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冲压发动机的工作原理。它为什么不能单独使用?

答案

参考答案:

冲压发动机没有专门的压力机,依靠飞行器高速飞行时的相对气流进入发动机进气道后减速,将动能转化为压力能,从而使空气静压提高的一种空气喷气发动机。组成进气道(扩压器)、燃烧室、尾喷管。

冲压发动机压缩空气的方法是,在进气道中将高速气流经过一系列的激波将速度停滞下来,气流的动能转变为压力能,提高空气压力。减速增压后的气流在燃烧室与燃油混合、燃烧,产生高温高压燃气,然后经尾喷管喷出而产生推力;由于冲压发动机在静止时不产生推力,因此要靠其他动力装置将其加速,达到一定速度后才能正常工作,所以冲压发动机通常和其他发动机组合使用,形成组合动力装置。现代冲压发动机按飞行速度可分为亚声速、超声速和高超声速冲压发动机。

冲压发动机的特点:构造简单,质量轻,推重比大,成本低,高速状态(Ma>2)下,经济性好,耗油率低;低速时推力小、耗油率高,静止时不能产生推力,不能自行起飞;对飞行状况的变化敏感,工作范围窄(Ma=0.5~6,高度0~40km)。常用于靶机和飞航式战术导弹,也可用作高超声速飞行器的动力装置。冲压发动机产生的推力与进气速度有关。飞行速度越大,冲压越大,因而产生的推力也就越大,因此冲压发动机较适合于高速飞行。在低速飞行时冲压作用小,压力低,经济性差。

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单项选择题

You really do have to wonder whether a few years from now we’ll look back at the first decade of the 21st century—when food prices spiked, energy prices soared, world population surged, tornados plowed through cities, floods and droughts set records, populations were displaced and governments were threatened by the confluence of it all—and ask ourselves. What were we thinking How did we not panic when the evidence was so obvious that we’d crossed some growth, climate, natural resource and population redlines all at once "The only answer can be denial," argues Paul Gilding, an Australian environmentalist, in a new book called The Great Disruption. "When you are surrounded by something so big that requires you to change everything about the way you think and see the world, then denial is the natural response. But the longer we wait, the bigger the response required."

Gilding cites the work of the Global Footprint Network, an alliance of scientists, which calculates how many "planet Earths" we need to sustain our current growth rates. G. F. N. measures how much land and water area we need to produce the resources we consume and absorb our waste, using prevailing technology. On the whole, says G. F. N. , we are currently growing at a rate that is using up the Earth’s resources far faster than they can be sustainably replenished, so we are eating into the future.

This is not science fiction. This is what happens when our system of growth and the system of nature hit the wall at once. We are now using so many resources and putting out so much waste into the Earth that we have reached some kind of limit, given current technologies. The economy is going to have to get smaller in terms of physical impact.

We will not change systems, though, without a crisis. But don’t worry, we’re getting there. We’re currently caught in two loops: One is that more population growth and more global warming together are pushing up food prices, causing political instability in the Middle East, which leads to higher oil prices, thus to higher food prices and more instability. At the same time, improved productivity means fewer people are needed in every factory to produce more stuff. So if we want to have more jobs, we need more factories. More factories making more stuff make more global warming, and that is where the two loops meet.

But Gilding is actually an eco-optimist. As the impact o the imminent Great Disruption hits us, he says, "our response will be proportionally dramatic, mobilizing as we do in war. We will change at a scale and speed we can barely imagine today, completely transforming our economy, including our energy and transport industries, in just a few short decades. " We will realize, he predicts, that the consumer-driven growth model is broken and we have to move to a more happiness-driven growth model, based on people working less and owning less.

According to Paul Gilding, faced with disastrous evidence, people would()

A. be frightened into rethinking the ways we treat the earth

B. refuse to admit the follies committed by human beings

C. set a redline for population growth and the exploration of nature

D. come up with a response required to cope with the worsening situation