SPS as a specific concept to be evaluated thoroughly in a joint study by DOE and NASA. This study began in 1977 and a report is expected this year. For those of you not familiar with this baseline SPS concept, let me outline its main elements so we can explore some of the most important requirements and associated problems. As you will be discussing, the baseline SPS is a huge, box-shaped rectangular structure, with one of its largest sides covered with silicon or gallium arsenide photovoltaic cells. A large circular antenna at one end of the box collects electricity from the photocells and relays it to an earth based antenna via a microwave beam. Let’s recall just how big this space box we are talking about is: its principal surfaces, one of which is covered with the photocells, measures 10 km x 5 km and the box is V2 km deep. It’s hard to visualize a single structure more than 6 miles long and 3 miles wide and 150 stories high, but that’s what we’re considering. The circular antenna is a full km in diameter, and the entire structure weighs about 50,000 tons. Can we design a structure this size to withstand the thermal, solar wind, and other forces in space? If so, can we build it, recalling that we’ve never fabricated a structure in space? Can we control it, keeping the antenna pointed at a fixed spot 23,000 miles away on earth and the photovoltaic cells pointed toward the sun, while keeping the entire structure on its proper station in geo orbit? How long will it take to do this? What will it cost? Will we be able to mass produce Si or GaAs cells economically that will perform at acceptable efficiency, with affordable orbital maintenance, over a lifetime of 30 years? Will the 100,000 klystrons proposed to convert the photocell output from DC to microwave AC function efficiently and reliably? Will that huge 1 km antenna produce a controlled microwave beam focused continually on the receiving antenna on earth without scattering energy throughout the atmosphere? At this stage, an honest answer to those questions might be, “Yes, we probably can do these things, but it’s going to require a well managed organization plus a lot of time and money.” There are a host of other questions. The baseline SPS plan calls for a new heavy lift launch vehicle, which would make even the giant Saturn V of Apollo days look small. It may weigh 24 million pounds at lift-off to put nearly a million pounds of payload in low earth orbit. From low earth orbit most of this payload must be transported on up to geostationary earth orbit, since the SPS is intended to operate 22,000 miles above the equator over South America. Another huge box, about one square mile in area and 180 stories high, is proposed as the cargo truck from low earth orbit to geostationary orbit, with a new electric ion engine as its propulsion system. Additional launch vehicles and spacecraft will be required for personnel, and operations such as cargo-handling and on-orbit maneuvering between earth and LEO, between LEO and GEO, and around the LEO and GEO operations bases. We’ve built a lot of launch vehicles, spacecraft, and space propulsion systems, including small ion engines, but these SPS requirements are orders of magnitude larger and more complex than anything we’ve experienced to date. Can we do all this and make it work? Again, we probably can, but it will take sound management, time and money. These open questions illustrate the uncertainties surrounding only one aspect of SPS — the technical feasibility of lifting the components of an SPS into GEO, constructing the SPS in orbit, orienting it properly, operating it economically, and maintaining all parts of the system in working order. Naturally the ground receiving antenna, the ground control system, and the conversion and distribution of power are also critical to success, but once you find the place to locate the ground facilities,
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