Of all the many spaceflight concepts NASA has studied, the most enormous was the Solar Power Satellite (SPS) fleet. Czech-born physicist/engineer Peter Glaser outlined the concept in a brief article in the esteemed journal Science in November 1968, and was awarded a patent for his invention on Christmas Day 1973. In October 1976, the U.S. Department of Energy (DOE) and NASA began a three-phase, four-year joint study of the SPS concept. Total study cost was $19.6 million, of which DOE paid 60%.

Glaser had noticed that a satellite in geosynchronous Earth orbit (GEO), 35,786 kilometers above the equator, would pass through Earth's shadow for only a few minutes each year. It was well known that a satellite in equatorial GEO moves at the same speed the Earth rotates at the equator (1609 kilometer per hour). This means that, for people on Earth's surface, the satellite appears to hang motionless over one spot on the equator. Glaser also understood that electricity did not have to travel through wires; it could be beamed from a transmitter to a receiver.

Glaser mixed these three ingredients and came up with a satellite in equatorial GEO that would use solar cells to convert sunlight into electricity, convert the electricity into microwaves, and beam the microwaves at a receiving antenna (rectenna) on Earth. The rectenna would turn the microwaves back into electricity, then wires would carry it to the electric utility grid.

The great advantage an SPS enjoyed over a solar array on Earth's surface was, as mentioned, that it would spend almost no time in Earth's shadow. Earth's rotation meant that an Earth-surface solar array could make electricity at most about half the time. The rest of the time it would sit dormant under the night sky.

The problem with the SPS concept was that, if a solar satellite was to contribute a meaningful amount of electricity to the interlinked U.S. utility grids - and, by DOE's reckoning, "meaningful" meant gigawatts - then it would have to be colossal by normal aerospace engineering standards. The SPS silhouetted against the Sun in the NASA artwork at the top of this post is typical: it would have measured 10.5 kilometers long by 5.2 kilometers wide and had a mass of 50,000 tons.

Paired with a rectenna a couple of kilometers across, such an SPS would contribute five gigawatts to the U.S. electricity supply. DOE estimated that 60 such satellites with a total generating capacity of 300 gigawatts could contribute meaningfully to satisfying projected U.S. electricity demand in the 2000-2030 period.

Image: NASA. Image: Boeing/NASA.

There was, of course, no way that NASA could launch such huge satellites intact, or even in a few modular parts. It would need to construct the SPS fleet in space, most likely in GEO, from many parts. This called for an armada of highly capable space transport vehicles and an army of astronauts and automated assembly machines.

The "Space Freighter" pictured in the Boeing painting above was, as its name implies, meant to serve as the main cargo launcher for SPS construction. Fully reusable to cut costs, it would have comprised at launch a delta-winged, unmanned Booster with a piloted, delta-winged Orbiter on its nose. After separating from the Orbiter, the Booster would have deployed turbofan jet engines and flown to a runway at its launch site.

Had it been built, the Space Freighter would have utterly outclassed all other launchers. Its Orbiter would have delivered 420 metric tons of cargo to a staging base in low-Earth orbit (LEO). For comparison, the largest single-launch U.S. payload ever put into LEO, the Skylab Orbital Workshop, weighed 77 metric tons. Skylab was launched on a two-stage Saturn V rocket.

Engineers speak of "gross liftoff weight" (GLOW) when they describe large launchers. The Space Shuttle had a GLOW of about 2040 metric tons and the three-stage Apollo Saturn V, about 3000 metric tons. Estimated GLOW for the Space Freighter was a whopping 11,000 metric tons.

Image: NASA. Image: NASA.

Alert readers will notice discrepancies in the paintings that illustrate this post. These occur because the images are based on design concepts developed by different engineers in different phases of the multi-year SPS study. The Boeing Space Freighter Orbiter design is different from the Space Freighter Orbiter design depicted above. This Orbiter, probably a NASA design, has skinny main wings, forward canard fins, and a payload bay near its front; not, as in the Boeing design, at mid-fuselage. It would, however, have had the same capabilities as the Boeing Space Freighter.

The NASA painting above depicts a hexagonal LEO staging base with a central "control tower." Access tubes link the control tower to docking modules at the hexagon's six vertices. Between the access tubes are color-coded triangular "marshaling yards" with socket-like bays for storing standardized Space Freighter cargo containers.

The staging base control tower has mounted on its roof a "space crane" descended from the much smaller Space Shuttle Canadarm, which was under development at the time DOE and NASA conducted their joint SPS study. The control tower space crane is positioning a cargo container so that an automated chemical-propulsion Orbital Transfer Vehicle (OTV) can dock with it. After docking and space crane release, the OTV would automatically transport the container to a construction base in GEO.

Another, smaller space crane rides a track around the edge of the hexagon. It is shown unloading a cargo container from the newly docked Space Freighter Orbiter.

The painting includes many other details. It shows, for example, what appears to be a conventional Space Shuttle Orbiter approaching the staging base in the background. Rockwell, prime contractor for the Space Shuttle, proposed that second-generation Space Shuttle Orbiters serve as dedicated crew transports for the SPS program. The company envisioned that replacing the Orbiter's payload bay with a pressurized crew module would enable it to transport up to 75 astronauts at a time.

Next to the crew transport is a cluster of cylindrical modules for housing the staging base crew and astronauts in transit between Earth and GEO. A piloted OTV for transporting astronauts to and from the GEO SPS work-site - identical to the automated OTV, except for the presence of a pressurized crew module - is shown docked with the LEO staging base at lower right.

Image: NASA. Image: NASA.

Image: NASA Image: NASA.

In the SPS study, NASA sought to balance automation and astronauts. Automation was, its engineers noted, good for repetitive actions such as fabricating the tens of kilometers of trusses needed to support SPS solar cell blankets.

The basic "beambuilder" depicted in the upper image above would turn tight rolls of thin aluminum sheeting into sturdy single trusses. The more complex multiple beambuilder system in the lower image would combine and link together single trusses to make the major structural members of the satellite.

Astronauts would supervise and maintain the beambuilder robots and join together the trusses they fabricated. Automated OTVs would deliver thousands of aluminum rolls to the GEO work-site, which the astronauts would then load into the beambuilders.

DOE and NASA expected to added two SPSs to the "fleet" in GEO each year starting in 2000. Each SPS would need about 200 Space Freighter launches and hundreds of OTV transfers between the LEO staging base and GEO. Propellants for the OTVs, as well as 50 metric tons of orbit trim propellants for each SPS per year, would demand even more Space Freighter launches.

Image: NASA. Image: NASA.

Despite extensive reliance on automation, the 30-year SPS project would require the presence of nearly 1000 astronauts in space at all times. Most would be based in GEO.

In addition to construction workers, personnel needed in space would include physicians, administrators, OTV pilots, life support engineers, general maintenance workers ("janitors"), cooks, space suit tailors, and computer technicians. Personnel needed on the ground - at the launch/landing site, at the rectennas, and at widely scattered factories for manufacturing SPS parts, OTVs, spares, foodstuffs, and propellants - would outnumber astronauts by at least 10 to 1, NASA and DOE estimated. Building and operating the SPSs could become a major new U.S. industry.

Image: NASA. Image: NASA.

As beambuilders and astronauts completed trusswork sections, automated OTVs would begin to deliver rolls of solar cell "blankets" to the SPS work-site. The NASA painting above shows in the background an automated OTV laden with bluish rolls of solar cell blankets (upper right).

Meanwhile, an automated system feeds blanket sections to a piloted "cherry picker" at the end of a small space crane. The cherry picker's "pilot" - who wears only shirt-sleeves in his pressurized cab - uses manipulator arms to link one end of a solar cell blanket to a truss.

More than 50 square kilometers of solar cell blankets would be spread over the trusswork of each SPS in this way. The end result of this intensive human and machine labor is depicted in idealized form immediately below.

Image: NASA Image: NASA.

Image: NASA. Image: NASA.

The lower painting above shows Glaser's invention at work. The intense sunlight of space strikes solar cells, which are hidden from view (the image does, however, provide a good look at the backside of a completed SPS). Millions of silicon or gallium arsenide cells efficiently convert the sunlight into electricity.

The kilometer-wide steerable microwave transmission antenna at the lower end of the SPS converts the electricity into microwaves and focuses a microwave beam on a rectenna on Earth, nearly 36,000 kilometers away. The beam appears in the illustration as a ghostly cone; in reality, the microwaves would be invisible.

Image: NASA. Image: Boeing/NASA.

DOE and NASA envisioned building the 60 rectennas required for the SPS system from coast to coast along the 35° latitude line. Cities on or near that line include Bakersfield, California; Flagstaff, Arizona; Albuquerque, New Mexico; Amarillo, Texas; Oklahoma City, Oklahoma; Little Rock, Arkansas; Memphis and Chattanooga, Tennessee; and Charlotte, North Carolina. If one flew between these cities, one would overfly rectennas on the ground in different settings - forest, farm fields, mountains, swamp, desert - every 50 kilometers or so.

The 1970s saw growing awareness of environmental problems and the dangers of terrorism. DOE and NASA took pains to seek public input so that they could attempt to calm public fears. Most people polled worried about the microwave beams linking the SPSs with their rectennas on Earth. Some expressed concern about the environmental impact of the beams, while others feared that terrorists might seize control of an SPS and turn its beam on a city.

NASA pointed out that the beam would be de-focused to reduce risk to the Earth's upper atmosphere, aircraft, and people working at the rectennas. As depicted in the painting, limited agriculture could take place under the rectennas, directly in the path of the microwave beams.

In addition, the microwave transmitter on the SPS could be designed to shut off if its beam drifted. DOE and NASA expected that each rectenna would have around it a "buffer" zone of uninhabited land; if the beam drifted a small distance before it turned off automatically, only the ring-shaped buffer would be affected.

Image: NASA. Image: NASA.

In this final image of this post, we see the SPS fleet near the end of 2015; that is, halfway through the 30-year construction program, when 30 satellites would form a bright line across the southern night sky as viewed from the contiguous United States. A DOE document explained that each satellite would shine a little brighter than Venus. The satellites would appear about as far apart as the stars making up Orion's belt. Widely available 7-x-50 binoculars would reveal each satellite's rectangular shape.

The string of satellites would remain still against the background of moving stars and planets. In reality, of course, the stars and planets would remain still relative to the rotating Earth and the SPSs would keep up with Earth's rotation.

Every six months, in spring and autumn, each SPS would pass through Earth's shadow near midnight for several days in succession. During its brief shadow passage, a satellite would not produce electricity. One by one, starting with the eastern satellites, the SPSs would redden and grow dark. After about 10 minutes in eclipse, each would return to its full brightness.

The DOE/NASA SPS study generated thousands of pages of planning documents. Future Beyond Apollo posts will describe some of the SPS-related plans in detail.

References

"Power from the Sun: Its Future," Peter Glaser, Science, Vol. 162, 22 November 1968.

The Solar Power Satellite Concept: The Past Decade and the Next Decade, JSC-14898, July 1979.

Some Questions and Answers About the Satellite Power System (SPS), DOE/ER-0049/1, U.S. Department of Energy, Office of Energy Research, Satellite Power System Project Office, January 1980.

Satellite Power System Concept Development and Evaluation Program, Volume I: Technical Assessment Summary Report, NASA Technical Memorandum 58232, NASA Lyndon B. Johnson Space Center, November 1980.

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