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SPACE ELEVATOR


Photocell Climber Space Elevator
Tech Level: 13
Asteroid-Anchored Space Elevator
Tech Level: 15

"The space elevator will be built about 50 years after everyone stops laughing".
--Arthur C. Clarke


The Space Elevator concept, also occasionally called a Beanstalk, has recently been given a boost thanks to the development of material technology which exhibit the tensile strengths needed to make the concept work.

Originally proposed in a popular science article in the Soviet periodical Komsomolskaya Pravda by Yuri Artsunatov in 1960, the concept was taken up by many science fiction writers over the years, most significantly Arthur C. Clarke in his 1977 novel The Fountains of Paradise. Since then, it has seen many incarnations, such as in Kim Stanley Robinson’s Red Mars novel, the game universe of 2300 AD, the Dirty Pair and Maps anime series, and countless science article speculations.

It is currently under serious preliminary study at NASA’s Institute for Advanced Concepts (NIAC.) While proponents say that an elevator could be built within 15 years or so, fundamental research into various component technologies still needs to be done, and political and economic factors will probably delay actual attempted construction of a space elevator for many decades yet.

 

SPACE ELEVATOR BASICS

The basic principle of a Space Elevator is fairly simple to envision. Tie a string to a baseball and twirl the string above your head. The string will remain taut and straight as long as the twirling motion is in effect. The Earth is spinning far faster than your hand could ever manage, about 1000 miles per hour. If you anchored an incredibly strong wire to Earth’s surface at the equator, then attached the other end to a large enough mass (say, a small asteroid) to keep it taut, you end up with a perfectly-straight railroad track right into space.

The Space Elevator’s center of mass would be at geosynchronous orbit, approximately 22,300 miles above the equator, helping to keep the entire construct fixed over a stable position on Earth. The geosynchronous point is also where the cable would be under the most stress (see below), so it would have to be thickest there and taper down exponentially as one moves away from it in either direction.

Once the cable is set up, elevators can ride it up and down via magnetic rails, delivering cargo straight into orbit. The Earth-end of the Elevator cable is usually envisioned as being attached to the top of a mountain or a super-high artificial tower. However, though both of these options could simplfy setting up the Elevator, they are not strictly necessary. One scheme, primarily involving the photocell climber elevator, details anchoring the cable to a specially-built but standard-height off-shore platform.

The Space Elevator is a simple, straightforward idea with one very important complication: the structural stresses put on the elevator cable would be truly enormous, far beyond what normal materials, even advanced alloys and composite laminates, is capable of dealing with. A Giga-Pascal (GPa) is a measurement of tensile strength. Quartz fiber has a tensile strength of about 20 GPa’s, while diamond filaments would only be slightly higher. The GPa’s required for Space Elevator cable material is around 62 GPa’s, threen times that of diamond filaments.

Also, the heavier you make the cable, the more weight it has to support and the greater stress it has to endure. So, ideally, the builders of a Space Elevator would not only have to have an incredible strong, flexible material, but said material also has to be incredibly light weight.

Fortunately, one material meeting these requirements has recently been synthesized, albeit only in microscopic quantities: carbon nanotubes composites, which would have a theoretical upper tensile strength of 200 GPa’s, over 100 times that of steel cable at only a small fraction of the weight. Small fibers of this material can be set down side by side, then interconnected to form a growing ribbon.

One interesting feature of the system is that the Space Elevator cable would constantly be moving through Earth’s electromagnetic field. If conductive material (or even superconductive material, if such is available when the Elevator is constructed) is run through the center of the cable, it would constantly generate electricity through this movement. This could provide not only ample power to operate all the Space Elevator systems and stations, but also provide a magnetic field elevator railcars can ride up the Elevator.

Using 1 g acceleration/deceleration for the elevators, a trip from the ground to the geosynchronous point would take about two days.

The advantages of a Space Elevator are enormous. Once the technology matures, orbital interface travel from surface to space could eventually be reduced to pennies a ton for cargo, or a rate equivalent to a passenger train for human riders. Today it costs about $22,000 per kilogram to put cargo into Low Earth Orbit because of the enormous energies standard rockets must generate in order to reach orbital velocity. Using today’s energy costs, it would take about 75 cents per kilogram for a Space Elevator to do the same thing.

A space elevator would pay for its initial set-up costs within a few decades. The builders could even make money by selling the delta—V of the space elevator to outbound ships, flinging them into space from the far end of the cable much like an oversized Rotovator. A ship "flung" form the end of the cable would be travelling at about 6.79 miles per second, fast enough to reach Mars in a few weeks if pointed in the right direction.

In most visions for a passenger-carrying Elevator, a weigh station is usually built at the geosynchronous point, where the local net acceleration forces along the cable cancel each other out, resulting in zero gravity conditions.


PHOTOCELL CLIMBER SPACE ELEVATOR
Tech Level: 13


This concept uses photocells "pushed" by a ground-based laser to slowly build a Space Elevator "from the ground up." The Elevator cable anchored to the ground is counter-balanced by an equal length of cable beyond the geosynchronous point, built up by photocell-pushed "climbers." These climbers would also be used to launch payloads up the elevator.

The following is taken from the article, "The Space Elevator Comes Closer to Reality," by Leonard David, from the Space.Com website. It can be found in its completion here: http://www.space.com/businesstechnology/technology/space_elevator_020327-1.html.

"Getting the first space elevator off the ground, factually, would use two space shuttle flights. Twenty tons of cable and reel would be kicked up to geosynchronous altitude by an upper stage motor. The cable is then snaked to Earth and attached to an ocean-based anchor station, situated within the equatorial Pacific. That platform would be similar to the structure used for the Sea Launch expendable rocket program.

"Once secure, a platform-based free-electron laser system is used to beam energy to photocell-laden ‘climbers’. These are automated devices that ride the initial ribbon skyward. Each climber adds more and more ribbon to the first, thereby increasing the cable's overall strength. Some two-and-a-half years later, and using nearly 300 climbers, a first space elevator capable of supporting over 20-tons (20,000-kilograms) is ready for service.

"For a space elevator to function, a cable with one end attached to the Earth's surface stretches upwards, reaching beyond geosynchronous orbit, at 21,700 miles (35,000-kilometer altitude).

"Twenty tons of cable and reel would be kicked up to geosynchronous altitude by spacecraft to get the project started. "If budget estimates are correct, we could do it for under $10 billion. The first cable could launch multi-ton payloads every 3 days. Cargo hoisted by laser-powered climbers, be it fragile payloads such as radio dishes, complex planetary probes, solar power satellites, or human-carrying modules could be dropped off in geosynchronous orbit in a week's travel time," (Bradley Edwards of Eureka Scientific in Berkeley, California) said."


ASTEROID-ANCHORED SPACE ELEVATOR
Tech Level: 15


The more "traditional" vision of a space elevator.

A small asteroid is diverted from deep space and locked into high orbit above earth. The end of the elevator cable beyond geosynchronous orbit is anchored to it as a counterweight (think of the baseball in the above example.) The mass of the asteroid moving in a higher orbit keeps the cable under tension and the cable straight. This way, the overall length of the cable can be greatly shortened.

A shorter cable may be desirable for economic reasons; today, carbon nanotubes cost about $500 per gram of mass, or roughly $500 million dollars per ton. A Space Elevator cable will, of course, weigh many thousands of tons. If this price does not significantly go down by the time a Space Elevator is ready for construction, diverting an asteroid may actually be a far cheaper deal than building an equal length of cable beyond geosynchronous orbit.

The asteroid could also have the added advantage of being used as a source of raw materials to build space facilities for the Elevator, such as the geosynchronous station, or complete additional cables for more "tracks" along the Elevator.


RELATED INFORMATION

In Print:

The Fountains of Paradise by Arthur C. Clarke
Red Mars by Kim Stanley Robinson
Beanstalk, a supplement for GDW's old 2300 AD RPG

On The Web:

The Space.com Aricle featuring the photocell climber elevator:

http://www.space.com/businesstechnology/technology/space_elevator_020327-1.html



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