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starship-design: Space Elevators Maybe Closer To Reality Than Imagined
TECH SPACE
Space Elevators Maybe Closer To Reality Than Imagined
dreaming of cloud nine is maybe not so impossible after all
by Richard Perry
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.
Red Mars
The next great opinion-forming novel was "Red Mars", by Kim Stanley
Robinson in 1992. A captured asteroid is mined using nanotechnology to
extend a graphite cable 37,000km down to the surface.
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
Space tethers have been discussed in international workshops annually
since 1983, and by the time that "Red Mars" was written had identified
the issues of material strength and production.
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 NASA Institute for Advanced Concepts (NIAC) commissioned Dr Bradley
C Edwards to study all aspects of the construction and operation of a
space elevator, and Phase I of the report was published in late 2002.
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.
Production Issues
Carbon NanoTubes are proposed to be the main material for the tape.
These were first produced in 1991 (the year before "Red Mars" was
published), with 3cm ropes being produced by 1998. The strength of these
laboratory-produced NaanoTubes confirmed people's predictions that this
material would have the strength that a space elevator would require.
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.
Destruction
The study highlights most of the risks that can be identified. Meteor
strikes, hurricanes, terrorist attack, even to the falling of the ribbon
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.
Power Systems
For powering the elevator, Clarke had to bring in nuclear fusion and
superconductors. This NIAC study proposes that power requirements for
the initial deployment of the tape would be minimal and met by solar
arrays or batteries. The deployment itself would actually generate
excess power.
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
Taking the design process to the ultimate stage, that of time and cost,
reveals some real-world numbers. The first cable would cost around
$40billion (50% of that being contingency), whilst a second cable would
cost only $14billion. The construction time for the first elevator is
scheduled to take 10 years, with another ten elevators built in the
following decade.
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
--
L. Parker
chief cook, bottle washer and sometime sysadmin
cacaphony.net