Settlement design

louis · 2021-04-15 14:33:06

How much steel do we need?

I think a better question might be: how far can we avoid using steel?

It might make more sense to Mars cement, Mars concrete, and basalt for construction.

We can certainly cannibalise any abandoned Starships.

Great effort should be put into 99% recycling of steel.

Here's a helpful presentation.

http://www.marspapers.org/paper/Moss_2006_2_pres.pdf

What about the methane-electric arc method?

RobertDyck wrote:

You guys go on and on. Here's a question: how are you going to smelt steel? I have proposed the Direct Reduced Iron Method. It uses less heat. Requires grinding ore to fines, uses carbon monoxide and hydrogen to convert oxide ores to pure iron. Carbon from carbon monoxide is dissolved into the iron. Using pure CO requires less energy, but results in far too much carbon in the steel. Metal with that much carbon is brittle. Removing the carbon requires heating the metal to completely melt it, and bubbling oxygen through to burn off carbon. If you mix hydrogen with CO for the first step, that hydrogen also binds with oxygen from the ore, converting ore to metal. Result is steam, which doesn't dissolve in steel. Hydrogen takes longer and requires more energy to make, but if you get the balance right then the final product won't have too much carbon. Although this works at lower temperature, it still requires between 800°C and 1,200°C; usually between 900°C and 1,000°C. This method requires high grade ore with very few impurities, basically pure iron oxide such as hematite concretions. Iron produced this way still has to be processed further to eliminate final impurities to become steel.
So how are you going to produce that much heat? Enough to heat literally tonnes of ore to well over +900°C? One advantage of this method is temperature is hot but low enough that a nuclear reactor can directly produce the heat without melting the reactor. Any conversion of heat from a reactor to electricity, then electricity back to heat, is very inefficient. It's far more efficient to directly use the heat. Temperatures for the Direct Reduced Iron Method is below the melting temperature of steel. You could use exotic materials for the reactor such as nickel/chrome allow to withstand higher temperature, but the point is to use the heat directly.
So again, if you want to go solar, how are you going to smelt steel?

Calliban · 2021-04-15 15:15:14

We will need a lot of steel on Mars. Remember that every inhabited structure there will be a pressure vessel. That includes any place where we grow food. We have looked into using basalt fibre rope on Mars for tensile elements and it does show promise. But it too requires high temperatures to melt the basalt before extruding it through a die. About 1400°C from memory. And the basalt fibres will attach to a steel frame, which will transfer load from window panels. Hot rolled, low carbon alloy steels are excellent for continuous tensile and bending loads, because within elastic limits they have a forgiving stress cycle and tend not to creep.

Being able to produce steel using direct nuclear heat is hugely advantageous. We still need to source hematite or some other pure iron oxide ore. But with so much of the energy requirements met by direct fission heat, steel can become a cheap structural material. It really needs to be cheap, as we are going to need a lot of it.

Temperatures of 1000°C are achievable using high temperature reactors. It is a technical stretch, as stainless steel melts at 1400°C, but is achievable using triso fuel. Actually, a pebble bed reactor using helium coolant could produce temperatures in that range using natural uranium as fuel. The same pebble bed reactors using CO2 coolant could also generate electricity needed to make hydrogen and CO, using a direct S-CO2 cycle. Two relatively compact nuclear power plants that both run on Martian non-enriched uranium. The high temperature reactors lend themselves to natural reactivity control and radiant loss of decay heat. Very simple systems, that should be quite easy to build in their entirety using materials found on Mars.

Last edited by Calliban (2021-04-15 15:46:16)

RobertDyck · 2021-04-15 16:48:54

Calliban wrote:

We still need to source hematite or some other pure iron oxide ore.

Remember the "blueberries" discovered by Opportunity rover? They're hematite concretions. And they were embedded in soft jarosite rock. That means a rock crusher can be set to crush softer matrix rock, but not hematite. Then sift. Then tumble the hematite to remove an soft material still stuck on. Once you have pure hematite, use a stronger rock crusher to crush it to fines.

Opportunity:

Curiosity also found hematite:

Curiosity collected the powder by drilling into a rock outcrop at the base of Mount Sharp in late September. The robotic arm delivered a pinch of the sample to the Chemistry and Mineralogy (CheMin) instrument inside the rover. This sample, from a target called "Confidence Hills" within the "Pahrump Hills" outcrop, contained much more hematite than any rock or soil sample previously analyzed by CheMin during the two-year-old mission. Hematite is an iron-oxide mineral that gives clues about ancient environmental conditions from when it formed.

RobertDyck · 2021-04-15 17:07:05

You can also mine anorthite as aluminum ore. Bauxite is usually the ore of choice on Earth because it's easy to process. But bauxite is the end result of millions of years of a tropical rain forest. No rain forest, no bauxite. Moving water such as rain, rivers and streams weather igneous rock to become clay. Plants of a tropical rain forest extract nutrients they need from clay. Left-overs are bauxite: primarily aluminum oxide, silicon oxide, and iron oxide. There may have been past microbial life on Mars, but there has never been a tropical rain forest. So no bauxite.

However, anorthite is igneous. You can process it by reversing the pH. For Bauxite, you start by dissolving in a strong alkali (sodium hydroxide) then move the liquid to another tank leaving any sediment behind. In the other tank you neutralize pH with a weak acid: bubble CO2 gas through to create carbonic acid. When pH gets close to neutral, aluminum hydroxide precipitates out. Then collect the white crystals from the bottom of that second tank, rinse with clean water, then calcinate. That means heat it so it's hotter than boiling water, but not so hot the crystals melt. Blow air across while it's hot; oxygen from air will combine with hydroxyl (OH) to form water. That will leave as steam. This converts aluminum hydroxide to aluminum oxide aka alumina. Then electrolysis using cryolite as a catalyst. Electrolysis requires low voltage DC, but one hell of a lot of current. This requires a lot of power. Often the chemical processing is done at the mine site, exporting white alumina. Electrolysis is done somewhere electricity is plentiful and cheap. Often a commercial aluminum operation will purchase an entire hydro-electric dam.

For anorthite, reverse the pH. Instead of strong alkali, use strong acid: hydrochloric acid. And neutralize pH with a weak akali: bubble ammonia gas through. Everything after that is the same.

I presented this at the 2004 Mars Society convention in Chicago. I thought I made a great wonder discovery! And it should work with bytownite, which is 70 to 90% anorthite. MGS had discovered bytownite on Mars. Of course ideal is mineral anorthite, which is 90-100% anorthite. But after the convention I discovered there's a company in Sweden doing this now. I have re-invented the wheel! Well... the good news is it works.

tahanson43206 · 2021-04-15 17:21:44

For Noah ... the posts immediately above are an example of the very high quality, high value conversations that are distributed randomly throughout the archive of the forum. I will try to come up with some search terms to make it easier to find this sequence, but you may wish to add search terms that work better for you. Just let me know and I'll add them to this post, or you can certainly create a finder post like this one.

For kbd512 and RobertDyck re series immediately above ...

SearchTerm:Smelting of materials found on Mars to make aluminum and steel
SearchTerm:Steel manufacture on Mars from raw sources
SearchTerm:Aluminum manufacture on Mars from raw sources
SearchTerm:basalt on Mars

(th)

louis · 2021-04-15 17:33:43

Well the need for steel is debatable:

https://www.euronews.com/2019/05/10/wat … nasa-prize

The winners of the NASA prize produced something that might work on Mars made from biodegradable and recyclable biopolymer basalt composite, which includes elements grown from crops.

Of course a key attractive feature here is the ease with such buildings can be put up using robot construction techniques. I've seen video of a Chinese company that can create these sorts of large house-like structures (for use on Earth) in under 48 hours.

Calliban wrote:

We will need a lot of steel on Mars. Remember that every inhabited structure there will be a pressure vessel. That includes any place where we grow food. We have looked into using basalt fibre rope on Mars for tensile elements and it does show promise. But it too requires high temperatures to melt the basalt before extruding it through a die. About 1400°C from memory. And the basalt fibres will attach to a steel frame, which will transfer load from window panels. Hot rolled, low carbon alloy steels are excellent for continuous tensile and bending loads, because within elastic limits they have a forgiving stress cycle and tend not to creep.
Being able to produce steel using direct nuclear heat is hugely advantageous. We still need to source hematite or some other pure iron oxide ore. But with so much of the energy requirements met by direct fission heat, steel can become a cheap structural material. It really needs to be cheap, as we are going to need a lot of it.
Temperatures of 1000°C are achievable using high temperature reactors. It is a technical stretch, as stainless steel melts at 1400°C, but is achievable using triso fuel. Actually, a pebble bed reactor using helium coolant could produce temperatures in that range using natural uranium as fuel. The same pebble bed reactors using CO2 coolant could also generate electricity needed to make hydrogen and CO, using a direct S-CO2 cycle. Two relatively compact nuclear power plants that both run on Martian non-enriched uranium. The high temperature reactors lend themselves to natural reactivity control and radiant loss of decay heat. Very simple systems, that should be quite easy to build in their entirety using materials found on Mars.

Calliban · 2021-04-16 07:37:40

I have carried out some background reading on high temperature gas reactors suitable for providing direct nuclear heat for steel production via fuel gas reduction of powdered iron oxide.

Pebble bed reactors have been operated at temperatures of 950°C in the past, so the idea is workable from this point of view. However, it turns out that as uranium heats up to these temperatures, 238U resonances absorb a higher proportion of neutrons. This is why the modular pebble bed reactor was expected to operate with 8% enriched uranium in its pyrolytic graphite pebbles.

It is possible to operate a high temperature reactor on natural uranium (0.71% enriched) alone. However, it would require zoning the core, with low temperature regions on the outside (which provide most of the reactivity) and a high temperature chimney region on the inside. The flux profile of the reactor would need to be parabolic rather than flat, to allow good power rating in the central hot regions of the core. The neutron economy of a natural uranium reactor is tight at the best of times. To allow for additional parasitic neutron losses in the central core region, the core radius would need to be relatively large, thereby reducing the number of neutrons escaping from the core. Fuel burn up will be low, even by the standards of a natural uranium reactor, because neutron absorption in the central core means that the reactor cannot tolerate substantial additional losses due to neutron absorption in fission products. For this reason, I would propose a twin reactor concept. The high temperature pebble bed would discharge pebbles at relatively low burn up. It will provide the heat needed for steel production. Close by, a lower temperature S-CO2 pebble bed would receive the discharged pebbles and complete the burn up of the fuel, up to about 5000MW-days per tonne HM. The S-CO2 reactor would generate the electric power needed to produce the fuel gas, along with excess electricity for other processes.

It turns out, that designing a reactor that is capable of burning the thin gruel that is natural uranium is no easy task. It requires lateral thinking.

NB. It is difficult to imagine why a Martian colony would be unable to import low enriched uranium. But given how crazy politics is going, I don't think we can automatically count on it being available. However, it is difficult to see any reason why launching heavy water would be banned. If a Mars colony can import heavy water but not LEU, then natural uranium heavy water reactors offer better power density than graphite moderated reactors. The aqueous homogenous reactor, with its unsurpassed neutron economy could use natural uranium sulphate fuel dissolved in a heavy water moderator. We would surround the core tank with breeding zones containing thorium. A 233U fuelled AHR, could function as a thermal breeder. Alternatively, plutonium produced in the AHR central core, could be used to produce metallic fuel for sodium fast breeder reactors. We could build these on Mars quite early on, as we can of course rely on design services of Earth based contractors.

Breeding ratio of SFRs with metallic tube in duct fuel, may be as high as 1.8. Each year, the reactor would replace about one third of its fuel, discharging fuel and blanket modules containing 80% more fissile isotopes than it received as fresh fuel. If we assume 1 year inside the reactor and 1 year of cool down prior to electro-refining, fissile fuel stock increases by a factor of 1.8 every 2 years. After 20 years (10 cycles) the stock of fissile fuel will have multiplied to 357 times its initial value. In a fully integrated fuel cycle, 1 tonne of uranium can ultimately yield 3000MW-years of thermal energy. If a Mars colony can build breeder reactors, then it can be entirely self-sufficient in energy very quickly and reactor doubling time in rapid enough to allow rapid growth in reactor capacity, supporting rapid growth in the size of the colony and it's industrial capabilities. The discharged fuel from a single initial 50MWe heavy water natural uranium reactor, can provide the starting plutonium for a breeding cycle that will yield enough fuel to support a population of 1.7billion (@3.7kWe per capita) after 40 years. Provided we can get started with nuclear energy on Mars, nuclear fuel supply will not be a constraint.

Last edited by Calliban (2021-04-16 09:12:25)

RobertDyck · 2021-04-16 09:21:56

Don't transport water from Earth. If you need heavy water, then process water from Mars ice to produce heavy water there. Transport tools, not bulk materials. And as soon as possible build up industry capable of manufacturing tools on Mars from in-situ materials.

Wikipedia: Thorium-based nuclear power
You can Google "thorium reactor" or "thorium reactor India", there's a lot.
I wrote about thorium in post #154 on page 7, including a map of thorium on Mars produced by Mars Odyssey.

Oldfart1939 · 2021-04-16 09:31:13

Heavy water on Mars will be easier to obtain as Mars water has ~ 6X the Deuterium abundancy as water on Earth.

Calliban · 2021-04-16 09:57:06

RobertDyck wrote:

Don't transport water from Earth. If you need heavy water, then process water from Mars ice to produce heavy water there. Transport tools, not bulk materials. And as soon as possible build up industry capable of manufacturing tools on Mars from in-situ materials.
Wikipedia: Thorium-based nuclear power
You can Google "thorium reactor" or "thorium reactor India", there's a lot.
I wrote about thorium in post #154 on page 7, including a map of thorium on Mars produced by Mars Odyssey.

This statement from Wiki is especially interesting:

'Mining thorium is safer and more efficient than mining uranium. Thorium's ore monazite generally contains higher concentrations of thorium than the percentage of uranium found in its respective ore. This makes thorium a more cost efficient and less environmentally damaging fuel source.'

The cost of mining on Mars will be much higher, so the fact that thorium exists in more concentrated ores than uranium, may stand in it's favour. One thing to remember is that whilst uranium does contain fissile 235U (along with 99.3% fertile 238U), thorium is pure fertile material, it has no fissile content. This means that you need some fissile uranium or plutonium to start a thorium cycle. Hence my example of an AHR running on natural U with thorium blankets.

Breeding ratio in thorium cycle reactors does seem to be lower. That won't necessarily be a problem, as long as there is sufficient uranium on Mars to build a few GW of starter cores for a thorium cycle. If thorium AHRs can achieve a breeding ratio of 1.1, then starting with 1GWe natural uranium thorium AHR, there will be enough breeding to support 117GWe after 50 years. According to Kbd512, that is enough generating capacity for over 30 million people on Mars.

Last edited by Calliban (2021-04-16 10:01:39)

SpaceNut · 2021-04-16 18:59:05

So far we have wondered all over power creation and from the many sources that could be but are not to much help for the total topic of creating a settlement design which we know is not a one time building process. That it takes many building steps plus temporary structures to allow for the build while occupying those for the final build is completed. Once the finish home is made all of the temporary construction is recycled and saved for other projects to come as the city grows outward.

The power we need will also be temporary with steps taken to achieve the final systems no fail design. This will be the methods for all that we need for mars as the equipment and people to make things happen to grow so with the need for all of this to transition through temporary states as we build to a sustainable settlement.

louis · 2021-04-16 20:03:33

I agree there will be temporary phases before we get to a more permanent settlement. Initially there are going to be a lot of imported habs. These may be connected by pressurised all-purpose walkways. Initially quite a lot of the city's internal transport will be out on the surface in pressurised rovers, so if you want to get from the various farm habs, back to your residential area, you may get into a pressurised rover that can take you there.

After that I'm thinking we will see something more organic and ISRU - interconnected buildings made of Mars cement or concrete, perhaps tiled internally in basalt and covered in regolith on the outside to aid radiation protection.

Further development may depend on steel structure if we are going to build large domes. I quite like the suggestion made here by someone of large vaulted spaces, using cut and cover. So you have arched glass above allowing in some natural light. Down below you would have houses, offices, parks, trees and a kind of street scene, though vehicles would be more like electric golf buggies.

The absence of large private vehicles will I think be a big difference between a Mars City and an Earth one. The million person city might be say 36 miles square with no point being more than 10 miles away from another. Travel could be via electric scooters, electric bikes, electric buggies and a small number of robot buses or taxis travelling along dedicated routes. Most journeys could be completed within 10 minutes, and the maximum would be about 30 mins.

SpaceNut wrote:

So far we have wondered all over power creation and from the many sources that could be but are not to much help for the total topic of creating a settlement design which we know is not a one time building process. That it takes many building steps plus temporary structures to allow for the build while occupying those for the final build is completed. Once the finish home is made all of the temporary construction is recycled and saved for other projects to come as the city grows outward.
The power we need will also be temporary with steps taken to achieve the final systems no fail design. This will be the methods for all that we need for mars as the equipment and people to make things happen to grow so with the need for all of this to transition through temporary states as we build to a sustainable settlement.

RobertDyck · 2021-04-16 21:54:23

louis,

Are you aware of steel reinforcing bars (rebar) in concrete? The reason is concrete has great compressile strength, but very weak in tension. That means if you attempt to crush it, or sit a great weight on it, concrete will resist. But if you pull it, concrete will break easily. Bending involves compressing on one side, tension on the other. Steel has good tensile strength, so holds concrete together. But it can crack. To prevent cracks, you can prestress. That means the steel rebar or steel cables embedded in the concrete are put in tension. So attempting to pull will fight against that tension.

Bruce Mackenzie suggested brick. His idea was a brick structure using Roman technology: arches. Air pressure inside would blow the brick apart. To prevent that, Mars dirt is piled on top with more weight per unit area than air pressure inside. So there's still net compression, no tension. That has several issues. Air pressure pushes in all directions, including sideways from walls. This requires the dirt to extend a significant distance from the walls. You will need a door of some sort to get in. The tunnel to the door will not have tens of feet of dirt pressing against it; in fact the door frame will have none. So any tunnel piercing the hill or dirt pile must be made of something with tensile strength. That means it can't be brick or concrete. Notice the Mars Homestead has a two story atrium with vaulted ceiling, made of brick. But that structure is well inside the hill. Apartments have a window, so apartments must be made of something else. Rover garage, entrance airlock, tunnel to surface structure such as greenhouses; all are made of something else. Options are steel, aluminum alloy, or fibreglass.

The other issue with brick is making it air tight. Pressure finds any way out that it can. A crack or small opening in the mortar will cause air pressure to leak into surrounding Mars dirt. That will result in a small tunnel to the surface. To prevent that, you have to spray the inside of the brick with a polymer sealant.

Notice I didn't include concrete. Because concrete doesn't have anywhere near the tensile strength. Rebar will hold it together, but any crack will form a pressure leak. Basically concrete has the same problems as brick. Mars Homestead did use concrete, but it requires rebar, and concrete was used to hold up the weight of Mars dirt over fibreglass tunnels. That way fibreglass shell outer surface can be accessed for repair, and doesn't have to withstand the dirt overburden. But don't expect concrete to be air tight.

kbd512 · 2021-04-17 04:00:18

Calliban,

I was thinking about ways to reduce energy consumption, and while that's nearly impossible to achieve in a way that would materially affect the total power requirements, we certainly can reduce the amount of electricity we have to generate through greater use of low and mid quality heat. I propose we incorporate the Kalina cycle to improve our waste heat and mechanical / electrical conversion efficiency if we decide to use low temperature reactors, as this technology transfers to both solar thermal and geothermal, as well as a variety of manufacturing process that includes fractional distillation / cement production / steel production, once we have sufficient power to start using those resources. Beyond that, low temperature / low pressure reactors don't require complex control mechanisms or fuel cycles and have very modest operator control / staffing requirements.

The Kalina power plant in Husavik - why Kalina and what has been learned

There is apparently some variation on this multi-component working fluid technology that also works at higher temperatures:

HEAT CONVERSION SYSTEM SIMULTANEOUSLY UTILIZING TWO SEPARATE HEAT SOURCE STREAM AND METHOD FOR MAKING AND USING SAME

I tend to favor the AHR design for flash evaporation / distillation of water and hydrocarbon products. The AHR is more of an "in-machine" than PWRs and BWRs, due to the very high burn-up rates achievable using a homogenous fuel mixture, versus fuel and control rod assemblies, and simple chemical or even centrifugal separation methods used to remove neutron poisons. As compared to PWRs and BWRs, far less radioactive material has to come "out", the technological hurdles associated with fuel manufacturing are much lower, and the reactor does not have to be taken offline for refueling. Similar to the molten salt reactor designs, if the fissile solution leaves the core, then continued fissioning is impossible, as criticality is entirely dependent upon the presence of the neutron moderator in or around the core to slow and reflect neutrons back into the core to sustain fission. Since AHRs do not use contain control rods or fuel rods, operate at modest pressures / temperatures, and can be constructed of low cost materials with low embodied energy, they're a prime candidate for mid-grade process heat.

The exact same alloy that Starship's propellant tanks are constructed from is the alloy required for AHRs, which means steel from inoperable Starships can be repurposed for power generation / water pipes / liquid (Propane / Ammonia / Water / LOX / LN2 / LCO2 ) storage. From an energy and cost perspective, designing cargo lander variants of Starship to be a single-use design, rather than a reusable transport, is beneficial to the growth of the fledgling colony by supplying the raw materials for power generation and construction. If it's possible to mass manufacture cheaper single-use cargo variants of the Starship using the same assembly methods as the crewed variants, then that would supply around 90t of high quality stainless steel for reactor fabrication, enough for 3 reactors (20t of steel per 1MWt reactor using the legacy 2.1m diameter design, plus 10t for piping and accessories). If we stuck with molten salts and near-atmospheric pressures (3atm for MSR vs 60atm for AHR), then that steel requirement could easily be reduced by a factor of 5. Either way, 1MWt AHRs are small and light enough that complete reactors could be transported, fueled after emplacement in a prepared site, and used to supply startup power for the colony. This design has the benefit of being a lot smaller and lighter than 250MWe MSRs, although most of the power generated will be thermal, rather than electrical.

My thought process on this is that we supply an initial load of U233 to initiate criticality, followed by in-situ breeding of U233 from Th232, to make proliferation of nuclear weapons from space-based reactors impractical to achieve, even though you and I know that in the real world all actual nuclear weapons are created with Pu239 bred from irradiated U238. After that, we'll continue operations by supplying the colonists with fertile rather than fissile isotopes, both to keep costs down and to assure that even if someone managed to steal an entire shipment, they still don't have the material to make a nuclear weapon of any kind. Starting with U233 would assure that nearly all of the high level waste products only remain an issue for a few centuries or less. We could contract with the Norwegians to supply U233 from their breeding program as a form of like-kind ESA support for the colony. Within 10 years after the fission products associated with fissioning U233 have been removed from the reactor cores, approximately 83% of that material will have decayed into stable isotopes of various elements. 1t of U233 can generate 1GWyr worth of power, and it ultimately requires long term storage for 170kg of long-lived fission products. We can vitrify the remaining fission products for indefinite storage.

The US has buried 2,700t of Th232 from rare-earths mining, also a major reason why we don't mine our own rare earths anymore. We dig up truck loads of Th232 at the rare earths mines we have in America. We could easily import all fuel from Earth for 25 years before the tonnage demand associated with nuclear power begins to match the tonnage demand for solar panels to supply equivalent power under optimal conditions. Since nobody else has any use for Th232, we can relieve them of the requirement to store many thousands of tons of Th232 associated with their mining activities. Heck, they'd probably give it to us for the cost of shipping, just to get rid of it. If the colonists can get their own Thorium mine up and running within 20 years, then there's no scenario where solar power can supply equivalent power without requiring at least 10 times, and more likely 100 times, more material input on a year-over-year basis. All we've really talked about is basic life support functionality, so actual power requirements for agriculture and construction will be much higher than what we've discussed here, which is why the need for an extremely energy-dense fuel that produces reliable power is so great.

Anyway, those are just some initial thoughts on the concept.

louis · 2021-04-17 07:38:57

Yes I like the Roman brick arch idea. I've often argued for that as part of a simple cut and cover trench with arches over. It might be very useful for creating low pressure CO2 farm habs using artifiical light.

Re doors and airlocks, I've wondered before now whether ice could form a barrier. So you might have A double air lock with ice in two separate large "U bend" pipes. When you want to open one "door" you melt the ice and pump out the water. When you want to close the "door" you pump in water and freeze it. Obviously you'd need some quick freeze and quick melt technology. The great advantage would be you save on material usage. They might be suitable for seldom visited automated farm habs.

RobertDyck wrote:

louis,
Are you aware of steel reinforcing bars (rebar) in concrete? The reason is concrete has great compressile strength, but very weak in tension. That means if you attempt to crush it, or sit a great weight on it, concrete will resist. But if you pull it, concrete will break easily. Bending involves compressing on one side, tension on the other. Steel has good tensile strength, so holds concrete together. But it can crack. To prevent cracks, you can prestress. That means the steel rebar or steel cables embedded in the concrete are put in tension. So attempting to pull will fight against that tension.
Bruce Mackenzie suggested brick. His idea was a brick structure using Roman technology: arches. Air pressure inside would blow the brick apart. To prevent that, Mars dirt is piled on top with more weight per unit area than air pressure inside. So there's still net compression, no tension. That has several issues. Air pressure pushes in all directions, including sideways from walls. This requires a the dirt to extend a significant distance from the walls. You will need a door of some sort to get in. The tunnel to the door will not have tens of feet of dirt pressing against it; in fact the door frame will have none. So any tunnel piercing the hill or dirt pile must be made of something with tensile strength. That means it can't be brick or concrete. Notice the Mars Homestead has a two story atrium with vaulted ceiling, made of brick. But that structure is will inside the hill. Apartments have a window, so apartments must be made of something else. Rover garage, entrance airlock, tunnel to surface structure such as greenhouses; all are made of something else. Options are steel, aluminum alloy, or fibreglass.
The other issue with brick is making it air tight. Pressure finds any way out that it can. A crack or small opening in the mortar will cause air pressure to leak into surrounding Mars dirt. That will result in a small tunnel to the surface. To prevent that, you have to spray the inside of the brick with a polymer sealant.
Notice I didn't include concrete. Because concrete doesn't have anywhere near the tensile strength. Rebar will hold it together, but any crack will form a pressure leak. Basically concrete has the same problems as brick. Mars Homestead did use concrete, but it requires rebar, and concrete was used to hold up the weight of Mars dirt over fibreglass tunnels. That way fibreglass shell outer surface can be accessed for repair, and doesn't have to withstand the dirt overburden. But don't expect concrete to be air tight.
http://www.marshome.org/images2/albums/ … age014.jpg

Calliban · 2021-04-17 12:06:03

Kbd512,

I had never heard of the Kalina cycle before, but yes it does sound suitable for first generation AHRs. I had heard of Organic Rankine Cycle. The K-cycle appears to offer a more compact power cycle, so it would be beneficial. A low temperature steam cycle is obviously possible, but the low pressure turbine would have high capital cost per unit power produced. The fact that the AHR can reuse stainless steel fuel tanks would make it a lot easier to fabricate on Mars. There needs to be a heat exchanger and vapour drying equipment. Another tank could be used for that as well. If large amounts of liquid CO2 can be pumped out of the ground, then it too is a candidate secondary side fluid, maybe even open cycle. But we cannot assume that this exists in abundance until drilling confirms its existence.

I don't know how long we would continue using AHRs before transitioning to MSRs. A Martian city would certainly need plenty of low grade heat for agriculture, and the simplicity and inherent safety of the AHR may make it ideal as a small modular power supply, supplying heat and smaller quantities of electric power. When breeding with thorium, the AHR has the advantage that thorium salts dissolved in secondary surrounding blankets, can easily be pumped through the blanket zone, given controlled amounts of irradiation and then stored in tanks away from the core, allowing protactinium to decay without additional neutron bombardment.

One of the most attractive features of the AHR is the possibility of continuous fuel reprocessing, with fission product removal and separation. This improves neutron economy but also raises options for valuable isotope separation. We have discussed in the past the possibility of Sr-90 powered fusion drive systems for interplanetary transport. Using Martian AHRs as a source, allows this technology to expand without politically objectionable launches of radioactive materials from Earth. Cs-137 and Cs-135 could be used similarly as a radiothermal heat source for rover vehicles and remote facilities. Also, for second generation AHRs, technetium recovered from fission product wastes can be used to produce technetium steel for AHR vessels and heat exchangers. This has superb corrosion resistance and should allow AHRs to operate at higher temperatures. Noble metal fission products, like ruthenium, platinum and palladium, can be used as catalysts in Martian chemical engineering.

Last edited by Calliban (2021-04-17 12:23:28)

GW Johnson · 2021-04-17 12:30:15

Building buildings on Mars is very, very different from building buildings on Earth. Precisely because Martian buildings must be strongly pressurized to be habitable, and Earthly buildings are entirely free of this requirement. Pressurization means your structures must resist high tension and/or high bending. Bending involves high tension, too.

Our masonry materials (cemented bricks and stones, concrete) have good compressive strength, but lousy to vanishing tensile strength. The partial (PARTIAL !!!) exception is reinforced concrete. The steel rebar in reinforced concrete carries the tensile loads, but ONLY because the Young's Modulus of steel is much higher than the Young's Modulus of the concrete matrix. That means you cannot use other materials of lower modulus than steel as your substitute rebar.

You can load the rebar to the point where the concrete cracks, but no further. Once cracked, the concrete's compressive strength degrades, sometimes is lost completely. On Mars, if you lose building pressurization, the compressive strength is required to prevent collapse under its own weight, lower gravity notwithstanding. Lose compressive strength because you cracked the concrete matrix, and the building falls. The rebar WILL NOT prevent that.

These concrete and cement structures really are porous. They will not be airtight without an appropriate coating on the inside. If you crack them, you tear that coating. Just something else to consider.

Also, consider the cold soak temperature effects we see on Mars that we don't even see at South Pole Station (coldest ever seen ~-115 F = ~ -82 C). Mars has -130 F or colder, quite commonly. It can get to -200 F in some places. I don't know what effect that might have on the concrete matrix, or other masonry, but it is catastrophic on the usual mild carbon steel we use for rebar. It becomes as brittle and fragile as a glass window pane.

Your rebar will have to be 304 stainless, 304L if you intend to weld it (and most rebar is welded). That is a high-nickel steel that does not heat treat, although it cold-works very hard very quickly. It is NOT magnetic, so that means of material handling gets ruled out.

Just more food for thought.

Bear in mind that these masonry and concrete materials (and the related soil mechanics for foundations) were not the focus of my aerospace education, nor of the strength of materials classes that I had in college as an aerospace student. I learned most of this on the job working in one or another aspect of civil engineering.

GW

Last edited by GW Johnson (2021-04-17 12:32:31)

Calliban · 2021-04-17 13:11:26

In naval applications, steels need to retain high ductility at temperatures far beneath zero Celsius. The common solution is to increase the percentage of manganese in the steel. These steels are easy to weld and not too expensive on Earth.
It does mean that we have to find manganese on Mars. According to Wiki, 14% manganese in steel allows it to remain ductile down to -200F.
https://en.m.wikipedia.org/wiki/Mangalloy

I had never really thought about youngs modulus being a limiting factor on what gets used as reinforcement in concrete. It explains why polymers have never caught on. Aluminium and magnesium alloys are ductile at cryogenic temperatures. But in general they are not desirable options, due to creep under static load and fatigue life and cracking under variable loads. Every aircraft has a documented flight history for exactly these reasons.

tahanson43206 · 2021-04-17 13:41:00

For Noah re posts 192, 193 and many others

Your topic is accumulating a ** lot ** high level wisdom/practical advice

I have said it before and I'll keep saying it ...

This forum ** MUST ** possess a storage facility that does not flow under the bridge never to be seen again unless SpaceNut goes on a foraging spree.

The ** only ** way that is going to happen is if Mars Society management agrees with the proposition that investment in something like a Wiki for accumulation of knowledge is a good idea.

At this point I seem to be the only member calling for such an addition to the NewMars infrastructure, but perhaps in time other members will toss in a bit of feedback one way or the other.

(th)

Calliban · 2021-04-17 14:29:06

I would second Louis's suggestion that urban spaces on Mars should be largely pedestrian and compact. If you look at the urban landscapes that human beings consider to be cultural treasures, they are all compact and largely pedestrian. Most of them were built before the industrial revolution, at which point mechanised transportation was not available. Think places like Venice, Bruges and the renaissance cities and hill towns of Italy.

Compact cities make sense on Mars, as pressurised volumes are expensive relative to Earth. They are places that must be kept pressurised and kept warm. But they are also spaces where precipitation can get precisely controlled and zero if necessary. I think this opens a lot of options that you could make these p,aces more livable. Roof gardens. Streets with seating that allows spillover from restaurants and bars. It also allows adobe and rammed soil to be used as building materials beneath domes as we don't need to worry about precipitation causing erosion or degrading strength.

RobertDyck · 2021-04-17 14:37:52

Mars Society wiki: Marspedia

There were proposals for NewMars to have a Wiki, but...

RobertDyck · 2021-04-17 15:03:35

GW Johnson,

I saw a TV documentary that concrete requires nitrogen to set properly. This is done on Earth by mixing concrete so it contains a certain amount of air. Atmosphere on Earth is 78% nitrogen. Air bubbles in wet cement must be enough to provide nitrogen to set, but not so much that it creates voids. A tricky balance. Mars atmosphere is very thin, and only contains 2.7% nitrogen by volume, as measured by Viking 2 lander in 1976. I understand modern rovers have measured similar but slightly different values. This is not enough for concrete to set properly.

::Edit:: I tried to verify the nitrogen thing. Haven't found anything. Was this propaganda to try to sell the service of a construction company?

Besides, wet cement requires water. There is cement that sets in winter, but pressure on Mars is so low that water freezes at 0°C and boils at about +6°C, depending on exact pressure. Pressure on Mars varies significantly by location, season, and time of day. So anything made with cement, whether concrete or mortar for brick, requires a pressure tent for construction.

You said stainless steel 304L will cold-work very hard very quickly. Does that mean it suffers metal fatigue?

Last edited by RobertDyck (2021-04-17 16:51:48)

Calliban · 2021-04-17 16:40:48

kbd512 wrote:

Calliban,
I tend to favor the AHR design for flash evaporation / distillation of water and hydrocarbon products. The AHR is more of an "in-machine" than PWRs and BWRs, due to the very high burn-up rates achievable using a homogenous fuel mixture, versus fuel and control rod assemblies, and simple chemical or even centrifugal separation methods used to remove neutron poisons. As compared to PWRs and BWRs, far less radioactive material has to come "out", the technological hurdles associated with fuel manufacturing are much lower, and the reactor does not have to be taken offline for refueling. Similar to the molten salt reactor designs, if the fissile solution leaves the core, then continued fissioning is impossible, as criticality is entirely dependent upon the presence of the neutron moderator in or around the core to slow and reflect neutrons back into the core to sustain fission. Since AHRs do not use contain control rods or fuel rods, operate at modest pressures / temperatures, and can be constructed of low cost materials with low embodied energy, they're a prime candidate for mid-grade process heat.

One interesting way of heating and watering Martian greenhouses and purifying grey water simultaneously. Have a wasteheat pipe carry water at 30°C running through a concrete sump within greenhouses. The sump will carry grey water. As the waste heat pipe heats the water to 30°C, it will evaporate, before condensing on the roof of the greenhouse. The condensed water would then drip feed the plants.

The waste heat pipe should be made from high silicon cast iron, which will resist corrosion even in acidic and oxygenated environments. A walkway made from cast iron grill covers would be above the sump. The water running through the waste heat pipe will be a closed loop that draws its heat from the condenser of an AHR power plant. With a technetium steel reactor vessel, this could operate on a steam cycle. It would raise steam at a temperature of ~200°C and wasteheat would be dumped into the condenser under the LP turbine, at 30°C. The waste heat would then be used for greenhouse heating and water purification.

Human waste in blackwater can be converted into methane and nutrient rich feed water for plants, using and anaerobic. These can be operated at 36°C or 70°C. The second is favoured for human waste, as the higher temperatures are useful in destroying pathogen bacteria like ecoli. This is therefore a good application for direct nuclear heat from a low temperature AHR. Other organic wastes could also be fed into a nuclear heated anaerobic digester, producing methane for metal production or as rocket propellant. The liquid and sludge that emerge from the digester are nitrogen rich fertilisers.

Last edited by Calliban (2021-04-17 16:52:19)

tahanson43206 · 2021-04-17 16:50:22

For RobertDyck re #196

Thank you for the reminder of the Mars Society concept of what a Wiki might be like.

This page was last edited on 20 January 2019, at 10:39.

My impression is that the Mars Wiki has a place alongside the main Wikipedia, as a location of highly vetted, accurate information that is exposed to the general public.

My impression is that the Mars Wiki as currently imagined and implemented has no place for the contributions of NewMars forum.

My concept of the NewMars version of a Wiki is similar in quite a few ways.

For one thing, like the Mars Wiki, it would be viewable by the publc.

Like the Mars Wiki, it would be curated.

And like the main Wikipedia, the content can be updated without censure by editors who have earned credibility.

My concept is that editors of the NewMars Wiki would be vetted by simply having secured and retained membership in the NewMars forum.

However, the ** purpose ** of the NewMars Wiki would be dramatically different from the main Wikipedia and the Mars Society version.

The purpose of the NewMars Wiki would be to serve as a repository of practical knowledge and best practice that would be consulted by future Mars Settlers, by those who would supply them, and by those who would wish to study planning for an enterprise on the scale of Mars Settlement.

The Universe is NOT changing properties with the blowing of the wind across the desert! The ability of humans to ** understand ** the Universe is changing, albeit slowly, and there is a lot of backsliding by individuals, but the march of scientific certainty continues to inch forward, excruciating step by step.

There is a wealth of practical advice posted in this forum, and posted again, and again, and again ....;

The patience of contributors is astonishing.

There should be need for only ONE location for the physical properties of an element, or a molecule.

There might be a discussion about the relative merits of one approach to solving a problem vs another, and the strength of each option will wax or wane as individuals learn more and alter the repository to reflect that increased knowledge or improved understanding.

The constant flow of posts can continue just as it has for 20+ years, but a fixed, easily searchable repository ** should ** assist old members and new ones alike in coming up to speed on some topic that interests them.

When an old or a new member asks a question, the reasonable first response might be to find out if that subject has been covered in the Wiki.

If it has not, or if the question exposes a weakness in some aspect of the repository, than that aspect should be improved, and the questioner can be a part of working out an answer that is satisfactory.

(th)

louis · 2021-04-17 18:02:28

Interesting idea about using compacted regolith within domes...that would allow dome roofs to be much lighter and therefore more viable.

Experiments have shown we could create compressed Mars bricks (no baking required).

Calliban wrote:

I would second Louis's suggestion that urban spaces on Mars should be largely pedestrian and compact. If you look at the urban landscapes that human beings consider to be cultural treasures, they are all compact and largely pedestrian. Most of them were built before the industrial revolution, at which point mechanised transportation was not available. Think places like Venice, Bruges and the renaissance cities and hill towns of Italy.
Compact cities make sense on Mars, as pressurised volumes are expensive relative to Earth. They are places that must be kept pressurised and kept warm. But they are also spaces where precipitation can get precisely controlled and zero if necessary. I think this opens a lot of options that you could make these p,aces more livable. Roof gardens. Streets with seating that allows spillover from restaurants and bars. It also allows adobe and rammed soil to be used as building materials beneath domes as we don't need to worry about precipitation causing erosion or degrading strength.

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