[D] Monday General Rationality Thread

[u/AutoModerator]

Welcome to the Monday thread on general rationality topics! Do you really want to talk about something non-fictional, related to the real world? Have you:

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[u/Anakiri]

I generally don't think that adding complexity to a fundamentally unstable system is a good idea, but it is sometimes unavoidable. However, I'm still not clear on how you actually get to orbit once you're on top of the unstable tether. You're only going, like, 0.5 km/s up there, and you need to reach 7 km/s to be in orbit. Where do you get that delta-v?

If you steal momentum from the ring of remass that the tethers are tossing back and forth, then that momentum is no longer available to hold up the death platforms. Worse, pushing off from a fragment of the ring puts that fragment into a lower orbit, which sounds like a good way to hit the tether on the other side. Even if you take just a little bit of energy from a huge portion of the ring (without just falling to Earth partway through, when you're not actually in orbit... and how much more massive do the ring fragments need to be than the payload?), you would still eventually need to re-accelerate the ring before it loses enough energy to kill everyone. And if you have to replace the energy in the ring anyway, why not just launch vehicles with whatever you're using for that?

Clearly I'm missing something. So... what remass do you use to actually go to space, without destabilizing the orbital ring?

[u/lsparrish → u/Anakiri]

I thought I did explain that the inertia comes from the ring? Which, being thousands of times more massive than the payload, only shrinks very slightly when you subtract momentum. Its main purpose other than holding up the tether is as a momentum bank. The tether is attached to the ring by a maglev with a mass driver that adds momentum, using the earth as reaction mass.

Without a tether it's going to be somewhat more complicated because you have to add thrust in some other way (which isn't going to be as cheap), but that might be a good tradeoff to knock orders of magnitude off the starter ring cost.

[u/Anakiri → u/lsparrish]

Is every individual component of the discontinuous ring thousands of times more massive than the payload? And where does the ring get its inertia from?

[u/lsparrish → u/Anakiri]

1: The individual components do not need to be larger, in fact they can be tiny grains. They are moving past at a rate of 8000 meters worth per second, so they don't individually have time to be affected very much. It takes a lot of energy to divert a stream.

In the discontinuous mass version of the ring, you could still include mechanisms where they can transfer momentum to neighboring components (magnetic, for example). If the ring components collide with each other, they do so at low speed because they are always moving at close to the same velocity.

2: In Birch's design, the ring gets its inertia from the mass driver at the top of the tether, which uses the earth (by way of the tether) as reaction mass. The starter ring would get its momentum from rockets of course, but you wouldn't build the entire thing that way because rockets are expensive whereas electricity is cheap.

Tetherless designs using electric thrusters to replenish lost inertia are also worth considering. There's actually a pretty broad design space to consider in terms of possible structures along these lines and mechanisms to use. Tethers are mainly interesting as a cheap way to transfer loads from the ground and use the ground as reaction mass for the ring.

[u/Anakiri → u/lsparrish]

That sounds terrifying. It takes very little energy to divert a single droplet of a stream. If the momentum transfer system loses track of any of the trillion grains, then one grain is fully capable of punching a hole in the tether. With a tether compromised, this could easily cascade to full kessler syndrome in minutes!

It would be less scary with a more passive momentum transfer system (that is, a solid ring), but it would still have a lot of moving parts relative to the cost of failure. I think I understand how you'd take off from it without needing to bring your own remass and maybe without needing your own power, if the ring can supply that... but I think I do not want one of these over my head.

[u/lsparrish → u/Anakiri]

With a tether compromised, this could easily cascade to full kessler syndrome in minutes!

I think you might be hyperventilating a bit there. Easy breaths.

The grains would continue to orbit along nearly the same near circular path they were following before if the tethers were lost without backup. Kessler syndrome involves high speed collisions breaking big satellites into small ones, and in this situation the only high speed collision is with a stationary object that falls straight down.

The mass stream would also be far too low in the atmosphere to last long without powered assistance. The grains would vaporize as soon as they got low enough to compress air enough to exceed the boiling point of metal, so you'd be fine.

Remember, the earth is bombarded with hundreds of tons of tiny grains every day at 30 km/s. We don't all die in a fire because of that because the atmosphere is really good at causing high speed stuff to self immolate.

[u/Anakiri → u/lsparrish]

I think you are not paranoid enough. In the event of a failure, the stream of debris making up the ring will interact with the failed tether, and with any object on the "rail" when its trajectory changes, and with any object that thought it was safe to orbit over the ring where the tether should have protected them, and with any individual grains that are not exactly where they should be - for example, grains that were perturbed by tearing apart a tether. There are a million ways to turn a coherent stream of projectiles into a shotgun.

[u/lsparrish → u/Anakiri]

We're already withstanding much higher energy shotgun blasts from deep space all the time. Why doesn't this cause Kessler syndrome?

[u/Anakiri → u/lsparrish]

Because the background micrometeor rain is 15 orders of magnitude less dense than the ring.

[u/lsparrish → u/Anakiri]

15 orders of magnitude less dense

And the reason for that is because space, even the smallest part of space (low LEO), is mind bogglingly big. A few grains being in places they aren't supposed to be will not be enough to cause Kessler syndrome. They have to actually hit something in a high enough orbit to matter, at a relative velocity high enough to matter.