Concordia
The open industrial rotor in vacuum: tank farms, rail yards and process modules hung on a ringed steel framework with few walls.

How Concordia was built

A city weighs about two billion tonnes. No rocket fleet carries that. Concordia was built the other way round: first a factory at L5, fed by lunar rock and captured asteroids, then the city that the factory made.

What the tags mean

Why you cannot launch a city

The station's finished mass is about 2 x 10^12 kg, two billion tonnes: four rotors at 300 million tonnes each, Ehricke (the industrial rotor) at 100 million, and about 750 million tonnes of shielding. The International Space Station weighs 419,725 kg. Concordia is about 4.9 million of them. Design value

Launch got cheap, and it was not enough. In 2018 NASA's Harry Jones put the Space Shuttle at $54,500 per kg to low Earth orbit and Falcon 9 at about $2,720. Established physics By the 2030s fully reusable heavy launchers carry more than 100 tonnes a flight for about $2 million, refuelled in orbit by tankers: about $20 per kg to low orbit and about $100 per kg delivered to L5. At that price two billion tonnes is 20 million full loads and about $200 trillion. Story

So cheap launch carried the start, not the bulk. The haulers, the first mining machines, the seed factory and the construction crews all flew up from Earth, and Earth still sends what only it makes well: electronics, medicines and specialist parts. Everything bulky is made from material that is already in space. Open in the simulation: See the 21 km silhouette

A sunlit station silhouette from far off, with radiator banks and a port on the end against black space.
The finished station: about two billion tonnes, 4.9 million International Space Stations, and 21 km long. Almost all of it was made from material already in space. Open this view

Step 1: feedstock flows, mid 2030s

How Concordia was built: five steps, factory first A vertical timeline of five steps. 1, mid 2030s: asteroids first, as metal-rich bodies are bagged and hauled to L5 for iron-nickel and carbon-rich ones follow for carbon, nitrogen and hydrogen; then lunar mining, refining and a mass driver bring the bulk. 2, the station starts as its own factory with a construction yard, power, radiators and an industrial plant, and workers live in small construction rings. 3, the 21 km superstructure and shielding go up around the yard. 4, rotor 1 is built stationary, tested, filled with air and spun up in small steps; first residents arrive in 2041. 5, rotors 2, 3 and 4 follow and the fourth rotor reaches full spin in 2044. 12345 Feedstock flows The station is its own factory Superstructure and shielding Rotor 1 spins up Rotors 2, 3 and 4 Mid 2030s First residents, 2041 Fourth rotor at full spin, 2044 Asteroids first: metal-rich bodies bagged and hauled to L5, then carbon-rich ones. Lunar mass drivers bring the bulk. Yard, power, radiators, industrial plant. Workers live in small construction rings. The 21 km truss goes up around the yard. Shielding cassettes are added as it grows. Built stationary, pressure-tested, filled with air, then spun up in small steps. The rings become spaceport lounges and spares.
Five steps, factory first.

Two streams of raw material start arriving at L5 in the mid 2030s.

The whole station with the material-flow overlay: lunar feedstock and asteroid volatiles flowing to the stores, the city and the yard.
The material-flow overlay: lunar feedstock and asteroid volatiles arrive at the stores in Ehricke, then flow on to the city, the yard and the hull. Open this view

From asteroids, first. The first metal arrives without a single landing. Small metal-rich near-Earth asteroids, tens of metres across, are bagged whole and hauled into the L5 region, where their iron-nickel goes to the yard without smelting. The Moon lacks carbon, nitrogen and hydrogen, so small carbonaceous asteroids follow the same route. The captured carbonaceous bodies run roughly 10 percent water and a few percent carbon by mass. That is a 2045 measurement, not a theory. Story How the haulers work is on Mining the Moon and asteroids.

From the Moon, at scale. Lunar mining and refining, and an electromagnetic mass driver, come up behind the asteroid haulers and soon carry most of the tonnage. The Moon has 1/6 of Earth's surface gravity (1.62 m/s2 against 9.80) and an escape velocity of 2.38 km/s against Earth's 11.2 km/s. It supplies most of the oxygen, silicon, aluminium, iron, glass and shielding mass.

Step 2: the station is its own factory

The first thing built is not a home. It is a factory: the zero-g construction yard, power and radiators, and the first industrial plant. Today those are Ehricke (the industrial rotor) and the shipyard. The seed factory's first product was more factory: AI-designed machines that build the next machines, so output doubled every few months (who paid for it, and why).

A large spacecraft hull held inside a vast construction volume by tension lines, rings and tugs.
The zero-g yard, one of the first parts built. The factory comes before the homes: it makes the girders, tanks and cassettes the city is made from. Open this view

Workers live in small rotating construction rings, a few hundred metres across. The factory turns incoming feedstock into girders, glass, fibre, tanks and cassettes, so that later steps draw on stock held in the station, not on Earth. The industrial rotor still holds stores from both sources: sintered regolith billets, aluminium and iron ingots, glass and basalt fibre and oxygen from the Moon; water ice, carbon feedstock, nitrogen compounds and metal concentrates from asteroids.

Open in the simulation: Ehricke Industrial Rotor, Wide Open in the simulation: Zero-G Shipyard: Hero View

Step 3: the superstructure and the shield

The 21 km stationary truss goes up around the yard. It is a deep spatial truss, not a pressure vessel. Shielding cassettes of compacted regolith and slag are fixed to its outside. For scale, one metre of regolith at 1.6 t/m3 over the 470 km2 of hull is roughly 750 million tonnes. Mass on that scale has to come from the Moon.

Hull truss and shielding tiles seen from 1 km: a lattice frame around pale cassettes, with one darker replacement panel.
The truss is the frame and the cassettes are the shield. From 1 km the hull reads as a patchwork of pale tiles, each one replaceable. Open this view

Step 4: Tsiolkovsky (Rotor 1) and the first spin-up

Tsiolkovsky is built stationary. The prebuilt megaframe goes in while the rotor is still, with only construction gravity. The rotor is pressure-tested, then given atmosphere, then spun up.

The foot of the urban megaframe, with massive steel piers rising from the ground between buildings and parkland on the rotor floor.
The megaframe was built while the rotor was still, then loaded for full spin. Open this view

Spin-up is slow on purpose. The first full-speed turn of a structure this large is one of the riskiest events in construction. Speed rises in small increments and dwells at each step to expose resonances. The next increment starts only when the structure behaves as the model says. Temporary construction equipment comes out before full gravity. The campaign takes days to weeks. Story

The air for one rotor is about 45 million tonnes. Most of its nitrogen comes from asteroids. The first residents arrive in 2041.

Step 5: Noordung, Bernal and O'Neill

Rotors fill one at a time, so a defect found in rotor 1 can be corrected before the others take people. Rotors 2, 3 and 4 (Noordung, Bernal and O'Neill) follow. The fourth rotor reaches full spin in 2044. The construction rings are not taken apart: they are towed 200 km out to the next build site, where the yards have started the next station (who pays).

A paved street with a stately stone frontage on the left, a green square ahead and a tower district on the horizon.
Noordung: a civic street with a stately frontage and a green square. Open this view
A mid-rise street with a bike lane, terraced frontages and the city curving up into the distance.
Bernal: a mid-rise terrace street, red roofs and a bike lane. Open this view
A narrow street between tall arched stone frontages running into the dense center of a district.
O'Neill: the dense core, with arched frontages closing in. Open this view
Go deeper: The numbers behind the build
  • Finished station: about 2.05 x 10^12 kg. Four rotors at 3 x 10^11 kg each (about 45 Mt of air included in each) make 1.2 x 10^12 kg, the industrial rotor 1 x 10^11 kg, and the shielding example 7.5 x 10^11 kg. The full table is on Concordia by the numbers.
  • Atmosphere for all four rotors: 150 to 200 million tonnes, in almost 200 km3.
  • Shield example: 1 m of regolith at 1.6 t/m3 over 470 km2, about 750 million tonnes.
  • Launch price: about $2 million a flight for 100 t is $20/kg to low orbit; four tanker flights per load bound for L5 make it about $100/kg there. 2.05 x 10^12 kg / 100 t = 2 x 10^7 loads; x $100/kg = $2 x 10^14, about $200 trillion. At the 2018 Falcon 9 price it would have been $5.6 x 10^15.
  • Escape energy per kg: Earth 0.5 x (11.2 km/s)2 = 62.7 MJ; Moon 0.5 x (2.38 km/s)2 = 2.8 MJ, about 22 times less.
  • Partial gravity: long-term data on living at 0.2 to 0.6 g is still being gathered. Engineering challenge Children, pregnancy and routine healthcare stay near 1 g in the rotor cities. Design value

Sources and further reading

  • Mike Brown, "SpaceX Starship: Elon Musk outlines an ultra-low price tag for launches", Inverse, 2019 (about $2 million per flight and at least 100 t to orbit, the stated goal used here as the launch baseline): inverse.com.
  • Harry W. Jones, NASA Ames, "The Recent Large Reduction in Space Launch Cost", ICES 2018: ntrs.nasa.gov.
  • NASA, Moon fact sheet (gravity 1.62 m/s2, escape velocity 2.38 km/s): nssdc.gsfc.nasa.gov.
  • NASA, "International Space Station facts and figures" (mass 419,725 kg): nasa.gov.
  • Colaprete et al., "Detection of Water in the LCROSS Ejecta Plume", Science 330, 2010: doi.org/10.1126/science.1186986.
  • Five generations of space habitats Every space habitat has proved one new thing. The first showed people can live in orbit. The second showed someone will pay for it.
  • Materials from the Moon and asteroids Concordia is made almost entirely of the Moon and of asteroids. The Moon supplies oxygen, metals, glass and bulk stone.
  • The industrial rotor Ehricke, the industrial rotor, is the factory floor. It is a skeleton of rings and girders 3.5 km long, turning opposite the city rotors, with refineries, tank farms, container...
  • Why we build in space Concordia exists because it pays. Sunlight is steady, the Moon is easy to leave, and some industries work better without a floor. A city follows once the industry is there.