Space Elevator

TIL it's estimated that over 48 tons of meteors hit the atmosphere every day.

Regarding actual space elevators though, while they're not sci-fi to the extent of something like FTL travel - ie. they're technically not physically impossible - they're still pretty firmly in the realm of sci-fi. We don't have anything close to a cable that could sustain its own weight, let alone that of whatever is being elevated. Plus, how do you stabilize the cable and lifter in the atmosphere?

A space elevator on the moon is much more feasible: less gravity, slow rotation, no atmosphere, less dangerous debris. But it's also much less useful.

Almost all discussions around space elevators focus on the cable itself, how to manufacture and deploy it, and completely forget about the issues that would arise afterwards:

1) How do you attach the climber to the cable without affecting its structural integrity? By squeezing it really hard? A material that's optimized for longitudinal tension strength is probably not very tolerant of lateral compression.

2) How do you provide power to the climber? A regular electric cable can't support its own weight, so either you have to attach it to the climbing cable, or you have to make it from the same material.

3) Is it even worth it? The climber needs to cover a distance of ~36,000 km, so even at 200 km/h it takes 7.5 days from the bottom to geosynchronous orbit. How many climbers and what payload can the cable support at the same time? Refer to issue #1 regarding limits in speed and mass per climber.

The throughput in tonnes/day is absolutely abysmal in relation to the immense upfront infrastructure cost per elevator. Compare this to SpaceX's Starship, which is getting closer and closer to fully reusable 100 tonnes to orbit in minutes. Space elevators will stay science fiction forever, not because they're infeasible, but because they're useless.

If you somehow manage to get magnetic fields involved, so you are not afraid of friction with the cable itself, at 1.3 max apparent acceleration/deceleration (after a turnover) and including earth’s gravity you get 116min to geostationary.

If you account for various inefficiencies like taking it slow in the lower atmosphere Ant whatnot, it still should be in the matter of hours. So totally feasible and even comfortable.

> If you somehow manage to get magnetic fields involved, so you are not afraid of friction with the cable itself, at 1.3 max apparent acceleration […]

This means that half-way after 58 minutes, the climber is traveling at 0.3 * 9.81 m/s² * 60 * 58 ~= 10.2 km/s ~= 36,720 km/h (!!!) relative to the cable. A tiny imperfection or wobble is going to make the climber crash into the cable, destroying both.

A climber with a mass of 10 tonnes requires 10^4 kg * 1.3 * 9.81 m/s² ~= 127.5 kN of force to accelerate at 1.3 g. At the ~56 minute mark, the climber reaches a speed of ~9,888 m/s. This means it requires a power output of 127.5 kN * 9888 m/s = 1.26 GW (!!!) to achieve this acceleration, plus overhead for the power electronics and transmission. Even at a voltage of 1 kV, that's around 1,500,000 A (!!!) of current that you have to transmit and invert.

If you have a way to reliably transfer that amount of power without touching the cable which is moving at 10 km/s relative speed, or with touching but without immediately melting the cable or the collector, let me know :-)

> So totally feasible

lol no

> A tiny imperfection or wobble is going to make the climber crash into the cable, destroying both.

A maglev train is several centimeters from the rail; if someone made the carbon nanostructures (the only known material strong enough are atomically precise carbon nanotubes or graphene, but the entire length has to be atomically precise you can't splice together the shorter tubes we can build today) this badly wrong, the cable didn't survive construction.

> Even at a voltage of 1 kV, that's around 1,500,000 A

Why on earth would you do one kilovolt? We already have megavolt powerlines. That reduces the current needed to 1500 A. 1500 A on a powerline is… by necessity, standard for a power station.

We even already have superconductor cables and tapes that do 1500 A, they're a few square millimeters cross section.

> A maglev train is several centimeters from the rail […]

No maglev train I ever heard of travels at 36,000 km/h. This is about two orders of magnitude faster.

> We already have megavolt powerlines.

That's transmission over long distances, but you need to handle and transform all that power in a relatively small enclosure. Have you seen the length of isolators on high-voltage powerlines? What do you think is going to happen to your circuit if you have an electrical potential difference of 1 MV over a few centimeters?

Yes, you can handle large voltages with the right power electronics, but you need the space to do so. For comparison, light rail typically uses around 1 kV, while mainline trains use something like 15 kV. But a train is also 10 to 100 times as heavy as the 10t climber in my calculation, so you need to multiply the power (and therefore the electric current) by 10 to 100 as well.

> No maglev train I ever heard of travels at 36,000 km/h. This is about two orders of magnitude faster.

You think the problem is the speed itself, and not the fact that trains are close to sea level and at that speed would immediately explode from compressing the air in front of them so hard it can't get out of the way before superheating to plasma, i.e. what we see on rocket re-entry only much much worse because the air at the altitude of peak re-entry heating is 0.00004% the density at sea level?

> What do you think is going to happen to your circuit if you have an electrical potential difference of 1 MV over a few centimeters?

1) In space? Very little. Pylons that you see around the countryside aren't running in a vacuum, their isolators are irrelevant.

2) Why "a few centimetres"? You've pulled the 10 tons mass out of thin air, likewise that it's supposed to use "one kilovolt" potential differences, and now also that the electromagnets have to be "a few centimetres" in size? Were you taking that number from what I said about the gap between the train and the rails? Obviously you scale the size of your EM source to whatever works for your other constraints. And, for that matter, the peak velocity of the cargo container, peak acceleration, mass, dimensions, everything.

> For comparison, light rail typically uses around 1 kV, while mainline trains use something like 15 kV.

Hang on a minute. I was already wondering this on your previous comment, but now it matters: do you think the climber itself needs to internally route any of this power at all?

What you need for this is switches and coils on one side, a Halbach array on the other. Coils aren't that heavy, especially if they're superconducting. Halbach array on the cargo pod, all the rest on the tether.

Right now, the hardest part is — by a huge margin — making the tether. Like, "nobody could do it today for any money" hard. But if we could make the tether, then actually making things go up it is really not a big deal, it's of a complexity that overlaps with a science faire project.

(Also, I grew up with 25kV, but British train engineering is hardly worth taking inspiration from for other rail systems, let alone a space elevator).

Dielectric strength of vacuum is 20kv/inch. Thus your megavolt needs 50 inches of separation at an absolute minimum. And you're operating this in space where you have ionizing radiation. Free electrons with a big voltage differential? You're describing a vacuum tube.

> 20kv/inch

Breakdown voltage is pressure dependent, not a constant.

Your figure is for (eyeballing a graph) approximately 2e-2 torr and 150 torr, less between, rapidly increasing with harder vacuum. The extreme limit even in a perfect vacuum is ~1.32e18 volts per meter due to pair production.

For a sense of "perfect" vacuum: if I used Wolfram Alpha right just now, the mean free path of particles at the Kármán line is about 15 cm, becomes hundreds of meters at 200km.

Though this assumes a free floating measurement, the practical results from https://en.wikipedia.org/wiki/Wake_Shield_Facility would also matter here.

> And you're operating this in space where you have ionizing radiation. Free electrons with a big voltage differential?

Mm.

Possibly. But see previous about mean free paths, not much actual stuff up there. From an (admittedly quick) perusal of the literature, the particle density of the Van Allen belts is order-of 1e4-1e5 per cubic meter, so the entire mass of the structure is only order-of a kilogram: https://www.wolframalpha.com/input?i=%284%2F3%29π%282%5E3-1%...

If this is an important constraint, this would actually be a good use for a some-mega-amps current, regardless of voltage drop between supply and return paths due to load. Or, same effect, coil the wires. And they'd already necessarily be coiled to do anything useful: Use the current itself to magnetically shield everything from the Van Allen belts.

Superconductors would only need a few square centimetres cross section to carry mega-amps, given their critical current limit at liquid nitrogen temperatures can be kilo-amps per mm^2.

But once you're talking about a 36,000 km long superconducting wire with a mega-amp current, you could also do a whole bunch of other fun stuff; lying them in concentric circular rings in the Sahara would give you a very silly, but effective, magnetic catapult. (This will upset a lot of people, and likely a lot of animals, so don't do that on Earth).

No, I wasn't eyeballing, but perhaps someone else was. I went looking for the dielectric strength of vacuum and I found a chart with values for a bunch of different things including vacuum.

And I don't understand the connection to the Van Allen belts--I'm talking about sunlight knocking electrons off your conductors.