Concordia
Two curved end walls of Noordung and Bernal (Rotors 2 and 3) turning opposite ways, with a lattice service frame and mast between them.

Safety: what changed since the 1970s

Gerard O'Neill and NASA's 1975 summer study showed that a city in space could be built. Concordia asks a second question: what happens when part of it breaks? The answer changed the shape of the station. There are no windows, the shield stands still, the city is four sealed cities, and no single failure can reach everyone.

What the tags mean

Two questions, fifty years apart

The 1970s asked whether a city in space could be built and pay its way. Gerard O'Neill and the 1975 NASA summer study answered yes, and Concordia keeps most of their answer.

Concordia adds a second question: what happens when part of it breaks? A million people cannot be evacuated, so the station shuts trouble in one place. The rule: rock is cheap, and a failure that hits everything at once is the expensive thing.

Topic 1970s Concordia
Daylight Windows and hinged mirrors (Island Three) An opaque shell and a display sky
Shield A separate ring 1 to 2 m out (torus) A stationary truss hundreds of metres out
Air One room per cylinder (Island Three) Four rotors, each split into districts
Atmosphere About half sea-level pressure (torus) Earth sea-level air
Spin machinery Counter-rotating pairs Independent rotors with backup catchers
Incoming mass Payload stream named a hazard Cargo caught far away, never aimed at the city
1room of air in Island Three's cylinderEstablished physics
4sealed rotors at Concordia, each split into districtsDesign value
1 to 2 mgap between the 1975 torus and its shieldEstablished physics
hundreds of mgap between Concordia's air and its shieldDesign value
Three habitats in cross-section: Island Three, the Stanford torus and Concordia Three end-on cross-sections, one above the other, each drawn to its own scale and schematic. Top, O'Neill's Island Three of the 1970s: a cylinder whose wall alternates three land strips with three windows, with a hinged mirror outside each window, and one room of air. Middle, the 1975 NASA Stanford torus: a rotating tube with a separate shield that does not rotate, separated from it by only a meter or two (the gap is exaggerated here). Bottom, Concordia: a rotor of 2,500 m radius with an artificial sky 450 m up, inside a stationary polygonal shell at 3,100 m radius, with no windows. The shield stands still, hundreds of metres from the air, and each of the four rotors is sealed and split into districts. Island Three, 1970s O'Neill's cylinder Half the wall is glassthree windows, three land strips Mirrors make the dayhinged, outside the windows One room of airthe whole length of the cylinder Stanford torus, 1975 NASA summer study Shield stands stilla separate ring, hatched here 1 to 2 m gapexaggerated in this drawing 4.5 t/m² shield, half sea-level airthe study's own figures Concordia, 2041 radius to scale, 2,500 m rotor in a 3,100 m shell No windowsthe sky is a display, 450 m up Shield stands stillhundreds of metres from the air Four sealed rotorseach split into districts Each section to its own scale. Island Three and the torus are schematic.
Island Three, the 1975 Stanford torus and Concordia in cross-section. Each is drawn to its own scale, and the first two are schematic.

A closed shell and a shield that stands still

Island Three is half window. Its wall alternates three strips of land with three of glass, and three hinged mirrors outside swing open to make day and shut to make night. Concordia has no exterior windows. Residents get a display sky 450 m up, powered by solar platforms far from the hull. Nothing transparent holds air, and no moving mirror can fail and end daylight. Design value

The stationary shield is not new. The 1975 torus already had one, "separated by only a meter or two" from the tube. Concordia stretches it. The stationary shell stands hundreds of metres outside the air, an open truss of removable cassettes that crawlers swap like roof tiles. The space inside is a service vacuum where robots work, so nobody in a suit has to. Island Three's cosmic-ray shield is its own air: O'Neill's 1974 design counted on the depth of the atmosphere. A hit on Concordia's skin costs shield, never air. Design value

The risk was always small and slow: the 1975 study put a 1 m meteoroid hole at once in 10 million years, and a day or more to lose most of the air through it. Established physics Concordia makes the outer layer replaceable and the inner layer unreachable.

The hull close up: a patchwork of shielding cassettes of several sizes between raised ribs, each one a replaceable tile.
The hull from 200 m: shielding cassettes set between ribs, swapped one at a time. Open this view
Looking down on the opaque back of the artificial sky from the upper city, with process vaults, nutrient vessels and grow galleries.
The back of the artificial sky: the window is a display. Open this view

Open in the simulation: Exterior Hull: Close Inspection Open in the simulation: Shielding overlay on the hull

Four cities, many rooms

Containment is nested. The station holds four rotors, each split into several pressure and fire districts. Large buildings add refuge halls, and refuge capsules run along the stationary spine. Design value

The 1975 study rated splitting a cylinder into separate pressure volumes "Difficult". Established physics Concordia answers it two ways. It is four vessels instead of one. And district walls are built into the megaframe before anyone moves in, disguised as ridges, transit complexes and civic buildings, with large passages that close behind pressure-rated barriers. A district barrier holds a pressure difference only after an emergency has closed it, so it never carries the city the way a rotor's skin does. Engineering challenge

Fire is the frequent threat. Materials are chosen not to burn, cooking is mostly electric, and suppression drones and water mist answer the first smoke. Design value

The air is different too. The 1975 torus breathed about half sea-level pressure with roughly 45 percent oxygen. Fire risk climbs with oxygen, and NASA certifies spacecraft materials only up to 30 percent. Established physics Concordia breathes Earth sea-level air.

How far does one failure reach? Two columns. Left, one room: a single long volume with one breach, and the whole volume is affected. Right, nested rooms: Concordia is a spine and four rotors, two spinning each way. One rotor is split into six pressure and fire districts divided by barriers. The district count is illustrative, because the engineering specification says only several. A breach in one district leaves the other districts untouched, and the district itself is divided into blocks, some with refuge halls. Refuge capsules sit along the spine. One room One breach reaches everyone Nested rooms Rotor: six districts(illustrative count) District: blocks, some with a refuge hall Affected by the breach Pressure barrier Refuge hall Refuge capsule on the spine One breach stays in one district Schematic. The engineering specification says "several" districts per rotor; six is illustrative.
How far does one failure reach? A single long room against nested rooms. The district count is illustrative.

Open in the simulation: A refuge capsule in the spine Open in the simulation: Pressure overlay

Parts that fail one at a time

Each of these failures stays small.

If this fails What happens
One tension band Its neighbours carry the load; the tear does not spread
A magnetic bearing's control Catchers hold the rotor while it is slowed
One rotary coolant coupling The others carry on; thermal storage rides through Engineering challenge
A radiator panel is punctured The loop isolates it; routine maintenance
Sunlight or the solar platforms Two 250 MWe fission masts restart the essentials
A bad software command Hard interlocks: no one command can vent every habitat, stop every bearing, cut all cooling or all power
A bridge during a speed change Traffic stops, doors close, the bridge retracts first

Each rotor is brought to speed once, empty, in small steps. The rotors then fill one at a time, so a defect found in Tsiolkovsky (Rotor 1) in 2041 is fixed before the others fill. Story See Building Concordia. The two places are described on the axis and spine and Aft Works.

The aft bearing gallery of Tsiolkovsky (Rotor 1): the spine tube passing through ringed bearing structure, trusses and lit bands.
The aft bearing gallery of Tsiolkovsky (Rotor 1), where the mechanical catchers sit behind the magnetic bearing. Open this view
Looking along the gap between two rotors: ringed end wall on the left, dark wall on the right and the spine tube crossing between.
The service gap between Noordung and Bernal (Rotors 2 and 3), where nothing rotating crosses. Open this view

Open in the simulation: Aft Works · Tsiolkovsky Aft Bearing Gallery Open in the simulation: Counter-Rotating Interface, Noordung and Bernal Open in the simulation: Structural overlay

Go deeper: What fifty years of flight taught Established physics
  • Apollo 1, 27 January 1967. A fire in a pure-oxygen cabin on the pad killed three astronauts. Oxygen fraction is a fire decision, so Concordia breathes ordinary air.
  • Mir, 25 June 1997. A cargo ship hit the Spektr module. The crew sealed it off and Mir kept its air. A breach can be shut inside one compartment, the idea behind pressure districts.
  • Aloha Airlines Flight 243, 28 April 1988. Part of the cabin roof tore away in flight and the aircraft landed. One failed panel must not unzip a shell, so Concordia's airtight liner is separate from its tension bands.

Nothing aimed at the city

The 1975 study already named the payload stream of a mass driver as a hazard, "the danger of damage if any of the payloads strike a colony or a spacecraft". Concordia answers by geometry. Cargo is caught tens to hundreds of kilometres out and never aimed at the habitat. Anything over about 10 cm is watched for and intercepted in layers. Design value See radiation and debris.

Open in the simulation: Defence overlay

What Concordia kept

The 1970s designers got the big things right:

The idea of a landscape wrapped around the inside of a spinning shell is theirs too.

A 1975 NASA painting by Don Davis of the Stanford torus interior: a curved valley of houses, trees and a lake under glass panels.
The 1975 torus interior, painted by Don Davis for NASA Ames (NASA image ARC-1975-AC75-2621, from SP-413). Cropped.
Groundside Park inside Concordia: a lawn path past flowering trees, with the ground climbing away across the valley to a bright sky.
Concordia's Groundside Park: the same idea, with the ground climbing away along the ring to a bright sky. Open this view

See five generations and why space.

Sources and further reading

Real-world sources

Concordia design documents

  • Concordia engineering specification, sections 6 to 9, 14, 17 to 19, 35 to 38, 43, 45, 48, 49 and 52.
  • Structure and pressure A rotor is a pressure vessel 5 km across and 2.5 km long. It holds in a full atmosphere while spinning under the weight of a city.
  • Radiation and debris Space is full of fast particles and drifting rocks. Concordia answers with mass: a thick layered shell of regolith and water, shelters for solar storms, and a layered watch for...
  • 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.
  • Counter-rotation and momentum A spinning city is a giant gyroscope, and a gyroscope fights every push that tries to tilt it. Concordia spins its four city rotors in opposite pairs, so the total spin is zero.