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  3. The great advance in rocket theory 40 years ago showed that liquid-fuel rockets were far superior in every respect to the skyroc

The great advance in rocket theory 40 years ago showed that liquid-fuel rockets were far superior in every respect to the skyroc

admin2010-02-21  48
问题    The great advance in rocket theory 40 years ago showed that liquid-fuel rockets were far superior in every respect to the skyrocket with its weak solid fuel, the only kind of rocket then known. However, during the last decade, large solid-fuel rockets with solid fuels about as powerful as liquid fuels have made their appearance, and it is a favorite layman’s question to inquire which one is "better". The question is meaningless; one might as well ask whether a gasoline or a diesel engine is "better". It all depends on the purpose. A liquid-fuel rocket is complicated, but has the advantage that it can be controlled beautifully. The burning of the rocket engine can be stopped completely: it can be re-ignited when desired. In addition, the thrust can be made to vary by adjusting the speed of the fuel pumps. A solid-fuel rocket, on the other hand, is rather simple in construction, though hard to build when a really large size is desired. But once you have a solid-fuel rocket, it is ready for action at very short notice. A liquid-fuel rocket has to be fueled first and cannot be held in readiness for very long after it has been fueled. However, once a solid-fuel rocket has been ignited, it will keep burning. It cannot be stopped and re-ignited whenever desired (it could conceivably be stopped and re-ignited after a pre-calculated time of burning has elapsed) and its thrust cannot be varied. Because a solid-fuel rocket can be kept ready for a long time, most military missiles employ solid fuels, but manned space flight needs the fine adjustments that can only be provided by liquid fuels. It may be added that a liquid-fuel rocket is an expensive device; a large solid-fuel rocket is, by comparison, cheap. But the solid fuel, pound per pound; costs about 10 times as much as the liquid fuel. So you have, on the one, hand, an expensive rocket with a cheep fuel and on the other hand a comparatively cheap rocket with an expensive fuel.
The most important consideration for manned space flight is that the rocket be ______.
选项 A、inexpensive to construct
B、capable of lifting heavy spacecraft into orbit
C、easily controlled
D、inexpensive to operate
答案C
解析 文中提到manned space flight needs the fine adjustments only be provided by liquid fuels，而前文讲了液体燃料火箭rocket engine can be stopped completely：it can be re-ignited when desired，可以判断manned space flight对火箭最重要的要求是易于操控，选项C正确。
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