New Mars Forums

Addressing the point made by SpaceNut.  Ultimately, it is transportation of people, goods and wastes, that limits the geographical size of a city.  I found this article to be enlightening.
https://www.d5render.com/posts/walkable … ples-guide

For an entirely pedestrian city to be practical, it must be possible to walk from any one part to any other part in a reasonable amount of time.  This is one of the reasons why preindustrial cities tend to be quite small and compact.  Ancient Rome is a notable exception.  But in general, inhabitants must be able to access all of the amenities of the city in easy walking distance of where they live.  In practice, this tends to result in pedestrian urban areas being very built up, with narrow streets and compact terrace houses.  Amsterdam, Venice and the North African medina towns demonstrate this.

The grand medina in Fez, houses some 150,000 people, along with thousands of commercial businesses and small manufacturers, all on just 1 square mile of land.  Population density is 550 people per hectare, which amounts to some 18 square metres per person.  What makes this even more incredible, is that the mud based buildings are rarely higher than two stories.  The arrangement works by making the most efficient use of space.  Street space in most cities is actually greater than that devoted to buildings.  But pedestrian streets can be as small as 2' wide and it is common for streets not to exceed 3' in width.  This is enough for people to pass each other.  The houses in Fez are also small, as they were built in more minimalist times.  On Mars, we can do better than the medieval builders of Fez.  We will be building cities in dry enclosures on a planet with only 2/5 of the gravity.  So we can safely build 3-4 storey structures out of rammed soil bricks and mortar bound stone, without needing impractically thick walls of the ground floor.  With stronger materials, we can build higher.  We can even have streets on multiple levels.  That would be impractical with cars, but can work in a pedestrian city.  The low gravity of Mars allows us to free up space for street restaurants, cafes and small gardens, whilst maintaining very high population density.

Another option occured to me when walking around the Dutch cities.  On Earth, areas with high rainfall need a sloped roof, to prevent the weight of water from overburdening roof supports.  On Mars, we will be building our cities under frames covered with rock and soil.  Essentially, artificial caves.  There will be no precipitation unless we deliberately introduce it.  Gravity is only 2/5 that of Earth.  It should be possible to build all structures with flat roof space.  This roof space can be developed as a greenspace, provided that soil is not too thick and heavy.  This would provide an open enironment that is above the cramped streets below.  In this way, we can effectively stack a garden on top of our city.  A place where people can walk and enjoy greenery.  The plants will contribute to cleaning and freshening the air.  This would be impractically heavy on Earth.  But the lower gravity of Mars could allow it.

I noticed a lot of houses did have cellars in Amsterdam.  Many of these had steps descending into them from street level and most appeared to me to have been converted into flats.  They appeared to be beneath the canal water level and presumably beneath the ground water level, which is never far beneath the surface in Holland.  When originally built, these cellars would have been used for storage.  Being beneath the water table would not necessarily have resulted in frequent catastrophic flooding, provided that evaporation balances the rate of seepage through the walls.  But it would have made these cellars rather damp places, vulnerable to fungal infestation.  To be habitable or suitable for storage of perishable items, tanking would be required, in addition to forced ventilation.  That can get expensive, as even small leaks can result in high humidity that fungus will take advantage of.

My own cellar in the UK has exactly this problem.  My house is 200 years old and was once a bakery.  Flour was stored in the damp cellar, hanging from iron hooks in the floor joists above.  I can only conclude that this arrangement was tolerable because flour was used quickly upon receipt, without giving it chance to rot.  I have made some improvements over the years.  A concrete floor.  I have repointed some of the original lime mortar within the granite walls with less permeable cement.  But damp is still a problem.

Some more pics of Amsterdam.

Utrecht (5)

Utrecht (3)

Some pictures from Utrecht.

Quite a lot to read through here, so I will comment again when I've had chance to read it all.  Gas turbine blades have always been made from high temperature nickel alloys.  Since the 90s, they have been grown as single crystals with mineral rods embedded to provide cooling channels by dissolving the rods in weak acid after casting.  So I'm not sure why the reference suggests that using nickel alloys is impractical or expensive.  It is standard aerospace practice.  Take any COTS GT and you find nickel alloy components.  For non-moving parts, steels can still be used at 700°C.  Strength will be reduced substantially and corrosion in hot CO2 will be more of a problem.  But is can be done.  There are specialist oxide dispersion strengthened mechanical alloys that were specifically developed for operation in this temperature range.

I will be back in Holland week after next.  Not somewhere I thought I would be going back to so soon, but it is where my son wants to go for holiday.  We are planning on visiting different places this time.  We are staying in Haarlem and will be getting the train to Utrecht, Delft and Den Hague.  I will take pictures as last time and post them here.

I have always found the Holland to be quite inspiring.  It is a place where about half of the land is reclaimed from the sea and much of the remainder was boggy marshland before humans terraformed it.  The sea is held back by a system of soil dikes.  Behind the dikes, water drains into ditches, and is pumped to sea level by pumps, originally wind driven.  This seems quite analogous to what we plan to do on Mars.  In that case, land will be recovered from vacuum by constructing a roof structure and covering with soil to counterbalance internal pressure.  The resulting habitable land will be relatively expensive.  Under the roof, the challenge will be to develop towns that are comfortable to live in despite high population density.  The urban architecture of pre-industrial Europe gives us solid examples of how to do that.

Much depends I think on how we construct space suits.  If we go with an MCP design, then you have an elastomer fabric covering skin.  Other, tougher garments can be worn over this.  As Robert noted, Mars dust grains are more rounded than lunar equivelent.  So I don't see that we need anything special compared to Earth based clothing.  On the moon, the situation is quite different.  That dust will destroy most fabrics quickly.  CNT or BNNT would appear to be necessary there.  Sharper dust will also be more toxic to the lungs if tracked back into the hab.

I have a GoreTex coat that is about 10 years old now and has seen heavy use.  It is still in good condition and is still reasonably waterproof.  That is a necessity in the northern parts of Britain.  The surface is easy to wipe down as it is relatively impermeable.  So it shoukdn't be difficult to keep clean on Mars.  The moon is a different case entirely.  It was noted that the original Apollo space suits were destroyed by a few days expusure to lunar dust.  I hate to think what it is going to do to astronaut lungs long term.  Will it be as bad as asbestos?

Gerard O'Neill's Island 1 was intended to present an appealling image of a space habitat so that people could become emotionally invested in the project.  But if our goal is to set up space manufacturing capabilities and the purpose of the space habitat is to house the workforce, we do not need to begin with such large habitats.  They are things that we can build up to over time.  Starship is designed to house up to 100 passengers in a habitable volume of 1000m2 for at least 6 months.  That is a 10m3 volume for each passenger.

If we used Starship as a guideline for the required habitable volume of a minimal space habitat, then a workforce of 1000 people would require 10,000m3 of habitable volume.  That is a sphere some 26.7m in diameter.  We can be more generous without the space habitat becoming unrealistically huge.  Lets assume 10x the Starship volume per worker.  That is 100m3 each, or 100,000m3 in total.  That is equivelent to a spherical habitat some 57.6m in diameter.  Say 60m.

Let us assume that such a habitat rotates to produce artificial gravity.  Human beings can endure about 3 rotations per minute before inner ear problems start to become problematic.  There is some evidence that humans can adapt to higher rates over time.  But lets assume for the time being that 3 rev/min is what we will design to.  That is 0.314rad/s.  Centripetal acceleration can be calculated as:

A = w^2 × r.

Solving for r = 30m, gives a centrifugal gravity of 2.96m/s2 at the outer edge of the sphere, or 0.3g exactly.  This is slightly less than Martian gravity.  The closer one gets to the centre of the station, the shorter the radiys of rotation and the lower the centripetal acceleration.  Lunar levels of gravity (1.635m/s2) would be experienced some 16.58m radius from the centre.  This means that about 60% of the internal volume of the sphere would have gravity greater than lunar.  Habitation areas will be concentrated around these outer sections.

The total volume of a 60m diameter spheres is 113,097m3, or 113.1m3 each.  Some of this volume will need to be apportioned to life support functions.  Food will be produced using a mixture of hydroponics, algaeculture and extracted chloroplasts.  Using acetate salts, food can be produced in very compact volumes without sunlight.  Gone is the need for the extensive agricultural areas that O'Neill anticipated in the 1970s.

Such a habitat would not have any wide open spaces.  But there is sufficient volume for every crew member to have their own small bedroom, equipped with a personal ensuite.  A cubic room with dimensions 2.4m aside, would have internal volume 13.8m3.  If every crew member had such a room, they would collectively account for 12.2% of internal habitation volume.  There would be canteens within such a habitat, as well as lounges, gym and cinema.  There could even be small green spaces beneath artificial lighting.

How much would such a habitat weigh?  This is difficult to estimate.  For Island 1, mass was dominated by cosmic ray shielding.  This amounted to 5000kg of silicate slag per square metre of hull area.  For a sphere 60m in diameter, this equates to a total shielding mass of 56,500 tonnes.  This is only 1.44% of the 3.9 million tonnes of shielding that would have been required by Island 1.  As our habitat will be constructed for the workforce of a lunar ore processing facility, it is reasonable to assume that this mass will be derived from lunar materials.  We could either use the first 56,500 tonnes of lunar ore as shielding or just work without shielding until we have 56,500 tonnes of silicate wastes from the ore refining itself.  Much will depend on the mass that can be provided by an initial lunar mining operation.

I am going to call this 60m, 1000 person habitat concept, Island 0.1.  Back in the 1970s, O'Neill had assumed that the colonists in Island 1 would be working families, with children attending school in the habitat, with both parents working.  This seems less realistic for an early habitat supporting space manufacturing, though it isn't impossible.  I think it more likely that individual workers would sign up for a 2 year contract, which would include transport to and from the habitat, food and accomodation, in addition to wages.  The habitat would be attached to the ore processing and manufacturing areas.  These would mostly be low or zero gravity.  Operations would either be automated or controlled remotely from within the habitat.  Only maintenance that cannot be carried out robotically, would require humans to leave the safety of the shielded habitat.  This allows exposure to cosmic rays to be minimised by limiting exposure time.  So the majority of the space factories will not require cosmic ray shielding.

I am presently looking into buying a computer with enough capacity to handle fire modelling CFD applications.  The cost is likely to be $5000 - $10,000.  The problem is that for a 3-Dimensional geometry, especially structures, the number of cells increases very quickly, which eats up a lot of memory.  To run a simulation in a reasonable amount of time, a high end Intel Xeon W-series processor is needed.  A minimum of 1TB of free memory space per simulation and preferably, 64 cores to be able to run the simulation in hours rather than days.  It gets expensive very quickly.  But it is such a significant advantage to me as an engineer, that I am minded to take the hit and make the investment.