Wednesday, July 27, 2016

Getting in Virtual Shape

This may not shock anybody, but building an entire virtual Moon colony is a really big job with many challenges. Trying to become more methodical and organized about it has led to a lull in visible output right now. It has also made the stack of tasks on my desktop look that much more daunting. For now. I hear that once a project is properly organized, things go much more smoothly. That would be really nice.

The Moonwards.com website was registered a year ago this week. I had been playing with the idea for a few months before that, and was encouraged to actually try it by people i met at the International Space Development Conference in Toronto in May of 2015. My idea of how that would go was really different at the time. Maybe it is typical when someone has an idea they find really exciting that they expect other people to find it equally exciting and hop on board just because one tells them about it. Now i realize that the rich, complex landscape of Moonwards in my head is only visible to me, and although many people find it interesting, they need much better reasons to devote their time to it than my enthusiasm and a small website. Turns out i actually have to figure out how to do this myself, and create the basic framework of it mostly on my own. Then it will draw people in and the actual doing of it will be shared among a group.

Saturday, July 23, 2016

Walking Rovers on the Moon

Moon dust is super nasty. It is composed of particles of shrapnel, essentially, created during meteor strikes. The only bits that aren't all sharp edges are the blobs of glass from the portion that was thrown up as lava. You want to stay away from it as much as possible.

And when exploring a new world, you need to be able to handle all terrain, from steep crater walls of loose material, to fields of boulders, to deep crevices, to ponds of deep fine dust.

Also your vehicle has to do everything. It will have arms so it can do all the manual labor needed to build, manufacture, repair, prospect... whatever comes up, most of which won't be very predictable.

So the rovers for Cernan's Promise will take the ATHLETE rover model one step further. They chuck the wheels altogether and walk around, and some of the legs also can be arms. (I can't help but think that is the eventual idea for ATHLETE too.)

I rather like this video for getting across what i mean :)

The spider-bot that wants to be a rover

My thought is to reduce that concept to 6 legs, put mechanisms on two of them so that the bottom joint folds out of the way and hands are deployed for doing any kind of work.

The Space Show on Conquering the World

(Note: the following is one of various posts that were copied here from the Moonwards.com forum. It was originally posted there on June 6th, 2016.)

Here is a link to Friday's episode of The Space Show, where i talked about how the Moon could conquer Earth, and in general about how military acts in space can be extremely effective and hard to prevent.

It is a dark theme i talk about in order to propose a bright future. I believe that it would be by far the healthiest thing for the world to undertake our first extensive venture in space together, in a manner that is as inclusive as possible. This is the only way we can develop the cooperation and trust needed to keep space safe at the same time we create the possibility for it to become very dangerous. No system of laws, treaties, or regulations are going to be enough on their own. They will only work if we build a sense that we have a common interest world-wide in peace, and that can only happen if we feel we've done it together and we are all on the same team.

The timeline for Cernan's Promise will be based on that approach. I have been focused over the last 8 weeks on building rather complicated models (that i am in fact unsure how to display well on the website, they are that large), preparing for, attending, and doing follow-up emails for the ISDC in Puerto Rico, and getting used to being the new junior moderator on space.stackexchange. I have added some content to the site over the last week and hope to add much more in the next two or three. One of those bits of content is the Residency Program.

It has been outlined in the Mission Timelines already (which are screaming for an update, but hey, one does what one can). Basically, any nation can send an astronaut for life to Cernan's Promise for $100 million, and you need at least 200 such astronauts to provide sufficient income for development. There is an overt effort to get astronauts from as many countries as possible as part of this effort, and there are later rounds done on the same model that add more residency spots at increasingly cheaper prices. Once the settlement has a few thousand residents and the price for further spots has decreased to a level a significant percentage of Earth's population could afford, the market is opened to all.

To get across the urgency of doing whatever the first big thing we do in space is, as a world, together, making people realize that we could pay a grave price indeed down the road if we don't is absolutely necessary. So, i've started going on about it whenever the opportunity arises.

Hybrid Ice Shuttles

(Note: the following is one of various posts that were copied here from the Moonwards.com forum for the sake of preserving the early days of the project. It was originally posted there on April 22nd, 2016.)

A nuclear shuttle for transportation of ice from the pole to an equatorial base is limited by the thrust of its engine. That mass must accommodate the mass of the engine and all the other dry mass, and the propellant. What is left in the mass budget then is the payload. Note that fuel required for the return trip must be subtracted as well.

A reasonable delta-v budget for a one-way trip (including some gravity losses) is 3200 m/s.
For all my examples, I am going to use a hypothetical but realistic nuclear engine, at 2500kg, 100kN and an Isp of 950s

Using hydrogen:

We can calculate the required mass ratio:



Also, using a thrust to weight ratio of 2, the launch mass of the shuttle can 30,800 kg. (Scale as desired by adding more engines).
From the mass ratio, we get that 21,800kg of that can be used for the dry mass, payload and return trip fuel.

Using water:

First, why use water? After all, the Isp of water is only 45% of hydrogen. But, even though the mass ratio gets a lot worse, water allows us to get more thrust from the same engine. In a NTR, thrust is simply inversely proportional to the Isp.

So the same configuration with a T/W ratio of 2 allows us to increase the launch mass to 70,800kg. The mass ratio is 2.2, but that still gives us 32200kg for dry mass, etc. Absolutely an improvement.


But we can do better!

The extra thrust we get from the water is only required for the first part of the burn. After the shuttle has depleted some of its propellant, it is light enough that if you then switch to hydrogen, the T/W ratio is still good. (Also note that an high T/W ratio is not that important when you have gained a significant fraction of orbital velocity, making it even more favorable to switch propellant mid-burn.)

If, for instance, we use water for the first km/s of the ascent burn, and for the last 200 m/s of the landing burn, and hydrogen for the remaining 2km/s mid-flight. That results isn 37,300kg for the dry mass etc.

Thrust when needed, efficiency when possible.

My reply:

This is great, Sigvart, you have started my day off quite well for me. And you have taught me an essential new emoticon :[)

That idea i talked to you about the other day, of moving the colony to Aristillus, is one i really didn't want to take on, at least not right now. This issue of transport of material between the pole and the base was what had driven me to consider it. Aristillus is at 33 degrees N latitude, meaning a one way hop would take about 10% less fuel than a trip to Lalande on the equator. As a double sub-orbital hop to get to Lalande and then fly back takes so much fuel, you and Russell had warned me very little space would be left for payload. But water (and carbon chemicals, and ammonia, and the rest) is so darned useful. Even though i think setting up a fuel depot is a low priority, i keep thinking of amazing things to do with water. Lots and lots of water. And those other things.

There are two things i think will make the colony flourish, especially once people are there. If you do it well, broadcasting can bring in 2 or 3 billion dollars a year. And you sell permanent residence in the colony for $100 million a pop. I see residence sales working in the early days mostly because it should be an international sponsorship process. You announce to the world that any nation can send permanent residents of their choosing for that price, and you focus on assembling an initial population that is as representative as possible. Those residents are then representatives of their countries, like Olympic athletes, like astronauts are today. Lots of countries would be attracted to the prestige of that - years and years of national pride and inspiration for one relatively low price, all things considered. And you'd get an almost ideal initial population as well. The best and brightest. The diplomats, the figureheads of a way of life, a way of seeing life and our place in the universe.

Yeah - so, water. For those things above to work, they must come to live in a beautiful place, a place just about anyone would be happy to live. People's impression of how worthwhile it is to live in space, how rewarding the experience could be, will be based on what they see in these broadcasts and the feeling they pick up from those residents. There are so many cool things you can do with water to make the colony more inviting, but you need a lot of it. There is, of course, the classic What If? lunar swimming pool scenario. There are water walls and water domes - 5 m of water is pretty good radiation shielding. There are natural ponds that both provide a sense of nature and do some of the work of recycling waste water.

To do those things on a proper scale requires several thousand tons of water. Some of it will therefore wait for the installation of coil guns at both the equator and the pole. Because really, you can't go too far with this idea. Lakes. A lunar lake, if you are putting in an actual town, you've gotta have one. A dome over a whole crater - good ol' Teacup Crater gets that treatment first. That will take 350,000 tons of water. After that you can stand below that dome and see the sky all around, and sort of feel like you are outside, unencumbered by a space suit, for the first time on another planet.

Oh - Aristillus. It is also a KREEP hotspot. Perhaps it can get the second full colony, but i think we are too far along to move the colony again, and actually, i think there is a decent argument against it. The likelihood of an ilmenite deposit is much higher at Lalande, based on Clemintine data of TiO2 on the surface. Concentration of TiO2 is much higher at Lalande than at Aristillus. ilmenite gets water and iron when reacted with hydrogen. Aristillus also looks a lot older and doesn't have the wonderfully clean walls that Lalande has, that look so very much like the digging depth to get to bedrock isn't much. And the equator has a big advantage if you start doing stuff with tethers, whether you go through EML1 or make a lunavator. So Lalande it is. Which is also nice because i've become quite fond of Lalande after looking at it so much.


Things are shaping up nicely :)

The Sunken Hall (Extensive Edit)

(Note: the following is one of various posts that were copied here from the Moonwards.com forum for the sake of preserving the early days of the project. It was originally posted there on April 2nd, 2016.)

New and improved! All the external main stuff looked at. Here are a bunch of images of this key ingredient to the colony concept, an excavated space reinforced with fused basalt material and turned into a nice, bright, quite large habitation space. Man building this took a lot longer than i thought it would. But i think i'm getting faster with these things. I was going to talk about each image but that effort would probably be better applied to the main page of the website, with the whole marker and tooltip setup. Though this model is really getting into the territory where it would be much better to have a 3d model people can rotate and move around in, or you don't see how it all works together or the details. In fact i'm not sure it would be helpful to put up all these images, as there are so many.

At any rate, a couple of highlights. The berms of material around the sides of the area with the arched roof ensure there is no direct view of the sky from the sides, cutting the radiation dose down to something safe, even over decades and even if you spend half your time in that area. The bottom image shows how a view of the sky is visible between the beams of that area, which i call the dugout. You get a significant dose of radiation from that direction due to that, but because you are so well protected from other directions it doesn't endanger you. All the beams have the same gap, so you can walk from end to end of that area, always have a view of the sky above you, and always have natural sunlight falling on you (minus the UV and gamma ray spectrum). The area is very well adapted for growing plants. The idea is gone over more in this post. Also through the windows at the end of both the tunneled sunken hall and the end of the dugout, a mirror has been set up that reflects in a view of the Earth. The Earth moves only a little in the Moon's sky, and does so quite slowly, because of the way the Moon is tidally locked to the Earth. Those mirrors will always keep the Earth in view.

None of the interior has been modeled yet, of course. I just put in the two airlocks and the structure. Doing the interior will be fun. The space is 40 m long, 30 m wide at the top and 15 m wide at the bottom, by 20 m high, in the dugout area. The hall area (the tunneled part) is 78 m long, 27 m wide, and 13 m high. There is all kinds of space to play with, and all kinds of things that are possible in that space that have never been possible before in, uh, space. I have been playing with ideas for exercises that take advantage of the room. On the Moon if you have enough distance, you can jump high enough to load your muscles and bones the way they are loaded on Earth. That might be very helpful to reducing health issues due to low gravity. You can leave large enough areas open to give something of a feel of the outdoors, or at least large enough not to feel positively claustrophobic after a few weeks.


That outside door you see on the airlock is actually 6 m tall by 5.3 m wide. The other airlock, in the dugout, also has a door that is much larger than an ordinary door. Don't let them fool you as to scale, they were just made very large so they can accommodate anything that might need to be brought inside. Use the little green men in the image below for scale.









A couple of notes:

The transparent areas will indeed all be windows. I don't care what they cost, the views are critical to maintaining mental health. And i don't think they will be that expensive, actually. They can be made of laminated sheets of sapphire, which isn't terribly expensive these days. The images show pretty large spans without reinforcement - actually the biggest open spans would be the squares shown with a reinforcing brace halfway vertically and horizontally, making the unsupported area about a meter square. That's about 1.1 tons each would have to bear if a full atmosphere was inside (and to start it would likely be more like two thirds of an atmosphere). Most would have 3 braces, vertically and horizontally, for spans of 50 cm. For the area of windows shown, for each centimeter thick the glazing is, it will mass 3.2 t. I really don't know how to assess how thick it needs to be to bear the pressure of an atmosphere across a span of a meter or 50 cm. If all of it was 2 cm thick, that is 6.4 t. That really seems worth it for how gorgeous it will be. One of the main undertakings in the early days will be broadcasting and film, a gorgeous place really helps with the success of that. Aside from how mentally refreshing that view would be. The windows looking out over Lalande crater would show you a big section of the crater interior. The dugout windows would only show you the Earth in the mirror, but you'd be able to see it throughout the whole dugout space. And besides, it occurred to me later probably most of that outer wall should be mirrored, for a wider view of the sky.

The arched beams over the dugout are held down at each end by those great big long solid beams with the series of holes along them. The holes show where anchors would been bored deep into the regolith, 3 from each hole at different angles. There is a technique for that that comes from putting together two promising techniques. Both are of course untested, but seem promising. Microwave Sintering of Lunar Soil and In-Situ Rock Melting Applied To Lunar Base. You melt through the regolith with a microwave emitter on the end of a long tube, down as deep as you can go, hopefully at least 20 m. The lava produced is forced out by displacement, and a hollow tube through the regolith is the end result. The microwave emitter sits at the bottom of the tube for a while melting a puddle of regolith around it. Any voids left are back-filled with lava. Deep strong anchors under the long beam on either side of the dugout is the end result. The thick basalt fiber cables running along the top of each beam are attached to these anchors. Similar anchors under the floor of the hall are also made and used to anchor the arches above.

The Polar Ice Shuttle

(Note: the following is one of various posts that were copied here from the Moonwards.com forum for the sake of preserving the early days of the project. It was originally posted there on Mar 19th, 2016.)

by Sigvart Brendberg:

The location of the Cernan's Promise lunar base has several important benefits, however, access to lunar volatiles is unfortunately not one of them. Therefore, there is a need for transporting ice from the poles to the equator. Currently, the most realistic way to do that is a reusable shuttle, performing a suborbital hop. The optimal trajectory, as described by Hop David here, has a Delta-v cost for take-off and landing at 3131 m/s. (from now on set to 3200 m/s for a little margin).

LH2/LOX

The propellant for the shuttle should be derived from the mines ice, as it has to be independent of imported resources (which would make it a net loss). For a pure chemical system, LH2/LOX is what we have. Most modern upper stages has a total-to-dry-mass ratio around 12, so a ratio of 10 for our shuttle should work as a reasonable assumption. For the Isp of a LH2/LOX system, ~455s, the required one-way mass ratio is 2.05 . That means that the shuttle should be able to transport a payload of 28 % of its launch mass. Further optimizations are however possible, most notably the fact that oxygen can be produced locally at Cernan's Promise, so the important part of the shuttle payload is hydrogen. Even the oxygen for the return trip can be refuelled there. Thus, assuming a fuel to oxidizer ratio of 4.5, it is able to deliver 37% of its launch mass in hydrogen, in total supplying the base with 330% of its launch mass in water. (Not a typo!)

Nuclear/LH2

Nuclear rocket engines are ready, in fact, they have been ready for over 40 years! I chose to use the RD-0410 prototype engine for reference: <engine Isp="910s" thrust="35.3kN" mass="2000kg" src="Soviet Union"> I assume technological progression can easily squeeze 50kN out of that prototype. If we want a trust to weight ratio of 1.5 on the shuttle, the total mass should be 20 tons, with a dry mass of approximately 3800 kg. Payload mass: 43%, water: 380%.

Nuclear/LH2/LOX

What the nuclear engine mostly suffers from, that makes it not that much more payload efficient over all, is that its relatively low thrust gives it a poor total-to-dry-mass ratio. Introducing LOX to the exhaust gives an almost three times higher thrust, while reducing the Isp to 650s. That should bring the total to dry mass ratio back to around 10. If it uses the LOX for the whole trip to the base, that gives a payload ratio of 46%, but high thrust is not required for the whole duration of the burn, so shifting to the high gear for about half of it gives 50% ! In the end, 450% water.

My reply:

I really like the thinking of transporting just the hydrogen. I worry a little that oxygen production will be slow enough that it will take a long time to produce enough to combine it with all the hydrogen to produce water.

The paper i've used for very roughly estimating the characteristics of the solar furnace is this one by Constance Senior, written in the 1991. Since here the relevant topic is how long it would take to produce the oxygen to make water out of the delivered hydrogen, i went looking for something else and found a pretty great paper, which will also be part of a new solar furnace post - this one by John Matchett, written in 2006. 71 pages of pyrolysis goodies.



The sample size was very small, under 40 grams. The applicable test is really only the last one, which used regolith simulant. 10% of that vaporized away in a little under an hour. The size of the lens relative to the sample was maybe a bit bigger than the mirror on the solar furnace, but the solar furnace will probably manage a higher energy flux, maintain a better focus, and will be operating in a vacuum where less heat is transported away. So i'm going to call it even and ignore various differences in terms of scale and setup for the sake of coming up with starting-point numbers. And again, a number of differences besides this will be discussed in a solar furnace post.

The tank on the solar furnace holds about 1.5 m3, but it will be hard to fill it completely to the top. I'm going to call it 1.3 m3. It will be loose regolith, which has a bulk density of about 1.3 g/cm3 - might be able to pack in more mass by putting in mostly chunks instead of powder, but we'll go with that. So that's 1.7 t per charge. At the bottom page 15 of the pdf of the Senior paper, she estimates yields of 0.02 to 0.2 g of oxygen per gram of soil. She postulated a very different design, and didn't have the benefit of experimental results from a prototype, instead using extrapolations of results from related experiments. I'm going to pick a figure in the middle of 0.1 g O2 / g of regolith. I'll also say that we evaporate away half of the charge before emptying the tank and refilling it. So that would mean we get 10% of 0.85 t of regolith out as O2, which is 85 kg. If the figure of 10% of the charge vaporizing in an hour held true, that would only take 5 hours. To account for the tasks of processing, maintenance, and put in a healthy margin for error, i'll say 10 hours. At that rate, working around the clock (which will be standard), over one lunar day we could do that 34 times, which is a total of 2890 kg of O2, and i'll round that down to 2.8 tons of O2.


If i take my understanding of your terms (danger!), then 50% payload for nuclear using hydrolox fuel is half of 20 tons, so 10 tons. Some of that is going to be pressure cylinders to hold the hydrogen, probably a few of them, because of the way they need to be processed on either end. Those cylinders need to handle long-term storage in the lunar environment - several days at least. I'm going to say they account for 1 ton of the payload. So that's 9 tons of H2, which needs 72 tons of O2 to convert it all to water. At 2.8 tons per month, it would take 26 months to produce enough oxygen. And also we need to have the oxygen ready to refuel the shuttle, so that's another 3400 kg of O2 (i think).

Sigvart:

Just a little comment on the shuttle calculations.

That "maximum total mass" is just because of the limited thrust of the nuclear engine I used as an example. That number is really just used to find an acceptable number for the total-to-dry mass ratio for the all nuclear system. Of course, the shuttle can be made large (or smaller). What matters are the percentages at the end. The results should be (roughly) scalable.

For the nuclear/LH2/LOX, the thrust limitations can pretty much be ignored.

However, the great differences does not seem to lay in the propulsion system design, as producing the required oxygen at Cernan's Promise is boosting the payload 800% ! (+ a little more for not having to transport the oxygen for the return trip.)

I am really looking forward to see what throughput the solar furnace ends up with, but I suspect that the limiting production rate would be the electrolysis at the poles anyway. (To be determined though).


Another consideration that may have an impact on the design is that event though hydrogen has a much better Isp than water, there may be benefits of using water for the first km/s or so: Despite the lover Isp, it is way denser, so for some substitution, the Delta-v actually increases due to the much better mass ratio (using the same tank space). Secondly, for the same reactor power, water has a higher thrust than hydrogen (inversely proportional to Isp). That extra thrust allows for a larger launch mass, and therefore also a smaller part of that mass being the propulsion system. (Better total-to-dry mass ratio). On top of that, straight water requires less processing to produce, of course.

Me:

Oh - a potential solution: reduction of ilmenite by hydrogen to produce water, iron, and titanium dioxide.

FeTiO3 + H2 > Fe + TiO2 + H2O

http://pubs.acs.org/doi/abs/10.1021/ie00057a005?journalCode=iecred
http://www.uapress.arizona.edu/onlinebks/ResourcesNearEarthSpace/resources07.pdf


Gotta go do something else, but i wanted to get that down.

Leap day for celebrating leaps!

(Note: the following is one of various posts that were copied here from the Moonwards.com forum for the sake of preserving the early days of the project. It was originally posted there on Mar 1st, 2016.)

Well, now that the Space Show interview is done, time to tear apart the Planning page and redo it as something more focused and useful. I'm deciding how to do that now and it will take some Javascript, so it won't be fast, especially as i only know basic JS right now and haven't practiced much. This time i'll do the work on CodePen and upload working versions to GitHub, so that if people have anything to say about it or want to add or change it, they can. Just register here and at GitHub (if you aren't already there).

New people coming in here see a long list of posts by mostly me, and recently a handful by Sigvart. Please don't let that hold you back from saying or asking what is on your mind. I'm learning, the site is young, and there is much to do.

I chose the day i'd be on TSS because it was leap day. For the past few leap days i've thought to myself, this is a day for a very special holiday, one that is more intense because it only happens once every 4 years. I think it should be a day for celebrating leaps - the turning points in human history when something happened that led to great changes. David Livingston mentioned Copernicus yesterday as such a thing. I also think of the steam engine, powered flight, radio, e=mc2, the discovery of the Americas, the invention of photography... It would be a great day for celebrating those special moments when an event enables the human spirit to suddenly expand, and those special moments are brought by great thinkers.

The space advocacy movement is a strong nucleus of people who believe in those leaps, and i think by making Feb 29 a holiday for celebrating them, Leap Day would become a way of reaching out to the general populace and encouraging them to value leaps as well. Because the more we focus on them and value them, the easier it is for them to happen.

The leap we 'space cadets' pine for is a full, proper, committed leap into space. We have lacked a way to really present that to people who don't know a lot about the science and engineering behind that, people who therefore have misgivings about how much would be gained for such a vast investment. What better way than to put on a party that honors the spirit of it?

So, i say we do something next Leap Day to really mark that message, and celebrate it as hard as we can. I vote for a parade. Okay, it is a cold time of year in a lot of places, but that has the advantage that there are no competing festivals. Besides, the end of winter has always lacked a decent holiday to brighten the cold and gray. And St. Patrick's really doesn't do it for me.

Yep, i say we have a parade. Something interactive for extra fun, and subtly educational. Something that gets the blood moving.

So, what city should we pick for the first such parade? San Francisco, home of Google, Tesla, and Silicon Valley?

Friday, July 22, 2016

Let us get a Lunavator thread

(Note: the following is one of various posts that were copied here from the Moonwards.com forum for the sake of preserving the early days of the project. It was originally posted there on Feb 28th, 2016.)

From Sigvart Brendberg: 

Lunavators is an approaching topic, so I just threw up the basics here if anyone is interested

A major concern in space travel is not distances, but change in velocity, commonly called delta v. A spacecraft is limited by its delta-v budget, governed by the rocket equation that has a nasty exponential nature. For example, a spacecraft taking off or landing on the Moon typically needs to be half propellant, with regular chemical engines. Thus, spacecraft are normally mostly fuel, and less of everything else.

Tethers, when completed, provides an efficient way of cheating the troublesome equation, what if you could just climb down to the Moon? The bottom part of the elevator is anchored to the surface, so the momentum of the spacecraft is simply transferred to that enormous chunk of rock.

The problem: Why does not the Lunavator simply fall down? That is after all our everyday experience. The classical approach to this problem is to use the spin of the planet to counteract gravity. (Try to spin around with a rope). However, that does not work for the Moon, as it spins way to slowly. Instead, the elevator extends to a point between the Earth and the Moon called EML1, a place where Earth's and the Moon's gravity cancels out, and together with the inertia creates a place where a thing would remain fixed relative to both the Moon and Earth. A nice place for a space station. From there, a counterweight can be lowered down into the gravity well of Earth. The lower end is also of interest because it moves slower than orbital velocity, making it possible to just "jump off" and re-enter the atmosphere. In other words, a free return from the Moon. Of course, it also works in reverse, making it possible to climb from a transfer orbit to the surface of the Moon. The total one-way delta-v saved is about 2500 m/s.

The math is not that horrifying:

The cable is stretched, so the cable strength we look for is ultimate tensile strength. Also, the density of it is important, the lighter the better. The combined metric for how good a material is as a tether, strength divided by density, gets the unit m^2/s^2, sometimes called Yuri.

This is the same unit that we use for the requirements for the tether, acceleration over a distance. Simply put, you take the acceleration profile over the whole tether span, and finds the area under the graph by integration. This is also a quantity in m^2/s^2.

This can be put into the tether equation, R = e^(requirement in Yuris / tether strength in Yuris). R is here the ratio of the cross sections at the start and at the end of the tether. If this ratio is very large, the material is too weak.

One promising material is Zylon. At a tensile strength of 5800 MPa and a density of only 1560 kg/m^3, it ends up at 3.7 mega-Yuri. That is good compared to the potential between EML1 and the lunar surface, 2.7 mega-Yuri. The end result is a taper ratio of 2.1, perfectly acceptable.

My reply:

That anchored lunar elevator concept is especially exciting, as it would basically be a highway into space. However it would weigh so much that a lot of infrastructure would need to exist in order to build it. Maybe it should be the thing in the Phase 3 Mission 1 section, instead of taking over the world. I was reading a bit about this the other day and it was mentioned that because the Moon's orbit isn't exactly circular, and other variants such as the tug of the sun, Jupiter, and Venus, and irregularities in the gravitational field of the Moon, the EML1 point moved around a fair bit. I went and found where - it was this Orbiter forum thread (where Hop David shows up, surprise, surprise). So the station will require significant orbital maintenance and will need to move continuously to stay in the right relationship to all the elements. But it still is a much easier project than an equivalent elevator from the surface of the Earth. 
The rotating 'skyhook' version, credited to Hans Moravic, allows for a system with much smaller mass that does something similar on a much smaller scale. It can be in a low orbit around the Moon and by rotating the whole structure as it orbits, one end can touch the lunar surface with no relative velocity. 
 
The clever additions by Robert Hoyt to this design allows for a tether 200 km long with a counterweight on one end and a travelling station or hub that moves up and down the tether to transfer momentum from the Moon to the Lunavator. (That's what he called his specific design, though i agree it's so catchy we should just use it for all lunar tether systems.) 
Not that i have yet read his paper properly, but i can't find what material he was using in his calculations. I imagine it was Zylon. At any rate, the tether mass in his design is listed as only 4706 kg. (I like it that he didn't round that to 4700 kg.) It is so light that just one Falcon Heavy could launch not only the tether but also the rest of the system, considering that there is no need to land on the Moon. So maybe the first Lunavator should indeed be done that way. Still - there is soooo much basalt to be had on the Moon, and it may be up to the job, if not as good as Zylon. Sure, it is twice as dense. But getting it where you want it takes about a fifth of the delta v. 
There are few sources on basalt tensile strength and what they say is highly variable. The particular source of basalt makes a big difference, from what i've read. This page by Dr Alexander Novyskyi seems to give a fair treatment of the subject. There is a chart on that page shown below that makes clear how much the particular basalt used and the process technology affect final strength. 
That product 2nd from bottom has a pretty good tensile strength, the two above it aren't too shabby either. And note this quote from the accompanying article: 
'An important step in obtaining quality is melt degassing process which involves the removal of gas held in the melt. This process takes place upon exposure of the melt at temperatures above 1720 K. The duration of exposure is about two hours.' 
Degassing you say? Well, the Moon will have no problem with that at all. In fact, this brings up again the speculation that lunar glasses may be significantly stronger than Earth glasses in general because they are completely anhydrous. This article by James Blacic rather gushes about that. (There is a lot of other fascinating stuff in that booklet too.) 
Which development route to take... much to ponder...
Sigvart: 

The whole rotating tether set-up have momentum, and we pick up something that stands still on the surface. Afterwards, the combined mass of the two must have orbital velocity, in order to not crash into the ground. We want the resulting orbit to be circular. We can manage to do that by either drop mass when picking up the payload, or initially having an elliptic orbit. Of course, we can chose to only lift payloads that are much smaller than the tether mass to approximate the orbit as a circle. The core principle does however remain, we balance cargo up and down from the surface.

If the tether orbits at an altitude of 100 km, the end of the tether has to rotate at 1720 m/s in order to be able to catch the payload. Let us see what the acceleration is then:

(1720 m/s * 1720 m/s)/100000 m = 29.6 m/s^2

That is over 3 g. Ouch.
And when we pick up the payload, we have lunar gravity too. (In total 31.2 m/s^2)

This is great opportunity to present another fundamental property of tethers:

Tether mass is proportional to tether-end acceleration. (for the same payload)

So even if a skyhook is shorter than an anchored tether, it has to be much ticker.


Next: Taper ratio.

The potential (in m^2/s^2, Yuri) caused by the rotation is simply the tether length times half the tether-end acceleration. For the low lunar orbit tether:

29.6 m/s^2 * 1/2 * 100000 m = 1.48 mega-Yuri

Quite substantial. In comparison, the potential caused by the lunar gravity over the 100 km is just 0.155 mega-Yuri.
Although a smaller item, we must consider that too. Taper ratio with Zylon fibre:

R = e^(1.64 MY / 3.7 MY) = 1.56

An improvement over the anchored tether, but the almost 20 times larger acceleration causes a 20 times larger cross section.

A longer tether, for example orbiting at 200 km, has a tether-end acceleration of 15.65 m/s^2 and a taper ratio of 1.58 (very close to the ratio for 100 km).

500 km: acceleration 6.48 m/s^2 and R of 1.65


In conclusion, the mass of the skyhook tether is almost independent of the orbital altitude (unless very high or low), but a higher altitude reduces the acceleration giving more time to make the catch.

Long range plans of all sorts

(Note: the following is one of various posts that were copied here from the Moonwards.com forum for the sake of preserving the early days of the project. It was originally posted there on Feb 28th, 2016.)

I added lots of stuff to the mission timelines over the last few days, and today tossed in a bouquet of ideas of many sorts. I am on the Space Show on Monday afternoon (2pm PST, 5pm EST, 10pm GST), and wanted to broaden the menu as much as i could in advance. So, i hashed out very, very brief outlines of missions going out far into the future, out into the times when the Moon will be a boomtown with extraordinary scope. The timeline roughly goes to 30 years or so after the first mission lands, accelerating sharply in development speed as it goes. I think this needs to be expected, as by the time the current timeline ends, robotics will be highly advanced. Most labor by then will be executed by independent machines acting almost entirely without supervision or interference, and very little of the mass of those robots will come from Earth. It is only the briefest of outlines of those missions, but it gives a sense of the giddy potential there.

Something to note. I named a few of the installations further out, big things like space stations deserving of a name. Those names come from the family names of people who have contributed to Moonwards so far. This is one little way i can recognize these people, do a little something for them that might actually be seen a lot in the future if things go well. Thus we have Brendberg Station for Sigvart Brendberg, Lynch Spaceport for Brian Lynch, Garcia Lunavator for my husband Aldo Garcia, and Holder Lunavator for me, because hey, we're a pair, and so that the pattern is maybe more clear if people are missing it. As time goes on, i'll give names in the same way to various things in the project. Of course this is only a taste of how in the future credit markers will be on everything. I thought about calling them 'plaques', like how buildings and monuments have plaques talking about their creators.

And now to explain this lunavator thing. Basalt fiber cable is something the colony should have little trouble manufacturing in bulk early on. There is a lack of good documentation regarding the strength of such cable, but the little there is suggests this stuff makes great rope. Wikipedia says its ultimate tensile strength is 4.8 GPa, compared to 5.8 GPa for Zylon, and 3.8 for Kevlar. That's really high. It is a preliminary number and must be treated with caution, but not so much caution that i can't get a little starry-eyed over it. At any rate if basalt rope doesn't cut it for this, further out something else will. Might be harder to make on the Moon though, for a while.

This document, from Robert Hoyt of Tethers Unlimited, looks at a really great version of what you could do on the Moon with an orbiting tether on page 9. The whole document makes delicious reading, an excellent treatment of tether launch systems. I love me a good tether. The paper was done under grant from NIAC, the NASA Institute for Advanced Concepts, so its quality is high. The idea is a tether with a total length of 200 km makes clever use of a counterweight and a station on the tether that travels its length in order to manage angular momentum. This way the tether can be made to meet the surface of the Moon at essentially no relative speed, pick up or drop off a payload, and swing back up into space. And it can do that again and again. Its capacity to move payload is high. It will orbit the Moon once every 90 minutes, so if everything is clicking, it grabs a payload once every 90 minutes. If what it can handle is 1 ton, then it can launch 16 tons every day, day in and day out. Even better, there is an extremely clever way to transfer momentum between the tether and the Moon so that the Lunavator doesn't need to expend propellant to adjust its orbit. It just takes a break every now and then to focus on swinging itself up to the orbital speed it needs.

200 km doesn't seem so very long if you are making basalt rope in really large quantities locally. Sure maybe a Lunavator made of basalt cable would need to be much larger than one made of more ideal materials, but if it can be made of basalt, i like the idea anyhow. Not only would it mean you don't have to spend payload space on an awful lot of cable, it means you can do continuous maintenance on the complex using your own resources. The tether tip velocity to pick up a payload is 0.75 km/s, much better than the 1.68 km/s tether tip velocity needed for a sling launcher to put something in lunar orbit. If a payload is released at that speed, then it has to add on the 0.93 km/s it needs to stay in orbit before it hits the ground. But it has a fair bit of time to do that, meaning big crude rocket engines made on the Moon would be good enough. This gives the Moon a way to ease in to both tether manufacture and rocket manufacture. I even pondered the idea of giant liquid-oxygen-based resistojets as a way for lunar robots to build rocket motors that are good enough to get the job done, albeit inefficient and crude. Otherwise known as simple and reliable. Or maybe more conventional engines from Earth could be made beefy enough, with such a light workload, to work for several hundreds of trips. They could fly back to the Lunavator to be captured, set down on the surface, and reused.

As the Lunavator tether is thickened and reinforced, and experience is gained, it can start hefting payloads at full lunar orbital velocity, or at lunar escape velocity. It can start doing this with small payloads and work up to larger ones. It would allow ion engines alone to be used for stuff that doesn't happen quickly, ones that stay in space and get refueled and serviced there. It is a way to launch lots of mass, which is the only way to do the really interesting things. It is the way the Moon will build its first space station, on a shoe-string budget compared to what it would cost an Earth enterprise to do it. A really nice space station that people would pay lots of money to be able to use.

You see, the Moon is an excellent example of how things change once you reach a tipping point. The Moon goes from being a money pit to a boomtown really quickly once you are able to do the right things - things that are based on known technology and well understood physics. A sense of the possible is all we need.

The Garden Space with Sky Views - Yes we can :)

(Note: the following is one of various posts that were copied here from the Moonwards.com forum for the sake of preserving the early days of the project. It was originally posted there on Feb 16th, 2016. An updated version of this will be posted soon.)

Okay, finally we have a reasonable design for a sunny space that uses spaced beams that are quite tall to limit radiation exposure to a livable amount even if you are hanging out there up to half the time.

Alright, suppose the little rectangle with the rays emanating from it is you. The beams start 4 m above you for a nice sense of space. In this version, the beams represented by the grey rectangles are 6 m high by 50 cm wide. The complete analysis was done using beams 4 m high by 50 cm wide. In both cases, each beam is separated by 1 m. The orange rays shows a representative set of paths radiation that passes through your body can take.

In the model of Cernan's Promise, the beams will look roughly like this:



See how it works? Straight up, there is very little material blocking radiation, you get a full dose. But because the spaces that have only glass or a transparent membrane are separated by lots of tall beams, starting a short angle away there is enough material that the dose is lowered to a small amount, and when you average the two you are a fair bit better off than if there had only been a membrane. Especially when you get into the details - those beams also protect against all micrometeorites except the ones coming straight down, which is rarely what they do. And unlike if you just dug down and put a membrane, the whole volume is usable. You don't have to stay down low in order to get a low dose.

Here is a map of how the doses work out. It bears a lot of contemplation. Because the energy in cosmic radiation is so incredibly high, it causes atomic nuclei to explode when struck and the bits that fly off have enough energy to cause other nuclei to explode too, and it takes a while before that process causes the original energy to be dispersed enough that you are getting net protection from the material shielding you instead of actually getting a lot more radiation because of the atomic shrapnel flying around.

So the map shows the area of a dome around a person through which the heightened doses due to an insufficient blocking thickness would pass. Those areas are in pink and red. The orange part offers some protection, and the yellow is better protection, good enough for permanent habitation. The green is as low as sea level on Earth. The area this dome examines is an angle of 60 degrees, roughly. Beyond where the mapping ceases, the protection has a fringe of yellow and then quickly turns to green for all the rest.

So imagine if the tall east-west sides of the beams were covered with a mirror surface. If you stand between two and look up you see nothing but stars. The are angled just slightly so that here at the equator the sun tracks around over the long day and the beams are always parallel to its rays of light, the shadows stay minimal. The Earth would be visible much of the time between the beams. When it isn't, it is still coming in from the south end, where there is a mirror pool outside the sunken hall for that very purpose. And the north wall reflect that image from the mirror pool so it is visible from inside the tunnel where people will spend most of their time.

If you'd like a few numbers, here are the basic ones (and please remember the previous post - these numbers are estimates):

Pink - covers 556 triangles, of which a half sphere has 10,240 - radiation between 0.5 and 0.13 Sv/yr. Average taken as 1 Sv/yr, cautiously on the high side. Dose of this area with constant exposure 0.027 Sv/yr
Red - covers 1120 - between 0.2 and 0.5 Sv/yr, average used is 0.4 Sv/yr (The unfilled triangles along the central axis are assumed to be red and counted in this number.) Dose of this area with constant exposure 0.022 Sv/yr
Orange - 2024 tris, average of 0.13 Sv/yr. Dose of this area with constant exposure 0.013 Sv/yr
Yellow - 964 tris, 0.07 Sv/yr. Dose of this area with constant exposure 0.003 Sv/yr
Green - 248 tris, Earth sea level equivalent, taken as 0.01 Sv/yr. If the rest of the half sphere is all green, that is another 0.003 Sv/yr. I padded that to 0.005 Sv/yr.

Which gave a round number of 0.07 Sv/yr total if you were always in that space. Spend half your time there, that's 0.035 Sv/yr. The maximum allowed for nuclear plant workers is 0.05 Sv/yr. So we're good. And we can soak up some sun and see the actual sky, and have healthy growing plants. Ahhhh....


Now let us remember what we don't know. Humans have never spent extended periods of time exposed to galactic cosmic rays. Never. The Apollo astronauts got the biggest doses in history but weren't exposed for more than 9 or 10 days each. These rays are actually particles, and they really aren't like radiation as we know it. In fact their name is a hold-over from when we thought they were radiation because they seemed like it, but actually they are protons, neutrons, and sometimes entire atomic nuclei stripped of their electrons, whipping through space at incredibly high speeds, coming from god knows where but it isn't from around here. Probably from supernovas. Our bodies repair damage from radiation if it is a small enough amount and happens slowly enough. It would in all probability react exactly the same way to cosmic rays. So we just have to keep our exposure down to levels our body can repair as part of its normal daily routine of cleaning, patching, and sprucing up. But we need to know much more before our first buff astronauts bathe in the sun in their hammocks among the tomato plants.

Radiation and windows - a cosmic ray blind design

(Note: the following is one of various posts that were copied here from the Moonwards.com forum for the sake of preserving the early days of the project. It was originally posted there on Feb 13th, 2016.)

First, i want to emphasize that how much radiation one would actually get on the surface of the Moon is UNKNOWN. I've scrounged for figures repeatedly on this, and went with numbers for initial designs that numbers from new sources make sound silly, which is why an initial dome design had such thin walls. Then i noticed the reference i'd happily started using seemed to have slipped a decimal over one spot to the left in the chart backing up all their statements, repeated in this video. You can tell because the starting figure for radiation on the open surface definitely shouldn't be 0.02 Sv per year. It should be 0.2 Sv/yr. That came out when i found the other reference the video mentions.

Finally today i found a paper that puts it in perspective. An exceedingly relevant snippet from the abstract of Radiation exposure in the moon environment by Reitz, Berger, and Matthiae:

On Earth, the contribution to the annual terrestrial dose of natural ionizing radiation of 2.4 mSv by cosmic radiation is about 1/6, whereas the annual exposure caused by GCR on the lunar surface is roughly 380 mSv (solar minimum) and 110 mSv (solar maximum). The analysis of worst case scenarios has indicated that SPE may lead to an exposure of about 1 Sv. The only efficient measure to reduce radiation exposure is the provision of radiation shelters.
    Very recently some data were added by the Radiation Dose Monitoring (RADOM) instrument operated during the Indian Chandrayaan Mission and the Cosmic Ray Telescope (CRaTER) instrument of the NASA LRO (Lunar Reconnaisance Orbiter) mission. These measurements need to be complemented by surface measurements.*
Models and simulations that exist describe the approximate radiation exposure in space and on the lunar surface. The knowledge on the radiation exposure at the lunar surface is exclusively based on calculations applying radiation transport codes in combination with environmental models.

*That bit translates in my mind to "the results don't correlate and/or don't make sense, so we don't believe them".

The radiation figures cited should be considered in the light of these calculations being astonishingly complex, mainframe computers needed, and the models haven't really been checked against reality, so there is no saying how accurate they are.

For one last bit of pause, consider how this article by Eugene Parker sees it:


This estimate is from 2006, but its high end figure is a third of the high end in the Reitz, Berger, and Matthiae paper. It's a total non-linear, non-intuitive mess.

Nevertheless, the design approach now being used does well in any of these scenarios. The part of it i'm working on now even provides a way to have a glazed roof over large portions of a sunken hall, provided you have a recessed area for when there are solar flares, and you get the proportions of the beams right. The proportions it has right now uses the average of 0.25 Sv/yr from the paper quoted above, which matches the charts from the other reference works, as long as you shift the decimal in the spots where it has drifted.

I am posting this early, the analysis of the radiation that penetrated it isn't done, but this helped me think it through. The illustration should be ready in a few hours.

Heat Islands

(Note: the following is one of various posts that were copied here from the Moonwards.com forum for the sake of preserving the early days of the project. It was originally posted there on Feb 11th, 2016.)

I suffer from a severe rabbit-in-headlights reaction when i open the Lunar Sourcebook. Yes, i know it is a venerable tome that contains about everything i need. But it is also extremely detailed and dense and designed for people who are trained in the field. David Kring's Powerpoint summary of its information on the nature of the regolith is much more my speed. I was musing over it again this morning. It tells a tale of regolith density at depths over a meter that (once again) is not the idea i had. There are so many things to know, you see, and though i've scanned a lot of material, some things take a lot longer to register clearly than others.

So, alright, as we can see on slide 22 of that presentation, below the first foot of regolith, relative density rises to over 90 percent. And on slide 27 we see that there it conducts heat as well as porcelain does. So forget what i said about it being a good insulator. It will suck up heat like crazy. However, we are still okay, in fact this could be regarded as a good thing - as long as you think more long term. Let us talk about heat islands.

Heat diffuses outwards in all directions based purely on the statistical likelihood of a particle with more kinetic energy striking one with less and thus transferring some of its energy in the collision. We experience that kinetic energy of molecules as heat (well, unless a very large number of them are moving in the same way at the same time). When you pour heat energy into a medium, the heat moves away more slowly the closer the temperature of the surrounding material is to the temperature at the heat source. So the longer heat is poured into the ground around the sunken hall we are building, the slower it will move away. At night the heat closest to the hall is just as likely to move towards it as away from it. When dawn arrives, heat starts pouring in again, and at first it will sink in to the ground faster than it did at sunset, because the ground is cooler than it was then.

The regolith deeper than 80 cm remains at a pretty constant temperature all day and all night. That temperature is about -20 degrees Celsius (-4 Farenheit). Once you expose that layer to the heat of the sun, it will start warming up for the first time in maybe a billion years. Then at night you want to limit heat loss with reflective insulation so that heat builds up. Thus we seek a recipe in which enough sunlight pours in during the day, into the right geometry of material, to create a heat island where the temperature stays reasonably close to something pleasant all the time. And we'd like to reach that temperature within a few years, so that people can arrive to a temperate clime. To get that done we are talking about dumping in a lot of heat, but it should be possible to achieve a desirable balance in a few years with a good design and possibly a few tweaks.

On Earth this approach to temperature control has probably best been explored as Passive Annual Heat Storage. The rule of thumb in that method is that if you thermally isolate a volume of dry soil extending 6 m from an underground home and heat it to the desired temperature, the temperature in the home will stay within a degree or two of that year-round using only the passive heat of the sun, through south-facing windows in a normal-looking house, other than that the rest of it is buried under a thick layer of soil. Beyond that 6 m point the temperature slowly falls to the annual average, which on Earth in mid-latitudes is 5 or 6 Celsius. If you look at the 'Look Inside' preview of the book at the Amazon link above, there is a case study where such a home's earthen thermal store reached a good temperature within 2 years, and was close after only a year.

On the Moon the temperature gradient we have to deal with is much steeper, -20 to 20 Celsius, 40 degrees instead of 15 or so. But the conditions for doing that are very favorable. Sun that averaged over time is twice as strong as what most places on Earth receive, the ability to control heat loss quite precisely and completely with just a few sheets of reflective foil.

My guess - and i say this with 68 arbitrary units more confidence than i've said other things - is that even the smallish sunken hall planned in Phase 1 can have a pleasant area of windows (or, you know, window-like objects) and be kept at a pleasant, even temperature using only reflective shutters. A set of them hooked up to a simple program that opens and closes them on some schedule would be enough, and at least a couple could always be open for that key view of the full Earth bounced into the hall from a mirror.

Moving day

(Note: the following is one of various posts that were copied here from the Moonwards.com forum for the sake of preserving the early days of the project. It was originally posted there on Feb 10th, 2016.)

After much contemplation, Cernan's Promise will be moved today to a spot half a kilometer north of its current location. Several changes in approach go with this move. Turns out planning an entire Moon colony, even just for education and inspiration, has a steep learning curve full of false starts, half-baked ideas, and a wide variety of mistakes. As hikers learn over time where best to pitch a tent, my feel for how best to make use of what the Moon offers is slowly growing.



I have now modeled this section of the rim of Lalande Crater much better, and made much better use of LRO photos to do so. I have done my best to make the boulders and outcrops shown in the photo accurate, and properly assess their scale. This is made more difficult by the fact that there exist only 2 photos of this area from LRO. In fact, to roughly estimate the downward slope heading away from the crater, i made some use of the photo taken by Apollo 16. As near as i can tell, the boulders in the foreground above are about 20 m high at their tallest, and the mound behind about 15 m at its tallest. Those 2 areas, and the large number of other boulders nearby, are the justification for the move.

There are several reasons.


  • How to store heat and energy through the night has been my main focus recently. Many recent posts discuss the matter. In my view, you want the right balance of two things.
    • Enough powder and fine-grain regolith to prevent heat from diffusing quickly, because even packed tightly such material is about half voids and it conducts heat so slowly it is decent insulation.
    • Enough dense rock to store heat in vast quantities so that power use for heating at night and cooling during the day is minimal. If you have enough such rock, you need no heating or cooling at all. Just let in the right amount of sunlight during the day.
    • There were boulders to be had near Teacup Crater, but this area has a true bounty. Teacup's boulder resources didn't seem adequate once i thought about it a lot. A big factor here is that the more i think about it, the more i suspect that the fine regolith layer at the rim of Lalande is very deep. Though it is mixed with a lot of boulders, it would be a mistake to assume there are enough in a particular area without evidence. Digging down to the mega-regolith layer to access superior thermal mass resources will be difficult, unless it is done through the inside wall of Lalande Crater. Though it is only a guess, i think there is sufficient evidence to postulate that the fine-grain regolith layer could be 100 m deep.
  • The inside wall of Lalande Crater is paydirt. We want to get down in there and start digging as fast as we can. I'd already identified that collection of large boulders i have started calling Gibraltar as the place to anchor ourselves so that is easier and safer. Now i think it is important to capitalize on them that way even more than i thought before, and even sooner. The slope inside the rim here is about 40 degrees on average, and steeper in spots. Lunar regolith compacts extremely well and flows extremely poorly, but you really, really don't want your rovers to get stuck, or heaven forbid, be struck by a boulder that has started rolling downhill. Gibraltar will live up to its name by having a winch with a long strong cable bolted to it, and anchor points for further cables. Advancing deep into Lalande will require the loose rocks be cleared, a winch will come in very handy.
    • Some of those loose rocks are paydirt on their own, because chances are they are composed of purer minerals than powder regolith.
    • About 300 m below the rim and 500 m east there is a ridge that looks very much like an outcropping of a strata of solid rock. Most likely, that is highland material extending from the hills 2 km or so to the west of the rim.
  • The adequacy of regolith as radiations shielding is much worse than i thought. The 1.2 m of packed regolith i'd envisioned in the early dome idea would only have made human residents sick. Cosmic radiation is so powerful that atomic nuclei are smashed when hit with such a particle, creating a cascade effect that takes over 5 m of loose regolith to block. That's not blocking all the radiation, just the extra radiation due to the particle cascade. To then protect against the dose that you'd get on the open surface, you need another 2.5 m of regolith. This issue was what made me decide to dig out the habitat, however difficult that is, because the lenses really help with that and it is by far the safest thing. The deeper you can dig the better, because the smaller the portion of sky you see, the less radiation you get. The closer to the rim of Lalande you can dig, the better, because then you can tunnel to it - an excellent place to expand the colony.


Far out proposal

(Note: the following is one of various posts that were copied here from the Moonwards.com forum for the sake of preserving the early days of the project. It was originally posted there on Feb 9th, 2016.)

From Sigvart Brendberg:

Two layers of reflective aluminium foil around the 18x18x18 meter Thermal Energy STorage cube (TESTcube?) is 3900 m².

What if this foil was used for something else?

Say you have all of it in one sheet at approximately the EML1 point. (not exactly as we must compensate for radiation pressure).

That is a 4.9 MW flash light!

It would of course be tilted 45 degrees at dusk and dawn, so that reduce the power to 3.5 MW, and also, pointing the beam directly at the base is a challenge. Was 300W/m² what the plants needed? Well, then it is enough for a 12000 m² garden.

I know Moonwards is not intended to be a sci-fi setting, and the main focus is keeping things as realistic as possible, but I just had to say it.

My reply: 

Hey, if it works technically, it deserves to be examined and depicted on the site. I have heard a bit about such ideas, but never looked for articles about it because i had the idea it would be for farther in the future. Of course i hope that Moonwards will grow into a 3d world depicting a variety of lunar bases at various points in the future. It's just that i began with a point and a set of installations that seemed like they should be done first, based on my limited knowledge. 
I was concerned about the aiming, like you mention. I also wondered about how flat the reflectors would need to be to get the majority of the light to fall neatly on the base. If those can be managed, then absolutely it should be modeled and included. Here you have a big advantage over me until i set aside modelling, writing, and coding the website to learn a lot more math and physics. If you can establish what the parameters should be and sketch out a design, i'll make the model. Or help you make the model if you wish, or something like that. 
There has to be a few papers out there on this concept. I can't think of how one might search for them. Maybe ask in the Pod Bay, TildalWave might have an idea. A couple of simple searches on NTRS didn't yield anything. A search for 'reflector earth moon' (couldn't think of better) yielded Russians to Test Space Mirror as Giant Night Light for Earth, from the New York Times, 1993. Surprisingly close for such a simple search, actually. 
There are of course other reasons to create an installation at the EML1 point, so such a system there could be refined and expanded over time. Again, the challenge is to get things going, but if Moonwards catches on and we have the resources, modelling such a station would be a good project. My sense right now is that once Cernan's Promise is basically set up - it will constantly be expanded, but once all its principal installations have a decent model - then it is time to model a polar base, and an EML1 station. 
Also, in my mission to goad people into taking space more seriously, i of course did the half-baked calculation in this post, in which i mused how much area of mirror would be needed on the Moon to cause chaos on Earth. Perhaps you would like to play with that idea for your EML1 installation too. After all, if we propose mirrors in space big enough to light lunar crops overnight to feed the lunar populace, what might those do if they were turned towards the Earth and focused on a much tighter area? :]]]

Lighting a Garden

(Note: the following is a set if posts that were copied here from the Moonwards.com forum for the sake of preserving the early days of the project. They were originally posted there between Feb 5th and March 10th, 2016. Some posts in the thread that were unrelated have been omitted.)

So i was looking into the power needs to light a garden through the night enough to keep the plants healthy. And i went to the NASA Technical Reports Server looking for papers on the topic. And sure enough, there were a number, going back decades. But they used odd specialized units for power - PAR, Photosynthetically Active Radiation, was measured in ╬╝mol/m2*s, being the number of photons received by each meter each second. They didn't actually say how many watts the lights were drawing. But after a while i found a paper that stated that. That paper was using BloomBoss UFO LED Growlights.


At which point i did a facepalm. Of course. What was i thinking, not going to the small marijuana grow-op scene for detailed and accurate information on how to grow plants in artificial light. And a wide range of products designed for exactly that. That NASA researcher was a practical guy.


Yes, he actually has dreadlocks.

Anyhow, the light unit above is conveniently designed to illuminate 9 square feet for 90 W, so that's 10 W per ft2, for easy scaling calculations for your... indoor garden. At 11 ft2 per m2, that's 11000 ft2 for the 1000 m2 garden i'd proposed, and thus 110 kW to illuminate the garden - if it was all illuminated at the same time. If it is divided into 2 halves and each is illuminated 12 hours a day, that is 55 kW, or 36 kW if divided in thirds and illuminated for 8 hours. What a deal!

The paper referenced illuminated its plants 18 hours a day for some reason. Given that half the time the garden will be illuminated by good old sunlight, i bet 8 hours is fine. Now, to get these results, the lights were no more than 24 inches from the leaves (60 cm). To have an interesting garden with plants of many different heights, including bushes and trees, it would probably be best to put the lights on strings and carefully weave them among the plants. The distribution of the different LEDs would need to be precise to get the best results. The format is 8 red LEDS plus a blue LED. Overall the effect is of pink light - quite relaxing. A reflective tent of
reinforced mylar will need to be extended over the whole garden when night falls to keep the light where it can do its work.

So, a garden is going to be rather easier than i'd feared. You do need the nuclear power plant to make it possible in the early days, but we already knew that makes a ton of sense. Once you bring a couple of payloads of water from the poles, and have the first sunken hall completely pressurized and balanced at a comfortable temperature, you can start right in on it. Once growing nicely, this garden would greatly reduce the payload mass of food needed from Earth (pro tip - eat bugs!) and be a great boon to the hall's ambiance.

Reply from Robert Walker:
Right, I got a similar figure for my article about astronauts getting all their oxygen from growing food. 20 watts to illuminate 0.2 square meters for a modern high efficiency LED grow light. It's the LEDs that reduce the power requirements so much compared with earlier experiments. 
At the same time - if they grow all their own food, then they also automatically must produce all their own oxygen too. Because we turn nearly all the oxygen in the food into the CO2 that we exhale. Feces are just a tiny part of the total mass there, especially once you remove the water from them, most of the mass of our food is either exhaled as CO2 or its water, far easier to recycle. 
So to complete the cycle, if we grow plants and they take up the CO2 that we exhale, they must also automatically produce enough oxygen for us to breath in to turn the food back into CO2 again next time we harvest it. 
Only 50% of the plant mass is used as food, but that doesn't matter. Even with burning up all the plant wastes, the excess oxygen after taking account of that still must exactly balance the amount of oxygenthe astronauts need to breath to eat the food. 
Also the amount of growing area needed is far less than most would realize. If you use hydroponics and rapidly growing crops, then you can grow nearly all the food from a tiny area. 
With the BIOS-3 experiments, it was 13 square meters per person, for 78% of their dry food requirements and nearly all their oxygen. 
See my article here for more details and links to some of the research:  Could Astronauts Get All Their Oxygen From Algae Or Plants? And Their Food Also?
Alright, now i've read the article properly, and i have a few thoughts. There is a lot of useful information and i imagine i'll need to review the article once a garden space is being designed. And a few of the linked articles.


  • To me, even the first sunken hall and it's garden space (which i've started calling a dugout) have to look great, like a place a person would definitely want to be. First, this is to draw people to spend time in the Virtual Colony once complete, but also i think this would be well worth any extra money spent in a real colony. So i'd put the algae in an aquarium type of construction, a tall narrow aquarium in which the swirling spirulina are seen in mottled shades of fresh green, as a pump swirls them around and sun filters through the tank. Water should be pretty cheap for the colony as long as there is no problem with the shuttles or the polar water mine. Using extra for something like this isn't too extravagant. And i often think about what it would be like to be in the dugout. I think such a tank would be quite pretty.
  • I make fermented drinks from organic fruit juice and sugar here at home. It has proven to be an important element of my diet, as i have health issues that can become much worse if i don't eat properly. And after all, before soda was made commercially, people made drinks like this all the time. It's very simple. My drinks are often just as fizzy as pop, and it's all CO2. I don't let them ferment long enough to have a significant alcohol content, but they are still quite fizzy. Of course, residents might well like the idea of home-brewing beer. Maybe that wouldn't make much difference to the oxygen - CO2 budget, but it is another possible pathway. Anything sweet can be used for this, including some plant waste.
  • Having eaten spirulina, i can tell you its flavor is okay, but i wouldn't want to eat it in large quantities. And it is not the kind of flavor you can easily combine with other things, or disguise. If a good portion of it could be used for other things besides eating, that would be preferable. It would be more tolerable in a diet together with the other plants you listed, like dwarf wheat, mixed greens, and potatoes.
  • Surely it would be easy to add a few insects to this system early on too. I have had grasshoppers, they aren't bad. Meal worms apparently have a nice nutty flavor, and you can avoid the ick factor by drying them and grinding them into a flour. I wonder how they would affect the oxygen / carbon dioxide budget.
  • The dugout is big, 30 x 60 m at the top, narrowing to 10 x 60 m at the bottom. I strongly feel building big is very important to success, and that the robotic technique based so much on melting basalt with sunlight makes that entirely feasible. So there is lots of space for a variety of plants. I quite agree the thing is to use some system to grow plants in a number of layers to best use space. I would really like to come up with a design that is effective and also attractive, which conveyor belts don't bring to mind at all. Maybe some system of trays rotating around an axis. There should be enough room to spare for this to work, and it would be much more attractive, i think.


That's all i can think of for now. Thanks for giving me a lot of ideas and a sound basis to start from. If you have input as to how to design the gardens, please hold forth!

Good links and resources

(Note: the following is one of various posts that were copied here from the Moonwards.com forum for the sake of preserving the early days of the project. It was originally posted there on Feb 3rd, 2016.)

I am finally going to list a few of the things i've encountered lately:

Integrated Space Plan - One big graphic documenting all space projects and plans and how they fit into the long-term goal of colonizing the solar system. All the entries are linked to an information page on that topic, which contain further links to many resources specific to that. (I have thought about doing graphics like this myself and this is a great example.) Fav link: A real lunar crane design from NASA Langley

High Frontier game - I got an email from the developers of this a while ago. It is a game about solar system colonization that is realistic and educational. I hadn't looked at it in a while and it is coming along nicely. It isn't a collaborative framework like Moonwards is trying to be, and the game is goal-oriented in a way i'm not interested in for the interactive content i hope will later be part of Moonwards. It serves a different purpose, and seems on track to do so well. This is the demo video linked to in that email, which i enjoyed.

Mining and Manufacturing on the Moon - now only available on the Wayback Machine. Shucks. It has a bunch of great little nuggets in it, presented in a digestible overview format. Some of the links are broken, though.

Lunar In Situ Materials-Based Surface Structure Technology Development Efforts at NASAMSFC - a ton of experimentation with possible construction techniques has been done at Marshall SFC over the years, and this is a great summary. Favorite line: 'several hundred meters of 0.010-in. to 0.030-in. diameter glass fiber, shown in Figure 10, have been successfully
pulled from a bath of molten JSC-I, demonstrating the ability to manufacture glass fibers.'
So that is some good stuff. I find it hard for my head to not start spinning when i get into research. In no time i have 20 tabs and a dozen documents open, and a week later i can't find half of them again. I'm really trying more now to file important links and docs for easy access.

Lunar crane pic from the video above:

Energy storage as heat

(Note: the following is one of various posts that were copied here from the Moonwards.com forum for the sake of preserving the early days of the project. It was originally posted there on Feb 2nd, 2016.)

There are 2 new images today on the main page. The solar furnace has been updated to something i am finally happy with, at least at this stage of concept development. The dome has been completely chucked out in favor of an even larger sunken gallery. Right now the model is only a hole in the ground, but i have big plans and will work on that model this week. Also in the text for the numbers i mention the crane is about to be replaced with a model on rails. Let me explain.

After mulling a bit the scale necessary to store energy for overnight use with the Potential Energy Storage System i'd very roughly postulated, it seems like something that would take a long time to develop to the point where it could play a significant role. I could be wrong about that as i am still a long way from taking the time to learn calculus the way i want to - i have to stick to working on the website and its content - and so the needed calculations are beyond me. And maybe the proper approach simply hasn't occurred to me. Maybe lowering absolutely gigantic weights very slowly on really sturdy rails would make the task more manageable. Meaning i also have to learn a lot of physics... At any rate i am indebted to Sigvart for giving me a better handle on the scale involved here. So now i'm all keen on storing heat energy that steam turbines can tap day and night for a smoother energy supply that requires less construction. Well, at least i think it would... The first missions would still rely on nuclear power, the cost and mass of a few plants sufficient to power the early days has no competition, they win hands down. But getting to the point where such plants can be built on the Moon, or even fueled on the Moon, is a complicated endeavor. So something is needed to bridge the gap between when more nuclear plants to power the growing colony is too expensive, and when the colony is capable of producing such plants itself. An alternative to nuclear has also got to be a good idea.

The core of everything proposed so far is that the strong constant sunlight on the Moon is the best power source for everything, and figuring out how to use it directly to make stuff and do stuff is the best way to go. So let me lean on that again. What if a gigantic tank of fused magnesia was filled with regolith (and enough of a gas to facilitate heat transmission through it, because without it regolith has really low thermal conductivity). And what if there was a whole network of pipes through that tank - iron ones i guess, it is the easiest metal to purify from the regolith by far. And those pipes carried a working fluid, i guess some salt that stays liquid through the appropriate temperature range, it would be nice if that could come all from the Moon but probably it would need to be imported. Then you make a nice long mirrored parabolic trough and send a set of those pipes down its focal point and back to the giant tank, heating up that regolith nice and hot, over 1000 degrees C maybe. On the other side of the tank you have a series of steam-driven turbines. They have their own circuits through the tank, in which water turns to steam, comes back, turns the turbines, and then goes to radiators to lower their temperature low enough to condense to water, and thence get circulated back into the hot tank.

The pluses i see in this is that as long as the main heat mass, the giant hot tank, was kept above 100 degrees C, your steam turbines could run all the time. They run at lower power output at the lower temperatures at the end of the long cold night and perhaps with a hit on efficiency due to having to operate over a wide range of temperatures and heat gradients, but they run. And maybe the power density you can expect in this concept is good. The vacuum works in your favor, slowing heat loss so thatmuch more of it can be directed to the turbines than if such a system was used on Earth. Just envelope the main tank in many reflective layers, same as there is on the tanks connect to the solar furnace, and suspend it above the ground on legs as small as possible while still structurally sound. The vast majority of this system can be fabricated right on the Moon without a lot of infrastructure. There needs to be a way of purifying magnesia to make the tank out of a material that can handle the temperature range. A way to do that has to be found for other reasons too, because there is certainly plenty of magnesia to be had and it would be very useful. I think maybe heating regolith to well over the temperature at which crystals start to form in it, and whipping it around in a centrifuge to separate it into its constituent layers might be enough, but i don't know. The molten salt needs to be imported, seems to me the rest can be 3d printed or cast from materials not that hard to get on the Moon at an early stage of industrial development.

Scale? Beats me. The big, big thing about this is that over the night all plants will die if they don't have a light source at least a few hours a day that is pretty intense. Energy can be saved on that by narrowing the light frequency to the bands most useful to plants. I have to go check numbers on that, but let me guesstimate again, for now (cuz it's getting late and i have to do stuff and want to post this before tomorrow, later i can make improvements. Shall we say 300 W/m2 for say 6 hours out of every 24 for good health? That seems pretty reasonable, it could maybe be cut down a lot more if you are just giving them enough to get them through the night and you choose the right plants and the right wavelengths of light. Let's go even with 500 W/m2 to account for inefficiency in the lights and so on. For a garden of 1000 m2 that you light in patches for 6 hours each, rotating through them all night long, you need 125 kW of power, all the time, just for that. So how big a system would be needed for that?

Any takers on that calculation?

PS - initially this was going to be about why i decided to put the crane on rails. One reason is that this system is long and the crane would be useful in building it. But there are other things planned for that crane. I'll get to that tomorrow. The sunken gallery will be extended with it too (probably saw that one coming though). And there was other stuff - scaling up the MIPs, and installing other infrastructure.

Reply from Sigvart Brendberg:
That is brilliant Kim!

"Any takers on that calculation?"

Yes.

First, we need to establish the energy storage needed. For a power consumption of over 150 KW, we need about 200 GJ of storage. I am going to use that number for reference for future calculation, if there are no dramatic changes in the required power supply. This is for a up-and-running base, and of course it has lot smaller requirements in the beginning.

The energy efficiency of converting the stored heat into electricity is not that great, about 10%. We therefore need about 2 TJ of storage. At a temperature difference of a 1000 degrees, we obtain a bit less than 300 tons of rock, at a specific heat capacity of 700 J/kg*K (basalt-ish). This number is scalable, in case you want another temperature. (For instance 600 tons for a temperature difference of 500 degrees).

That is quite a bit, but regolith is (literary!) cheap as dirt.

You are going to need radiators! A heat source is not enough to produce energy, you need a temperature difference. For complicated calculus reasons the optimal operational temperature for the radiators is 64/81 of the temperature of the heat source. Or, it is optimal for radiator mass at least, you have a better energy efficiency with larger radiators and a lower temperature.

In case you are interested, a realistic energy efficiency of the conversion is given by the endoreversible heat engine efficiency: n = 1 - sqrt(Tc/Th) , where Th is the temperature inside the regolith heat storage system, and tc is the temperature of the radiators.

This is a typical trade-off situation, determined by how much of the radiators that can be manufactured locally.

The storage efficiency is more complicated, so I am going to do the numbers later. Seems like it follows the square-cube law, so the heat loss decreases with size.
Ah yes, radiators. I mentioned them briefly but didn't give them much thought.

So, metals production. Some amount of iron production can be had simply by passing big magnets through a large enough quantity of regolith, especially if it is of the right kind. One of the advantages of Lalande Crater is that any type of regolith you wish should be available close by. Otherwise, molten metal oxide electrolysis, which would produce puddles of whatever metals are present in the regolith feedstock as it split the oxygen off oxide molecules. Choosing the right feedstock would mean the resulting alloy could be mostly iron and titanium, from ilmenite. That sounds pretty good.

My understanding is that what you want is to make the radiators as light as possible, so they heat quickly and that way they radiate heat faster. Perhaps the approach would be to roll out the hot iron or iron/titanium alloy into sheets as thin as possible, and then have a 3d printer that uses a metal powder feed sinter a network of pipes onto that.

In early days you can just import them, they are pretty light and last a long time. Probably better to go with a design that can be repaired, even if they are less efficient. Space is one thing there is no lack of.

Response from Sigvart:

An attempt to sum up the findings so far:

One of the major problems with a lunar base is the energy storage during the 14 days long lunar night. Two of the proposed solutions to this problem is the Potential Energy Storage System (PESS) and the Thermal Energy Storage System (TESS).

First off, the PESS has one unique feature; No energy loss during storage. Rocks just hang there, without radiating away or anything. Any loss is during the transformation to and from electric energy. Overall efficiency is about 50% but may be considerably different from that.

One interesting aspect is the storage efficiency of 0.5 kWh per ton per 100 m (compensated for energy losses). Even if we have a short distance with a massive weight, or a long distance with a modest weight, the required cable mass Is going to be the same. The properties of the basalt ropes would be really interesting in that way.

Another thing about the PESS Is that it requires high grade energy in order to fill the energy reservoir. Electricity and electric motors seems the most likely option, although a direct mechanical transfer is possible to imagine.

The TESS is a much more dynamical system, with almost no linear properties. The main advantage of the TESS is perhaps the low grade energy it requires, heat. At a work-in-progress efficiency rating of 10%, heating the reservoir with solar power gives about the same gain as solar cells, but only requiring mirrors. Another important option is that you can simply add a nuclear reactor to the system, making an efficient way of storing the power from it.

High temperature end energy loss: 
The storage stack, design whatsoever, is going to lose heat through two major processes. One is, if parts of its surface are adjacent to vacuum, black body radiation, governed by the Stefan-Boltzmann law, radiating energy proportional to the forth power of the absolute surface temperature. The other process is conduction through the walls of the storage stack, into the surrounding medium. This increases linearly with temperature. (Well, not really linearly, that is if the regolith instantly conducts the heat away, which is not going to happen. The surroundings are going to heat up, cancelling some of the temperature gradient. The actual mechanics are complicated, and strongly dependent on the exact design).

That means that the core temperature is going to increase during the charging, until it reaches a point where the heat source (mirrors, reactors) has an equal production as the heat loss from the stack. The TESS is now “topping off”. During this scenario, it is more efficient to cap the temperature a little lower (because it essentially loses the same amount of energy as it receives), and instead let the turbines run. The energy produced can be used to charge a long term system, like the PESS.

Low temperature end energy loss:

The system must have a temperature gradient in order to produce energy. That means that one end of it must remain relatively cool. Cooling is provided either by radiators, or by pipelines in the regolith. In that later case, the surrounding material would heat up with a too high energy consumption, reducing the overall efficiency.

Efficiency at different high-end temperatures, low-end temperature fixed to 300 degrees Kelvin. Efficiency along the y-axis, temperature in Kelvin along the x-axis:


The temperature of both extremes during normal operation is going to look something like this. Temperature along the y-axis, time along the x-axis. (days since sunset)


((edited slightly for grammar and spelling. Thanks, Sigvart - kim))
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There is a lot worth pondering here. For brevity and clarity, i'm going to put my thoughts in bullet points and lists

Weight fabrication:

  • The longer the vertical drop, the more energy can be gotten from the same mass. As i can't think of a way to make weights that don't require a lot of fabrication, that seems important. 
  • Even if i could, at a bulk density of 1.6 g/cm3 with no wasted space, it takes a hectare of weights 10 m high to store the equivalent of 100 kW of continuous energy for 1 lunar night. Getting those weights to and from the line to be lowered and raised quickly becomes a challenge. If weights could be made mostly of solid ilmenite the same volume would store close to 300 kW all night. 
  • I have a lot of cable anxiety. That is partly why i proposed the chain design in this post. The connection between links can be improved to have a much higher cross-sectional area, they fold together for storage with virtually no wasted space, and need very little extra infrastructure to get them to and from the line.
  • Direct mechanical energy in PESS: I had thought about spring-loading the stack of weights in storage in that chain-link design, to save a little on the energy involved in transferring them from storage to their monthly descent and ascent.


Integrating the nuclear reactors: Oh hey, yeah! They have to be hooked up to steam turbines anyhow. Dear god, i never accounted for that in mass of the nuclear power plant. Doh! When it is time to make 3d models of these systems, i'll need to make a very basic model of it for review by others so people can adjust things easily.

Configuration of Hot Tank:


  • I'm thinking cube shapes for the tanks. It has a higher surface area to volume ratio than spheres or cylinders, but as the system expands, new cubical tanks could be added on the walls of the first, best limiting the surface area of the whole set of tanks.
  • I wonder how much reflective insulation can help with radiative heat loss. 
  • To limit the contact area between the hot tanks and the ground, they need to be on posts
  • Basalt starts to melt at around 1300 K, and is completely melted at around 1750 K. The phase changes through that temperature range must affect efficiency.
  • This probably doesn't help much with being specific about efficiencies, but i thought it was worth mentioning as general design principles

Low temperature energy loss
I don't understand the point about using subterranean rock as a cooling medium draining too much energy. I do know that thermodynamics is extremely complicated and can just accept that, since i can't do the calculations myself and it may be quite hard to explain. If there is any need for nighttime heating, that would still be a valuable option. I have tried a little to look into how to fabricate radiators on the Moon, but i get stuck. There are too many factors i know nothing about. 3d metal sintering doesn't convince me though, unless there is a way to create metal powder. Getting the metal? I'm comfortable with that. Making metal powder? I have no idea.
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Response from Sigvart:

OK, using your specifications, I have done a crude size estimate of the storage cube. It turns out that the "^4" in the Stefan-Boltzmann equation is much more important than the loss of efficiency at a lower temperature gradient. 
Initial approach:
For 150 kW of power during the lunar night, we need 200 GJ of storage. At ~30% conversion efficiency of the thermal system, we need 670 GJ of energy storage. For safety margins, and for the fact that we can not drain the tank down to 0 K, I call that 1 TJ for an estimate. 
The specific heat capacity of basalt is 840 J/kg K. That means 750 tons of basalt is required for 800 K storage. The basalt densities vary a bit, but I call it 2 tons/m³.
That becomes a 9x9x9 cube. 
As the cube stands on posts I assume a total surface area of close to 500 m².
The radiation numbers are not pretty. The cube is in the beginning radiating heat away at over 10 MW. 
After optimization:
For a high end temperature optimization, the over all efficiency is best at 430 without any insulation. Still, it radiates away 3.6 MW at the beginning of the night. 
Here, aluminium foil can do miracles. A reflective layer of it a few cm away from the cube reflects 92% of the radiation back. 8% is absorbed, and half of that is re-emitted back to the storage cube. Heat loss at sunset down to 150 kW. 
Adding one more layer of foil outside that again means halving the heat loss. However, that is going to be much better if the inner layer has a black coating on the outside. We are then down to 20 kW (the leak is of course also decreasing with the temperature, so it drops during the lunar night). That should be acceptable. 
Conclusion: The storage cube is about 18x18x18 metres, and has a starting temperature of about 450 K (180 degrees C) at sunset.