2006

28/8

Uncategorized

All Aboard

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This week Fungus explores the not-so-fictional world of Space Elevators.


Science Fiction?

In Arthur C. Clarke’s 1978 novel, Fountains of Paradise, engineers construct a space elevator on top of a mountain peak in the mythical island of Taprobane (closely based on Sri Lanka, the country where Clarke now resides). His book brought the idea to the general public through the science fiction community. But Clarke wasn’t the first. As early as 1895, a Russian scientist named Konstantin Tsiolkovsky suggested a fanciful "Celestial Castle" in geosynchronous Earth orbit attached to a tower on the ground, not unlike Paris’s Eiffel tower. Another Russian, a Leningrad engineer by the name of Yuri Artsutanov, wrote some of the first modern ideas about space elevators in 1960. Published as a non-technical story in Pravda, his story never caught the attention of the West. Science magazine ran a short article in 1966 by John Isaacs, an American oceanographer, about a pair of whisker-thin wires extending to a geostationary satellite. The article ran basically unnoticed. The concept finally came to the attention of the space flight engineering community through a technical paper written in 1975 by Jerome Pearson of the Air Force Research Laboratory. This paper was the inspiration for Clarke’s novel. Extreme as the ideas may have sounded in their time they are quickly approaching fruition in the laboratories of today.

The Fundamentals

 

A space elevator is a theoretical structure designed to transport material from a planet’s surface into space. Many different types of space elevators have been proposed. They all intend to replace rocket propulsion with the movement of a fixed structure via a mechanism like an elevator in order to move material into or beyond orbit. Space elevators have also sometimes been referred to as beanstalks, space bridges, space lifts, space ladders or orbital towers.

The most common proposal is a tether, usually in the form of a cable or ribbon, spanning from the surface to a point beyond geosynchronous orbit. As the planet rotates, the inertia at the end of the tether counteracts gravity and keeps the cable taut via centrifugal force. Vehicles can then climb the tether and escape the planet’s gravity without the use of rocket propulsion. Such a structure could eventually permit delivery of great quantities of cargo and people to orbit, and at costs only a fraction of those associated with current means.

At this time orbital tethers are the only space elevator concept that is the subject of active research and commercial interest in space. However, there are two related concepts worth mentioning: a space fountain and a very tall compressive structure (i.e. a structure that stands on its own).

A space fountain would use pellets fired up from the ground by a mass driver, the pellets traveling through the center of a tower. These pellets would impart their kinetic energy to the tower structure via electromagnetic drag as they traveled up and again as their direction was reversed by a magnetic field at the top. Thus the structure would not be supported by the compressive strength of its materials, and could be hundreds of kilometers high. Unlike tethered space elevators (which have to be placed near the equator), a space fountain could be located at any latitude. Space fountains would require a continuous supply of power to remain aloft.

Compressive structures would be similar to those used for aerial masts. While these structures might reach the agreed altitude for space (100 km), they are unlikely to reach geostationary orbit (35,786 km). Due to the difference between sub-orbital and orbital spaceflights, additional rockets or other means of propulsion would be necessary to achieve orbital speed.

Orbital tethers

 

This concept, also called an orbital space elevator, geosynchronous orbital tether, or a beanstalk, is a subset of the skyhook concept. Construction would be a vast project: a tether would have to be built of a material that could endure tremendous stress while also being light-weight, cost-effective, and manufacturable in great quantities. Today’s materials technology does not quite meet these requirements, although carbon nanotube technology shows promise. A considerable number of other novel engineering problems would also have to be solved to make a space elevator practical. Not all problems regarding feasibility have yet been addressed. Nevertheless, some believe that the necessary technology might be developed as early as 2008 and the first space elevator could be operational by 2018.

Conceptual designs place the tower construction at an equatorial site. The extreme height of the lower tower section makes it vulnerable to high winds. An equatorial location is ideal for a tower of such enormous height because the area is practically devoid of hurricanes and tornadoes and it aligns properly with geostationary orbits (which are directly overhead).

Space Elevators in the Laboratory

In their quest to make space elevators a reality scientists have focused on five main areas of development. First was the development of high-strength materials for both the cables (tethers) and the tower.

&nbsp In a 1998 report, NASA applications of molecular nanotechnology, researchers noted that "maximum stress [on a space elevator cable] is at geosynchronous altitude so the cable must be thickest there and taper exponentially as it approaches Earth. Any potential material may be characterized by the taper factor — the ratio between the cable’s radius at geosynchronous altitude and at the Earth’s surface. For steel the taper factor is tens of thousands — clearly impossible. For diamond, the taper factor is 21.9 including a safety factor. Diamond is, however, brittle. Carbon nanotubes have a strength in tension similar to diamond, but bundles of these nanometer-scale radius tubes shouldn’t propagate cracks nearly as well as the diamond tetrahedral lattice."

Fiber materials such as graphite, alumina, and quartz have exhibited tensile strengths greater than 20 GPa (Giga-Pascals, a unit of measurement for tensile strength) during laboratory testing for cable tethers. The desired strength for the space elevator is about 62 GPa. Carbon nanotubes have exceeded all other materials and appear to have a theoretical strength far above the desired range for space elevator structures. The development of carbon nanotubes shows real promise. They are lightweight materials that are 100 times stronger than steel.

The second technology thrust was the continuation of tether technology development to gain experience in the deployment and control of such long structures in space.

Third was the introduction of lightweight, composite structural materials to the general construction industry for the development of taller towers and buildings. Buildings and towers can be constructed many kilometers high today using conventional construction materials and methods. There simply has not been a demonstrated need to do this that justifies the expense, thus far. Better materials may reduce the costs and make larger structures economical.

Fourth was the development of high-speed, electromagnetic propulsion for mass-transportation systems, launch systems, launch assist systems and high-velocity launch rails. These are, basically, higher speed versions of the trams now used at airports to carry passengers between terminals. They would float above the track, propelled by magnets, using no moving parts. This feature would allow the space elevator to attain high vehicle speeds without the wear and tear that wheeled vehicles would put on the structure.

Fifth was the development of transportation, utility and facility infrastructures to support space construction and industrial development from Earth out to GEO. The high cost of constructing a space elevator can only be justified by high usage, by both passengers and payload, tourists and space dwellers.

During a speech he once gave, someone in the audience asked Arthur C. Clarke when the space elevator would become a reality.

Clarke answered, "Probably about 50 years after everybody quits laughing." As in the past, he may have been closer to the mark than people were willing to give him credit for.

Also by

Share This

2006

28/8

Uncategorized

All Aboard

by | Print
{mosimage}

This week Fungus explores the not-so-fictional world of Space Elevators.


Science Fiction?

In Arthur C. Clarke’s 1978 novel, Fountains of Paradise, engineers construct a space elevator on top of a mountain peak in the mythical island of Taprobane (closely based on Sri Lanka, the country where Clarke now resides). His book brought the idea to the general public through the science fiction community. But Clarke wasn’t the first. As early as 1895, a Russian scientist named Konstantin Tsiolkovsky suggested a fanciful "Celestial Castle" in geosynchronous Earth orbit attached to a tower on the ground, not unlike Paris’s Eiffel tower. Another Russian, a Leningrad engineer by the name of Yuri Artsutanov, wrote some of the first modern ideas about space elevators in 1960. Published as a non-technical story in Pravda, his story never caught the attention of the West. Science magazine ran a short article in 1966 by John Isaacs, an American oceanographer, about a pair of whisker-thin wires extending to a geostationary satellite. The article ran basically unnoticed. The concept finally came to the attention of the space flight engineering community through a technical paper written in 1975 by Jerome Pearson of the Air Force Research Laboratory. This paper was the inspiration for Clarke’s novel. Extreme as the ideas may have sounded in their time they are quickly approaching fruition in the laboratories of today.

The Fundamentals

 

A space elevator is a theoretical structure designed to transport material from a planet’s surface into space. Many different types of space elevators have been proposed. They all intend to replace rocket propulsion with the movement of a fixed structure via a mechanism like an elevator in order to move material into or beyond orbit. Space elevators have also sometimes been referred to as beanstalks, space bridges, space lifts, space ladders or orbital towers.

The most common proposal is a tether, usually in the form of a cable or ribbon, spanning from the surface to a point beyond geosynchronous orbit. As the planet rotates, the inertia at the end of the tether counteracts gravity and keeps the cable taut via centrifugal force. Vehicles can then climb the tether and escape the planet’s gravity without the use of rocket propulsion. Such a structure could eventually permit delivery of great quantities of cargo and people to orbit, and at costs only a fraction of those associated with current means.

At this time orbital tethers are the only space elevator concept that is the subject of active research and commercial interest in space. However, there are two related concepts worth mentioning: a space fountain and a very tall compressive structure (i.e. a structure that stands on its own).

A space fountain would use pellets fired up from the ground by a mass driver, the pellets traveling through the center of a tower. These pellets would impart their kinetic energy to the tower structure via electromagnetic drag as they traveled up and again as their direction was reversed by a magnetic field at the top. Thus the structure would not be supported by the compressive strength of its materials, and could be hundreds of kilometers high. Unlike tethered space elevators (which have to be placed near the equator), a space fountain could be located at any latitude. Space fountains would require a continuous supply of power to remain aloft.

Compressive structures would be similar to those used for aerial masts. While these structures might reach the agreed altitude for space (100 km), they are unlikely to reach geostationary orbit (35,786 km). Due to the difference between sub-orbital and orbital spaceflights, additional rockets or other means of propulsion would be necessary to achieve orbital speed.

Orbital tethers

 

This concept, also called an orbital space elevator, geosynchronous orbital tether, or a beanstalk, is a subset of the skyhook concept. Construction would be a vast project: a tether would have to be built of a material that could endure tremendous stress while also being light-weight, cost-effective, and manufacturable in great quantities. Today’s materials technology does not quite meet these requirements, although carbon nanotube technology shows promise. A considerable number of other novel engineering problems would also have to be solved to make a space elevator practical. Not all problems regarding feasibility have yet been addressed. Nevertheless, some believe that the necessary technology might be developed as early as 2008 and the first space elevator could be operational by 2018.

Conceptual designs place the tower construction at an equatorial site. The extreme height of the lower tower section makes it vulnerable to high winds. An equatorial location is ideal for a tower of such enormous height because the area is practically devoid of hurricanes and tornadoes and it aligns properly with geostationary orbits (which are directly overhead).

Space Elevators in the Laboratory

In their quest to make space elevators a reality scientists have focused on five main areas of development. First was the development of high-strength materials for both the cables (tethers) and the tower.

&nbsp In a 1998 report, NASA applications of molecular nanotechnology, researchers noted that "maximum stress [on a space elevator cable] is at geosynchronous altitude so the cable must be thickest there and taper exponentially as it approaches Earth. Any potential material may be characterized by the taper factor — the ratio between the cable’s radius at geosynchronous altitude and at the Earth’s surface. For steel the taper factor is tens of thousands — clearly impossible. For diamond, the taper factor is 21.9 including a safety factor. Diamond is, however, brittle. Carbon nanotubes have a strength in tension similar to diamond, but bundles of these nanometer-scale radius tubes shouldn’t propagate cracks nearly as well as the diamond tetrahedral lattice."

Fiber materials such as graphite, alumina, and quartz have exhibited tensile strengths greater than 20 GPa (Giga-Pascals, a unit of measurement for tensile strength) during laboratory testing for cable tethers. The desired strength for the space elevator is about 62 GPa. Carbon nanotubes have exceeded all other materials and appear to have a theoretical strength far above the desired range for space elevator structures. The development of carbon nanotubes shows real promise. They are lightweight materials that are 100 times stronger than steel.

The second technology thrust was the continuation of tether technology development to gain experience in the deployment and control of such long structures in space.

Third was the introduction of lightweight, composite structural materials to the general construction industry for the development of taller towers and buildings. Buildings and towers can be constructed many kilometers high today using conventional construction materials and methods. There simply has not been a demonstrated need to do this that justifies the expense, thus far. Better materials may reduce the costs and make larger structures economical.

Fourth was the development of high-speed, electromagnetic propulsion for mass-transportation systems, launch systems, launch assist systems and high-velocity launch rails. These are, basically, higher speed versions of the trams now used at airports to carry passengers between terminals. They would float above the track, propelled by magnets, using no moving parts. This feature would allow the space elevator to attain high vehicle speeds without the wear and tear that wheeled vehicles would put on the structure.

Fifth was the development of transportation, utility and facility infrastructures to support space construction and industrial development from Earth out to GEO. The high cost of constructing a space elevator can only be justified by high usage, by both passengers and payload, tourists and space dwellers.

During a speech he once gave, someone in the audience asked Arthur C. Clarke when the space elevator would become a reality.

Clarke answered, "Probably about 50 years after everybody quits laughing." As in the past, he may have been closer to the mark than people were willing to give him credit for.

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