The thread Culture Orbital: Mass and orbit questions reminded me of something I’ve wondered about Niven’s Ringworld. It’s unstable if the ring is rigid, in that if the Ringworld isn’t perfectly centered, the near side will be more attracted to the Sun.
What if it isn’t rigid? If scrith was a little stretchy, its circumference would depend on how fast it was spinning. If it was a little off-center, the part of it moving a little closer to the Sun could speed up, and the part moving away could slow down, just like a body in a regular, stable orbit would do. That means the nearer part would be stretched a little more, and have less mass per unit arc length than the part farther away.
Could scrith be stretchy enough to make it stable? Would it be sufficient for it to have a Hooke’s law form of elasticity, or would it need to be engineered to have a different stress-strain relationship in the vicinity of its nominal operating range?
Yes, but we know that scrith does stretch, a bit, because the meteor that made Fist Of God stretched the scrith floor into a mountain before punching a hole in it.
The tensile strength required for it to hold it’s circumference when it’s spinning fast enough to provide 1G on the surface would certainly count as extreme.
Well, we know the Ringworld was unstable; it had massive attitude jets. I think, given what we know, the last sentence in your OP “would it need to be engineered to have a different stress-strain relationship in the vicinity of its nominal operating range?” can only be answered “yes.”
It has to be rigid in order for centrifugal force to create pseudo-gravity.
Remember that it’s spinning faster than orbital velocity. This is why things stick to the inner surface. They’re traveling so fast they want to rise to a higher orbit, but they are prevented from doing so by the rigid surface of the Ringworld.
If the segments can stretch apart, then the ring as a whole will expand until the segments reach the natural orbit for their velocity. Of course, in the latter case you no longer have any surface “gravity”, which kind of defeats the purpose of building a Ringworld … .
Incidentally, the attitude jet solution has always bothered me since it implies a long-term mass drain.
A better solution would be a system of counterweights on rails running around the outer circumference. They’re constantly shifting around to keep the rotation stable. The electricity for moving the counterweights comes from the sun.
Wasn’t this the main sub-plot of the Ringworld sequel “The Ringworld Engineers”, in that the ancestors of the engineers had to collect and repair the remaining attitude control jets and then basically overrev them to correct a big Ringworld instability issue caused by other Ringworld races removing some jets over the millenium to use as intersteller spaceship engines? To answer the Hamster King’s complaint, I believe the attitude jets use energy from the solar wind, so no real Ringworld mass loss.
Fist of God Mountain proves that scrith can stretch, but it would need more than that for ZenBeam’s idea: It needs not just to stretch, but to be elastic. That is to say, after the stretching stress is released, it needs to then un-stretch back to its original configuration. And I’m pretty sure that it needs to do so entirely without dissipation: I think that any nonzero amount of dissipation, no matter how small, will just mean that the timescale for instability is increased, but that it would still be unstable in the long run.
Would the basic idea in the OP work at all? Is that a solution for instability?
The thing drifts off-center, and, if rigid, drifts further and further off-center until a catastrophic impact against the sun.
If it’s flexible, to the maximum extent desired for our purposes, would the faster-orbiting parts actually expand and extend away from the sun, so as to provide an orbital counter-weight? I don’t have the math, but, intuitively, might not the stretchy parts sag toward the sun a little, making the problem worse?
I think the point was that the configuration was dynamically unstable regardless, much as a bowling ball balanced on another bowling ball is unstable. The least little perturbation causes forces that drive it further deviation from the ideal.
I haven’t seen the analysis, but I assume the need to be flexible means that the part perturbed closer to the sun (gravity source) does not automatically force the other side further from the sun center.What this means in the long run I don’t know - the flexible ring would have buckled pieces, eventually spots where centrifugal force does not exceed gravity, etc.? Huge chunks of the ring would suddenly be uphills, wate would spill down into other areas, but we haven’t analyzed the tidal force implications, etc.
Maybe that’s why some later novels were set in a smoke ring instead.
I thought the “jets” were Bussard ramjets (a Niven staple) using magnetic fields to collect ambient atomic hydrogen and then fuse it to helium and expel it out the exhaust end; as long as there is ambient hydrogen, the aether provides its own fuel and propellant.
Another problem with scrith being stretchy; the Ring would be uninhabitable due to massive, massive earthquakes. However stretchy scrith is, rock isn’t stretchy at all. It breaks, it doesn’t stretch.
It’s been a long time, but I thought **Ringworld Engineers **was written because a bunch of engineering students wrote Niven that the Fist of God impact would have had to make it’s rotation unstable (among other things), and the plot of the sequel was Niven trying to ret-con the engineering inconsistencies because he found the discussion intriguing. I remember it as the engines originally being use to start the ring rotating to speed, and the project was moving them to where they could be used to adjust attitude.
There’s no mention of the engines being used to spin up the ring, but there’s an acknowledgement that the ring did need to be spun up and that this would have used an inordinate amount of energy - which meant revising the original assertion that the ring was formed from the planetary matter in a single system (the sun wouldn’t have provided anything like enough energy in a reasonable timescale).
The students figured out that the ring was unstable with or without the Fist of God meteorite - as soon as it’s off centre for any reason, it will wander further off centre. FoG helped, of course…
The project (remounting the attitude jets) was necessitated because Halrloprillalar’s species, long after the Ringworld was built, pinched most of the motors to make spaceships. By the time of “Engineers”, more than 95% of them are missing - the best the repair crew can do is get back to one-twentieth of the original number, there is no time to make or mount more.
Here’s a discussion about the amount of energy required to spin the Ringworld; 23000 times a years output of the Sun, which is a lot. http://crowlspace.com/?p=982
You c0ould do it using fusion rockets - but the rockets would shine as bright as a nova, announcing to the entire galaxy that something big was being constructed.
Of course, the logical building method would be to assemble segments in orbit, then spin it from orbit to the speed where you experience 1 g on the interior; or assemble pieces in orbit and then winch the pieces together so the ring is closer in so they have a smaller radius and conservation of angular momentum speed up the ring.
However, you still need to get the pieces into place. Assuming the majority of the matter comes from farther out, you are not so much adding energy as subtracting it, and it would be retro rockets that light up the galaxy’s night sky.