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Author Topic: Space Elevator - Launch Pad  (Read 1289 times)
Maets
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« on: August 18, 2015, 02:24:05 am »

An elevated launch pad for trips into space with a lot less fuel required.

http://www.huffingtonpost.com/entry/inflatable-space-elevator_55d1f855e4b055a6dab0cb7d?utm_hp_ref=science&kvcommref=mostpopular





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« Reply #1 on: August 18, 2015, 02:55:45 am »

I suspect it'll take some time for materials science to catch up to this idea.
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« Reply #2 on: August 18, 2015, 02:40:23 pm »

But way more $$$.
And insurance.
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« Reply #3 on: August 18, 2015, 05:17:59 pm »

I'm not entirely convinced by this.

Firstly the idea that you save energy by starting higher is a bit dubious as you still need to get your vehicle and payload there in the first place and that takes a minimum amount of energy however you do it. Whether a 20 mile winch is a more efficient way to do this than a rocket is debatable with currently available materials.

The other issue is that the real issue with getting into space is not gaining altitude, which you can do easily with a balloon in any case, but achieving the required orbital velocity to stay there.

For a spacecraft to stay in orbit and not just fall back down to earth it needs to be orbiting fast enough that its centrifugal acceleration balances out acceleration due to gravity.

Astronauts in space stations experience weightlessness not because there is no force of gravity acting on them but because they are in permanent free fall.

A true space elevator needs to extend to at least the geosynchronous orbit altitude, which is some 35,000km and be anchored at the equator, at this point  it is travelling fast enough to achieve a stable orbit.
« Last Edit: August 18, 2015, 07:58:46 pm by Narsil » Logged







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« Reply #4 on: August 18, 2015, 08:19:27 pm »

I'm not entirely convinced by this.

Firstly the idea that you save energy by starting higher is a bit dubious as you still need to get your vehicle and payload there is the first place and that takes a minimum amount of energy however you do it.
Actually, you would save energy. Remember a rocket has to carry enough fuel to lift itself and its cargo, and enough additional fuel to lift the weight of the fuel. by using an electric elevator you're eliminating 100% of the fuel weight along with the energy needed to lift it.

The other issue is that the real issue with getting into space is not gaining altitude, which you can do easily with a balloon in any case, but achieving the required orbital velocity to stay there.

For a spacecraft to stay in orbit and not just fall back down to earth it needs to be orbiting fast enough that its centrifugal acceleration balances out acceleration due to gravity.

Astronauts in space stations experience weightlessness not because there is no force of gravity acting on them but because they are in permanent free fall.
You also have to overcome aerodynamic drag, which is a function of air density, which is a function of altitude. By starting your launch at 20 000 m altitude, you've eliminated a huge portion of that drag along with the fuel needed to overcome it. This is why the balloon-launched rocket system is still being researched.

A true space elevator needs to extend to at least the geosynchronous orbit altitude, which is some 35,000km and be anchored at the equator, at this point  it is travelling fast enough to achieve a stable orbit.
I don't think they're claiming this is a "true" space elevator, as in The Fountains of Paradise; they're proposing to use this almost exactly like a balloon-launch system with a much greater lift capacity.
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Maets
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« Reply #5 on: August 18, 2015, 09:40:44 pm »

The big thing is that the energy to lift the rocket that first 20,000 is not carried by the rocket but supplied as needed from the ground.  Same goes for the fuel needed for the first lift.  Effectively unlimited energy available from the ground at no added cost to the rocket requirements.
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« Reply #6 on: August 18, 2015, 10:37:19 pm »

Pretty interesting, people talking about 20-mile cables, considering the size they'd have to be, and the equipment you'd have to build to use it; not to mention the cable's own weight. 20 miles of something that big is going to be pretty danged heavy. You'd have to haul that weight, plus the weight of whatever you're lifting. Not to mention swaying, and structure flex.
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« Reply #7 on: August 18, 2015, 11:11:10 pm »

Pretty interesting, people talking about 20-mile cables, considering the size they'd have to be, and the equipment you'd have to build to use it; not to mention the cable's own weight. 20 miles of something that big is going to be pretty danged heavy. You'd have to haul that weight, plus the weight of whatever you're lifting. Not to mention swaying, and structure flex.


It won't be a cable-hoist lift - as you say, enough elevator cable for a 20-km shaft would be completely unmanageable. Elevator designs on this scale almost universally use self-propelled "climber" cars which drive themselves up and down a vertical track and draw power through a 3rd-rail or inductive-coupled supply system. The several hundred km of elevator cable that would be required would actually outweigh a self-propelled climber by at least an order of magnitude.

As to sway and flex, they're talking of a dynamic skeleton for the structure, with tendons that can be rapidly tensed or slacked to resist movement of the tower.
« Last Edit: August 18, 2015, 11:15:06 pm by von Corax » Logged
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« Reply #8 on: August 19, 2015, 03:28:19 am »

Pretty interesting, people talking about 20-mile cables, considering the size they'd have to be, and the equipment you'd have to build to use it; not to mention the cable's own weight. 20 miles of something that big is going to be pretty danged heavy. You'd have to haul that weight, plus the weight of whatever you're lifting. Not to mention swaying, and structure flex.


It won't be a cable-hoist lift - as you say, enough elevator cable for a 20-km shaft would be completely unmanageable. Elevator designs on this scale almost universally use self-propelled "climber" cars which drive themselves up and down a vertical track and draw power through a 3rd-rail or inductive-coupled supply system. The several hundred km of elevator cable that would be required would actually outweigh a self-propelled climber by at least an order of magnitude.

As to sway and flex, they're talking of a dynamic skeleton for the structure, with tendons that can be rapidly tensed or slacked to resist movement of the tower.

Correct me if I'm wrong but I don't think we currently have a material, such as steel, or synthetic fibre which has enough tensile strength to span this length of vertical distance.

https://en.wikipedia.org/wiki/Space_elevator
Quote
A space elevator is a proposed type of space transportation system.[1] Its main component is a ribbon-like cable (also called a tether) anchored to the surface and extending into space. It is designed to permit vehicle transport along the cable from a planetary surface, such as the Earth's, directly into space or orbit, without the use of large rockets. An Earth-based space elevator would consist of a cable with one end attached to the surface near the equator and the other end in space beyond geostationary orbit (35,800 km altitude). The competing forces of gravity, which is stronger at the lower end, and the outward/upward centrifugal force, which is stronger at the upper end, would result in the cable being held up, under tension, and stationary over a single position on Earth. Once the tether is deployed, climbers would repeatedly climb the tether to space by mechanical means, releasing their cargo to orbit. Climbers would also descend the tether to return cargo to the surface from orbit.[2]

The concept of a space elevator was first published in 1895 by Konstantin Tsiolkovsky.[3] His proposal was for a free-standing tower reaching from the surface of Earth to the height of geostationary orbit. Like all buildings, Tsiolkovsky's structure would be under compression, supporting its weight from below. Since 1959, most ideas for space elevators have focused on purely tensile structures, with the weight of the system held up from above. In the tensile concepts, a space tether reaches from a large mass (the counterweight) beyond geostationary orbit to the ground. This structure is held in tension between Earth and the counterweight like an upside-down plumb bob.

On Earth, with its relatively strong gravity, the required specific strength for the cable material is very high. Current technology is not capable of manufacturing cable materials that are both strong and light enough for a space elevator on Earth. However, in 2000, the recently discovered carbon nanotubes were first identified as possibly being able to meet the specific strength requirements for an Earth space elevator.[2] This sparked a surge of interest and development in space elevators focusing on carbon nanotubes and the similar boron nitride nanotubes. In 2014, diamond nanothreads were first synthesized.[4] Since they have strength properties similar to carbon nanotubes, diamond nanothreads were quickly seen as a candidate material as well.[5] Nanotubes and diamond nanothreads both hold promise as materials to make an Earth-based space elevator possible.

There's also the problem of orbiting debris.  At geostationary orbit that is not much of a problem, since the "cable" of the elevator is moving at the same speed as objects in geosynchronous orbit. But below that orbit objects move faster, and by low Earth orbit, they move much faster than the speed of the cable (local tangential speed = height*radius, as radius goes to zero, so does the tangential speed).

Impact from flying debris becomes a huge headache, and you may find yourself in a shooting gallery from hell.

Quote
In 2000, another American scientist, Bradley C. Edwards, suggested creating a 100,000 km (62,000 mi) long paper-thin ribbon using a carbon nanotube composite material.[12] He chose the wide-thin ribbon-like cross-section shape rather than earlier circular cross-section concepts because that shape would stand a greater chance of surviving impacts by meteoroids. The ribbon cross-section shape also provided large surface area for climbers to climb with simple rollers. Supported by the NASA Institute for Advanced Concepts, Edwards' work was expanded to cover the deployment scenario, climber design, power delivery system, orbital debris avoidance, anchor system, surviving atomic oxygen, avoiding lightning and hurricanes by locating the anchor in the western equatorial Pacific, construction costs, construction schedule, and environmental hazards.
« Last Edit: August 19, 2015, 03:42:04 am by J. Wilhelm » Logged

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« Reply #9 on: August 19, 2015, 04:04:15 am »

I'm not entirely convinced by this.

Firstly the idea that you save energy by starting higher is a bit dubious as you still need to get your vehicle and payload there is the first place and that takes a minimum amount of energy however you do it.
Actually, you would save energy. Remember a rocket has to carry enough fuel to lift itself and its cargo, and enough additional fuel to lift the weight of the fuel. by using an electric elevator you're eliminating 100% of the fuel weight along with the energy needed to lift it.

The other issue is that the real issue with getting into space is not gaining altitude, which you can do easily with a balloon in any case, but achieving the required orbital velocity to stay there.

For a spacecraft to stay in orbit and not just fall back down to earth it needs to be orbiting fast enough that its centrifugal acceleration balances out acceleration due to gravity.

Astronauts in space stations experience weightlessness not because there is no force of gravity acting on them but because they are in permanent free fall.
You also have to overcome aerodynamic drag, which is a function of air density, which is a function of altitude. By starting your launch at 20 000 m altitude, you've eliminated a huge portion of that drag along with the fuel needed to overcome it. This is why the balloon-launched rocket system is still being researched.

A true space elevator needs to extend to at least the geosynchronous orbit altitude, which is some 35,000km and be anchored at the equator, at this point  it is travelling fast enough to achieve a stable orbit.
I don't think they're claiming this is a "true" space elevator, as in The Fountains of Paradise; they're proposing to use this almost exactly like a balloon-launch system with a much greater lift capacity.

True on both counts.  The ideal multi-stage rocket is a cone with an infinite number of stages which it sheds as it gains altitude. Carrying the fuel is a major problem.

The second problem is accelerating through a dense atmosphere.  Hence all the plans for multi-stage to orbit space planes in the past decades.  Insisting on a single stage to orbit is a pretty difficult task *cough cough -X33- cough cough*.
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« Reply #10 on: August 21, 2015, 04:09:12 pm »

I wonder if it would be possible to build one that would intentionally flex and sway in the middle and stabilize on top...
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« Reply #11 on: August 21, 2015, 04:23:59 pm »

I wonder if it would be possible to build one that would intentionally flex and sway in the middle and stabilize on top...

have the resonance right that way that the knot is at the top?
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« Reply #12 on: August 21, 2015, 07:25:03 pm »

I wonder if it would be possible to build one that would intentionally flex and sway in the middle and stabilize on top...

Resonance varies depending on where the elevator is at the time. There needs to be sway and constant tension because as the gondola climbs it is "pulled" in perpendicular direction from the cable by the "coriolis force.". Basically as the elevator moves up, it's own inertia is not "in synch" with the rotation of the Earth, so it needs to be "dragged into the right speed" by the cable or tower.  That is how you in fact "build up" your orbital speed as you approach Geosynchronous orbit. Naturally, the tower being much shorter, you never get even close to orbital speed, so you still have to launch a rocket or whatever, but the tip of the tower is not in a straight line with the base of the tower
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« Reply #13 on: August 22, 2015, 04:01:32 am »

I read a book a couple of years ago (author John Scalzi) called 'Old Man's War' 1995, where transfer from Earth to space station was via a space elevator called the Beanstalk, theoretically an alien technology but possibly based on the use of carbon nanotubes.
Carbon nanotubes being unstable and prone to breakage, real life physics is researching the the use of a combined engineered fibre called carbyne, which should, in theory, increase reliability and minimise fracturing.



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« Reply #14 on: August 22, 2015, 08:05:49 am »

If you did create a 100 km carbon nanotube, how would you move upwards on it? As it would be so thin, I don't see there would be any friction to help move a cable car along it. Nothing to really gain traction.
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« Reply #15 on: August 22, 2015, 11:33:42 am »

If you did create a 100 km carbon nanotube, how would you move upwards on it? As it would be so thin, I don't see there would be any friction to help move a cable car along it. Nothing to really gain traction.

 Huh

I'm hoping this is a joke (or at least a bait question to pull obsessive technical types like me  Wink ). Who suggested a single nanotube?  We're talking about fibre bundles... This is not an issue of having a single continuous strand miles long, but rather to have a chord made from fibres that are very strong and light, with a strength to weight ratio larger than anything we have available now in the market.  The talk is that the technology is advanced enough that right now we fall short by a factor of 2 or so, and so it is theorised it will be a short time before we can produce chords strong and light enough to span these distances.
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« Reply #16 on: August 22, 2015, 11:55:18 am »

When I hear it described, I usually hear about it being done with these superthin strings, to keep the weight down.
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« Reply #17 on: August 22, 2015, 12:48:51 pm »

The book did not mention the number of carbon nanotubes, but I got the impression it was quite a thick bundle, not a single tube. Super-thin strings of carbyne, bundled, sounds much more reasonable than a single tube.
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« Reply #18 on: August 22, 2015, 01:23:20 pm »

Perhaps but even making a single string that's strong enough could be difficult. Even if it's as thick as fishing wire, I don't see how you can move along it.
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« Reply #19 on: August 23, 2015, 12:18:31 am »

Perhaps but even making a single string that's strong enough could be difficult. Even if it's as thick as fishing wire, I don't see how you can move along it.

I think you have a wrong impression of what they are trying to accomplish. Partly because they need to explain a few engineering principles that are not available to non-engineers.

According to modern engineering mechanics, the thin nature of the strand has to do with the fact that tensile forces flow along the surface of cables. The ideal element in tension -according to mid 20th. C computer simulations and graduate-level theory on mechanics of materials- is actually an infinite number of parallel, infinitely thin strings to maximize the surface area.  More traditional theory (from antiquity and trial by error) shows that bundling strings together can provide more resistance by way of shearing forces between strands, like in an a twisted rope made of natural fibres.

The cable will have to be very very thick, and to maximize strength and minimize weight will need a variable cross section, being the thickest where the tension is maximum at geostationary orbit.  Think of it as a giant rope made from very thin strands.
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