Titanium

Immunity to Environmental Attack
Architectural titanium's unsurpassed corrosion resistance results from its stable, highly-adherent, protective surface oxide film. Because the metal is highly reactive and has a strong affinity for oxygen, the beneficial oxide film forms spontaneously when exposed to moisture or air. In fact, a damaged oxide film can generally restore itself instantaneously.

Lowest Thermal Expansion
Titanium's coefficient of thermal expansion is half that of stainless steel and copper and one-third that of aluminium. It is virtually equal to that of glass and concrete, making titanium highly compatible with these materials. Consequently, thermal stress on titanium is very low.

Light Weight
The specific gravity of titanium is 4.51 g/cm3 - about 60% that of steel, half that of copper and 1.7 times that of aluminium. Being such a lightweight metal, titanium imposes less burden on structure. It is easily fabricated and permits ease of installation.

Environmentally Safe
Due this its relative inertness in most atmospheres, titanium is considered environmentally friendly. It is 100% recyclable and the product of a renewable resource.

Greatest Strength
In addition to having excellent mechanical strength (comparable to mild steel), titanium is durable and shock resistant. Its modulus of elasticity (a measure of strain rate) is half that of stainless steel. This means titanium is more flexible than other architectural metals during earthquakes and other periods of violent movement.

https://www.azom.com/article.aspx?ArticleID=1299

You also build thing for ease of repair. Walkway's top most layer is replaceable. Handrails are replaceable. As things wear out, they are taken back, smelted down and recast and reused.

Design everything modular so it can be unbolted and replaced as needed and the old one recycled into new products.

You need to fundamentally abandon the idea of building a generation ship, and sending it off into deep space.

A generation ship needs to be self sustaining, which includes the capability to recycle and manufacture any part of the ship several times over. This kind of manufacturing capability you most likely need anyway when you arrive at your target.

The design criteria will contain some estimates on how good your recycling works, and you will as much spare parts/spare material along as your fuel budget allows, but effectively your crew will spend their journey rebuilding their ship over and over again.

The key is in the ecosystem you're taking with you.

Organic tech.

I contend that it's about an integrated system, not just about one aspect of that system.

The floor is a Tapestry Lawn:

^{Attribution: grassfreelawns.co.uk}

The various species of bee and other insect that you'll no doubt be wanting to take with you to populate your new world will love it, as will the worms and soil bacteria. (Moles, gophers, well - that's your choice)

• The ivy and other vines and creepers that you decorate the walls with acts as fine self-repairing handholds and homes for insects (a food source for the many birds/bats that are part of your habitat).




• While there might be some large common areas for trees, flowering plants (insects and birds), most of the oxygen production can be from wheat/barley and other food crops in your farming bays.




• Solid human-waste composted with dead plant matter would of course be used as fertilizer, as would urea, trace salts and water from the liquid waste to grow the plants necessary for life. Minerals necessary to re-manufacture metal or ceramic parts could be extracted at this point too.




• Fabrics that the inhabitants need would be made from compostable plant-fiber, faux leather from bacterial matting, furniture from wood (see futon).




• Your aquaculture bays would house and produce fish, oyster, shrimp, algae and various plants, as well as more varied marine habitats for transplantation to the new world - again recycling the O₂ and other nutrients.

Many seeds and corms and tubers would be preserved and held in reserve to re-grow after damage. All this would require maintenance by; gardeners, arborealists, microbiologists, chemists, habitat management specialists, waste recycling engineers ... etc. This would provide rich educational opportunities for the future children, helping to maintain hands-on skills throughout the voyage necessary for populating the destination-world.

Is there is any material for an engineered structure or set of systems that will resist oxidation for the many thousands of years required for the journey?

Aluminum, when in contact with oxygen, forms a thin layer of oxide which protects the underlying metal from further oxidation. Moreover, aluminum has a more favorable elasticity to density ratio when compared to iron, so it is already preferred for applications where weight is a concern (yes, aerospace is such an application)

The only way to overcome these problems would be to have a large number of spare de-oxygenated ships in tow to move into.

False. Fatigued metal can be recovered by a proper thermal cycle. You just need some spare parts and a thermal cycling facility.

For a long time but not indefinitely.

There is a calculation you'll have to do. Every time someone walks down a corridor, how much of the surface is worn away. Every time someone uses a handrail, how much is worn away. Damage can be repaired, fatigued metal can be heat treated. Wear of non-organic materials is lost to the system and extra must be carried to replace it.

While you can work on replaceable coverings for surfaces, even ones you can develop in your own labs from plants grown on the ship. Little by little the hard surfaces underneath will be worn away and need replacing.

You might be able to drag it out for thousands of years, but sooner or later you'll have to get new materials from the outside. No matter how hard wearing, no matter how carefully you maintain protective coverings, nothing is forever.

The same is true of your oxygen

While the air and water within a closed system can potentially be recycled forever. That's not true of any leak, any use of an airlock, any time air escapes the ship it will need to be replaced. Again, unless the system is perfectly sealed, which is unlikely, you'll occasionally need to get more air/water in.

Even if you could protect the exposed parts of the ship, things like electronics will degrade. You will need a power source and engines to keep the environment habitable and then slow down at the end of the trip, which also have to last for thousands of years.

The only practical solution is maintenance. As you suggest, keeping spares for such a long journey may be difficult. You could send a second, uninhabited supply ship perhaps. But really you need manufacturing and recycling facilities on-board.

You could consider advanced 3D printers that work at a molecular level, i.e. Star Trek style replicators. You could also consider nano-technology that is able to repair materials on a microscopic scale. But you will still need some industrial processes for things like energy production.

I think if you make the generational ship large enough, you can incorporate enough redundant and mutually reinforcing systems that they become reasonable.

Steps for creating a generational ship:

• Hollow out a M-type asterroid to use as a shell


• Get resources needed for life from C and S type asteroids as well as icy comets.


• Import Biomass to seed the living chamber (This is arguably the hardest part as all of the seed biomass would have to be imported from preexisting ecosystems intact)


• Use an Orion drive for propulsion, supported by less energetic nuclear propulsion systems for finer control

Ships could be made in size from a couple hundred meters to kilometers in size.

Size would be determined by estimated flight time, minimum viable ecosystem size, material strength, cost, etc...

Frame challenge: If you have the tech to build a generation ship, why is it taking you thousands of years to reach your destination? This is almost certainly harder than building a ship that can accelerate at 1g (or even some decent fraction thereof) and get you anywhere in at most a few hundred years (probably less).

For the math explaining how this works, see this related answer on the sister site Astronomy SE: https://astronomy.stackexchange.com/questions/14559/how-long-would-it-take-to-reach-the-edge-of-the-reachable-universe/14562#14562

You could have self-replicating, self-recycling maintenance nanobots which work at the molecular level. The only input you then need is some form of energy to keep them (and the rest of the ship) running.

At a larger scale you could rebuild or replace whole parts of the ship. All you need is an almost 100% efficient recycling plant.

In the end you’ll always lose some matter (even black holes do) but if you have an outside energy source (e.g. a star’s radiation) and your ship is really advanced it could turn energy into matter.

No.

Things break. You will lose your oxygen because the recycling system will fail. All machines fail given enough time. You will have to carry many sets of replaceable parts and hope that there is enough parts to get you alive to your destination. But too many parts and the tyranny of the rocket equation will bite you because you will need gigantic amount of propelent to accelerate. Orion drives won't save you: the stress of the nuclear explosions is dangerous and the plate or the shock absorbers can (will) break when you need them the most. Nuclear propulsion won't save you. The exposure to the hot neutrons that both fission and fusion release will damage your machinery.

Also biospheres can break too. I don't know the minimum size that an ecosystem must have to be able to support a human and even if this size is small enough to be placed inside a hollow asteroid biospheres can collapse as the mass extinctions the Earth have shown. A critical species may go extinct, cascading an ecological collapse, for example. Or another species can grow unchecked and destroy the ecosystem, like the algal blooms. And how will you get energy to feed your ecosystem once you go beyond the martian orbit? From machines that will break down once you are in the middle of the voyage?

Build a large enough 'rocket ship' that it doesn't matter precisely what it's made of as it will drag its own atmosphere with it. A roughly spherical one approximately 13,000km wide would do nicely. Perhaps harvest planets in order to do so. This will be reasonably resistant to wear and tear unless your passengers start doing really stupid things (disclaimer: anecdotal evidence suggests this may be more of a risk than you thought). Provided that doesn't happen, it will last much longer than your typical generation ship would be designed for, so you can move it around the universe much more slowly. You then 'merely' need to solve the problems of how to accelerate it (0.5g will get you most places after a reasonably small number of generations), and how to supply it with energy (A sufficiently large fusion source would work - no need to have one 1.4m km wide, perhaps bury several fusion plants under the surface). Probably wouldn't hurt to make the ship rotate, so make it rotate around a pole, and put the thrusters/engine/whatever at one pole.

Think Bigger

Carry extra mass. Build the ship in a huge asteroid or something. I know you objected because it would take too long "to get up to speed", but I disagree. A trip that already takes generations can afford to spend a long time accelerating, so long as you get up to speed quickly relative to the total length of the journey.

Has anyone discussed this?

When talking about scifi space concepts, the answer to this question is always Isaac Arthur.

https://www.youtube.com/watch?v=H2f0Wd3zNj0

Get new material after a while

If you can't withstand millenias of wear, then just get new material to replace it. Catch asteroids as you go or make pit stops on planets to get new metal, oxygen and fuel.

You have to repair the ship, and the hard part is hanging onto your mass.

A generation ship needs to be fully self-sustaining, and that means having manufacturing and recycling facilities for everything. This is tough if you only have a few hundred people, but if you have a million, plus good automation, it's not that implausible. Generation ships have to be big.

Stopping wear entirely isn't practical, everything wears out, especially when humans and other living things keep growing in it. You have to repair and maintain. Even the dust can be swept up and recycled. It has to be. On Earth, materials that wear away turn into dust and just blow away and become part of the planet, but in space, if you don't dispose of your dust, in a few thousand years your ship will be knee-deep in dust. And if you throw it overboard rather than recycle it, you'll soon run out of supplies and/or ship.

So really, you need to worry about how much of your mass escapes into space. Some of it will be air leaking out when you open an airlock to do maintenance. Some of it will be bits of the hull getting blasted off by micrometeors. Some of it will be garbage that you just can't recycle and have to throw overboard (need to keep that to an extreme minimum!) And some will just be a few atoms slipping out through the seams. But all that adds up.

Isaac Arthur's video on ark ships covers this in more detail. A spacecraft leaking mass effectively has a half-life rather than a linear rate of loss, and the more mass you lose, the shorter the half-life. No matter how much extra material you take with you, hanging onto the mass you have dominates. But eventually it leaks out, and the only way to maintain enough material to last indefinitely is to stop and replenish it every so often, or build your ship big enough that it holds onto mass by gravity rather than by sealing.

Assuming that second option isn't viable, building bigger still helps. Assuming you don't intentionally throw material overboard, leakage is proportional to the surface area of the ship, but total mass is proportional to the volume. Still, endurance of a few tens of thousand years is probably the best you can do. That's good for a couple of hundred light years of range at 1-5% of light speed, so it's viable. But range will still be a problem.

But resupply is practical. Nearly every star system probably has an Oort cloud, and comets are basically made of rocket fuel. Asteroids are pretty much made of spaceship construction material. If you can realistically travel through interstellar space, you by definition have the ability to reach an Oort cloud. All you need is a refinery on board capable of processing a couple of comets into fuel, then you can visit a nearby asteroid and pick up any heavy materials you might happen to need. And then you can continue on your way. Problems in resupplying are more about the delay and inconvenience of having to stop than about any impossibility in the technology. So, over long distances, the better you seal and recycle, the faster you go.

Of course it can, earth is one of these.

I think one way of building this ship is to build a recycling factory that will make the real spaceship out of the prototype parts. So you have proven that you are able to recycle all parts.

Of course the recycling factory should be onboard. The ship so big that all devices are multiple times on it (even the recycling factory).

In fact the only critical device you need is something that takes anything in and sort atoms to get pure material out. This technology is not that far away from us. The basic principle is what happens in a mass spectrometer.

Once you get that the only remaining problem is to keep all material on board so the spaceship does not vanish due to billions of imperceptible collisions.

How and I forgot some kind of energy harvesting system, solar panels ?

The best option is probably to just use interstellar asteroids or make long duration comets into generation ships with a little bit of gravitational slingshotting.

It will be faster than anything of similar size we could hope to propel with any engines ever, without the associated cost in energy.

And all we need to transport there is a small colony with the manufacturing capacity to build all its parts and further parts for expansion and replacements. Even the population needed for maintenance and other tasks could be created after a core team created/brought the first parts of the station.

Someone said something about lack of choice in trades: I don't think we should think in terms of medieval guilds. Rather, everyone will learn the basics of many skills and specialise in some that interest them or which no-one else is equally talented in. If a task is very unpopular, people will have to do it in turns until someone starts to like it or can be convinced to take it upon them.

The asteroid just needs an abundance of the most volatile or important materials, some of which can be transported there with rockets in advance, or maybe even by joining 2 or 3 smaller asteroids.

If a large enough asteroid flew by the planet, if would even be possible to cover it in a layer of liquid hydrogen or such. The advantage would be that when something gets set in motion, it won't be able to reach escape velocity due to the resistance. Everything stays in a liquid bubble and can be recycled.

On a side note, well mixed materials (used up resources) can be recycled over the millenia by simply exposing them to heat, wear, gravity, solvents, current, radiation, crystallisation and so on. It takes a while, but time is not the essence on a generation ship.

Metallic structures can really last centuries. The main problem would be the perishables (food,water, oxygen etc...)
If you imagine 3d printing is advanced enough, you would just need a big enough stockpile of raw materials, metals and stuff for electronics and can manufacture spare parts from that. Maybe you can mine some asteroids on the way to replenish your metal stockpile.

It would be much cheaper and consume less resources to keep everyone in deep hibernation.
Let robots do all the work, maintenance, navigation.

Maybe have a small rotating crew of humans be on duty just in case.

Even better, just carry seeds and "manufacture" humans once you arrive at your destination with artificial wombs. A tiny ship could potentially carry billions of people using this method.

The idea of an "ark" type generation ship where everyone is supposed to live for centuries in a closed space is really the most inefficient method.