Copyright 1995 by Ralph Nansen, reproduced with permission
Table of Contents
Chapter 12: Features of the Satellite System
Let me take you on a tour of a solar power satellite. We’ll look at one as defined for the Department of Energy in tha late 1970s. It was designed to provide 5,000 megawatts of electrical power to the earth, which is equal to five typical nuclear power plants. As we make the tour I will discuss how technology has evolved over the intervening years and how the new models will change in the future. One change will likely be a reduction in size to 1,000 megawatts capacity to better match utility needs.
As we approach the satellite we see a huge rectangular array of solar cells stretching into the distance, bathed in dazzling sunlight. A shining jewel in the blackness of space. Its frame is hidden in the shadow of the solar array, and at one end is a giant flat disk, textured with millions of small rectangular slots to focus the energy streaming toward the earth 22,300 miles below.
Size of the Satellite
Our first impression of the satellite would be its gigantic size. A few years ago while on a business trip to New York, I was suddenly struck with the magnitude of size. On a brilliantly clear day I sat in a reception room on the fiftieth floor of one of New York’s modern office buildings. I looked out over Manhattan Island and realized that a single satellite would cover half of the sea of buildings before me. It was a sobering vision that dramatically impressed me with how large a solar power satellite would actually be—a rectangle of nearly twenty square miles (fifty square kilometers), paved entirely with solar cells.
Size alone should not intimidate us since throughout history civilizations have built grand edifices. When I stood in the hot sun of the desert looking up at the pyramids of Egypt, I tried to imagine how such monstrous structures were built by people using only muscle power and rudimentary tools. As I walked the Great Wall of China I had to marvel at the effort to build this barrier along a frontier that wanders for 1,250 miles across the mountain tops of northern China. Built before the birth of Christ, this colossus crosses a mountain 5,000 feet high. Placed in the most inaccessible of regions and built before the existence of engines, it is wide enough to allow five horsemen to ride abreast on its top. The size and quantity of stones used in the construction represent a massive task even by today’s standards.
The American interstate highway system stretches for tens of thousands of miles in endless ribbons of concrete and asphalt. Much of it is beautifully landscaped, sweeping through scenic countryside and rugged mountains. I am astounded as I realize this great system was built in only two decades.
Size is really just numbers of smaller things added one at a time. In Manhattan, the size reflects thousands of buildings side by side, each built of millions of parts. The Great Wall of China is tens of billions of rocks laid in neat rows. In the case of solar power satellites, the size will result from billions of solar cells, each not much thicker than a sheet of paper, connected one to the other and hung on a frame in the weightlessness of space.
Even though the satellite is vast, it will not cast a shadow on the earth. Most of the year, the tilt of the earth’s axis causes the satellite to pass either above or below the earth’s shadow. During this same time, as the satellite’s orbit takes it toward the sun from the earth, its shadow would also pass above or below the earth. During the equinox period, when the earth’s axis is perpendicular to the sun, the satellite will pass between the earth and the sun. However, even then its shadow won’t pass over the earth because of an interesting phenomenon of light. As an object is moved away from the earth toward the very large sphere of the sun, the sunlight passing on one side of the object will converge with the light passing on the other and there will be no shadow.
Also, despite the satellite’s size, it won’t be visible from earth during the day. The question of visibility depends upon several factors including distance, size, and reflectivity. I have already discussed its size and altitude, so the remaining question is how much light will it reflect. During the daylight hours, the satellite is oriented toward the sun, away from the observer, while the dark side is facing the earth, just like the new moon. We cannot see the new moon during the day and will not be able to see the satellite either.
After dark, the story will be different. With the satellite on the opposite side of the earth from the sun, it will reflect light back toward the earth like the moon. However, since the surface is covered with solar cells that absorb light while converting sunlight to electricity, there will probably be just enough reflected light to make the satellite as visible as an average star. On a clear night, we should be able to see a starry string of pearls—with each satellite representing one pearl—glowing in the night sky.
When I speak to the public about solar power satellites, one of the questions I can always expect is this: “Skylab fell back to the earth; since the solar power satellite is so big, won’t it fall too?”
Skylab was America’s first large space station and was placed in a low-earth orbit. Low-earth orbit extends from about 75 miles altitude up to 400 miles. Manned systems in low-earth orbit are placed within a maximum of 400 miles because of the radiation in the Van Allen Belt above that altitude. The initial orbit of Skylab when it was launched in 1973 was about 270 miles above the earth, over 22,000 miles closer to earth’s gravity and atmosphere than the solar power satellite will be.
We think of the atmosphere as having a finite end, but it does not. It simply becomes less and less dense until finally it is gone. There was a very thin layer of atmosphere in the Skylab orbit, which produced some drag and gradually reduced its speed with atmospheric friction. With the speed decrease, altitude was lost. As Skylab drifted into lower altitudes the atmosphere became denser and eventually friction caused the satellite to burn as it reentered the dense atmosphere. If there had been enough fuel on board to make course corrections, it would still be in orbit.
In the case of a solar power satellite in geosynchronous orbit, the atmosphere is nonexistent so the satellite will not be subjected to atmospheric drag. Satellites at that altitude will stay there for hundreds of thousands of years. There is no danger of a satellite falling from geosynchronous orbit.
The Satellite Framework
As we move behind the satellite and look underneath the vast expanse of solar cells we see a spidery framework of triangular-truss beams that form the satellite’s skeleton. It’s a rectangular framework, divided into 500-meter-square bays, providing the foundation for mounting all of the other elements and defining the satellite’s shape and size. Triangular trusses have proven to be the lightest, most efficient skeleton for very large structures. The great rigid dirigibles built in the early part of the century used aluminum triangular trusses for their structure, and most radio and television antenna towers are truss beams. Such beams are ideally suited for the structure of the satellite, which must be very large and rigid, but will be lightly loaded.
An alternative structural approach we have been looking at for the new smaller models (1,000 megawatts) would use a tetrahedron space frame made of tapered aluminum tubes. A tetrahedron looks like a pyramid with a strut at each corner and around the bottom. All of the struts are the same length, and when you join these pyramids together by adding struts to the tops it becomes a space frame. You may have seen this type of structure used in modern buildings where the structure is left exposed because of its unique beauty. The Museum of Flight in Seattle is a glass-covered space-frame structure that is able to support full-sized airplanes suspended from the structure.
Space structures do not have to carry the crushing loads imposed by gravity; neither do they have to resist the wind or carry huge burdens of snow. In the benign environment of space there is no oxygen or rain to cause corrosion. Size bears very little correlation to weight. A graphic example of the difference between earth-based and space-based structures can be made using the Space Needle, a well-known landmark built in Seattle for the 1962 World’s Fair. The needle is a 550-foot tower with a revolving restaurant on top. Built of thousands of tons of steel and concrete, it took a crew of fifty men six months to assemble. A 550-foot section of the main structural frame of the satellite, while about the same physical size as the Space Needle, would weigh less than two tons and take about an hour for one man to assemble using an automated assembling machine.
There are several choices of materials that could be used for the structure, but the most likely will be aluminum. It is lightweight, easy to work, inexpensive, and has the great advantage of long life. The satellite structure would be designed to last indefinitely. Only the power generator and transmitter would require regular maintenance, probably on an annual basis by robotic means supported by maintenance personnel living on a nearby space station.
Advanced composite materials might reduce the weight and increase the rigidity of the satellite, but there are still some unanswered questions about their longevity in the space environment. NASA has been testing a number of new composite materials in space for several years, but until sufficient time has passed to determine life capabilities, aluminum remains the best choice.
Solar Cell Power
If the structure of the satellite is its skeleton, then solar cells are the muscles that do the work. When we look at the cells closely we see smooth, flat, blue-black wafers with a fine grid pattern on their surface to collect the electricity that is generated when sunlight releases electrons within the cell. They convert 16.5% of the sunlight to electricity and are only two thousandths of an inch thick, which is no thicker than a sheet of paper.
In 1978, my Boeing study team faced quite a challenge trying to decide which solar cells to use in our research. At that time there were several different types being developed, but the most common were single crystal silicon cells and gallium arsenide cells. After many sessions with some talented aerospace engineers, the decision was made to use single crystal silicon cells because of their proven performance, light weight, and demonstrated efficiency. The engineers at Rockwell International, who were also studying solar cell choices at the same time, chose gallium arsenide for their studies. Their decision was to explore the potential of more advanced, higher efficiency cells. Our selection was more conservative—we knew the cells were readily available and would work.
As we look at the satellite you can see that the solar cells are assembled into conveniently sized panels, one meter square. What you can’t see is how the cells in each panel are interconnected using 14 cells in parallel with each parallel group connected in series to the next group. This provides very high reliability since any four cells in parallel can be lost before the panel stops operating. If a meteorite struck the satellite, it would only experience power loss in the damaged cell area. Based on NASA’s meteorite density data, the study team projected a loss of less than 1% of the cells over a 30-year period. The new models will have even higher reliability as they will use the technique developed by the terrestrial solar cell industry of bypass diodes to provide for an alternate electrical path. Each panel would be interconnected like the squares of a quilt to fill each structural bay. These in turn are joined together to create the rectangular form of the satellite.
Great strides have been made in solar cell development since the 1970s, and there are now many good cell materials from which to choose. Single crystal silicon is still the most common, multilayer gallium arsenide are still the highest efficiency, but thin-film cells made from cadmium telluride, copper-indium-deselenide, or multilayer amorphous silicon are lighter weight and less expensive, but also less efficient. The decision for the future must be made whether to build a large satellite with many low-cost cells or a smaller satellite using fewer, but more expensive cells. Another way to reduce the number of cells required is by using concentrators, such as mirrors or lenses, to focus the sunlight from a large area to a smaller area of solar cells. Regardless of the type of solar cell material selected, the cells would be installed in a way similar to the efficient arrangement worked out in the 1970s studies—a smooth and shiny quilt in the darkness of space.
Keeping the Satellite in Place
By now you may have noticed that the only moving parts on the entire satellite are the rotary joint between the solar array and the transmitting antenna and the attitude control system, which keeps the satellite pointing toward the sun. Because the solar cells must always point toward the sun while the transmitting antenna points toward the earth, the connecting joint will make one revolution each day. At the same time, this joint must also provide for the transmission of eight billion watts of electrical output from the solar cells to the transmitter. Unfortunately, wires cannot be used as they would soon twist off due to the continuous rotating motion. Slip-rings, similar to those used in some electric motors and radar sets, but on a much larger scale, would work well here. Since the motion is slow, there should be very little wear. As a comparison, the engine of an automobile will make more revolutions during a 15-minute drive to the grocery store than the antenna will make in a hundred years.
The task of keeping the satellite pointing toward the sun will be performed by an attitude control system. The attitude control systems for the Space Shuttle and for some of the current satellite systems burn small amounts of chemical propellants in tiny rockets to turn and position the spacecraft to the desired position. However, on the solar power satellites we can take advantage of electricity to power ion thrusters to accelerate an inert gas (most likely argon) to very high velocity, thus creating a rocket-thrust reaction.
The concept of using electricity to power rockets was demonstrated many years ago when Hughes Aircraft Company developed ion thrusters for NASA. These rockets used electricity generated by solar cells. In addition to the development of ion thrusters, work has progressed on other types of electric propulsion systems.
The attitude control system would also be used to maintain the satellite’s precise position in geosynchronous orbit. Due to slight gravitational variations on the earth, one of the anomalies of geosynchronous orbit is that a satellite left without any control will wander from one location to another within the orbit. It is necessary to keep the satellite aligned with its earth receiving antenna while simultaneously ensuring that it will not interfere with weather and communications satellites already in geosynchronous orbit.
Argon, used by the attitude control system, would be the only material on the satellite that would have to be resupplied on a periodic schedule. Current geosynchronous satellites must carry all additional fuel needed for their useful life when they are launched since there is presently no capability to resupply them. The infrastructure necessary to resupply argon and carry out routine maintenance on the satellites would be an integral part of the concept development.
The Transmitting Antenna
As we turn our attention to the transmitting antenna we see that it is mounted on a rotary joint at one end of the satellite and faces the earth at all times. It seems small compared to the rest of the satellite, although a disk one kilometer (0.62 miles) in diameter is large by most standards. Closer inspection reveals the disk to be covered with aluminum planks. As we take a still closer look we see that the planks are covered with slots, which are actually slotted wave guides. Wave guides are hollow rectangular box sections that channel radio-frequency energy so it can radiate out through the slots in the front face of the guides. This design concept is identical to many large phased-array radars in use today. The difference would be in the specific frequency used and in the fact that radar sets transmit energy in pulses, receiving reflected signals between pulses. By contrast, energy from the satellite would be continuously transmitted. The term “phased array” means that the beam formation and steering comes from control of the radio-frequency waves across the face of the transmitter.
As we make our way to the backside of the antenna we see it is cluttered with supporting structure and with electronic equipment generating radio-frequency energy. The transmitter is the most complex part of the satellite system with two important functions to perform. It must convert electric energy generated by the solar cells to radio-frequency energy and it must form the beam.
Conversion of electricity to radio waves can be done in several different ways. One example is a device called a magnetron, which is similar to a vacuum tube and is used in today’s microwave ovens. Another means of conversion is called a klystron and is used most commonly in large radars with power outputs up to a million watts. Newly developed solid-state devices similar in concept to transistors are also a possibility and are being developed by the Japanese for their 10-megawatt, low-earth orbit demonstration satellite.
The satellite we are looking at uses high-power klystrons for the energy conversion, but new models will use much smaller magnetrons or solid state converters. The decision of which type of conversion method to use will be based on conversion efficiency and life expectancy. The goal is to achieve more than 80% conversion efficiency. The higher the efficiency, the smaller the satellite can be. A life expectancy of 40 years is typical for power generating plants, but in the benign environment of space, the satellites can possibly last 100 years or more.
Transmission of the energy to earth is the last function the satellite needs to accomplish to fulfill its role as a solar power plant. Control of the beam is accomplished by controlling the frequency phasing of the radio waves over the face of the antenna. This in turn requires controlling the phasing of each individual microwave generator in relation to its neighbor.
From our brief tour, you can see the elegant simplicity of the design, but before leaving the subject of the satellite itself, I want to address one often-raised question about the vulnerability of the solar satellite. Even though the cold war is just a memory and the threat of nuclear holocaust has diminished with the collapse of the communist world, the question still remains: “Isn’t the solar power satellite vulnerable to attack?”
In this case a solar power satellite can be compared to a commercial ship, only instead of traveling on the great oceans of earth, it will move through the high seas of space. Throughout history, the right of commercial ships to ply the oceans of the world in relative safety has been respected by other nations. An attack on the high seas, even of a commercial vessel, has always been considered a deliberate act of war. This same logic should and will be preserved as we move into the new frontier of space. As on the high seas, the rights of sovereign nations and their vessels in space will be respected and an attack would be considered an act of war.
Throughout the world, there are power plants situated in well-known locations. Some, like Grand Coulee Dam and Hoover Dam, are even major tourist attractions. None of these power plants are defended against attack. Even though most have some kind of fence against intruders, the best precautions will not keep out dedicated terrorists, as evidenced by antinuclear protesters who illegally enter nuclear test sites. Our power plants are neither more nor less vulnerable to terrorist attack than any other public place, as we have seen in the devastating World Trade Center explosion in New York City in February of 1993 and the bombing of the Federal Building in Oklahoma City in April of 1995. Power plants are as vulnerable to enemy attack by air or ballistic missiles as any potential target in our nation. The best deterrent is having a large number of power plants. No nation would have the combined weaponry and skill to bypass our defense systems to destroy them all.
The location of the solar power satellites is a strong deterrent to attack. Access to geosynchronous orbit is difficult to achieve, and even with modern rockets it is a journey of over five hours. High-powered laser weapons could conceivably be used in the future, but they require high technology and a large sustained power output that is difficult to achieve.
Vandals or terrorists would also find it very difficult to attack a solar power satellite. Even the receiving antenna on earth would prove to be a frustrating target because of its great size.
If the United States were the only nation to possess solar power satellites, we could be in a situation envied by other nations and in that case it might be prudent to consider defending them from attack. However, the best defense is to make energy from solar power satellites available to all who need it. The energy crisis is not only an American crisis but a world crisis as well. Global access to energy from space would benefit all nations and preclude a threat of war.
Sun Power: Chapter 13 Table of Contents