3 ms·

At 74km/h you would reach space (100km karman line) in an hour and a half but you would need to keep going for another 20 days at that speed reach geostationary

At 74km/h you would reach space (100km karman line) in an hour and a half but you would need to keep going for another 20 days at that speed reach geostationary orbit in order to not fall back to earth when you exit the space elevator.

NoMoreNicksLeft 3y ago
Can the speed be safely increased once you leave the atmosphere and there is no (appreciable) drag? With such a system in place, are there no other interesting places to stop along the way to geosync?
midoridensha 3y ago
The other problem people seem to forget is radiation. The Van Allen belts expose people to a lot of radiation; normally it's not a problem, because either people stay under the belts (as in stations like the ISS which are at LEO), or for the few times humans went beyond LEO, to the Moon, they blasted through the belts very quickly and thus had negligible cumulative radiation exposure. But if you're sitting on a space elevator taking days to go through the VA belts, you'll probably get a fatal dose. Wikipedia has an article that touches on this here: https://en.wikipedia.org/wiki/Space_elevator_safety https://en.wikipedia.org/wiki/Space_elevator_safety
philwelch 3y ago
Yeah, I wish this went on as far as an actual space elevator would, perhaps with extra visualizations of the heights of various orbits. The top would be slightly above geostationary orbit, since you would need the center of mass to be at the geostationary point in order to work.
- Sharlin 3y ago
  Or more than slightly if you don't want to haul up a big counterweight but just use the cable itself as a counterweight. The Brad Edwards design proposes a total cable length of around 100,000 km. One very nice side effect of a long cable is that you can slingshot spacecraft from the far end up to ~Jupiter's orbit with zero propellant needed.
  - philwelch 3y ago
    I would imagine rather than hauling up a big counterweight, you might be better served capturing a near-earth asteroid and hauling it in.
    - Sharlin 3y ago
      Yes. However, capturing a NEO might still be more difficult than just using a longer tether.
LeanderK 3y ago
but do you need to reach geostationary? couldn't you just switch to a rocket at this stage? Maybe a small one to bridge the gap? I would have thought that the first kilometres are the ones where you have to invest the most work so if you just bridge them using the space elevator it would be still an improvement. Maybe for cargo the 20 days are not really important
- midasuni 3y ago
  Getting to 100km or even 200km is easy. Staying there is hard. 80% of the energy in a rocket goes to adding sideways motion. An elevator upto GEO adds enough energy (taken from the earths rotational energy) to your payload to get it into orbit. If you let go lower down you need to add sideways velocity. In LEO you on an elevator would be travelling about 1,400mph and need to get to 18,000 to get into orbit. Sure there’s no atmosphere to work against but it’s still a lot of fuel to use in a rocket.
  - metalliqaz 3y ago
    > 80% of the energy in a rocket goes to adding sideways motion. I have not researched this at all, so I'm not doubting you, but doesn't most of the energy go into accelerating the remaining fuel? So, by the rocket equation, shouldn't you be way ahead of the game by starting at the edge of space?
    - jandrese 3y ago
      You still need to accelerate the fuel at that point. Free travel up to LEO doesn't buy you much. There is an excellent What-if on this: https://web.archive.org/web/20230211050159/https://what-if.xkcd.com/58/ https://web.archive.org/web/20230211050159/https://what-if.x...
  - outworlder 3y ago
    > In LEO you on an elevator would be travelling about 1,400mph That's a pretty significant speed already and it helps that you are already in space. See how tiny a Pegasus rocket is, and that one is launched from aircraft, slower, and nowhere near as high as what we are discussing. https://www.northropgrumman.com/space/pegasus-rocket/ https://www.northropgrumman.com/space/pegasus-rocket/ Getting to 100km from ground requires beefy first (even second) stages because you need to accelerate further stages plus the fuel, inside the atmosphere. We can basically pretend we are starting at the second or third stages. It now up to the rocket equation. So, to recap, there's already 1400mph of sideways motion, at a negligible air resistance, 100 to 200km up with vacuum optimized nozzles. It is a massive win. The question is if we could hoist what are still relatively heavy payloads.
nivenkos 3y ago
At that point gravity will act slower though, so it might be enough to take up a small liquid fuel craft too. Rockets will have abandoned solid fuel boosters long before then.
- drtgh 3y ago
  If you take a lift to a distance of 100 km and drop it, it will fall immediately reaching the ground in less than 10 minutes. Gravity remains strong. Satellites and space stations are always in free fall. The thing that keeps them in "space" is the velocity with which they were originally launched to orbit the Earth as centripetal force becomes the weight of the satellite, which slows down such falling process. Seeing it with cold eyes, they employ all the energy at a first for to lift the weight of that falling object. If a geostationary satellite has to stay over the same region of the planet, it has to be sent much much far away, until its orbital velocity become synchronized with the Earth's rotation. And even like that, keeps being in free fall due Earth's gravity. I mean, what we see in cartoons or sci-movies about to send a rocket absolutely vertical and cut engines, or a spaceship joining over a planet and conserve the same orbital position, could be done only with anti-gravitational engines. A small liquid fuel could not maintain them in place.
  - concordDance 3y ago
    > The thing that keeps them in "space" is the acceleration with which they were originally launched to orbit the Earth, which slows down such falling process. It would be more clear to say that what keeps them in space is that they're moving sideways so fast. Velocity, not acceleration, is the right term here.
    - drtgh 3y ago
      I updated velocity. I also added centripetal force become the weight of the satellite for to try to explain it better.
  - neaden 3y ago
    Yeah, the thing people miss about the space station isn't that it's high up, it is that it is fast. It orbits the earth roughly every 90 minutes! It is literally faster then a speeding bullet.

NoMoreNicksLeft 3y ago

Can the speed be safely increased once you leave the atmosphere and there is no (appreciable) drag? With such a system in place, are there no other interesting places to stop along the way to geosync?

midoridensha 3y ago

The other problem people seem to forget is radiation. The Van Allen belts expose people to a lot of radiation; normally it's not a problem, because either people stay under the belts (as in stations like the ISS which are at LEO), or for the few times humans went beyond LEO, to the Moon, they blasted through the belts very quickly and thus had negligible cumulative radiation exposure. But if you're sitting on a space elevator taking days to go through the VA belts, you'll probably get a fatal dose. Wikipedia has an article that touches on this here: https://en.wikipedia.org/wiki/Space_elevator_safety https://en.wikipedia.org/wiki/Space_elevator_safety

philwelch 3y ago

Yeah, I wish this went on as far as an actual space elevator would, perhaps with extra visualizations of the heights of various orbits. The top would be slightly above geostationary orbit, since you would need the center of mass to be at the geostationary point in order to work.

Sharlin 3y ago
Or more than slightly if you don't want to haul up a big counterweight but just use the cable itself as a counterweight. The Brad Edwards design proposes a total cable length of around 100,000 km. One very nice side effect of a long cable is that you can slingshot spacecraft from the far end up to ~Jupiter's orbit with zero propellant needed.
- philwelch 3y ago
  I would imagine rather than hauling up a big counterweight, you might be better served capturing a near-earth asteroid and hauling it in.
  - Sharlin 3y ago
    Yes. However, capturing a NEO might still be more difficult than just using a longer tether.

LeanderK 3y ago

but do you need to reach geostationary? couldn't you just switch to a rocket at this stage? Maybe a small one to bridge the gap? I would have thought that the first kilometres are the ones where you have to invest the most work so if you just bridge them using the space elevator it would be still an improvement. Maybe for cargo the 20 days are not really important

midasuni 3y ago
Getting to 100km or even 200km is easy. Staying there is hard. 80% of the energy in a rocket goes to adding sideways motion. An elevator upto GEO adds enough energy (taken from the earths rotational energy) to your payload to get it into orbit. If you let go lower down you need to add sideways velocity. In LEO you on an elevator would be travelling about 1,400mph and need to get to 18,000 to get into orbit. Sure there’s no atmosphere to work against but it’s still a lot of fuel to use in a rocket.
- metalliqaz 3y ago
  > 80% of the energy in a rocket goes to adding sideways motion. I have not researched this at all, so I'm not doubting you, but doesn't most of the energy go into accelerating the remaining fuel? So, by the rocket equation, shouldn't you be way ahead of the game by starting at the edge of space?
  - jandrese 3y ago
    You still need to accelerate the fuel at that point. Free travel up to LEO doesn't buy you much. There is an excellent What-if on this: https://web.archive.org/web/20230211050159/https://what-if.xkcd.com/58/ https://web.archive.org/web/20230211050159/https://what-if.x...
- outworlder 3y ago
  > In LEO you on an elevator would be travelling about 1,400mph That's a pretty significant speed already and it helps that you are already in space. See how tiny a Pegasus rocket is, and that one is launched from aircraft, slower, and nowhere near as high as what we are discussing. https://www.northropgrumman.com/space/pegasus-rocket/ https://www.northropgrumman.com/space/pegasus-rocket/ Getting to 100km from ground requires beefy first (even second) stages because you need to accelerate further stages plus the fuel, inside the atmosphere. We can basically pretend we are starting at the second or third stages. It now up to the rocket equation. So, to recap, there's already 1400mph of sideways motion, at a negligible air resistance, 100 to 200km up with vacuum optimized nozzles. It is a massive win. The question is if we could hoist what are still relatively heavy payloads.

nivenkos 3y ago

At that point gravity will act slower though, so it might be enough to take up a small liquid fuel craft too. Rockets will have abandoned solid fuel boosters long before then.

drtgh 3y ago
If you take a lift to a distance of 100 km and drop it, it will fall immediately reaching the ground in less than 10 minutes. Gravity remains strong. Satellites and space stations are always in free fall. The thing that keeps them in "space" is the velocity with which they were originally launched to orbit the Earth as centripetal force becomes the weight of the satellite, which slows down such falling process. Seeing it with cold eyes, they employ all the energy at a first for to lift the weight of that falling object. If a geostationary satellite has to stay over the same region of the planet, it has to be sent much much far away, until its orbital velocity become synchronized with the Earth's rotation. And even like that, keeps being in free fall due Earth's gravity. I mean, what we see in cartoons or sci-movies about to send a rocket absolutely vertical and cut engines, or a spaceship joining over a planet and conserve the same orbital position, could be done only with anti-gravitational engines. A small liquid fuel could not maintain them in place.
- concordDance 3y ago
  > The thing that keeps them in "space" is the acceleration with which they were originally launched to orbit the Earth, which slows down such falling process. It would be more clear to say that what keeps them in space is that they're moving sideways so fast. Velocity, not acceleration, is the right term here.
  - drtgh 3y ago
    I updated velocity. I also added centripetal force become the weight of the satellite for to try to explain it better.
- neaden 3y ago
  Yeah, the thing people miss about the space station isn't that it's high up, it is that it is fast. It orbits the earth roughly every 90 minutes! It is literally faster then a speeding bullet.