Hacker News Reader

The first one is an engineering problem. If the mass of the cable is immensely higher than the mass of "whatever it is" then it simply doesn't matter. High efficiency (ISP) engines unfortunately stereotypically output very little thrust, while high thrust engines stereotypically waste fuel. So rather than hoisting something heavy in only 9 minutes using incredibly inefficient engines, hoist it in a couple days and then using super efficient engines very slowly get everything back into place. Basically you shatter the old ISP vs thrust tradeoff by immense capital expense.

The second problem also frankly doesn't matter much. Aerobraking is cheap and efficient. I don't remember the specs but it scales pretty well, even when small, such that one pound of re-entry vehicle can land something like 20 pounds of "stuff" if you do it right, so rather than building a second elevator you simply "give up" on 5% of the first elevators capacity. Weight of heat shield is immensely lower than weight of fuel to get up there so don't even bother with reusable shields. Also once you get something up there, you should never return it with the exception (possibly?) of people. So keep a stockpile of shipping crates of re-entry capsules up there and never take anything down. Ever. Except maybe emergency medi-vac. If it costs almost nothing to get it up there, send up a solar powered foundry and a solar powered machine tool plant and start squirting out a stockpile of rough spare parts and panels and the like.

As for the edit, the hidden assumption is the cables "have to be" side by side. However the fuel cost to travel along an orbit is basically zilch compared to getting up there. And the materials science concerns of basically building a cablecar elevator are limited. So if you really, really insist on installing a "down" elevator, simply up the "up" in Africa and the "down" over south america and if you still manage to tangle them, you must have totally screwed up beyond all recovery anyway.

There's something which tells me it won't work, and I can't prove it yet. But I think it's provable with a relatively straightforward simulation.

I don't think that (1) is really such an issue. Two things to consider.

First, the energy required to keep the ... err... toppiece? stable is significantly less than the energy required to launch something into the same orbit.

Second, I think that the metaphorical centrifugal force will bring the toppiece back to where it began. That is, there is a stable equilibrium with the toppiece at its highest point, and bringing it out of that equilibrium will be resisted.

Edit: Certainly the total energy required, from the train and toppiece stabilization, is the same as the energy required to get something into orbit, but to stabilize would require thrusters, while climbing can be done electronically against the chain.

total amount if energy won't be the same as to get something into orbit with rockets, first because rockets have to carry their own fuel, second because it's far more efficient to move by displacing a very heavy reaction mass (the earth) rather than displacing a light reaction mass (which the rocket is carrying)

These are not addressed in pop-sci articles, but are actually considered quite seriously.

There is lateral force from the climber to the cable, yes. But if the net mass flow to/from the top is zero, you don't have to compensate. You only compensate for the net flow. You do that by applying orbital corrections to the station at the top.

If the climber is not rising at hypersonic speeds, the lateral force is pretty tiny - literally lost in the error margin, compared to the tension in most of the cable length.

The climbers would have to rise fast, and the journey would be long anyway, yes. Basically, we would have to marry high-speed train technology (for speed) with Orient Express or ocean liner amenities (against boredom). The view would be spectacular, and would be a major part of the entertainment.

It's probably best to build cables in pairs, one going up, the other down.

I don't think it's entirely unreasonable that you could jettison the climber at the top. It could even be a reusable component of the craft itself.

With that, you could potentially send up as many simultaneous climbers as the cable could support.

With some engineering, you could have dual-use climbers that double as habitat segments.

I'm probably a bit out of my depth here, but maybe somebody else can jump in and set me straight. Regarding point (1), I think that may be mostly solved by conservation of energy if the vehicle is large compared to its payload. The reason is that, as it climbs, it will have to be accelerated to orbital velocity (I'm assuming this will be done by the cable itself, or more accurately the mass at the end that keeps it in tension). It will then drop its payload, and climb back down. During the climb down, the cable will have to decelerate the climber back to surface velocity, so the energy that went into speeding it up on the climb will be mostly recovered on the way back down.

So it seems like the cable would sway backward as it lifts the climber, and be pushed forward almost as far when the climber descends again. If the mass at the end of the cable included some kind of constant low-grade thrust, maybe you could bank up angular momentum between runs to account for the mass of the payloads.

I'd love it if someone with more physics background could weigh in on it though.

> If you don't do this, then it'll "tug" on the cable counter its rotation.

There is tension on the cable, the counterweight at the top will pull the cable straight.

> I don't think anybody would engage in a project that cost trillions of dollars to get built, and then can only send 12 missions per year into orbit.

That's a good point, single space elevators don't scale very well if there's bidirectional traffic on a single cable. Jettisoning the climber at the top is certainly feasible in the beginning, but once we're really using space (which the elevator could finally enable us to do) we're going to need the elevator as a safe return vehicle. In the long run, we're going to need drastically more space elevators, and it might make sense to have a dual ribbon for each elevator, allowing full duplex.

Right now it's about getting someone to build the first one, though. Baby steps.

Regarding the second point, building two cables in parallel would certainly not double the project price. (It could however pose additional safety challenge).

Most of the economic analysis of elevators I've seen use one elevator to lift a second cable as their first flight, this is why building every one after the first is cheaper.

Just like Star Trek's replicators: Building the first one requires you to batter the rules of quantum physics into submission and bend them to your will.

Building the second one requires you to push the "Copy" button.

Having two cables in parallel would also allow to regain the energy that's held in the climber that's descending by using it to power the upwards motion of the other. This would further reduce the "launch" cost.

Regarding 1: most designs have a large space station as a counterweight at the geostationary position that helps keep the cable steady.

I'm having difficulty wrapping my mind around the magnitude of the forces involved. Wouldn't sending a climber up to the top ultimate result in that much force being "pulled" on the tether at the bottom? The entire tether would be under extreme stress, although maybe it's insignificant compared to the weight of the tether itself...

Can't you leave the climbers at the end to add to the counter-weight? Or am I misunderstanding something?