-rocket-structure/page-2#entry761740

insulated in space by multilayer film, and hold by polymer straps in an exoskeleton like a truss. This leaks <10W for several 10t of oxygen or hydrogen.

 

To that tank design (also useable for a few hours in an atmosphere with some foam), add a small cryocooler, this one or an other

http://saposjoint.net/Forum/viewtopic.php?f=66&t=2051&start=10#p23468

powered by solar panels. Nasa also considers powering the cryocooler by a small fuel cell fed by the propellants that evaporate: nice for missions away from the Sun.

 

So even if it's not done up to now, principle designs are easy and the desire is there, so you'll have them.

 

6) I still don't grasp how the return propellants are brought to Mercury (maybe your scenario isn't still detailed there?). Are they propellants for a chemical engine, for the Solar thermal engine...? Do you need 185t preset there? Do they arrive by sail, by the Solar thermal engine, how quickly...?

 

The return leg uses to be the hardest nut. Consider aerobraking at Earth, gravitational assistance at Venus, and maybe a combined chemical/Solar push to leave Mercury, similar to

http://www.scienceforums.net/topic/83289-manned-mars-mission/#entry806615

I'll put more data about sharing the effort among the chemical and Solar thermal engines there - sometime

http://www.scienceforums.net/topic/76627-solar-thermal-rocket/page-2#entry807522

 

If optimizing every operation, including presetting the propellants or vessel efficiently, and using new propulsion, maybe one can bring a crew back from Mercury, but it's hard.

 

7) Staying on Mercury during night would make life easier and need less real estate operations prior to going there, but this constraints very strongly any docking, gravitational assistance, or generally any synchronized operation. What is your choice? Still building quickly a house for daytime when landing there during nighttime?

[Moonguy]

1) The Tanker departs on a solar sail, so we only need the easiest departure point. Most cargo missions will depart from L3, but the Tanker takes on water from the Moon, so Tankers depart from orbits closer to the Moon but with easy departure velocities. That would be either L1 or L2. On reflection, L2 might actually be better as it does not interfere with operations (depots) in the L1 orbit.

 

2) The Tanker is an expendable unit. There is no need to worry about coordinating or synchronizing the Tanker's orbit for either picking up the crew (for the return to Earth) or for follow-on missions.

 

3) Also, the Tanker's orbit only needs to intersect Mercury's orbit at one of its nodal crossing points just once. A one-year circular orbit which intersects Earth's orbit and subsequently intersects Mercury's orbit at a node, while Mercury is passing through that node, should be fairly easy to establish. It just may take a couple of years for the sail to spiral into the appropriate orbit. That is why producing propellants from water is preferred over storing cryogens for 'years' before the crew can use them.

 

4) Water can be stored passively. Cryogens would need liquefaction equipment even with a small boil-off rate to avoid gas build-up. With no crew onboard for most of the time, the chances for problems increases.

 

5) The MOTS we discussed earlier is deployed before any crews launch to Mercury. The 185-ton mass includes the MOTS' dry mass, the dry mass of two manned Mercury Landers and 100 tons of water. The crew should be able to produce propellant sufficient for return to Earth from that for at least one mission. It depends on the mass of the Crew Module payload and that is still being worked out for this concept .

 

6) Another advantage to expending the Tanker is that the crew does not have to constrain their stay time on Mercury to the Tanker's orbital period. They can extend their mission stay time indefinitely if needed. Alternately, once the crew arrives at Mercury, they would have at least a year before the Tanker is anywhere near Mercury. Whether it would be near enough for use in a return flight. . .I just do not know for certain at this point.

 

At present I am working on a piece for the Facebook group site concerning Sheltering the crew on Mercury. I hope to have it posted before the weekend. You might want to check into that to get more details. . .

Edited July 22, 2014 by Moonguy

[Enthalpy]

O yes, apologies. You mean Moon's L1, L2, L3, don't you? I was wrongly thinking at Earth's L1 which made less sense. Moon's Lagrange points are indeed better than orbits, agreed.

 

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Frankly, you shouldn't worry about storing cryogenic propellants indefinitely in space. This is easy technology, everyone needs it, so you will have it. A consumer fridge works for years, so with more bucks and redundancy, the cryocooler can be reliable - more so anyway than the software that orients the sails and regulates the vessel's temperature. And even if storing water, electrolysis will take months so the cold propellants must be stored. Worse: the tanks must be superinsulated anyway, or the heat leak is so big that electrolysis can't replenish them fast enough. It's easier to cool the propellants than replenish them by electrolysis.

 

Let's take as an example 100t of 6:1 O₂:H₂, that's 200m³ of H₂, or a sphere of D=7.3m S=166m² exposed to 300K.

- Alone liquefying the produced hydrogen is worse than keeping liquid the available hydrogen.

- 2t foam (heavy!) leak 7kW which would evaporate 7mol/s. That's the whole 14t H₂ in 12 days, so obviously foam is impossible.

- So hold the tank by polymer belts and have 25 layers of film insulation (150kg). At 0.02 emissivity, the stack leaks well under 20W, and so do the belts.

 

40W at 20K mean a redundant 1kW cryocooler, easy.

Now if producing 14t H₂ in 3 months, alone liquefying them takes 800W at 20K, or a 20+kW liquefier. Electrolysis takes 500kW.

I prefer to leave such hardware on the Moon than bring it near Mercury - even more so because lifting the water from the Moon already needs propellants.

 

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The scenario is still very obscure to me. You've spent much time on it and it's yours so it must be clear for you, but to newcomers it's esoteric. What about, say, a list of the modules, telling where each comes and goes and when? Then you might tell what its propulsion is, which speed increment it needs, how long it takes in the case of a solar sail, and what the masses are at beginning and end.