07-08-2012, 12:45 PM
SPACE ELEVATORS AN ADVANCED EARTH-SPACE INFRASTRUCTURE FOR THE NEW MILLENNIUM
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Introduction: What is a Space Elevator?
A space elevator is a physical connection from the surface
of the Earth to a geostationary Earth orbit (GEO) above the
Earth ≈35,786 km in altitude. Its center of mass is at the
geostationary point such that it has a 24-hr orbit and stays
over the same point above the equator as the Earth rotates on
its axis. The vision is that a space elevator would be utilized
as a transportation and utility system for moving people,
payloads, power, and gases between the surface of the Earth
and space. It makes the physical connection from Earth to space
in the same way a bridge connects two cities across a body of
water (see cover art and fig. 2).
Future Directions
Five primary technology thrusts were identified as critical
to the development of space elevators in the 21st century. All
have many other near-term applications for new products and
services on Earth and in space. They are as follows:
1. Develop advanced high-strength materials like the
graphite, alumina, and quartz whiskers that exhibit
laboratory strengths >20 GPa. Continue development
of the carbon nanotube materials that exhibit strengths
100 times stronger than steel. Introduce these new
lightweight, high-strength materials to the commercial,
space, and military markets for new and improved
product developments (see 3.1 Materials).
2. Continue development of space tether technologies for
space transportation systems to gain experience in the
deployment and control of long structures. Utilize
higher strength materials as they become available.
Continue analysis and plan for demonstration of
momentum exchange and low-Earth orbit (LEO) space
elevator facilities for low-cost, in-space transfer to
GEO (see 3.2 Tension Structures).
Technology Demonstrations
Technology demonstrations were identified for tethers,
towers, and electromagnetic systems as being critical to a
technology progression toward space elevator construction
capabilities during the 21st century. Figure 1 illustrates one
logical course of events over an indefinite period of time
leading up to the full-scale development of Earth to GEO space
elevators.
More details on many of these technology demonstrations
as well as other related potential developments and benefits
are discussed in this publication. The intent is to show that
these technology demonstrations and developments can
provide incremental benefits and are logical to pursue for their
own merit in addition to their obvious relationship to future
space elevator developments.
SPACE ELEVATOR CONCEPTS
The following sections provide an overview of the basic
Earth to GEO space elevator concept as well as a number of
other related space elevator concepts that have been envisioned
over the years. This basic concept for building a structure from
the surface of the Earth into space has been around for a long
time, but was not well known or even seriously considered
from an engineering standpoint until the latter part of the 20th
century.
Brief History
The idea of building a tower from the surface of the Earth
into space, the sky, or the heavens dates back to some of the
very earliest known manuscripts in existence. The writings of
Moses, about 1450 BC, in his book Genesis, chapter 11,
reference an earlier civilization that in about 2100 BC tried to
build a tower to heaven out of brick and tar. This structure is
commonly called the Tower of Babel, and was reported to be
located in Babylon, a city in ancient Mesopotamia. Later in
chapter 28, about 1900 BC, Jacob had a dream about a staircase
or ladder built to heaven, commonly called Jacob’s Ladder.
More contemporary writings on the subject date back to K.E.
Tsiolkovski in his manuscript “Speculations about Earth and
Sky and on Vesta,” published in 1895. No doubt the idea for
building a tower from the surface of the Earth into space has
been dreamed, invented, and reinvented many times throughout
modern civilization.
Space Elevator Basics
The most complex and demanding concept for a space
elevator is the Earth to GEO space elevator, the primary topic
of this publication. In this system, illustrated in figure 2, the
elevator center-of-mass station is at GEO; the tether structure
“hangs” down over 35,000 km to the Earth with no relative
horizontal velocity and connects to a tall tower constructed
from an ocean-based platform. The structure is designed
integrally with six tracks for electromagnetic vehicles to travel
continuously up and down the elevator structure. If a payload
is released in LEO, it would need a propulsion system to
increase its orbital velocity from that of GEO (3.1 km/sec) to
that of LEO (7.7 km/sec).3 Payloads released above GEO
would be released into a transfer ellipse to a higher altitude.
Release along the upper section of the tether at an altitude of
47,000 km would provide for Earth escape.4 At the base, the
tether structure in tension connects to a tall tower in
compression. The taller the tower the better, since it is the
lower section, as the structure approaches the Earth, that has
the greatest impact on the systems’ structural strength
requirements and diameter at GEO.