单项选择题

Space Travel in the Future Space flight may be about to undergo a transformation far more radical than anything planned by national or international space agencies. In the next fifteen years or so , there could be a fleet of fifty space-planes carrying a million people into orbit about the Earth each year, at $ 10,000 per head. A prototype of spaceplane could be up and flying within five or six years. Perhaps surprisingly the main obstacles to realizing this dream are neither technical nor commercial. Space transportation is expensive and risky at present because all launchers so far have used large throw-away components that are based on ballistic missile technology. But the technology already exists for a prototype of a fully reusable, aeroplane-like launcher, and its development costs need only be equivalent to about two space shuttle flights (about $ 1,000 million). The cost per prototype space-plane flight would be about 1 percent of the cost in the space shuttle. Costs that low will not be achieved without several years of operating experience and continuous development to create heat shields and rocket motors that meet the usual airliner standards of long life and low maintenance costs. According to recent market research in Japan, more than a million people a year would be prepared to pay such a price for a brief visit to a space station. If correct , this level of space tourism would provide the sort of commercial incentive and operating experience needed to achieve airliner standard. However, space policy is so dominated by politics that more than sound engineering and commercial arguments will be needed to transform a high-cost industry into a low-cost one. Many aviation engineers in 1961, was as a member of a spaceplanes to be feasible over thirty yeas ago. ( My first job, starting in 1961, was as a member of a spacepalne design team. ) They were not developed primarily because the main player in the filed, NASA, because preoccupied with its part in the Cold War and locked into a ballistic missile mindset. As a result, NASA has not encouraged studies of spaceplanes that could be built u-sing existing technology and tends to view predictions such as those outlined above as far-fetched. How then can the transformation be brought about Four recent events should between them trigger the required overthrow of the mindset. The first components have been manufactured for the International Space Station and NASA, in conjunction with the Space Transportation Association, has begun the first official study the Orbital Science Corporation and the Rockwell International Corporation, for development of the X -34 launcher. The X-34 has a reusable lower stage and an expendable upper stage, and is designed to reduce the cost of launching small satellites. Unpiloted and looking rather like a large, fat fighter aeroplane, it is launched from a converted Boeing 747. Having released the upper stage at about one half satellite speed, the rocket-powered lower stage glides back to base and lands. Following inspection, maintenance and refueling, the lower stage will be ready for the next flight a few days later. The first orbital test flight is scheduled for just two and a half years from now. In April 1995, NASA places competitive study contracts with Lockheed, McDonnell Douglas and Rockwell for the X-33 demonstrator, which is tended to lead to an unpiloted single-stage-to-orbit launcher. When the implications of such projects become widely appreciated, the case for a new and realistic way ahead for space will become overwhelming. While the X-34 cannot be described as a true space-plane , since it has an expendable upper stage, if it is successful it will provide unassailable evidence for the feasibility of a true spaceplane. A piloted two-stage spaceplane using existing technology will then be seen as among the all-time best aerospace buys. Its development cost would be recovered by saving just three shuttle flights. It is not so much that the spaceplane would be especially efficient, but rather that the shuttles are especially inefficient, and that NASA has so far succeeded in playing down this fact. British companies have proposed designs that are more suitable than the X-33 and X-34, but they are handicapped by a government, which would not prepare to invest in even seed corn studies of new launchers. The largest new space project planned at present is the International Space Station. The estimated cost is about $ 20 billion plus about the same again in launch costs, including some 28 shuttle flights. A large part of the cost is due to the complexity of integerating the various modules from the US, Canada, Europe, Japan, and Russia. If these modules were adapted to be flown separately as independent small space stations, not only would costs come tumbling down, but the science would be far better because each discipline—be it astronomy, atmospheric science, Earth observation or microgravity research —has a different optimum orbit. The resulting constellation of small space stations would require more frequent servicing and supply flights than the single large one. This would not be affordable using the shuttle but would be with the new spaceplane. The total cost could be reduced by at least 80 per cent to less than $ 8 billion. Then all that is needed is for some entrepreneur to realise that the best way to finance new space-plane and space stations is to carry tourists. And before too long a million people a year will be taking their once-in-a-lifetime, round-the-world cruise in orbit. Four recent events should between them trigger the required overthrow of the mindset. The first components and second components have been manufactured for the International Space Station.