Point well taken, I did miss some of what you meant. Plus, I cut my teeth dancing the Twist so it's a good reference.
We have a 20 year old 3 axis laser welder that has the ability to do the computational gymnastics to make amazingly fine pulse rate/feed rate adjustments so as to maintain weld integrity in tight corners (without overshooting the corner or piling up pulses). This requires the computer to look ahead; not complex at all by today's standards.
If two or three spiders and tugs work together, and especially if the printing head and its movements and output are accounted for well in advance and isolated mathematically from the spider's mass, I see no reason that the same type of computations cannot be used to make turns. Or, as you stated... disengage... then reposition when single piece printing is impossible. But even this move is subject to the same brittleness issues.
In order to have a zero force extrusion between the spider and the previously printed parts of a structure the dance will have to be darn near perfect... This stuff I'm talking about ups that a notch, but not two.
The reason I don't believe that the spiders will crawl along the threads to return to base fits with your thoughts about breaking the structure through torquing. Plus, any unnecessary contact would be very risky.
The mother-ship word may have been a poor choice of making what I still think is a valid point - unless these are to be single ship/spider only missions some other small ship may in the end be acting as a central control/supply and maybe even a repair base.
Also, I think a stabilized plant would be great, but that seems to be a huge jump up from what these are intended to be, and to cost. Individual stabilization of and control of the spiders kind'a goes without saying...
@Ralphy Boy, Hmm, you might have missed what I was trying to say. I think jhankwitz has a pretty good handle on a solution to the problem I presented. Let me illustrate: Place a small motor on your desk and apply power. That thing will Jump and Twist like Chubby Checker (oops, giving away my age). And you're not even in zero g. Motors also act like gyroscopes which exhibit a property called precession. Remember the 8th grade science experiment where you stand on a lazy Susan turntable while holding a wheel on an axle in your hands. Another student spins the axle as fast as they can. Then you tilt the axle and -- wheee, you go rotating.
If a relatively massive robot is making a spidery thin filament and tries to change orientation or even move an arm (think Newton's laws of motion) the reaction could snap the tiny filament. Tethered to a mother ship? That defeats the idea of putting minimal hardware into space. A possible solution is just to pack all the material you need into a print cartridge attached to the robot. Then, as jhankwitz suggests, have massive (or better, small very rapidly spinning) flywheels to keep things steady while motors and arms are working. If the robot needs to turn, it should disengage contact and use its vision sensors to reposition. Of course, another solution is to develop materials that expand and shrink like muscles to eliminate torque.
I would imagine that the resupply would not require the spider like printing head to do anything. Think of the way roads are laid... The Paver just stays on the path and lays asphalt, while dumps trucks run back and forth bringing more material.
Those dumpers (or whatever they end up being called, Tenders perhaps) could also help adjust the slight movements of the construction, or there could be Tugs that are dedicated to that job if it is necessary. Most of these would probable be self propelled or tethered to a mother ship.
It is possible that this construction method may eventually lead to an unexpected leap forward in human space presence in space... so long as we don't abandon it just as it gets going.
Seems that the idea of building spacecraft in orbital platforms, per Star Trek and others, is moving closer to a reality. Imagine seeing Kirk flip open a communicator. Not possible. Then look at modern cellphones. Now we are talking aboout "printing" large kilometer long space structures!!!
Rob, I won't spoil it for you, but let's say those apparently robotic aspects are not explained or shown when the alien's origins are revealed. Whether that aspect will be added later somehow, who knows?
Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.