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Space Elevators Maybe Closer To Reality Than Imagined
Los Angeles - Jul 22, 2003
Space elevators have an image problem, mainly due to two prominent science fiction novels. They appear either ungainly impossible, or so potentially dangerous to the planet itself you would never dream of building one. With the science now indicating that they are potentially near-term transport systems, it's time to review the fiction in relation to the possible reality.
Three publications by Pearson in 1975/6/7 and work done by Moravec and published in the Journal of the Astronautical Sciences in 1977 were enough to prompt Arthur C Clarke to write "The Fountains of Paradise" and Charles Sheffield "The Web Between the Worlds" - both published in 1979.
Clarke wrote of a world developed to a point where the weather systems could be controlled to produce designer-sunsets. A lone architect designs a 40,000km elevator consisting of four tubes. With a pair each for up and down travel, and regenerative breaking used to minimize the power losses.
The first attempt to lower a wire to Earth fails when it gets entangled, and the design is changed to that of an inverted square tower. A small iron asteroid is moved into Earth orbit to act as a counterweight. The four sides of the track will feature superconducting cables backed by fusion power generators.
Ultimately, the tower stands for 1500yrs, growing to be 500m on a side with a city built at the 1500km level. Half a billion people eventually settle in orbit for a zero-g lifestyle.
In a later printing, Clarke claims his inspiration came from much earlier articles from 1966, but the resurgence of interest and writing prior to 1979 was timely. He also says that he may have been too conservative, and that the tower may be a 21st century achievement. The latest research proposes 'early' 21st century.
Elevator cars take several days to make the journey, and are thirty stories high. But the main image from this incarnation is when the cable is brought down by revolutionary action. It twists around the planet at 21,000km per hour, with horrific consequences.
"Red Mars" was part of a trilogy. In "Green Mars", a replacement cable is made using Carbon Nanotubes from another captured asteroid. Cars travel up and down the cable at the same time to minimize energy losses. It's no coincidence that both these cables are called 'Clarke'.
The "The Fountains of Paradise" elevator is used to promote the concept that many people would wish to travel to, and even live-in, low Earth orbit. In "Red Mars", the cable is the main transport system, and seen as an essential 'umbilical cord' for the new colony.
Tower of Babel
However, even as late as 1999, these workshops were becoming confused in their own clouds of science and fiction. The Advanced Space Infrastructure Workshop on Geostationary Orbiting Tether "Space Elevator" Concepts, held in June 1999 at the NASA Marshall Space Flight Center, for instance. The history section of the conference report tries to claim that the origins of space elevators could be traced back to Genesis 11.3 and references to the Tower of Babel.
They also concentrated on the non-fixed tethers, which do not go all the way to the Earth's surface and consequently require mach 16 aircraft vehicles to reach them. Even more worryingly, they considered the idea of building tall towers - up to 50km in height.
The significant point here is that as late as 1999, the materials issue had been acknowledged, but the thought processes had been allowed to dream back into 1950's style fiction. Basic desk research shows that the Tower of Babylon dates back to the time of King Nebuchadnezzar II who lived from 605-562 BC and rebuilt it to stand 295 feet high. It was nothing more then a ziggurat, honoring the god Marduk.
Clearly, the scientific thinking on space elevators had broken down and a more rational appraisal of the technology was long overdue.
Tapes and Lifters
The report very specifically addresses design and operations, which had until then escaped close scrutiny.
Firstly, the elevator would not be a cable. It starts as a 1-micron thick piece of tape 91,000km long, tapering from 5cm wide at the Earth's surface to 11.5cm wide near the middle. This tape would be taken up by shuttle together with some booster rockets. It would then be 'flown-down' to the surface whilst the booster rockets provide the required counterbalance beyond geosynchronous orbit.
Centripetal force throws the higher part of the tape away from the Earth, whilst the effect of gravity on the lower mass of the tape keeps it in tension. This first link is capable of supporting 1238kg before breaking.
That's enough to allow more 'lifters' to add additional tapes to increase the strength of the elevator to a useful amount. This takes a total of 207 lifters and nearly two and a half years to complete. In its final form, each new lifter is capable of carrying 13,000kg and then adding their own mass to that of the counterweight when their job is done.
Moving asteroids around the solar system is not a requirement for a space elevator, you can 'build' the counterweight using your own construction equipment. By flying the tape all the way down to the ground you do not need tall towers and fast aircraft to connect to your orbital transport system.
A main concern is how to produce 91,000km long tapes, when the present capability is only a few centimeters. The tapes they have defined in this study are Carbon NanoTube/expoxy composites. Standard composites use these in a 60/40 ration, but this design proposes only a 98/2 ratio to minimize the mass of epoxy required - the rest would be bare Nanotubes, required to be at least a centimeter in length. This reduces the design issues to the high-volume production of NanoTubes and how to operate the elevator itself.
In "Red Mars", the falling cable causes destruction, but with this design all you get is thousands of miles of carbon-based tape fluttering to the ground at the speed of a sheet of newspaper. Hurricanes are avoided by careful selection of the ground site, which also addresses the lighting strike risk.
A damaged cable ribbon is intended to be capable of in-situ repair, whereas a broken one only causes inconvenience until a replacement length can be flown down. If lifters become detached from the ribbon then parachutes or re-entry vehicle solutions are required.
The report mentions the very problems that affected the Clarke cable - those of a tangled cable as it is deployed at the rate of 200km per hour, and identifies the need for appropriate mechanical control of the tension.
The lifters that climb the tape to add new strands are powered by beaming power onto their solar panels. With this and additional power coming from the locomotive system beyond geosynchronous orbit, getting rid of excess power is actually more of an issue. This technology is under development by several companies.
So no exotic power systems are required for the construction or operation of the cable, and much of the technologies required either already exist or are being worked on as near-term objectives. Such a system is highly scaleable. Once in place, a space elevator can be used to build another, thereby increasing capacity in a predictable manner.
One of the aspects of the elevator in "Red Mars" is that it had to oscillate to avoid hitting the moon Phobos. This design identifies a similar need to avoid low Earth orbit satellites and space debris. The solution is to ensure that there is adequate warning to move the elevator, and using a sea-based anchor station to do this.
Real World Numbers
However, there have been lots of changes since the report was written. A current program is $7-10B, with a 15-year cycle to build. That assumes 2 years of research into the material sciences, with some additional testing and research on other aspects. After 3 years of design and engineering, the actual "cutting metal" and building of parts for the system will begin. That will take another 7 years, and then 3 years for launching, on orbit assembly, and final integration.
They take the opportunity to propose how to make use of this space asset, with a large space station capable of housing hundreds of people, and the construction of a Martian elevator on Earth. It would be lifted into Earth orbit and then thrown onward to Mars itself to allow for unmanned and later manned exploration. No great detail, simply a possible roadmap for the use to which tethers can be put for the next fifty years.
The space elevator has been a concept ahead of its time for too long and the implications of mass access to Earth orbit and beyond need to be considered. The remaining work of the report's writers is to further refine their studies, whilst existing commercial industry works on the production related issues.
In terms of funding, an elevator is not outside the realms of commercial business, although the business case for it needs to be confirmed. At present, this may be simply put - whoever owns the first space elevator will control economic access to space for a long time to come.
Already the commercial development of space elevators has begun. LiftPort is a new group of companies that has sprung into being as a direct result of this study. The rest as they say, is future.
Richard Perry is a director of Transorbital Inc Member of the Moon Society and the National Space Society
Liftport (commercial elevators)
Highliftsystems (elevator research)
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