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I have only been following space elevator research cursorily, but it seems to me that this proposal lacks the most compelling reason for building a space elevat

I have only been following space elevator research cursorily, but it seems to me that this proposal lacks the most compelling reason for building a space elevator: breaking the bonds of Earth's gravity well. The most expensive part of space travel now is getting from the earth's surface to a stable orbit, and a space elevator from the moon does exactly nothing to reduce this cost. It does still have some benefits, i.e. if I'm not mistaken you could still use this elevator to "slingshot" spacecrafts on interplanetary trips, and it would make it cheaper to get to and return from the moon. But it's unclear (to me) whether these benefits could actually be realized when the cost to get to the space elevator in the first place is still so prohibitive. I guess what I'm saying is that this is just the cost analysis, not the full cost/benefit analysis. The costs may be a lot less than a space elevator from the surface of the earth, but the benefits are a lot smaller too.

skj 7y ago
> and a space elevator from the moon does nothing to reduce this cost. How do you figure? My extensive Kerbal Space Program experience tells me that freeing oneself from the moon's gravity well is far cheaper than doing the same from the earth, and then you're approximately MGH of the way to freeing yourself from Earth's well.
- luxuryballs 7y ago
  Perhaps you jest but clearly we can’t launch anything from the moon until we bring it there, from Earth.
  - flopunctro 7y ago
    Unless it's something that was mined there, like Helium-3 or some other mineral resource that we may find it's cheaper to extract from the moon.
    - wallace_f 7y ago
      You need more dv to get to the moon than to leo. An elevator helps with this.
    - ElFitz 7y ago
      Or (I hope people have a sense of humor here), space Nazis [1] [1] https://en.m.wikipedia.org/wiki/Iron_Sky https://en.m.wikipedia.org/wiki/Iron_Sky
  - himlion 7y ago
    But once we have this space elevator it's much more feasible to manufacture on the moon.
    - Retric 7y ago
      The moon has negligible atmosphere, so you can use a relatively cheap electromagnetic gun to reach the moon’s escape velocity. If all your doing is sending raw material back that’s a cheaper option.
      - Ajedi32 7y ago
        True. But if you're doing raw material extraction on the moon, you probably also want a cheap way to bring mining equipment down to the lunar surface.
        mLuby 7y ago
        Just run the rail gun in reverse! (Just no fragile humans please unless it's a very long barrel.)
      - zaarn 7y ago
        If the equipment is G-force sensitive, an elevator could launch science equipment that previously couldn't be put into space at all.
        MagnumOpus 7y ago
        > science equipment that previously couldn't be put into space at all. The large amount of science equipment manufactured on the moon?
        zaarn 7y ago
        The large amount of science equipment that could be manufacturer or assembled on the moon.
      - 7y ago
- leereeves 7y ago
  Launching from the moon would be easier and cheaper than launching from the Earth, unless you include the cost of reaching the moon. But according to the table in [1], going from the Earth's surface to geostationary orbit (to rendezvous with a lunar elevator) requires about 14km/s delta-v, and from there to the moon's surface only about 4km/s delta-v. Of the 18km/s total delta-v required, a lunar space elevator only eliminates the last 4km/s. 1: https://en.wikipedia.org/wiki/Delta-v_budget#Earth%E2%80%93Moon_space%E2%80%94high_thrust https://en.wikipedia.org/wiki/Delta-v_budget#Earth%E2%80%93M...
  - Ajedi32 7y ago
    I haven't done the math, but are you sure that last 4km/s isn't way more significant than you're estimating due to the rocket equation? A small mass savings near the end of the journey might translate to significant mass savings near the start.
  - raducu 7y ago
    Also I think (a series of) rotating elevators to get from LEO to GSO are also possible?
  - dwaltrip 7y ago
    The end of the lunar space elevator wouldn't be traveling at geostationary orbital speed, it would be going 28x slower. A rocket could sync with the end of the elevator at this lower speed, which would significantly reduce the delta-v budget. [1] Not sure how practical all of this, but that is at least the theory. [1] Credit to mnw21cam: https://news.ycombinator.com/item?id=20996246 https://news.ycombinator.com/item?id=20996246
  - s1artibartfast 7y ago
    The really interesting part is that the elevator would not be in a true geostationary orbit traveling at ~3000m/s. instead it would be traveling at 100m/s at the same height. The delta-V to reach GEO without the radial velocity to maintain it is closer to 10 km/s if you use the partial escape velocity equation. This is pretty huge as mass required to achieve delta-V is nonlinear.
scardine 7y ago
Leaving moon's gravity well may be cheaper than getting to space from earth but it is far from free. With a space elevator in the moon you can use raw material from the moon to assemble huge stations and ships in space.
- smrtinsert 7y ago
  If I have learned anything from sci fi that turned out to be true its that we shouldn't touch the moon at all.
- kerkeslager 7y ago
  That's a good point.
- Shivetya 7y ago
  Not a snark, but how long before the is some sort of preservationist, nationalist, or other, declaration that the moon isn't they to be exploited. I do not know how much is in place already with regards to regulation but you can be damn sure nations and people will be tripping over themselves once someone does find a means to make money using the moon for resources. the fantasies of space elevators appeal the geek/nerd in many of us but as a world we are far from the need of one if not too far from being united to having one. throw in there are just enough parties with the means to damage or destroy the ground side of one if ever built
  - Ajedi32 7y ago
    I don't think there are that many parties on the moon with the means to "destroy the ground side". In fact, as of right now there are actually zero. (Though that would probably change should a lunar space elevator be developed.)
  - logfromblammo 7y ago
    If they can get up there to chain themselves to the bulldozers, they can protest all they want.
  - scardine 7y ago
theothermkn 7y ago
> [T]his proposal lacks the most compelling reason for building a space elevator: breaking the bonds of Earth's gravity well. This proposal actually addresses exactly that problem. The dangly end of this thing down around GSO will not actually be in orbit at GSO. Rather, it will be hanging, stationary, at that point. In other words, you don't need to get 'into geostationary orbit.' Rather, you just need to lob yourself up so that your apoapsis is as high as GSO. Once there, you hook onto the little eye loop, or whatever fancier attachment system they devise, and wait to be pulled up to the Moon, more or less. The key thing here is that you don't have to expend all the delta-v to get into GSO, but rather just enough to lob you to height. This, in itself, is a tremendous savings, given that you're shaving velocity off of a very large delta-v, where the penalties from the rocket equation are most severe. However, even if you only got a free ride from actual GSO to the Moon, it would still be an important savings.
- ahelwer 7y ago
  Sounds like pretty high consequences if you miss the hook!
  - marcosdumay 7y ago
    The thing is moving at 300km/h up there. You'll have plenty of time to correct any miss.
- codeulike 7y ago
  It wouldn't exactly be stationary, it would be moving at about Mach 1 relative to the ground https://what-if.xkcd.com/157/ https://what-if.xkcd.com/157/
  - s1artibartfast 7y ago
    We currently dock with the ISS which is traveling 10X faster.
    - codeulike 7y ago
amalcon 7y ago
From a geostationary intercept, a payload can theoretically ride a cable climber (power only, no reaction mass) and gravity all the way to a circular lunar orbit. Per a sibling comment, this technically only requires a transfer orbit or less, not a GSO. This can be quite a good jumping off point to various places around the solar system, as many of these payloads will want a gravity assist from the moon anyway. The American Delta IV Heavy has a payload to GTO of 14,220kg, and a payload to TLI of 10000kg. This means that getting to geostationary orbit is responsible for about 70% of the cost of reaching the moon. The ratio is a bit worse for the Chinese Long March 5. These are the two currently operational rockets for which my source lists a TLI payload. 70% is indeed the majority of the cost, but cutting 30% from your cost is still a pretty big deal. source: https://en.wikipedia.org/wiki/Comparison_of_orbital_launch_systems https://en.wikipedia.org/wiki/Comparison_of_orbital_launch_s... edit: misread tables
mnw21cam 7y ago
> The most expensive part of space travel now is getting from the earth's surface to a stable orbit, and a space elevator from the moon does exactly nothing to reduce this cost. That's not the point, and not actually correct. [1] explains a lot of this. Getting to low earth orbit means you have to climb 100km, and go really fast. The climbing 100km part of this is easy - the going really fast bit is what makes going to space so expensive. With this proposal, there will be a tether hanging down at the height of geostationary orbit which is travelling slowly around the earth - about 28 times slower than the orbital speed at that altitude. So, the elevator makes getting to the Moon cheaper by two mechanisms. Firstly, it means that a rocket just needs to get to geostationary orbit height, without having to do the speed bit as well. Secondly, you have a space elevator to get you the rest of the way. Now, admittedly, geostationary height is much higher than low earth orbit, so the saving of not having to build up speed is not as extreme as if the tether hung lower. Geostationary orbit has a speed of 3.07km/s, so if you only have to go 1/28th of that, you are saving nearly 3km/s of delta-v on your rocket. This is not to be sniffed at. The size of the rocket required to transport a set payload is exponential with the required delta-v, with a logarithmic base of the exhaust velocity of the rocket. So, if we can save 3km/s, and a decent rocket motor has an exhaust velocity near 3km/s, then the size of the rocket can go down by a factor of about e. This is a very nice saving. One could extend the analogy, if one were to consider a slightly longer tether, and imagine we can get it to hang down to an altitude of 100km without hitting any satellites, then it would be travelling across the sky at around 60km/h. You could use a very small rocket indeed to climb up 100km, grab onto the end, and then go all the way to the moon. At a push, the X15 rocket-plane, plus some decent guidance system could manage it. [1] https://what-if.xkcd.com/58/ https://what-if.xkcd.com/58/
marcosdumay 7y ago
The elevator is not positioned in an orbit, so one does not need to reach any orbit to get there. Up to a point, sending things up is easier than sending them into orbit. Despite the really bad article that carries no information, I estimate the trip to the elevator takes ~9.5km/s (plus atmospheric losses). That isn't much more than LEO, and takes you all the way into the Moon.

skj 7y ago

> and a space elevator from the moon does nothing to reduce this cost. How do you figure? My extensive Kerbal Space Program experience tells me that freeing oneself from the moon's gravity well is far cheaper than doing the same from the earth, and then you're approximately MGH of the way to freeing yourself from Earth's well.

luxuryballs 7y ago
Perhaps you jest but clearly we can’t launch anything from the moon until we bring it there, from Earth.
- flopunctro 7y ago
  Unless it's something that was mined there, like Helium-3 or some other mineral resource that we may find it's cheaper to extract from the moon.
  - wallace_f 7y ago
    You need more dv to get to the moon than to leo. An elevator helps with this.
  - ElFitz 7y ago
    Or (I hope people have a sense of humor here), space Nazis [1] [1] https://en.m.wikipedia.org/wiki/Iron_Sky https://en.m.wikipedia.org/wiki/Iron_Sky
- himlion 7y ago
  But once we have this space elevator it's much more feasible to manufacture on the moon.
  - Retric 7y ago
    The moon has negligible atmosphere, so you can use a relatively cheap electromagnetic gun to reach the moon’s escape velocity. If all your doing is sending raw material back that’s a cheaper option.
    - Ajedi32 7y ago
      True. But if you're doing raw material extraction on the moon, you probably also want a cheap way to bring mining equipment down to the lunar surface.
      - mLuby 7y ago
        Just run the rail gun in reverse! (Just no fragile humans please unless it's a very long barrel.)
    - zaarn 7y ago
      If the equipment is G-force sensitive, an elevator could launch science equipment that previously couldn't be put into space at all.
      - MagnumOpus 7y ago
        > science equipment that previously couldn't be put into space at all. The large amount of science equipment manufactured on the moon?
        zaarn 7y ago
        The large amount of science equipment that could be manufacturer or assembled on the moon.
    - 7y ago
leereeves 7y ago
Launching from the moon would be easier and cheaper than launching from the Earth, unless you include the cost of reaching the moon. But according to the table in [1], going from the Earth's surface to geostationary orbit (to rendezvous with a lunar elevator) requires about 14km/s delta-v, and from there to the moon's surface only about 4km/s delta-v. Of the 18km/s total delta-v required, a lunar space elevator only eliminates the last 4km/s. 1: https://en.wikipedia.org/wiki/Delta-v_budget#Earth%E2%80%93Moon_space%E2%80%94high_thrust https://en.wikipedia.org/wiki/Delta-v_budget#Earth%E2%80%93M...
- Ajedi32 7y ago
  I haven't done the math, but are you sure that last 4km/s isn't way more significant than you're estimating due to the rocket equation? A small mass savings near the end of the journey might translate to significant mass savings near the start.
- raducu 7y ago
  Also I think (a series of) rotating elevators to get from LEO to GSO are also possible?
- dwaltrip 7y ago
  The end of the lunar space elevator wouldn't be traveling at geostationary orbital speed, it would be going 28x slower. A rocket could sync with the end of the elevator at this lower speed, which would significantly reduce the delta-v budget. [1] Not sure how practical all of this, but that is at least the theory. [1] Credit to mnw21cam: https://news.ycombinator.com/item?id=20996246 https://news.ycombinator.com/item?id=20996246
- s1artibartfast 7y ago
  The really interesting part is that the elevator would not be in a true geostationary orbit traveling at ~3000m/s. instead it would be traveling at 100m/s at the same height. The delta-V to reach GEO without the radial velocity to maintain it is closer to 10 km/s if you use the partial escape velocity equation. This is pretty huge as mass required to achieve delta-V is nonlinear.

scardine 7y ago

Leaving moon's gravity well may be cheaper than getting to space from earth but it is far from free. With a space elevator in the moon you can use raw material from the moon to assemble huge stations and ships in space.

smrtinsert 7y ago
If I have learned anything from sci fi that turned out to be true its that we shouldn't touch the moon at all.
kerkeslager 7y ago
That's a good point.
Shivetya 7y ago
Not a snark, but how long before the is some sort of preservationist, nationalist, or other, declaration that the moon isn't they to be exploited. I do not know how much is in place already with regards to regulation but you can be damn sure nations and people will be tripping over themselves once someone does find a means to make money using the moon for resources. the fantasies of space elevators appeal the geek/nerd in many of us but as a world we are far from the need of one if not too far from being united to having one. throw in there are just enough parties with the means to damage or destroy the ground side of one if ever built
- Ajedi32 7y ago
  I don't think there are that many parties on the moon with the means to "destroy the ground side". In fact, as of right now there are actually zero. (Though that would probably change should a lunar space elevator be developed.)
- logfromblammo 7y ago
  If they can get up there to chain themselves to the bulldozers, they can protest all they want.
- scardine 7y ago

theothermkn 7y ago

> [T]his proposal lacks the most compelling reason for building a space elevator: breaking the bonds of Earth's gravity well. This proposal actually addresses exactly that problem. The dangly end of this thing down around GSO will not actually be in orbit at GSO. Rather, it will be hanging, stationary, at that point. In other words, you don't need to get 'into geostationary orbit.' Rather, you just need to lob yourself up so that your apoapsis is as high as GSO. Once there, you hook onto the little eye loop, or whatever fancier attachment system they devise, and wait to be pulled up to the Moon, more or less. The key thing here is that you don't have to expend all the delta-v to get into GSO, but rather just enough to lob you to height. This, in itself, is a tremendous savings, given that you're shaving velocity off of a very large delta-v, where the penalties from the rocket equation are most severe. However, even if you only got a free ride from actual GSO to the Moon, it would still be an important savings.

ahelwer 7y ago
Sounds like pretty high consequences if you miss the hook!
- marcosdumay 7y ago
  The thing is moving at 300km/h up there. You'll have plenty of time to correct any miss.
codeulike 7y ago
It wouldn't exactly be stationary, it would be moving at about Mach 1 relative to the ground https://what-if.xkcd.com/157/ https://what-if.xkcd.com/157/
- s1artibartfast 7y ago
  We currently dock with the ISS which is traveling 10X faster.
  - codeulike 7y ago

amalcon 7y ago

From a geostationary intercept, a payload can theoretically ride a cable climber (power only, no reaction mass) and gravity all the way to a circular lunar orbit. Per a sibling comment, this technically only requires a transfer orbit or less, not a GSO. This can be quite a good jumping off point to various places around the solar system, as many of these payloads will want a gravity assist from the moon anyway. The American Delta IV Heavy has a payload to GTO of 14,220kg, and a payload to TLI of 10000kg. This means that getting to geostationary orbit is responsible for about 70% of the cost of reaching the moon. The ratio is a bit worse for the Chinese Long March 5. These are the two currently operational rockets for which my source lists a TLI payload. 70% is indeed the majority of the cost, but cutting 30% from your cost is still a pretty big deal. source: https://en.wikipedia.org/wiki/Comparison_of_orbital_launch_systems https://en.wikipedia.org/wiki/Comparison_of_orbital_launch_s... edit: misread tables

mnw21cam 7y ago

> The most expensive part of space travel now is getting from the earth's surface to a stable orbit, and a space elevator from the moon does exactly nothing to reduce this cost. That's not the point, and not actually correct. [1] explains a lot of this. Getting to low earth orbit means you have to climb 100km, and go really fast. The climbing 100km part of this is easy - the going really fast bit is what makes going to space so expensive. With this proposal, there will be a tether hanging down at the height of geostationary orbit which is travelling slowly around the earth - about 28 times slower than the orbital speed at that altitude. So, the elevator makes getting to the Moon cheaper by two mechanisms. Firstly, it means that a rocket just needs to get to geostationary orbit height, without having to do the speed bit as well. Secondly, you have a space elevator to get you the rest of the way. Now, admittedly, geostationary height is much higher than low earth orbit, so the saving of not having to build up speed is not as extreme as if the tether hung lower. Geostationary orbit has a speed of 3.07km/s, so if you only have to go 1/28th of that, you are saving nearly 3km/s of delta-v on your rocket. This is not to be sniffed at. The size of the rocket required to transport a set payload is exponential with the required delta-v, with a logarithmic base of the exhaust velocity of the rocket. So, if we can save 3km/s, and a decent rocket motor has an exhaust velocity near 3km/s, then the size of the rocket can go down by a factor of about e. This is a very nice saving. One could extend the analogy, if one were to consider a slightly longer tether, and imagine we can get it to hang down to an altitude of 100km without hitting any satellites, then it would be travelling across the sky at around 60km/h. You could use a very small rocket indeed to climb up 100km, grab onto the end, and then go all the way to the moon. At a push, the X15 rocket-plane, plus some decent guidance system could manage it. [1] https://what-if.xkcd.com/58/ https://what-if.xkcd.com/58/

marcosdumay 7y ago

The elevator is not positioned in an orbit, so one does not need to reach any orbit to get there. Up to a point, sending things up is easier than sending them into orbit. Despite the really bad article that carries no information, I estimate the trip to the elevator takes ~9.5km/s (plus atmospheric losses). That isn't much more than LEO, and takes you all the way into the Moon.