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The concept of a stairway to heaven is a recurring theme throughout mythology. From the Far East came tales of magicians who could toss the end of a rope into the air, where it would stay there, hanging from seemingly nothing. In the ancient children's story, "Jack and the Beanstalk", a beanstalk grows up into the clouds and Jack climbs up the beanstalk to find adventure and riches in the clouds. In the Old Testament we have two examples. In the Book of Genesis, Chapter 11, Verse 4, early mankind used bricks mortared with bitumen in an attempt to build themselves a tower with its top in the heavens. Then later in Chapter 28, Verse 12, Jacob dreamed that there was a ladder set up on the Earth, the top of it reached to heaven, and the angels of God were ascending and descending on it.

Today, we know that one sure way to get to the heavens is to use rocket power. In fact, rockets have been so successful that other methods to reach the heavens have been nearly forgotten. How about those other ancient techniques? Can we build a tower or magic beanstalk and climb up to the stars? Or can we hang a rope ladder onto the nearest star and lower the ladder down to the ground?

Although it may be hard to believe, it may not be too long from now that instead of leaving Earth using rockets, we will emulate Jack and climb up a magic beanstalk to find our own adventure and riches in space. Or, instead of a beanstalk based on magic, perhaps we can build a modern version of the Tower of Babel.

Is it possible to build a tower up into space? Aerospace engineer Dani Eder has carried out feasibility studies to see if very high towers could be built as platforms to lift telescopes up out of the atmosphere, or as an aid in launching rockets into space. It turns out that under reasonable engineering limits such as cost and adequate safety margins, steel towers could be built up to sixkilometers high and aluminum ones to almost tenkilometers high. For comparison, Mount Everest is about ninekilometers above sea level (but on a very broad base). A tower built using presently available graphite-epoxy composite materials could reach a height of fiftykilometers, the altitude that officially designates the boundary of space. But it seems like the limits of the compressive strength of materials will prevent us from ever raising a tower up to the 400 kilometer altitude that the Space Shuttle flies at.

How about a modern version of the Hindu rope trick? Out at the very special distance of 36,000kilometers from the surface of the Earth (about six Earth radii), there now exist dozens of satellites in geostationary orbit. Here the rotation of the satellite in its orbit is 24hours, exactly equal to the rotation of the Earth below. Thus, as you stand on the Earth and look up into the sky, the geostationary satellites stay fixed in one position above you while the stars slowly rotate from east to west.

Suppose some friendly giant in a geosynchronous satellite were to let down a long cable—36,000kilometers long. If the cable were strong enough to hold its own weight, then the cable could reach down to the surface of the Earth. It would be a Skyhook, a magic beanstalk in reverse. Given adequate supplies stashed along the way, a light-weight spacesuit, and enough time, a person would be able to emulate Jack and climb up into space (resting along the way occasionally to enjoy the scenery) instead of having to use a rocket.

One of the first persons to think of the concept of a cable hanging down from geostationary orbit was a Soviet engineer and popular science writer named Yuri Artsutanov. In 1957 a fellow graduate of the Leningrad Technological Institute told him about a tiny whisker of material that was so strong that it could support a 400 kilometer length of itself in the Earth's gravity field. Artsutanov realized that a hanging cable that long would have part of its length far away from the Earth where the gravity field is weaker. Consequently the length could be even longer. Also, if the cable were tapered it could be made even longer still. He worked out the relatively simple equations, but he never published them in an engineering journal where they could be found. Instead, he published the idea as a popular article in the Sunday supplement section of Komsomolskaya Pravda (Young Communist Pravda) in 1960.

A Skyhook would be built from the middle out, starting with a cable-making machine at a Central Station in geostationary orbit. For balance, the machine would extrude two cables, one upward and one downward. The cables would be thin at first, then, when the length of the cable hanging down became longer, the thickness of the cable would have to be increased to provide enough strength to support the increasing weight below. The thickness of the upward-growing cable would also have to increase as the cable became longer, but for a different reason. Instead of the Earth gravity pulling on the cable, the pull is due to the centrifugal force from the once per day rotation about the Earth.

If the extrusion rates of the two cables are carefully controlled, then the net pull on the Central Station in geostationary orbit would be zero, and the cable laying machine would remain in geostationary orbit. Eventually, the lower end of the cable would reach the ground (or the top of some convenient near-equatorial mountain) 36,000kilometers below. At that time, the outgoing cable would be 110,000kilometers long. The outgoing cable has to be longer than the Earth-reaching cable because of the way the gravity forces and centrifugal forces vary with distance.

If everything was done smoothly and slowly, there would be no horizontal motion of the cables. In fact, the gravity and the centrifugal forces combine to produce a force that helps to maintain the cable exactly vertical. The bottom end of the long cable could now be anchored to the ground so it doesn't blow about in the winds, and a large counterweight (a small asteroid) would be attached to the outer tip. The counterweight, like a stone in a giant sling, would keep the cable under moderate tension to help keep it straight.

A nearly magical material would be needed to construct a Skyhook for the Earth. What is needed is a material that is both strong and light. The best would be a Skyhook made from a tapered fiber of perfect diamond crystal. Unfortunately, making tapered diamond fiber will require the use of a fabrication technology that is presently indistinguishable from magic, and we will have to wait a while for that. In the meantime, crystalline graphite fiber is the best candidate material. Theoretically it is twenty times stronger than conventional steel and four times less dense, making it potentially eightytimes better than steel for Skyhook cables. It is because of this high strength-to-weight ratio that you find graphite fibers used in tennis rackets, fishing rods, golf clubs, and other sports equipment. Weaving large cables of graphite fibers with strengths near that of the present tiny whiskers is the major technical hurdle that must be overcome if terrestrial Skyhooks are to became a reality. In the coming years we can expect the strength of the graphite cables to improve until they are adequate for terrestrial Skyhooks. Interestingly enough, they are already more than strong enough for constructing Skyhooks on the Moon and Mars.

Actual measurement of tiny graphite whiskers show a tensile strength of over twomillion newtons per square centimeter or threemillion pounds per square inch. With that strength, a one square centimeter cable of crystalline graphite could lift 210tons in the gravity field of the Earth. A one centimeter cross-section crystalline graphite cable weighs about 220kilograms per kilometer of length, so with a 210ton lifting capability, a graphite cable could support almost a 1000kilometer length of itself in the gravity field of the Earth. By building the cable with a taper to it, it can be made even longer. Fortunately, the gravity field of the Earth decreases with altitude, so that less taper is needed at the higher altitudes. With a taper of ten-to-one, a graphite cable could be built to go all the way out to synchronous orbit, some 36,000kilometers above the Earth's surface—and beyond.

The first cable to be lowered down would have a total mass of about 6000tons. It would have a diameter of about one millimeter at the Earth's surface and would be able to lift only one or two tons. The initial cable, however, could be used in a boot-strap operation to lift more cable up from Earth until it was strengthened a hundred times. Once the Skyhook is in place, then it could be climbed like the proverbial magic beanstalk. For smaller diameter cables, special electrically powered cars would be built to climb up on the outside. If the Skyhook design used a number of cables arranged in a hollow structure, the electrified tracks could be built inside the structure. As each car climbs the beanstalk from the Earth's surface into geostationary orbit, it would consume an appreciable amount of electrical energy. The cost of the electricity, two dollars per kilogram, would be much less, however, than the present cost of using rockets, which is five thousand dollars per kilogram.

As the cable cars climb up the Skyhook, they are always positioned above their anchor point on the Earth below, but like a stone in a sling, they have a higher absolute velocity through space than the anchor point. An object dropped from a cable car during the first few kilometers of travel would fall nearly straight down to the surface below. As the car climbs higher, the point of impact would move toward the east, since the object would leave the cable car at a higher horizontal velocity than the more slowly moving anchor point on the surface of the Earth below. At the 25,000kilometer point on the cable, an object dropped from the car would have so much horizontal velocity that it would sail over the horizon of the Earth and go into a highly elliptical orbit. At altitudes higher than 25,000kilometers, objects dropped out of a cable car would go into orbits that became more circular, and closer to a 24hour period, as the cable car approached 36,000kilometers.

The turnover point for the cable cars would be the Central Station at 36,000kilometers up. Here the gravity and centrifugal forces balance. If you drop something out of the cable car (or step out yourself), there will be no motion relative to the cable car. At this point the cable car is traveling horizontally at geostationary orbit velocity. Communication satellite payloads brought up on the cable cars would be simply floated out to become synchronous satellites.

Cars continuing beyond the Central Station would be pulled along the cable by the every-increasing centrifugal force, like a skater at the end of a "crack-the-whip" chain. The cable cars would have to brake to keep from flying out too fast. If the braking were done by an electric motor, the braking energy could be turned into electricity instead of heat and used to raise the next cable car on its way up.

On reaching the ballast stone, the cable car would be 150,000kilometers from the center of the Earth and moving with a horizontal velocity of elevenkilometers a second. If the cable car were to let go of the cable at just the right time, the car (now turned spacecraft) would be able to coast slowly to Saturn on a minimum energy orbit or travel rapidly to all the other planets nearer than Saturn.

By reversing the process, returning payloads can be brought back to Earth without the use of heat shields, braking rockets, or atmospheric braking. Also, a Skyhook is a conservative system. If electric motors are used to lift payloads up the elevator and brake payloads going down the elevator, and the mass flow is the same in both directions, incoming traffic would provide all the energy needed to power outgoing traffic.

An Earth Skyhook would be an engineering marvel. The job of building the 36,000kilometer section down to the Earth would be equivalent in difficulty to building a suspension bridge completely around the Earth. In order to lift appreciable loads, say 100tons at a time, the Skyhook would have to weigh about 600,000tons. Fortunately, the carbon needed for the graphite fibers can be found in special kinds of asteroids called carbonaceous chondrites. After the carbon was extracted from the asteroid, the remaining slag could be used as the counterweight.

The construction job would be staggering in scope. To build the 36,000kilometer Earthgoing section of the Skyhook in five years would require an average construction rate of cable and track of 20kilometers a day. After the Skyhook was built, the cable cars would have to travel at more than 6,000kilometers per hour (ten times faster than a jet airplane) in order to make the trip up to the Central Station in less than six hours. Some kind of magnetic levitation design for the track and cars would be needed, for no rubbing or rolling contact can be tolerated at those speeds.

For protection against space debris, the Skyhook would probably be constructed of many interconnected cables in a large open structure. Thus, orbiting debris smaller than a meter in size would cut only one cable at a time and the slack would be taken up by the others until repairs could be made. Even if cut completely through, the Skyhooks would be quite safe, since the entire structure is in orbit. If a large object like an out-of-control airplane or satellite accidentally cut the cable, the portion below the cut would fall to the ground, but the portion above would stay almost in the same place, rising only slightly because of the reduced load. After things had quieted down, a new starter cable could be dropped down from the cut-off end, and contact reestablished with the Earth's surface.

Mars is the best planet in the solar system for a Skyhook, having both a shallow gravity well and a high rotation rate. Since the 24.5 hour rotation rate for Mars is nearly the same as that of Earth, while its gravity field is only thirty-eight percent that of Earth, a Mars Skyhook using graphite would have to mass only fortytimes what it could lift. Mars also has a twenty-kilometer high mountain on the equator, Mons Pavonis, that can be used as an anchor point, and a small moon, Deimos, that is available at almost just the right orbit to act as the counterweight. As Arthur Clarke showed in his novel, The Fountains of Paradise, the problem of a possible collision between the Martian Skyhook and the moon Phobos can be avoided by deliberately exciting the first vibrational bending mode of the Skyhook so that the cable "twangs" to one side just as Phobos passes by. Similar techniques could be used on Earth to avoid the thousand or so larger satellites that orbit the globe.

The first precursors of these Skyhooks have already flown in space. Although the Space Shuttle is a remarkable vehicle that can haul large payloads into orbit, it has one major problem. The volume of space that the Space Shuttle can reach is limited to the small region just outside the atmosphere of Earth. The Shuttle cannot fly up to higher orbits. In fact, nearly all satellites launched by the Shuttle have to have a booster rocket to put them into their final orbits.

It turns out that there is a way for the Space Shuttle to put spacecraft into their proper orbits without using rockets. The Shuttle can use a long cable or tether to "fly" the spacecraft into another orbit. But what makes the spacecraft on the end of the tether "fly" to different altitudes is not the pressure of air, but the tides of gravity.

Because of the way that gravity and centrifugal forces work at orbital altitudes, it is just as easy to send a tether up to a higher altitude as down. The Shuttle, in its orbit about the Earth, has the pull of the Earth's gravity exactly canceled by the centrifugal force due to its orbital motion. It is in free-fall. If the Shuttle sends a spacecraft down on the end of a tether, the spacecraft will experience a stronger gravity pull, but its motion will be that of the Shuttle, so the centrifugal force is smaller than the gravity force and the spacecraft is pulled downward. If the Shuttle sends a spacecraft up on the end of a tether, the Earth gravity pull will be weaker, but the spacecraft, moving at the same speed as the Shuttle, experiences a centrifugal force that is stronger than the gravity, so the spacecraft flies outward.

The first test of a space tether was attempted in July 1992 using the Space Shuttle. Unfortunately the test had to be aborted early because a too-long bolt jammed the reel for the tether line. In the planned Tethered Satellite System (TSS) experiment, a conducting tether 2.5millimeters (a tenth of an inch) in diameter and twenty kilometers (twelve miles) long was to deploy an Italian research spacecraft upward from the Shuttle. The spacecraft was to be made electrically positive, so as to collect electrons from the ionosphere and pass them down the conductive tether to the Shuttle, which would put the electrons back into the ionospheric plasma with the help of an electron emitter. The motion of the conducting cable through the magnetic field of the Earth was expected to generate up to five thousand volts of electrical potential between the two ends of the tether, and produce up to fivekilowatts of electrical power. (The electrical energy, of course, would come from a decrease in the kinetic energy of the Shuttle, causing a decrease in its orbital altitude.) The next TSS experiment on the Space Shuttle is scheduled for 1995-6. It will involve a repeat of the electrodynamic experiment using a twentykilometer upward-going tether. This will be followed a year or so later by a 120kilometer downward-going tether trolling an aerodynamic experiment through the upper atmosphere of Earth.

The first successful tether experiment was carried out in March 1993 by a group led by Jim Harrison and Chris Rupp of NASA Marshall Space Flight Center in Alabama. The tether hardware was manufactured by Joe Carroll of Tether Applications of California, a one-person small business. Carroll wound the tether on its spool in the loft of his home, showing that you don't need to be a giant aerospace corporation to make workable space hardware. The tether was a twentykilometer long braided polyethylene string, that was three-quarters of a millimeter in diameter and massed sevenkilograms. The SEDS (Small Expendable Deployment System) tether experiment piggybacked on a U.S. Air Force Delta II rocket that launched a Navstar Global Positioning Satellite from Cape Canaveral. After the satellite had been released from the booster stage and the primary mission had been accomplished, the SEDS package was activated an orbit later, and the SEDS payload deployed over Guam. As the payload drifted away from the Delta booster, it pulled the tether off the spool though a tension controller. The constant pull of the tether applied a deceleration force to the payload that slowed it down and caused it to fall below the booster. By the time the full twenty kilometers of tether had been pulled out one orbit later, the payload had been slowed enough that it could no longer stay in orbit, and it fell into the atmosphere at the preplanned reentry point over Baja California. This experiment showed that something as simple, cheap, and as benign as a ball of string can replace a retro-rocket for precisely deorbiting payloads from space.

In the next SEDS experiment, the tether will remained fastened to the Delta booster stage, and the payload will hang down below the Delta as they both orbit the Earth. The objective of this experiment will be to collect data on tether lifetime due to micrometeorites cutting the tether. It is estimated that there will be a seventy percent chance of the tether being cut in the three-month long experiment.

The lifetime of tethers in space is limited by space debris. Recent measurements of crater number versus size on aluminum panels which had spent six years in space, indicate that most space debris is still caused by micrometeorites and not manmade space debris. Using this recent information on space debris, the lifetime of the TSS electrodynamic single-strand tether with its diameter of 2.5millimeters and length of twentykilometers would be predicted to be six months. For the follow-on aerodynamic experiment tether with a smaller diameter and a length of 120kilometers, the predicted lifetime is only a half a month. Longer tethers would have even shorter lifetimes unless their diameter (and consequently their mass) were increased.

The lifetime problem has been overcome with the design by the aerospace engineer, Rob Hoyt, of a failsafe multistrand tether that increases the single strand lifetime by factors of a hundred or more for a mass penalty of only two. The lifetimes of this multistrand tether are so long, and the degradation is so graceful and failsafe, even in the highly unlikely event of multiple cuts in close proximity, it looks like these multistrand tether designs will never need repair during their operational lifetime. With this tether design available, transorbital, lunar, and interplanetary propulsion scenarios can be contemplated without undue concern for manmade space debris or natural meteorites.

As the NASA engineers fly more of these space tether systems without incident, then perhaps some of the more risky tether experiments can be attempted. A payload can be sent upwards a number of hundreds of kilometers from the shuttle on the end of the tether. Normally a satellite at this altitude will be moving slowly, but the Space Shuttle will be pulling it along with a velocity appropriate to the Shuttle's much lower orbit. Since the upwards deployed satellite is moving faster than normal, if it is released from the end of the tether it will fly up into a higher elliptical orbit. The peak of this orbit could be high enough to catch onto a tether hanging down from a space station in geostationary orbit. Longer tethers could even launch a payload into an Earth escape trajectory.

Simple tethers may also be useful in planetary exploration. In one proposed approach, a surface sampling payload is sent out ahead of a spacecraft in low orbit around one of the airless bodies of the solar system, like the Moon, Mercury, the asteroids, or one of the smaller moons of the outer planets. The main spacecraft would then pull on the tether, slowing the payload down so it is shifted into an elliptical orbit that intersects the surface of the planet ahead of it. The payload would then drop like a well-walked wet fly and touch down for five to ten seconds on the surface of the planetoid. As the main spacecraft passed by, it would pull up the payload containing surface analyses and the valuable core sample extracted during its brief sojourn on the surface.

There is another version of the Skyhook, I call the Rotavator. It uses a cable that is much shorter than the geostationary orbit Skyhook. The Rotavator rotates as it orbits about the Earth, the ends of the cable touching down near the surface. This concept was the brainchild of Yuri Artsutanov, the same person who also first thought of the Skyhook. In 1969 Artsutanov published the idea of synchronously rotating cables as a popular article in the magazine Znanije-Sila (Knowledge is Force). The magazine illustrator's title drawing for the article shows a huge wheel rolling over the surface of a small Earth—an apt illustration of the concept, since the rotating cable acts like a pair of spokes rotating inside an invisible wheel. It was Hans Moravec, however, who published the first technical paper on the concept.

The Moravec design for a Rotavator uses a cable that is 8500kilometers long. This is two-thirds the diameter of the Earth, but only one-quarter the length of a 36,000 kilometer geostationary Skyhook. The central portion of the cable would be put into an orbit that is 4250kilometers high with a period of 183minutes. The cable would be set to spinning at one revolution every 122minutes. Three times each orbit, once every 61minutes, one of the ends of the cable would touch down into the upper portions of the Earth's atmosphere. These entry points would be the three ports of embarkation from the Rotavator Transportation System. Because of the large dimensions of the bodies involved, the ends of the cable would seem to come down into the upper atmosphere nearly vertically, with almost no horizontal motion. The cable, although made of one of the stiffest materials known, would still have some stretch to it. This means that a coupling vehicle at the end of the cable could use jets and aerodynamic forces to "fly" the elevator car at the end of the cable to a rendezvous point. This could allow the elevator car to arrive ahead of its nominal touchdown time and delay its return to orbit. There would be almost a full minute available for dropping off and picking up cargo and passengers.

A 8500kilometer long Rotavator made out of crystalline graphite and designed to touch down three times per orbit would have a taper of about twelve to one. To be able to lift a 100ton cargo into space it would have to have a total mass of about 7500tons. At touchdown the end of the cable would approach and leave the Earth with an acceleration of 1.4Earth gravities. Counting the one Earth gravity field of the Earth itself, there will be a total acceleration at liftoff of only 2.4Earth gravities, less than that experienced by the Space Shuttle astronauts.

A future scenario for a Rotavator Transportation System would probably work like this: You check in at any one of the major hypersonic airports around the world, clear Earth customs, and board a small capsule six meters in diameter and twenty meters long. The capsule will look like a section cut out of a modern widebodied jet aircraft. There are seats for about thirtypassengers, with cargo space below. There is a diminutive cockpit/control center in a bubble topside, containing an alert capsule crew. The crew checks on the sealing of the capsule as it is carried away on the bed of a truck to the corner of the field, where automatic rollers move the capsule onto the flatbed spine of a hyperlift cargo airplane. Fairings slide out to merge the body of the capsule into the midsection of the plane. Now aerodynamically restored, the aircraft taxis down the field and takes off. It reaches altitude over the nearby ocean and accelerates to Mach3. The capsule crew has little to do except monitor the radar displays while the crew of the hyperlift plane takes it higher in altitude and speed, heading southward for a rendezvous in space and time with the Rotavator. The rate of climb of the aircraft slows as the atmosphere becomes thinner. The plane passes through the fiftykilometer altitude that used to mark the difference between an pilot and an astronaut, but there is no pause as it climbs higher on its powerful oxygen-augmented jets.

There is crackling conversation between the aircraft crew, the capsule crew, and the tether grapple crew still hundreds of kilometers overhead, diving downward on the rapidly dropping grapple-craft at the end of the Rotavator cable. The aircraft crew unlatches the capsule and dives down and away, leaving the wingless egg to soar on through the nearly empty upper atmosphere in a long arcing trajectory that will end in a parachute recovery if something goes wrong. The capsule sails upwards toward the rendezvous point with its crew busy. Looking out the sides through the heavily tinted windows, you can see attitude control jets flash as the crew keeps the capsule in proper position and orientation for the pickup. You then look up through the ceiling ports to see a similar flaring of jets from the grapple-craft streaking vertically downward, trailing a long thin thread. The grapple-craft comes to a hovering stop a little way above the capsule, then slowly drifts downward.

Carefully, taking their time, the grapple-crew attaches the four grapple hooks to the lifting lugs at the top of the capsule. The capsule crew confirms attachment, then the grapple-craft adds its jets to those of the capsule to match speed with the cable. The free fall environment of the dropping capsule is slowly replaced with an upward acceleration. In ten seconds, the acceleration reaches 2.4gravities, and you are glad that you are strapped into the comfortable seat beneath you. Having started at 80kilometers altitude, in five minutes the capsule reaches 260kilometers altitude and a velocity of 1.2kilometers per second. The acceleration slowly drops as you continue your ride into outer space, traveling on the whip end of a fine thread. After thirty minutes the capsule is at the high point in its giant swing through space and you look down on the blue-white globe 8500kilometers below. There is a warning klaxon and an announcement from the capsule pilot. You strap in securely, there is a multiple click as the grapples release, then you and the capsule are in free fall, heading for the Moon with a velocity of ninekilometers per second. You settle down with a good book. It will be twelvehours before you get there.

After two books, two meals, and a nap (disturbed by the unfamiliarity of free fall), the capsule arrives in the vicinity of the Moon. Here it is again met by a grapple-craft crew on the Lunar Rotavator and is lowered almost to the lunar surface. There the capsule is handed over to a jet-tug that takes you to Copernicus Base. You are home once again, in familiar surroundings, and glad to get away from the oppressive gravity, air, and crowds of Earth.

A Rotavator on the Moon would have an enormous advantage over rockets for providing resupply and crew rotation needed for space industrialization. A Lunar Rotavator could be made with presently available materials, like the superfiber Kevlar. With a density of only 1.44 times that of water and a tensile strength of 280,000newtons per square centimeter (400,000psi), it has about five times the strength-to-weight of steel. Kevlar is presently being used in large quantities for bullet-proof clothing, radial tires, and parachutes. A 130ton Kevlar Rotavator around the Moon would be able to lift and deposit tentons every twentyminutes. Rotavators could also be used on any of the other moons in the solar system. Jupiter's Ganymede and Saturn's Titan are larger than the Earth's Luna, but a Kevlar Rotavator with a taper of six to one would suffice for these bodies.

A variant of the Rotavator concept is the Bolo satellite. The Bolo is a more modest version that is shorter in length, but rotates faster. It has been studied in detail by the former scientist-astronaut Philip Chapman as a possible near-term addition to a more general space transportation system. His basic design is a rotating cable with relatively large end-mass stations. Some versions have a central station at the center of mass of the system.

In the Chapman design for a Bolo Space Transportation System, one Bolo would be in low Earth orbit and the other would be in geostationary orbit. A payload would be launched from the Earth using either a rocket, the Space Shuttle, or some new hypervelocity single-stage-to-orbit vehicle. The payload would go into a low energy arcing trajectory that would rendezvous with the lower tip of the Bolo in low Earth orbit. The payload is attached to the Bolo and the launch vehicle returns to Earth. The payload is released from the Bolo at the highest point of its swing. This puts it into a transfer ellipse orbit with the apogee near geostationary orbit. At the top of the transfer orbit the payload would rendezvous with the second Bolo, whose center of mass is in geostationary orbit. If the payload is going on to the Moon or elsewhere, it waits until the direction is right and is slung by the Bolo into an escape orbit. If its destination is geostationary orbit, it is hauled up the cable of the Bolo to the Central Station and floated off to join the rest of the satellites ringing the Earth at 36,000 kilometers altitude. While awaiting the next payload, the angular momentum and energy of the Bolos would be replenished by means of on-board, high-efficiency solar powered thrusters.

A big advantage of the Bolo Space Transportation System is that the payload would not have to be launched with the normal full orbital velocity of almost eight kilometers a second that is needed to get into low Earth orbit. Instead, the payload only needs to reach six kilometers a second. This relatively small change in required launch velocity capability translates into a fifty percent increase in payload mass delivered to a Bolo rendezvous compared to insertion into low Earth orbit. More importantly, the reduced velocity requirement greatly increases the feasibility of constructing a single-stage-to-orbit (SSTO) aerospace plane than can take off from a normal runway and deliver payloads into space.

At maximum throughput, this system is capable of transferring over 4000 payloads of twenty-five tons each or 100,000 tons to geostationary orbit each year. This is thirty times the mass of the Bolos in the system. For a while, as we built space stations and solar power satellites, the mass flow outward will be greater than the mass flow inward. The energy and momentum in the payloads comes from the Bolos and it will be necessary to haul up propellant to keep the Bolos spinning and orbiting at the proper altitudes. To minimize the amount of propellant needed, it would be desirable for the mass flow down through the Bolo system to equal the mass flow up.

To supply energy to a Bolo system, a new breed of wildcatter might spring into being. It might be profitable for some entrepreneur to haul back an iron-nickel asteroid and sell chunks of it to the Bolo Space Transportation System operators—not for raw materials to build things, but as a source of energy to power the Bolos. Blocks of asteroid would be sent swinging down the Bolo system as payloads came swinging up. A simple drag brake, made from asteroidal material, might be sufficient to reduce the terrestrial impact velocity below the speed of sound and the asteroidal material would pile up into a mountain of nickel-iron ore that could be sold for scrap (by the same entrepreneur) after all the valuable gravitational potential energy had been extracted from it.

Similar spinning Bolo cables in solar orbits between the planets could act as transfer points or "momentum banks" to cut the travel time between the planets in the solar system. Instead of heading off on a low velocity trajectory toward a distant planet that may be in a bad position on the other side of the Sun at that time of year, the capsules would head at high speed for the nearest momentum transfer Bolo. As they approach the spinning cable, they would choose the point along the spinning thread that matches their approach velocity. Once attached, the capsule would then move along the cable, climbing up or down in the centrifugal field, until the capsule reached the point on the Bolo that had the velocity needed for the next leg in the journey. There would be a short waiting period until the direction was correct, then with a command to the attachment hooks, the capsule would be freed from the Bolo to go flying off into space toward its distant objective. As long as more mass is dropped inward down the gravity well of the Sun than is going out, no energy source would be needed to operate this interplanetary space transportation system once it was set into motion.

Hanging a cable down from the sky using the tensile strength of materials is just one way of making a magic beanstalk. There is another way. Like Jack's magic beanstalk, this beanstalk grows from the ground up, but unlike a Tower or a Skyhook, it does not depend upon either the compressive or tensile strength of materials. I call it the Space Fountain, for it holds objects up in space in the same way that a water fountain supports a ball bobbing at the top of its vertical jet of water.

The Space Fountain concept originated in early 1980 in the etheric depths of a computer net. Some scientists who usually work in artificial intelligence, Marvin Minsky of MIT, and John McCarthy and Hans Moravec of Stanford were speculating back and forth over the net about variations on the Skyhook concept with some scientists at Lawrence Livermore National Laboratory who usually work on laser fusion, Roderick Hyde and Lowell Wood. One of the ideas was a method of supporting the upper ends of a Skyhook at altitudes that were much less than geostationary orbit altitudes. This would be done with a stream of pellets that would be shot from a space platform hovering motionless up at 2000 kilometers altitude to another platform partway around the Earth. The pellets would be deflected by that platform to the next platform until the polygonal pellet stream made its way around the Earth back to the original station. The deflection of the pellets at each station would be sufficient to support that station in the gravity field of the Earth at that altitude. Since the stations would be only 2000 kilometers from the surface of the Earth instead of 36,000 kilometers, it would be more feasible to find materials strong enough to hang Skyhooks from the stations down to the surface of the Earth. There was still some concern expressed by the computernet debaters whether a strong enough material could be found to make a cable even 2000 kilometers long.

I joined the discussion on the net at about that time and suggested that instead of a dynamic compression hexagonal pellet stream held together with Skyhooks under tension, that a pellet stream be shot straight up from the surface of the Earth to support a pellet deflector station at the upper end that would reflect the pellet stream back down to the surface again. There was initially some skepticism by the others on the net that the idea would work, because of the Earth's atmosphere at the lower altitudes and the Coriolis forces due to the rotation of the Earth. Further hard work and detailed engineering calculations by Rod Hyde showed, however, that the concept was valid. Hyde has now worked out all the engineering design details for a Space Fountain right down to the design of the transistors to switch the currents in the projectile accelerators and decelerators.

In the Hyde design for a Space Fountain, a stream of projectiles is shot up the bore of a hollow tower. As the projectiles travel along the tower they are slowed down by electromagnetic drag devices that extract energy from the upgoing stream and turn it into electricity. As the projectiles are braked, they exert a lifting force on the tower which supports the weight of the tower. When the projectiles reach the top of the tower, they are turned around by a large bending magnet. In the turnaround process they exert an upward force on the station at the top of the tower, keeping it levitated above the launch point. [See Figure 5.]

As the projectiles travel back down the tower they are accelerated by electromagnetic drivers that use the electrical energy extracted from the upgoing stream of projectiles. The push exerted by the tower drivers also acts to support the weight of the tower. The projectiles reach the bottom of the tower with almost the same velocity that they had when they were launched. The stream of high speed projectiles is then bent through 90 degrees by a bending magnet so that it is traveling horizontally to the surface in an underground tunnel. The projectile stream is then turned in a large circle by more bending magnets and energy is added by electromagnetic drivers to bring the projectiles back up to the original launch velocity. The beam of projectiles is then bent one more time by 90 degrees to send it back up the tower again to repeat the cycle. Thus, the Space Fountain acts as a continuous mass driver with captive projectiles. The various parts of the external structure are stressed by the transfer of momentum from the pellet stream. Together, the stressed structure and flowing projectile stream form a rigid, stable structure that is not limited in height by the strength of materials.

Since the projectiles are slowed down or sped up just enough to balance the gravitational force on the tower at every point, there is no requirement anywhere for ultrastrong materials. In the lower parts of the tower there will have to be an airtight pipe supported between the Deflector Stations to keep out the atmosphere so that the drag on the projectiles is negligible. But after the first one hundredkilometers the only structure that would be needed is a minimal framework to hold communication and power lines, and the guide tracks for the elevator cars.

To first order, no energy is needed to support the Space Fountain. When the projectiles return to the base of the tower, they have essentially the same speed and energy as they started with. Their momentum has been changed, but not their energy. As a result, the input power required to support the Space Fountain is determined by the inefficiency in the electromagnetic motors and air drag on the projectiles.

Fig. 5 - Space Fountain


One of the major advantages to the Space Fountain concept is that it can be built slowly from the ground up. The driver loop and the bending magnets in the Base Station are constructed first, then the Top Station with its turnaround magnets is constructed right above it. The system is loaded with projectiles and tested out at full power with the Top Station sitting safely just above the Earth's surface. Once these major components have been thoroughly tested out, then the power is increased, and the projectile velocity rises until the Top Station starts to lift off the ground. More projectiles are added and the Top Station rises up a few hundred meters, pulling up out of the ground a section of vacuum pipe and the first Deflector Station with it. The next Deflector Station and section of pipe are assembled around the exit and entrance tubes to the driver, power is increased, and the Space Fountain rises into the air as fast as the additional sections can be attached.

A Space Fountain should be built with a good deal of redundancy in it. Instead of just one double projectile stream, there should be two, three, or six, each with a separate power supply. Each stream by itself should be able to support the basic Space Fountain structure with a small amount of safety margin. All of them working together would have sufficient power to haul heavy loads up into space while providing adequate safety margin for minor failures and other problems like heavy transverse wind loads at the surface.

Because the circulating power in the projectiles is so much greater than the driving power, and the round trip time for the projectiles is over three hours, the tower will continue to operate for many hours even if the main drive power failed, as long the control circuits were still operating (they can be powered by electricity extracted from the energy in the projectile stream).

The elevators that would take payloads up the Space Fountain could conceivably ride up tracks on the tower structure using electrical power supplied by the tower, treating the Space Fountain solely as a mechanical structure. A more attractive option would be to design the tower structure, the Deflector Stations, and the elevator cars so that the cars can interact directly with the projectile streams themselves rather than coupling to the tower structure at all. In this manner, both the momentum needed to hold the elevator car up in Earth gravity and the energy needed to raise it to a higher level will come directly from the projectile stream.

One straightforward design, which I used in my science fiction novel Starquake, had a Space Fountain with six separate pairs of projectile streams in a hexagonal pattern. Each Deflector Station was hexagonal with two triangular cutouts to let the triangularly shaped upgoing and downgoing elevators pass through. Each elevator rode on three pairs of projectile streams, dragging on the upgoing streams and pushing on the downgoing streams. Their couplers were strong enough that they could decouple from one or more projectile streams and ride on the rest. By doing this sequentially, they could pass over the stream couplers to the Deflector Stations.

What is most amazing about the design studies that Rod Hyde has done for the Space Fountain is that none of the design parameters requires the use of exotic materials. As Rod Hyde likes to point out, this is a Skyhook that we can build now. Yes, the structure is immense in mass and length compared to anything that we build now. Yes, it will take years to power it up and push it into the sky. Yes, it will take a city-worth of power to keep it running. But the payoff is enormous. The Space Fountain can carry a payload at any one time that is two percent of its total mass. If that payload moves at a reasonable speed of onekilometer per second once it gets out in vacuum, it can make the 30,000kilometer trip up the Space Fountain in eight hours. At that rate, the amount of mass transmitted into space by just one Space Fountain is six million tons per year, just for the cost of the electrical power to run it. This is indeed a magic beanstalk that could open up space for exploration, industrialization, and finally colonization.

A Space Fountain does not have to go straight up. The projectiles from the Base Station could be sent off at an angle in a large partial orbital arc that intersects the ground some distance away. A second Base Station could then receive the stream of projectiles, turn them around and send them back to the first Base Station, completing the loop. This concept has been studied in detail by Paul Birch and Keith Lofstrom. The Keith Lofstrom design is called a Launch Loop. It has a long straight section on top that is used to launch payloads into low Earth orbit. The projectiles used in the Launch Loop are bars of iron. The ends of the bars are interleaved like tongue and groove boards into a continuous ribbon of iron moving at twelve kilometers a second.

Surrounding the two high-speed projectile streams is a non-moving hollow double-track system that shields the moving projectile stream from the atmosphere. The track contains sensors, cables, control electronics, permanent magnets, electromagnets, and parachutes in case of catastrophic system failure. The track supports itself by hanging one centimeter below the ribbon of iron using the attractive forces from permanent magnets augmented by active electromagnetic control forces to maintain the spacing. The track is also designed to support vehicles that ride on the outside of the stationary track using electromagnetic levitation, while extracting kinetic energy by coupling magnetically to the high speed iron ribbon inside the track. The ribbon of iron bars is launched from the West Turnaround Terminal by a mass driver at about a fifteen degree angle to the surface. The ribbon climbs to about 120kilometers altitude where it is deflected by the West Deflector Station into a trajectory that follows the Earth's surface below.

The path of the iron ribbon is that of the orbit of a satellite at 120kilometers altitude modified slightly by the weight of the track that it must support. The twelvekilometer per second "orbital speed" of the iron ribbon is much greater than the true orbital speed of eightkilometers per second at this altitude, so the ribbon has a tendency to fly outward. This net upwards force on the ribbon means it can support a weight of over a kilogram per meter of length of non-moving track while remaining parallel to the Earth's surface. This "straight" portion of the Launch Loop continues on for 2000kilometers to the East Deflector Station, where the ribbon is deflected downward to the East Turnaround Terminal. There the ribbon of iron bars is turned around, brought up to speed with mass driver and launched on the return path.

The vehicles are hauled up on 120 kilometer long elevator cables to the West Deflector Station and placed on the acceleration track. They are launched from there to the east in order to utilize the rotation of the Earth to aid in reaching the desired terminal velocity. The vehicles slip-couple to the rapidly moving iron ribbon with magnetic fields and accelerate at threeEarth gravities. Depending upon their desired final destination, the vehicles can be launched with any velocity up to Earth escape velocity of elevenkilometers per second. The Launch Loop can be used for landing by simply reversing the process, with the kinetic energy of the returning vehicle being put back into the iron ribbon instead of being dissipated as heat. The excess energy can be used to launch another vehicle or turned back into electricity by using the electromagnetic mass drivers as electromagnetic brakes. A single Launch Loop could easily launch a five ton vehicle to escape velocity every hour with an input of 200megawatts of electrical power. At fivecents per electrical kilowatt-hour, that amounts to two dollars per kilogram for launching payloads into space.

An ultimate extension of the Launch Loop concept would be an orbital ring of projectiles. Like many beanstalk concepts, the idea was independently invented by a lot of people, but the person who has done most of the hard engineering studies is Paul Birch. In the Paul Birch design of an Orbital Ring system, a ring of massive projectiles is placed in a low Earth orbit. Riding on this ring, supported electromagnetically, are Ring Stations that stay in one place above some designated point on Earth. Hanging down from these Ring Stations are Skyhooks made from cables with high tensile strength to mass ratio.

Paul Birch has found that since the Ring Stations can be used to deflect the projectiles in the Orbital Ring sideways as well as vertically, it is possible to deliberately cause the Orbital Ring to precess around the Earth instead of staying fixed in inertial space while the Earth rotates beneath it. By making the precession rate large enough, the Orbital Ring can be made to precess at the once per day rotation rate of the Earth. The orbit is now "geostationary" without having to be either at the normal geostationary altitude or even in the equatorial plane. This means that using the Orbital Ring concept, a Ring Station can be positioned above any point on Earth that is desired, and anywhere on the globe can be served by a Skyhook instead of just the poles and the equator. A network of Orbital Ring systems crossing, for example, at the poles, could cover the whole planet with an array of Skyhooks and geostationary Ring Stations. Once a payload has climbed up the Skyhook and reached the Orbital Ring, it can then accelerate horizontally by coupling to the moving projectiles in the Orbital Ring. If there were Orbital Rings around each moon and planet, then transport around the solar system would be fast, easy, and inexpensive.

Tethers, Skyhooks, Rotavators, Bolos, Orbital Rings, Launch Loops, and Space Fountains are definitely forms of space transportation that are almost indistinguishable from magic. Yet soon we will see the first experiments with tethers hanging both upward and downward from the Space Shuttle. If those experiments are successful, then the NASA engineers will become more comfortable with these strange new rocketless forms of space propulsion. They will start to pay attention to their own studies, which show the great benefits to be obtained from the use of long tethers for inexpensively hauling large quantities of mass into and around in space.

Newer materials with higher strength to weight ratios are already coming out of the organic and inorganic materials laboratories, driven by the trend toward composite materials in aircraft, automobiles, and sports equipment. These newer materials will make Tethers, Rotavators, and Bolos technically feasible and perhaps even commercially viable now that good engineering solutions have been found for the space debris problem.

Even the projectile stream concepts might come into fruition within the foreseeable future. The first closed loop projectile systems would be used for energy storage. They would be completely underground and used to provide load leveling in an electrical power grid. Next will come long underground kinetic energy power transmission lines, then perhaps a completely enclosed, non-electric replacement for the overhead line or third rail in subways and electric train systems. We will then be ready to consider Launch Loops, Orbital Rings, and Space Fountains.

One use for the Space Fountain concept will be in constructing tall antenna masts for news events and military operations. Perhaps after a few years of experience with the Fountain Masts, the braver camera crews might be willing to ride up on the Top Station for better overhead shots. Once experience has been gained with smaller Fountain Masts, larger Fountain Towers, perhaps ten to twenty kilometers high, might prove to be commercially viable for radio and television broadcasting in the Plains states and the steppes of Asia. Fountain Towers might also prove to be an economical alternative to communication satellites for point-to-point television and FM radio communication between the various islands of some of the smaller nations in the Pacific Ocean.

The real test of confidence in the Fountain Tower concept will be when buildings many kilometers in height are constructed using Fountain Towers as support beams to hold up the building. There would naturally be multiple redundancy in the number of Fountain Beams in each corner of the building. Each Fountain Beam would have an independent control power supply and there would be enough inertia in the flowing streams of projectiles to stay up for hours even in the event of a main power failure. Finally the Fountain Towers would rise higher and higher until they went into space. We would then have a true Space Fountain, reaching upward into the heavens.

The arched fountain structures can start small. The first ones may be demonstrations for Congress. Fountain Bridges made using subsonic cables inside steel tubing will arc across the Potomac, following the path of George Washington's silver dollar. With the development of superconducting magnets and accelerators, projectile-supported Fountain Bridges can be made across the Channel, the Sahara, the Alps, the Bering Strait, and the waters that separate the Third World islands in the Pacific. Finally the Fountain Bridges will arc so high they will reach into space and become Space Bridges.

There the Space Fountains and the Space Bridges will connect to a system of Orbital Rings running from pole to pole and around the equator in a globe-encircling interlinked network. Built and maintained by the larger nations, the EarthLink net would allow rapid transportation anywhere on Earth and easy access to the solar system. For access to space, all a Third World nation would have to do is build its own Space Bridge or Space Fountain. One end would be firmly embedded in its own soil while the other would be attached to the EarthLink that unites all the nations of the world into one. Once off the Earth, the vehicles of each nation could then couple to the high speed projectiles inside the EarthLink rings to withdraw enough of the energy to send the vehicle on its way into Earth orbit, or on to the Moon and the planets. The projectile stream inside EarthLink would slow slightly as some of its energy is lost, taken away by the disappearing vehicle, but it will gain it back again when the vehicle returns with its massive cargo and passengers, and its equally important, but massless, cargo of knowledge.

Recommended Reading

Ivan Bekey, "Tethers Open New Space Options," Astronautics & Aeronautics, Vol. 21, pp. 32 ff (April 1983).

Ivan Bekey and P.A. Penzo, "Tether Propulsion", Aerospace America, Vol. 24, pp. 40-43 (July 1986).

Paul Birch, "Orbital Ring Systems and Jacob's Ladders," Journal of British Interplanetary Society, I, Vol. 35, pp. 475 ff (1982); II, Vol. 36, pp. 115 ff; III, pp. 231 ff (1983).

Arthur C. Clarke, "The Space Elevator: 'Thought Experiment' or Key to the Universe?," Advanced Earth Oriented Applied Space Technology, Vol. 1, pp. 39 ff (Pergamon Press, London, 1981).

Robert L. Forward and Hans P. Moravec, "High Wire Act," Omni, Vol. 3, No. 10, pp. 44-47 (July 1981).

Roderick A. Hyde, "Earthbreak: Earth-To-Space Transportation", Defense Science 2003+, Vol. 4, #4, pp. 78-92 (August/September 1985).

Hans Moravec, "A Non-Synchronous Orbital Skyhook", Journal of the Astronautical Sciences, Vol. 25, #4, pp. 307-322 (October-December 1977).

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