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Comment by vl | Hacker News Reader

Cool concept, but some things are just strange:

Power provided by toroidal nuclear fusion reactors in the outer shell of the living module, but why do you need such reactors if your primary propulsion is provided by Helium 3 - Deuterium Direct Fusion Drive? If you have direct fusion technology, you don't need toroidal reactors.

Rotating inner shells mechanically for 400 years is terrible design, it's much easier just to rotate entire structure. Once it's going it keeps rotating inertially!

Another comment points to error in speed calculation - at declared acceleration they should go at 0.1c, not 0.01c!

And what is missing of course is the calculation of how many years of current world's GDP is required to complete such project event if all yet-to-be invented technologies exist.

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If you would spin the whole structure you couldn't have multiple shells all with 1G on their surface. The required spin speed for 1G depends on the diameter. But their whole concept is built around multiple shells, which is clear from the name.

Regarding the GDP needed once you have a working "mine from the moon and send to orbit" economy it doesn't seem to be too bad. The assumption would be that a lot of technology is already developed for other projects. Launching it all from earth obviously wouldn't be possible even with vastly cheaper launch. That's why they put the build into the moon-earth L1 lagrange point to be easily reachable from the moon.

For propulsion and reactors, but there are multiple projects today working on all of this. Building a life support system for 400 years is still an unsolved problem however.

Re: orbital assembly. L1 point is bad in every respect. If most materials come from Moon, then the best assembly point is low Moon orbit (as benefit you get a boot to your launch speed to escape Earth gravity), if most material comes from Earth, then best assembly point is low Earth orbit. Hauling all material to L1 is going to be more expensive in either case (unless the ratio of materials is very exact, which is unlikely).

Re: spin. I still claim that the best design is to rotate entire living module as one. Most of the activity is going to be on the outer shell. Warehouses, etc will be in the lower gravity inside. No moving parts.

The only question is what to do with fuel and retro engines. Rotate them as well? Fuel tanks need to be stronger. Do not rotate? Then maybe living module can undock for the flight and rotate separately.

Belts and suspenders?

These are supposed to be generational ships. Now imagine you need to take the primary drive down for maintenance? What does the moving colony have for power?

I'd want tri-redundant systems at a least, for everything.

I'd want tri-redundant systems at a least, for everything, and zero gears to keep the rotation of the concentrical shells at the correct speed.

My washing mashine self destructed a few times. I don't want to put my children inside one for 400 years.

> I don't want to put my children inside one for 400 years.

I'm sure every parent has thought the opposite at least once.

Your washing machine probably used cheap nylon gears which self destructed. There are windmills older than 400 years old that still operate today despite using gears :)

This is false example - these windmills are actively maintained and repaired, how are you going to replace massive gears or bearings in space? Are you going to bring spare parts with you?

No, you will bring manufacturing capabilities with you. Decently sized warships carry a machine shop for a reason. A generation ship will have to be totally self sufficient, this will include not just food, air and water but also being able to manufacture pretty much anything they might need.

You mean wood gears like in https://www.alamy.com/gears-of-a-windmill-image245974580.htm... ?

I guess they were replaced a few times during the non harvesting season.

To replace the main gears of a rotating shell in space, you must "turn off" gravity for a week (or month).

My point is more narrow: if you have magical "direct fusion" technology (which is used in the main engines, as presentation says), you don't need regular toroidal nuclear fusion reactors, just use multiple redundant "direct fusion reactors". Which in turn are going to be more compact and don't need to be built as huge toruses around main cylinder.

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