4 ms·

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 (a

by 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).
- adwn 1y ago
  Oh, and 10 tonnes is bus-sized. For infrastructure at that scale, you want trains at the very least, and those are on the order of 1,000 tonnes. Multiply force, power, and current by 100 accordingly.
  - ben_w 1y ago
    The current method to get 10 tons to low orbit is to burn chemical fuel at a thermal power output of around a gigawatt. This consumes something like 20 times the mass of the payload as propellant, and only barely avoids catastrophic failure 95% of the time. GEO is harder. From what I've seen nobody currently directly launches more than 4.9 tons direct to GEO (Vulcan Centaur VC4). Starship is supposed do 27 to GTO (not GEO) when finished, but it's not finished. If a space elevator lasts long enough to amortise the construction costs (nobody knows, what with them not being buildable yet), they would represent an improvement on launch costs relative to current methods, even if you were limited to 10 tons at a time and each GEO being a 2 hour trip.
- ben_w 1y ago
  A more relevant criticism of that peak velocity is that it significantly exceeds Earth escape velocity (and is 6/7ths of solar EV from here) and therefore wastes energy: https://en.wikipedia.org/wiki/Escape_velocity#List_of_escape_velocities https://en.wikipedia.org/wiki/Escape_velocity#List_of_escape...

by seer 1y ago

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).
adwn 1y ago
Oh, and 10 tonnes is bus-sized. For infrastructure at that scale, you want trains at the very least, and those are on the order of 1,000 tonnes. Multiply force, power, and current by 100 accordingly.
- ben_w 1y ago
  The current method to get 10 tons to low orbit is to burn chemical fuel at a thermal power output of around a gigawatt. This consumes something like 20 times the mass of the payload as propellant, and only barely avoids catastrophic failure 95% of the time. GEO is harder. From what I've seen nobody currently directly launches more than 4.9 tons direct to GEO (Vulcan Centaur VC4). Starship is supposed do 27 to GTO (not GEO) when finished, but it's not finished. If a space elevator lasts long enough to amortise the construction costs (nobody knows, what with them not being buildable yet), they would represent an improvement on launch costs relative to current methods, even if you were limited to 10 tons at a time and each GEO being a 2 hour trip.
ben_w 1y ago
A more relevant criticism of that peak velocity is that it significantly exceeds Earth escape velocity (and is 6/7ths of solar EV from here) and therefore wastes energy: https://en.wikipedia.org/wiki/Escape_velocity#List_of_escape_velocities https://en.wikipedia.org/wiki/Escape_velocity#List_of_escape...