> So what good is it to lift a satellite that high if you will need to fire a large rocket at the top anyways? An electric motor used on a space elevator could be slightly more efficent than a traditional rocket engine (esp since it can work slowly to reduce irreversibilities), but this is negated by the extra work that will be needed to lift the rocket and its full fuel load to the correct altitude.
As a payload climbs, it picks up tangential velocity from the cable. Starts at 1050 mph (along with everything else) on the surface at the equator. It's not going nearly fast enough in the 200-600 mile LEO zone, but by the time it reaches GEO, it is in orbit just as that segment of cable is... with all the tangential component of its velocity borrowed from the angular momentum of the earth-cable-counterweight system.
(Yes, the math has been done, and at the proposed tensions there's more than enough restoring force to keep the cable taut and vertical.)
If you want LEO, use a small rocket to drop from GEO and circularize: the energy cost is well under 1% of that required to get the same mass from ground to LEO.
> I wish one of the elevator advocates would actually publish a more detailed analysis, showing their figures and assumptions instead of the hand-waving "it'll burn up in the atmosphere" claim.
And I wish I could hang on Slashdot and have all my answers spoon-fed, instead of tiring myself out with a few minutes on Google. But hey, we all need our dreams.
For starters, from http://www.isr.us/Downloads/niac_pdf/contents.html (online for several years, cited here frequently):
"If a cable is severed the lower segment will fall back to Earth while the upper portion floats outward. The worst case would be if the countermass breaks off the far end of the cable and the entire 91,000 km of cable falls back to Earth.
"Depending on the location of the break, the epoxy used, the dynamics of the fall, etc. the cable will re-enter the Earth's atmosphere at a velocity sufficient to heat the cable above several hundred degrees Celsius (figure 10.9.1). If the cable is designed properly, the epoxy in the cable composite will disintegrate at this temperature. This means the cable above a certain point will re-enter Earth's atmosphere in small segments or carbon nanotube / epoxy dust. About 3000 kg of 2 square millimeter crosssection cable (20 ton capacity) may fall to Earth intact and east of the anchor. Detailed simulations will be required to determine the possible sizes of segments that will survive and the health risks associated with carbon nanotube and epoxy dust. In terms of the mass of dust and debris that will be deposited, we can compare what will happen to what naturally happens now. Each year 10,000 tons of dust accrete onto Earth from space, the additional 750 tons of the first cable will increase that year's infall by 7.5%. A larger 1000-ton capacity cable would have a mass of 30,000 tons or roughly equivalent to 3 years of normal global dust accretion. Further investigations are required to determine the environmental impact of depositing this much dust along the Earth's equator."
The online version was expanded into 2002's "The Space Elevator" (try Amazon, revised edition now in the works)
For some detail on dynamics of breaks at various altitudes:
Less gee-whiz, but more impressive in its way, is how Clarke has gone out of his way to credit Tsiolkovsky, Artsutanov, McCarthy, Isaacs et al, Moravec, Pearson, and others who had and developed the idea before his novel...
>And if you're manufacturing the ribbon at the space end (which you'd probably like to, since most of the materials suggested are easier to make at a high quality in free fall)...
Handwavium detected. The only material known with adequate strength is single-walled carbon nanotubes -- maybe in a composite matrix, maybe braided, maybe continuous. The new length record for continuous low-defect CNTs is 4 cm; it's early days.
Nobody has done any work on CNT synthesis in free fall, and there's no prima facie reason to think that would make it easier: neither gravity nor gravity-driven convection is very relevant to the thermodynamics of hot carbon vapor.
You may be influenced by (1) earlier space elevator proposals (and the Red Mars treatment)with MUCH more massive cables, which favored using a carbonaceous asteroid as source and counterweight... and (2) a few decades of overselling about wonder materials made in free fall.
The latter may indeed become important once there's serious lab -> R&D -> manufacturing capability in orbit, but so far they're just tantalizing.
>The elevator ribbon has a very low mass per unit length (indeed, this is one of the characteristics that make the elevator physically possible, not just sci-fi)
Specifically: KSR's Mars cable is 10 m diameter, total mass 6 billion tons
Edwards' reference design: ribbon roughly 1 m by 1 mm, total mass ~700 tons
>The ribbon goes up as two full-length spools, which are bonded together as they are unspooled to create a single initial ribbon with a 20 ton lift capacity.
Nope -- the initial ribbon is much weaker, and is built up to 20-ton capacity over ~2+ years by climbers with their own spools bonding additional strips.
Yes, the 200-kph climbers in the Edwards scenario would spend ~4 days in the Van Allen belts -- for which "substantial" shielding could mean 10-12" of metal. (Apollo astronauts went through in 30 minutes).
Looks like freight-only until the climbers are much, much faster.
>It's raising the several billion dollars when any revenue is 10-15 years off that's proving to be the problem.
The critical factor "gating" the space elevator is the strength of bulk CNT material. IMHO research on that is already reasonably well funded -- i.e. it's keeping busy most of the people qualified to advance such research. It will snowball as strength approaches levels attractive for extreme engineering here on earth.
THAT will be the time to start raising billions. Until then, it makes sense to spend some millions on R&D for things other than the ribbon cable itself.
>We had "graphite whiskers" 48 years ago that had 20 GPa and we can not make strong ropes of these yet. These are easier to bind to than nanotubes. So 2 years is just much too optimistic for 100 GPa.
Two years is definitely too optimistic. But the comparison to graphite whiskers is (by implication) too pessimistic. CNTs by their nature tend to grow much, much longer w/r/t their diameter than "flakes" of graphene do w/r/t their width... i.e., it's energetically much more favorable for new C atoms to just keep popping into place on the end than to go anywhere else.
That means they "want to be" really, really long... even macroscopically long with very few imperfections. And preserving native bond strength as much as possible over long ranges is ultimately what strength of materials is about.
Nanotubes hard to bind to? Sure -- the high native strength is there precisely because nearly all the C bonding capability is used up, leaving only VdW forces. So you'll have to functionalize the CNT surface: replace an occasional C atom with one that's part of a "hook" molecule that a composite matrix can get a grip on. People are doing that. The challenge is in quantifying "occasional" -- placing enough hooks for good stress transfer while not weakening the CNT too much.
We'll just have to see how long that takes. But it's quite a different challenge from working with graphite whiskers or any other single-crystal-like material so far, because -- again -- CNTs "want to be long" much, much more than any of them do.
> Until they figure out how to cope with all that the space elevators will be cargo lifters, nothing more.
Agreed, although a 90% reduction in cargo lift cost along with vastly increased total lift (24/7/365 service) is a huge payoff in itself.
Depending on how much of the early SEs' capacity you use for replication, it's striking how fast you could ramp up to many small ones -- or to bigger ones that *can* handle the mass of shielding -- while driving toward a 99% reduction in lift cost.
In principle you could hop off in the LEO range, below the Van Allen belts, and thrust up to orbital velocity there. What's the trade-off between the mass of the thruster needed for that... and the mass of shielding plus (much smaller) thruster needed to ride higher, then drop and circularize?
My theory is that the first human-carrying climber to GEO will feature a big igloo on the cargo bed, with the ice stopping radiation in the Van Allen belts -- and useful as a heat dump or reaction mass for projects at the top.
Sooner or later, SEs will become strong enough for slick ultra-fast climbers like those in the 1999 NASA study , but it's foolish to make them a prerequisite for building the first generation.
(1) if you're scaling up to make 24,000 miles of it, presumably the cost of 38,000 miles more should be a relatively small increment.
(2)you have to pay Shuttle-level rates to lift *whatever* mass serves as the counterweight -- so why not make that mass extremely useful? (I.e. the length beyond geosync gives you several kps free on the way to Moon, Mars, etc. when you detach at the far end.)
> And what if the tether breaks and drops on Earth: a wire so small you can almost not see it, but stronger than any other material. It will acts as a knife and cut through almost everything.
Yessir, a thin strong tether under tension would cut...
Ooops, you said "if the tether breaks," didn't you? No tension.
> Something I never heared anybody about: Where does the kinetic energy come from that the cargo gains when ascending into orbit?
You're swinging a weighted rope around your head. An ant crawls out your arm, then out to the end of the rope -- where it's moving at tip speed. Where did its kinetic enregy come from?
From slowing your rotation down a tiny, tiny, tiny bit.
>The devastating space elevator fall is bad science fiction.
Hardly fair to Kim Stanley Robinson: given the comparatively massive Mars SE cable he posited, his physics was good enough for SF.
What's "bad" is applying that scenario to a low-mass ribbon.
Based on a presentation at the SE conference this week, you're right about the Van Allen belts. Most likely humans will still use rockets until the 200-ton and bigger elevators are running (could be just a couple of years after the first one, depending on how much capacity you sell vs. lifting material to build bigger).
At that point, instead of a sleek Bullet Train to Heaven, you may see a big "igloo" of ice maybe 2' thick, which should be adequate shielding.
Lots of water will be handy in orbit in any case. Beyond drinking and shielding, it makes a good dump if you don't want HUGE radiators...
Slashdot Mirror
> So what good is it to lift a satellite that high if you will need to fire a large rocket at the top anyways? An electric motor used on a space elevator could be slightly more efficent than a traditional rocket engine (esp since it can work slowly to reduce irreversibilities), but this is negated by the extra work that will be needed to lift the rocket and its full fuel load to the correct altitude.
As a payload climbs, it picks up tangential velocity from the cable. Starts at 1050 mph (along with everything else) on the surface at the equator. It's not going nearly fast enough in the 200-600 mile LEO zone, but by the time it reaches GEO, it is in orbit just as that segment of cable is... with all the tangential component of its velocity borrowed from the angular momentum of the earth-cable-counterweight system.
(Yes, the math has been done, and at the proposed tensions there's more than enough restoring force to keep the cable taut and vertical.)
If you want LEO, use a small rocket to drop from GEO and circularize: the energy cost is well under 1% of that required to get the same mass from ground to LEO.
> I wish one of the elevator advocates would actually publish a more detailed analysis, showing their figures and assumptions instead of the hand-waving "it'll burn up in the atmosphere" claim.
l (online for several years, cited here frequently):
/ br eaks/index.html
And I wish I could hang on Slashdot and have all my answers spoon-fed, instead of tiring myself out with a few minutes on Google. But hey, we all need our dreams.
For starters, from http://www.isr.us/Downloads/niac_pdf/contents.htm
"If a cable is severed the lower segment will fall back to Earth while the upper portion floats outward. The worst case would be if the countermass breaks off the far end of the cable and the entire 91,000 km of cable falls back to Earth.
"Depending on the location of the break, the epoxy used, the dynamics of the fall, etc. the cable will re-enter the Earth's atmosphere at a velocity sufficient to heat the cable above several hundred degrees Celsius (figure 10.9.1). If the cable is designed properly, the epoxy in the cable composite will disintegrate at this temperature. This means the cable above a certain point will re-enter Earth's atmosphere in small segments or carbon nanotube / epoxy dust. About 3000 kg of 2 square millimeter crosssection cable (20 ton capacity) may fall to Earth intact and east of the anchor. Detailed simulations will be required to determine the possible sizes of segments that will survive and the health risks associated with carbon nanotube and epoxy dust. In terms of the mass of dust and debris that will be deposited, we can compare what will happen to what naturally happens now. Each year 10,000 tons of dust accrete onto Earth from space, the additional 750 tons of the first cable will increase that year's infall by 7.5%. A larger 1000-ton capacity cable would have a mass of 30,000 tons or roughly equivalent to 3 years of normal global dust accretion. Further investigations are required to determine the environmental impact of depositing this much dust along the Earth's equator."
The online version was expanded into 2002's "The Space Elevator" (try Amazon, revised edition now in the works)
For some detail on dynamics of breaks at various altitudes:
http://www.mit.edu/people/gassend/spaceelevator
Bottom line: please sketch a scenario in which ~100,000 km of ribbon all hits the atmosphere in ten seconds? You are not allowed to crank up _g_.
IOW, try getting within 3-5 orders of magnitude yourself before accusing others of hand-waving...
Less gee-whiz, but more impressive in its way, is how Clarke has gone out of his way to credit Tsiolkovsky, Artsutanov, McCarthy, Isaacs et al, Moravec, Pearson, and others who had and developed the idea before his novel...
http://www.islandone.org/LEOBiblio/CLARK1.HTM
> Spider Robinson's book, "The Web Between the Worlds", is quite enligtening.
;-)
Chareles Sheffield's is even better
>And if you're manufacturing the ribbon at the space end (which you'd probably like to, since most of the materials suggested are easier to make at a high quality in free fall)...
Handwavium detected. The only material known with adequate strength is single-walled carbon nanotubes -- maybe in a composite matrix, maybe braided, maybe continuous. The new length record for continuous low-defect CNTs is 4 cm; it's early days.
Nobody has done any work on CNT synthesis in free fall, and there's no prima facie reason to think that would make it easier: neither gravity nor gravity-driven convection is very relevant to the thermodynamics of hot carbon vapor.
You may be influenced by (1) earlier space elevator proposals (and the Red Mars treatment)with MUCH more massive cables, which favored using a carbonaceous asteroid as source and counterweight... and (2) a few decades of overselling about wonder materials made in free fall.
The latter may indeed become important once there's serious lab -> R&D -> manufacturing capability in orbit, but so far they're just tantalizing.
>The elevator ribbon has a very low mass per unit length (indeed, this is one of the characteristics that make the elevator physically possible, not just sci-fi)
Specifically: KSR's Mars cable is 10 m diameter, total mass 6 billion tons
Edwards' reference design: ribbon roughly 1 m by 1 mm, total mass ~700 tons
>The ribbon goes up as two full-length spools, which are bonded together as they are unspooled to create a single initial ribbon with a 20 ton lift capacity.
Nope -- the initial ribbon is much weaker, and is built up to 20-ton capacity over ~2+ years by climbers with their own spools bonding additional strips.
Yes, the 200-kph climbers in the Edwards scenario would spend ~4 days in the Van Allen belts -- for which "substantial" shielding could mean 10-12" of metal. (Apollo astronauts went through in 30 minutes).
Looks like freight-only until the climbers are much, much faster.
>It's raising the several billion dollars when any revenue is 10-15 years off that's proving to be the problem.
The critical factor "gating" the space elevator is the strength of bulk CNT material. IMHO research on that is already reasonably well funded -- i.e. it's keeping busy most of the people qualified to advance such research. It will snowball as strength approaches levels attractive for extreme engineering here on earth.
THAT will be the time to start raising billions. Until then, it makes sense to spend some millions on R&D for things other than the ribbon cable itself.
Elevator cable in Red Mars: ~6 billion tons
Elevator cable in Edwards proposal: ~700 tons
Do the math...
>We had "graphite whiskers" 48 years ago that had 20 GPa and we can not make strong ropes of these yet. These are easier to bind to than nanotubes. So 2 years is just much too optimistic for 100 GPa.
Two years is definitely too optimistic. But the comparison to graphite whiskers is (by implication) too pessimistic. CNTs by their nature tend to grow much, much longer w/r/t their diameter than "flakes" of graphene do w/r/t their width... i.e., it's energetically much more favorable for new C atoms to just keep popping into place on the end than to go anywhere else.
That means they "want to be" really, really long... even macroscopically long with very few imperfections. And preserving native bond strength as much as possible over long ranges is ultimately what strength of materials is about.
Nanotubes hard to bind to? Sure -- the high native strength is there precisely because nearly all the C bonding capability is used up, leaving only VdW forces. So you'll have to functionalize the CNT surface: replace an occasional C atom with one that's part of a "hook" molecule that a composite matrix can get a grip on. People are doing that. The challenge is in quantifying "occasional" -- placing enough hooks for good stress transfer while not weakening the CNT too much.
We'll just have to see how long that takes. But it's quite a different challenge from working with graphite whiskers or any other single-crystal-like material so far, because -- again -- CNTs "want to be long" much, much more than any of them do.
> Until they figure out how to cope with all that the space elevators will be cargo lifters, nothing more.
Agreed, although a 90% reduction in cargo lift cost along with vastly increased total lift (24/7/365 service) is a huge payoff in itself.
Depending on how much of the early SEs' capacity you use for replication, it's striking how fast you could ramp up to many small ones -- or to bigger ones that *can* handle the mass of shielding -- while driving toward a 99% reduction in lift cost.
In principle you could hop off in the LEO range, below the Van Allen belts, and thrust up to orbital velocity there. What's the trade-off between the mass of the thruster needed for that... and the mass of shielding plus (much smaller) thruster needed to ride higher, then drop and circularize?
My theory is that the first human-carrying climber to GEO will feature a big igloo on the cargo bed, with the ice stopping radiation in the Van Allen belts -- and useful as a heat dump or reaction mass for projects at the top.
Sooner or later, SEs will become strong enough for slick ultra-fast climbers like those in the 1999 NASA study , but it's foolish to make them a prerequisite for building the first generation.
No doubt the cable will be expensive, but
(1) if you're scaling up to make 24,000 miles of it, presumably the cost of 38,000 miles more should be a relatively small increment.
(2)you have to pay Shuttle-level rates to lift *whatever* mass serves as the counterweight -- so why not make that mass extremely useful? (I.e. the length beyond geosync gives you several kps free on the way to Moon, Mars, etc. when you detach at the far end.)
> And what if the tether breaks and drops on Earth: a wire so small you can almost not see it, but stronger than any other material. It will acts as a knife and cut through almost everything.
Yessir, a thin strong tether under tension would cut...
Ooops, you said "if the tether breaks," didn't you? No tension.
> Something I never heared anybody about: Where does the kinetic energy come from that the cargo gains when ascending into orbit?
You're swinging a weighted rope around your head. An ant crawls out your arm, then out to the end of the rope -- where it's moving at tip speed. Where did its kinetic enregy come from?
From slowing your rotation down a tiny, tiny, tiny bit.
> Not stupid, but boring. It just doesn't feel like bona fide space travel to me.
That's the idea. A lot of us *want* space travel to become boring.
It's "exciting" that in 43 years since Gagarin, fewer than 500 human beings have been to LEO or above.
It's "exciting" that we're so thrilled to see Spaceship One do what X-15s were doing 45 years ago (albeit more elegantly, with private money).
It's "exciting" that a Shuttle launch costs $350M-$500M instead of the $10M-$20M hoped for in the 1970s.
Enough excitement, OK?
>The devastating space elevator fall is bad science fiction. Hardly fair to Kim Stanley Robinson: given the comparatively massive Mars SE cable he posited, his physics was good enough for SF. What's "bad" is applying that scenario to a low-mass ribbon.
>Not for free. The momentum has to come from somewhere...
Yes, it's a tiny subtraction from the angular momentum of the earth+SE system. Six sextillion rotating tons.
Close enough to "free" for me.
Based on a presentation at the SE conference this week, you're right about the Van Allen belts. Most likely humans will still use rockets until the 200-ton and bigger elevators are running (could be just a couple of years after the first one, depending on how much capacity you sell vs. lifting material to build bigger). At that point, instead of a sleek Bullet Train to Heaven, you may see a big "igloo" of ice maybe 2' thick, which should be adequate shielding. Lots of water will be handy in orbit in any case. Beyond drinking and shielding, it makes a good dump if you don't want HUGE radiators...