Looking for a Cheap Launch
One important consideration in planning space power is the expense of putting a satellite into orbit. Right now, it costs a thousand times more to put an object into space than to fly it across country by commercial airliner, even though the two jobs require roughly the same amount of energy-about 10 kilowatt-hours per kilogram of payload. Two factors account for the extra cost: the army of engineers and scientists required for a successful space launch, and the practice of discarding much of the launch vehicle after each flight.Launch costs are likely to drop, however, as the demand increases for hoisting large volumes of material into space on a regular basis: the more frequently a launch system is used, the lower the cost per use. Moreover, NASA is seeking a new generation of reusable launch vehicles. The agency recently sponsored a competition among aerospace contractors for a space vehicle with the potential for airline-like operation. The winner was Lockheed Martin Skunk Works, legendary innovators in aircraft design from the U-2 to the Stealth fighter. Lockheed Martin plans to build and test the $1 billion wedge-shaped reusable X-33 -a one-half size, one-eighth mass version of a launch vehicle called Venture Star that would replace the space shuttle for ferrying cargo into low orbit. The target launch cost is $2,200 per kilogram-one-tenth that of a shuttle launch. At that price, space power could become cost-effective if satellites pull double-duty as communications relays and solar-power sources.
A solar power satellite should quickly pay back the energy needed to put it into orbit. Start with the conservative assumption that solar power satellite technology would produce 0.1 kilowatt of electricity on the ground per kilogram of mass in orbit. In that case, the energy expenditure of 10 kilowatt-hours per kilogram to lift the satellite into orbit would be repaid in electricity after only 100 hours-less than five days.
One way to keep launch costs down is to use an inflatable structure as the solar collector. Doing so would maximize the collector’s surface area-important to gathering the greatest amount of solar energy-without imposing a major weight burden on the launch vehicle. Deflated solar collectors could be folded into a compact space on board the spacecraft; once in orbit, gas from a pressurized container would inflate the structure.
Balloons in space are an old story. In fact, the 1960-vintage satellite known as Echo I was a balloon used to bounce radio waves back to Earth. NASA is now studying the feasibility of inflatable structures in space for antennae, sunshades, and solar arrays, although not explicitly for solar power satellite systems. An important experimental milestone was the successful deployment by Space Shuttle Endeavour astronauts in May 1996 of the Spartan Inflatable Antenna Experiment-a 14-meter antenna inflated by a nitrogen gas canister in orbit.
It is not such a very large step from such an experiment to a solar-collecting satellite that could be assembled in orbit from inflated segments. Were NASA to make research on inflatable space structures a high priority, the knowledge base to make cost-effective low-mass power satellites could evolve rapidly.