D.V. Smitherman, Jr.
NASA Marshall Space Flight Center, Huntsville, Alabama
This conference publication, "Space Elevators: An Advanced Earth-Space Infrastructure for the New Millennium," is based on findings from the Advanced Space Infrastructure Workshop on Geostationary Orbiting Tether "Space Elevator" Concepts, held in June 1999 at the NASA Marshall Space Flight Center, Huntsville, Alabama. Subsequent consultation and review of the document with the participants was made prior to publication to clarify technical data and ensure overall consensus on the content of this publication.
1.1 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 at approximately 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.
The Earth to GEO space elevator is not feasible today, but could be an important concept for the future development of space in the latter part of the 21st century. It has the potential to provide mass transportation to space in the same way highways, railroads, power lines, and pipelines provide mass transportation across the Earth's surface. The low energy requirements for moving payloads up and down the elevator could make it possible to achieve cost to orbit <$10/kg. The potential for low-cost mass transportation to space makes consideration of the technology paths required for space elevator construction very important today. The technology paths are beneficial to many other developments and can yield incremental benefits as progress is made toward making space elevator construction feasible.
2.1 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 of, invented, and reinvented many times throughout modern civilization.
The first published account describing a space elevator that recognized the utility of geosynchronous orbit did not occur until 1960. Yuri Artsutanov, a Leningrad engineer, published a non-technical story in a Sunday supplement to Pravda, which did not become known in the West. Later, in 1966, a group of American Oceanographers led by John Isaacs published a short article in Science on a pair of whisker-thin wires hanging from a geostationary satellite. Again, this did not come to the attention of the space flight engineering community. Finally in 1975, Jerome Pearson, working at the Air Force Research Laboratory, also independently invented the space elevator and published a technical paper in Acta Astronautica. This publication brought the concept to the attention of the space flight community and later inspired Sir Arthur Clarke to write his novel, The Fountains of Paradise, about a space elevator based on a fictionalized Sri Lanka, which brought the concept to the attention of the entire world. Pearson later participated in the NASA Marshall tether workshops beginning in 1983, and brought the space elevator concept into the space tether technical community. The bibliography in this publication contains many contemporary writings on this and related subjects since 1960.
3.2.1 Low-Earth Orbit Space Elevator Concepts
The LEO space elevator is an intermediate version of the Earth surface to GEO space elevator concept, and appears to be feasible today using existing high-strength materials and space technology. It works by placing the system's midpoint station, and center of gravity, in a relatively low-Earth orbit and extending one cable down so that it points toward the center of the Earth and a second cable up so that it points away from the Earth. The bottom end of the lower cable hangs down to just above the Earth's atmosphere such that a future space plane flying up from the Earth's surface would require 2.5 km/sec less change in velocity than a single-stage-to-orbit (SSTO) vehicle launched directly to LEO. The space plane and LEO space elevator combination would likely be able to carry 10 to 12 times the payload as an equivalent-sized SSTO launch vehicle without the LEO space elevator. The length of the upper cable is chosen so that its endpoint is traveling at slightly less than Earth escape velocity for its altitude. This is done so that a spacecraft headed for higher orbit, the Moon, or beyond, can be placed in the proper orbit with only minimal use of its onboard propellant.
Figure 4. LEO Space Elevator concept
The overall length of a LEO space elevator from the bottom end of its lower cable to the top end of its upper cable is anywhere from 2,000 to 4,000 km, depending on the amount of launch vehicle velocity reduction desired. For example, a 2,200-km-long system provides a 1.6-km/sec reduction in launch vehicle velocity, while a 3,000-km-long system gives a 2-km/sec reduction, and a 3,800-km-long system offers a 2.3-km/sec reduction. It should be possible to launch a LEO space elevator in segments using existing launch systems. Once on orbit the LEO space elevator would then use its own onboard propulsion system to raise itself to the necessary orbital altitude while reeling out the upward and downward pointing cables as it went. Another advantage of this system is that as the market expands and materials improve, it could continue to grow in length and diameter, further reducing launch velocity and increasing system payload capacity. It even appears possible to grow the LEO space elevator into the full-length, 35,000-km-plus space segment length of the Earth surface to GEO space elevator if that were desired.
The concept illustrated in figure 4 is a long, freely orbiting, vertically oriented tether structure that completes 12 orbits per day. The fact that it is a freely orbiting system and not attached to the Earth at its lower end allows the system to be placed in an inclined orbit aligned with the plane of the ecliptic. This has advantages for traveling to the Moon and other planets as it would avoid plane change maneuvers and would greatly increase the number of launch windows for a given timeframe. Another advantage of the inclined orbital plane is that if a resonant orbit is used, the lower end of the system will pass within range of most of the world's major airports twice a day on a fixed schedule. Once the velocity required to reach the lower end of the LEO space elevator is down to the Mach 16 range or less, horizontal takeoff and landing space planes operating out of those airports appear to become both technically and economically feasible.
Another possibility is to combine the LEO space elevator with a vehicle utilizing an Earth-based electromagnetic launch rail or mass driver. Due to the size of the investment required to build a ground accelerator of this size, it would most likely require the higher flight rates made possible by an equatorial orbit and an equatorial launch site in order to make such a large, high-speed ground accelerator economically viable. A variation on this idea would be to use a vertically oriented, 4g ground accelerator mounted on a 4.5-km-tall tower to accelerate a launch vehicle to approximately 600 m/sec as a way of further reducing the launch vehicle's change in velocity requirements and increasing its payload fraction. In this way it might be possible to keep the cost of the ground accelerator down to an amount that would be profitable at a much lower flight rate, thereby allowing the LEO space elevator to be in a resonant orbit in the plane of the ecliptic.
In addition to allowing spacecraft leaving the upper end of the cable to be released at near Earth escape velocity, people traveling to the Moon or Mars would be able to experience those gravity levels on the LEO space elevator prior to departure. A station located at approximately 340-km altitude would experience Mars' gravity level while another station at 900-km altitude would be at a gravity level similar to the Moon. These stations would also be good for people returning to Earth from long stays in low or zero gravity as they would allow them to gradually reacclimate themselves to full Earth gravity. The Earth arrival/departure terminal at the bottom of the lower cable is at about one-half Earth gravity (figure 4).
There are three major issues associated with LEO space elevator operations that will require some type of propulsion system included in the design. These are atmospheric drag caused by the lower end of the cable, movement of payloads up and down the cable, and changes to the system's center of gravity and orbital altitude that are the result of arriving and departing spacecraft.
Figure 5. LEO Space Elevator for Lunar Transfers
The majority of the atmospheric drag is caused by the lower end of the cable between the lower endpoint station at 150-km altitude and the Mars station at 340-km altitude. In the example shown (figure 4), a unit payload of 5 metric tons in a 12-orbit-per-day system using T-1250 graphite fibers, a fiber volume of 65 percent, and a safety factor of 2.5, the diameter of the lower cable segments will be on the order of 6 mm. This produces a continuous drag force of approximately 10 Newtons (N) for the cable and an additional 2 N drag when a payload is transiting this segment of the cable. Because of the near-exponential dependence of air density on altitude, and the large area of even a thin tether, most of the drag on a long tether system is caused by the lower 30 km of the tether itself. Hence, hoisting the lower end up a modest amount between uses can greatly reduce average drag. One way to do that is to use a "funicular" at the bottom, a car at either end of a long rope, so they somewhat counterbalance each other. Between uses the cars can be stored at intermediate heights, while during use the lower one reaches down to 150-km altitude.
Movement of people and cargo to various locations on the LEO space elevator will be via elevator. These mass movements will cause the LEO space elevator's center of gravity to move, and as a result, change the system's orbital altitude. The arrival and departure of spacecraft will cause even greater changes in the center of gravity. Consequently, it will be necessary to constantly "fly" the LEO space elevator to maintain its orbital altitude within a certain range. The smaller of these center of gravity movements may be dealt with by raising and lowering the upper and lower endpoint terminals and with local adjustments of the midpoint station. Large center of gravity changes will require a propulsion system on the LEO space elevator to raise or lower its orbit. In the cases of launch vehicle arrivals from Earth, departures of a spacecraft to the Moon or higher orbits, or transfers of large payloads up the cable, it will be necessary to use the propulsion system to speed the LEO space elevator up and raise its orbit. In the cases of lunar arrivals, departures to Earth, or large payloads moving down the cable, it will be necessary to use the propulsion system to slow the LEO space elevator down and lower its orbit. Sizing of the propulsion system will be determined by the amount of center of gravity travel and the flight rate. Lower flight rates will allow more time between arrivals and departures, thereby allowing for a smaller, lower thrust propulsion system, while higher flight rates will require a larger, more powerful system. As the system matures and the mass flow moving down the cable matches the mass flow moving up the cable, the propulsion system will only be needed for drag makeup.
There are two prime candidate technologies for this propulsion system: ion propulsion and electrodynamic tether propulsion. Electrodynamic tether propulsion is unlike most other types of space propulsion in use or being developed for space applications today. There are no hot gases created and expelled to provide thrust. Instead, the environment of near-Earth space is being utilized to propel a spacecraft via electrodynamic interactions. A charged particle moving in a magnetic field experiences a force that is perpendicular to its direction of motion and the direction of the field. When a long conducting tether has current flowing through the cable, this force is experienced because charged particles are moving along the wire in the presence of the Earth's magnetic field. This force is transferred to the tether and to whatever is attached to the tether. It can be an orbit-raising thrust force or an orbit-lowering drag force, depending on the direction of current flow. Putting current into the cable makes it an orbit-raising thrust while drawing current from the cable makes it into an orbit-lowering drag force. The principle is much the same as an electric motor; reverse its operation and it acts as a generator.
The principle advantage of this propulsion system over any of the other types of propulsion systems (including ion) is the lack of any need for a propellant to serve as a reaction mass. In other words, solar arrays may be all that is needed to produce the energy required. This means lower recurring cost. Today, large-scale re-boost by electrodynamic tether is not a proven technology, but a technology demonstration is being developed and will be tested on orbit in the near future.
3.2.3 Lunar Space Elevator Concepts
Another near-term application of the space elevator concept could be demonstrated at the Moon. The one-sixth gravity at the Moon makes it theoretically possible to construct tethered connections from the surface of the Moon to the LaGrange libration points L1 and L2, on the near and far side, respectively, using existing materials (Kevlar, Spectra, or PBO graphite epoxy).
It has been envisioned that on the near side of the Moon such a structure could become the transportation system for moving materials to L1 in support of solar-powered satellite construction and propellant storage platforms. The regolith located at the base of the elevator contains oxygen which could be extracted. Additional gases from ice deposits at the lunar poles might also be transported around the Moon to this point for transfer to L1. At L1, solar-powered satellites would become part of a space utility system for production and transfer of power to the surface of the Moon and other stations within the Earth/Moon system. Likewise, a propellant platform at L1 would act as a service station for reusable in-space transportation vehicles.
On the far side of the Moon at L2, a similar system could be envisioned for lunar and space infrastructure support. On the surface of the far side of the Moon, ideas have been proposed for large space observatories, and as a remote location for the long-term storage of hazardous materials like the nuclear waste generated on Earth that must be stored safely for thousands of years.
Figure 7. Earth Orbiting and Lunar Space Elevator concepts
3.2.4 Mars Space Elevator Concepts
At Mars, proposals have been studied for tethered elevator type structures in a low-Mars orbit, and extended from the two moons in orbit around the planet, Phobos and Deimos. Both moons are in the same orbital plane around Mars at near equatorial inclinations. Tether structures extended toward and away from Mars on each of these moons have been shown to provide a means of payload transfer to and away from Mars that would significantly reduce propellant requirements.
The material strength required for a system like this appears to be within the limits of current technology. In one possible design, a Kevlar tether is used to transfer a 20,000-kg payload from a low-Mars orbit to a Mars-Earth transfer orbit. Such a system in orbit around Mars could be one way to establish a permanent transportation infrastructure for ongoing exploration and development of the Mars system.
Figure 8. Mars Space Elevator Transportation System
3.3 Compression Structures
The third technology area is in the continued development of tall towers for Earth applications, and eventually for space applications. This requires the introduction of lightweight composite structural materials to the general construction industry for the development of tall tower and building construction systems. The goal is to foster the development of multi-kilometer height towers for commercial applications (i.e., communications, science observatories, and launch platforms).
3.3.1 Tall Towers
Today, the world's tallest self-supporting building is the CN Tower in Toronto, Ontario, Canada. It was built from 1973-1975, is 553 m in height, and has the world's highest observation deck at 447 m. The tower structure is concrete up to the 447-m observation deck level. Above the observation deck is a steel structure supporting radio, television, and communication antennas. The total weight of the tower is 300,000 tons. The height of existing towers and buildings today are not limited by construction technology or by materials strength. Conventional materials and methods make it possible even today to construct towers many kilometers in height. When considering how high a tower can be built, it is important to remember that it can be built out of anything if the base is large enough. Theoretically, you could build a tower to GEO out of bubble gum, but the base would probably cover half the sphere of the Earth. The height of existing towers and buildings today are not limited by building technology or by materials strength; it is simply that there has not been a good economic reason to build towers any taller than have been built so far.
One approach in determining the maximum height practical for various tower construction materials is to look at the maximum height of a column that can just support its own weight. This is done by dividing the column materials' strength by its density.
Strength, lbs. per square inch (psi) / Density, lbs. per cubic inch (pci) = height, in inches.
The two most common tower construction materials yield the following results:
Structural steel = 60,000 psi / 0.3 pci = 200,000 in. = 5 km theoretical
Aluminum = 60,000 psi / 0.1 pci = 600,000 in. = 15 km theoretical.
If new composite materials were introduced to the conventional construction industry, then even greater heights would be possible. Using the same analysis yields the following result.
Carbon/epoxy composite = 300,000 psi / 0.066 pci = 4.5 million inches = 114 km theoretical.
Real designs will use a lower design stress and have structural overhead for horizontal members to provide stability and strength against buckling. Instead of a single column, it will be a tapered tower with a height-to-base width ratio of 20 (i.e., a 20-km-tall tower would require a 1-km-wide base). By increasing the base width and distributing the load of the upper sections over more area and more members in the lower sections, then even taller tower heights are conceivable.
This tall tower concept uses a fractal truss design with the main columns made up of smaller trusses, which in turn are made of smaller trusses. This approach minimizes wind load, provides reasonable component sizes, and would lend itself to a robotic assembly method. For stability against buckling, the height-to-base width ratio of 20 is used.
3.3.3 Tall Tower Applications
Tall towers that extend up through the Earth's thick atmosphere appear to have numerous applications for government and commercial purposes and appear to be feasible in the near term from a materials capability standpoint. Two concepts illustrated in this section help explain the wide variety of uses that tall towers could perform.
Figure 10 illustrates a tall tower concept 50-km in height constructed from composite materials. Its primary use is to launch payloads from a rotating tether to LEO or to a LEO space elevator shown over the horizon.
Other uses for towers of this height include the following:
- Communications boost: A tower tens of kilometers in height near large metropolitan areas could have much higher signal strength than orbital satellites.
- Observation platform: A permanent observatory on a tall tower would be competitive with airborne and orbital platforms for Earth and space observations.
- Solar power receivers: Receivers located on tall towers for future space solar power systems would permit use of higher frequency, wireless, power transmission systems (i.e., lasers).
- LEO communications satellite replacement: Approximately six to ten 100-km-tall towers could provide the coverage of a LEO satellite constellation with higher power, permanence, and easy upgrade capabilities.
Figure 11 illustrates a launch arch concept that uses a series of tall towers in combination with an electromagnetic launch assist rail. At 15 km in height, this system has the potential to significantly improve the performance of future reusable launch vehicles by providing a permanent first stage and by launching above 83 percent of the Earth's atmosphere.
Other potential uses for such a system include the following:
- Variable-gravity (g) launch: A rail designed for low-g launch assist could use a similar configuration designed for high-g launch. Propellants and raw materials could be delivered to LEO with minimal upper stage requirements.
- Entertainment: Tourism to the edge of space where passengers could see the darkness of space and the curvature of the Earth's horizon.
Advantages of these types of structures include lower gravity, no weather-related interference, accessibility to upgrade of mounted systems, and permanence. These features over the long term could provide significant economic advantages over conventional launch systems and some LEO satellite systems. Many other ideas that have yet to be envisioned are always possible from new technology developments like this.
3.4 Electromagnetic Propulsion
The fourth technology area is in the continued development of electromagnetic propulsion systems. Electro-magnetic propulsion is important to the space elevator concept because of the need for a high-speed, non-contact transportation system to quickly traverse the space elevator's great length.
Technology development would include the application of electromagnetic systems to a variety of transportation systems including MagLev for propulsion of trains, MagLifter for launch assist of new, reusable launch vehicles, and mass driver and rail gun systems for propulsion of payloads to orbit at high-g levels.