5 ms·

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 ari

by adwn 1y ago

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.

seer 1y ago
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.
- adwn 1y ago
  > 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
  - ben_w 1y ago
    > 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.
    - adwn 1y ago
      > 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.
      - ben_w 1y ago
        > 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).
LorenPechtel 1y ago
#1 is one of the things they typically get wrong in stories. Climbing the cable is a nightmare, especially as it gets thicker as you go up. Thus do not climb the cable! Rather, when the cable is built a whole bunch of anchors are built into it. You are not climbing the cable, you are climbing a track on the side of the cable. The cable's job is to support the track plus any load on it.

by adwn 1y ago

seer 1y ago

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.

adwn 1y ago
> 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
- ben_w 1y ago
  > 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.
  - adwn 1y ago
    > 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.
    - ben_w 1y ago
      > 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).

LorenPechtel 1y ago

#1 is one of the things they typically get wrong in stories. Climbing the cable is a nightmare, especially as it gets thicker as you go up. Thus do not climb the cable! Rather, when the cable is built a whole bunch of anchors are built into it. You are not climbing the cable, you are climbing a track on the side of the cable. The cable's job is to support the track plus any load on it.