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  3. A rocket thruster based on an engine designed to power a lunar lander on an expedition to the Moon has been successfully tested

A rocket thruster based on an engine designed to power a lunar lander on an expedition to the Moon has been successfully tested

admin2009-06-24  57
问题     A rocket thruster based on an engine designed to power a lunar lander on an expedition to the Moon has been successfully tested by the Northrop Grumman aerospace company in the U.S.
    The thruster runs on a mix of liquid methane and liquid oxygen, which has the potential to be more efficient than other engines, but has never before been used to power or steer a spacecraft.
    Methane engines are a candidate for powering the liftoff vehicle NASA is developing to return astronauts from the lunar surface. Methane thrusters could be used for steering in space.
    Missions such as landing on and taking off from the Moon put stringent requirements on engines. Importantly, rockets must be liquid-fuelled so they can be shut down and restarted if needed.
    The Apollo lunar landers used exotic mixtures called "hypergolic" fuels, which ignite when they come into contact with a matched oxidizer. Because they are liquids at or near room temperature, hypergolic fuels don’t require heavy cryogenics or pressurized tanks, and can be stored longer than liquid hydrogen and liquid oxygen (LOX) without boiling away.
    But the specific impulse—a measure of propulsion power—of hypergolic engines is only 260 to 310 seconds, compared to 425 to 455 seconds from liquid hydrogen and LOX. And hypergolic engines require the use of compounds such as nitrogen tetroxide and hydrazine, which are extremely toxic for astronauts and ground crews.
    Methane requires cooling to -161.6℃, close to the temperature of LOX, but well above the -252.9 degrees C needed for liquid hydrogen, reducing the mass of insulation and cooling equipment.
    Liquid methane is also denser than liquid hydrogen, so fuel tanks can be smaller than those for liquid hydrogen. And Northrop’s test engine has already beaten the specific impulse of hypergolic fuels, although it can’t match that of a liquid-hydrogen engine.
    The Northrop tests are a step toward answering NASA concerns about the ease of igniting methane, crucial for engine function, and have earned the company a 10-month contract for further engine development. "The engine far exceeded performance requirements", said Northrop programme manager Mark Trinidad. It was fired more than 50 times, a key capability for thrusters, which are used repeatedly.
    Meanwhile other teams are also working on more powerful methane/LOX engines suitable for lunar liftoff. Last year, NASA engineers fired a methane-LOX engine for 103 seconds and XCOR Aerospace test-fired a methane-LOX engine that generated 33,400 newtons (7,500 pounds) of thrust in shorter bursts lasting about one second.
    The XCOR engine is soon to be test-fired in a vacuum—a necessary test for space use. The company is developing another lower-powered methane/LOX engine for the MarsFlyer aircraft being developed by Aurora Flight Systems, which may one day take to the skies over the red planet.
选项 A、Liquid methane and liquid oxygen.
B、Methane.
C、Liquid hydrogen and liquid oxygen.
D、LOX.
答案A
解析
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