Hello, signing on

[Drashner1]

Rather than slow the ship down to build a new cylinder, you could send an advance party with a fast ship to build it as the generation ship sails past. This would be much more economical on fuel and propellant. You could even cobble the fast ship together from the engines of the generation ship (which aren't being used, after all).

Assuming you have the manufacturing technology to do so, not to build the smaller ship but to pack enough equipment and people into it to to get the job done. Neumanns in the early days were't as advanced (the need for supervision places a cap on how many can be deployed for a given job) but they were good enough for the colonies that did succeed. For a story it would be a fine line between having enough technology to be able to send off industrial seeds to harvest/transport resources and build more habitats and not having technology good enough to keep your current systems going. There might not be any overlap at all.

Note however that:

a) Neumann tech from this early in the timeline was presumably much much less capable (exactly how much is something we would need to work out and possibly not relevant to developing this story.

b) Even if neumanns (or whatever is used) any sort of advance party would need to slow itself down from the ship's speed (and magbrakes don't work very well at this low a speed, so this would be expending some amount of fuel at some point. Also, this would at the least require detecting the target in interstellar space well in advance), hopefully locate all the required materials at the target to build what is desired and then provide the fuel to get it back up to speed with the ship, build the desired ship/hab - and then get the whole thing up to speed to rendezvous with the main ship as it passes by. TBH I don't think even Y11k tech could pull that off, or at least not reliably.

On a related note, rather than bringing 'raw' asteroid/comet material at launch, it might be better to bring along a big mattercache of material mined and refined in the solar system before launch. This would let you know exactly what you had (vs having to go with what you think you have in the asteroid) and would make construction and recycling easier in most cases.

Just some thoughts,

Todd