原文：
After two delays over four days, the space shuttle Discovery and its seven-member crew are set for launch Friday night on a 13-day mission to the International Space Station. The crew will deliver the treadmill named for comedian-satirist Stephen Colbert to the station, along with laboratory hardware, a new crew sleep station, and other supplies. Astronauts are scheduled to conduct three spacewalks for maintenance tasks. And the mission will return station crew member Timothy Kopra, leaving astronaut Nichole Stott on the station, as part of a regular crew rotation. The launch, scheduled for 11:59 Eastern Daylight Time, originally was to have taken place early Tuesday morning, but at the last minute bad weather forced launch director Peter Nickolenko to scrub the attempt. The team aimed for a Wednesday morning launch. But that attempt was canceled when controllers couldn't tell if a critical valve had closed, as it was supposed to do, as the fueling process ended. The valve controls the flow of fuel when the shuttle's main tank is being loaded or emptied. Flight rules dictate that when sensors fail to signal the valve's status, controllers can't test the valve directly by opening and closing it with a full load of super cold fuel in the tank. If the valve is balking because it's gunked up with debris from wear and tear, it could get stuck in the wrong position. Instead, the shuttle's external fuel tank must be emptied. And temperatures and pressures in the valve's surrounding plumbing must be at ambient levels before controllers can try to open and close the valve several times to make sure it works. So technicians emptied the tank and controllers ran the test. The valve passed. So did the indicator that failed to signal the valve's status when the tank was full. The team ran a second, indirect test to make sure the valve had closed. The valve passed that one, too. But that gave some engineers pause, acknowledges Mike Moses, who heads the mission management team. Some thought "we should be no-go," he said during a prelaunch briefing Thursday afternoon. The reason: If the fuel tank is refilled and the sensor fails again, no one knows if the valve has shut. Team members have come up with several indirect ways to verify the valve's status ? measurements they can compare with the valve's performance on previous flights. If these then-and-now measurements match up, this would give the launch team a high level of confidence that the valve is behaving properly, Mr. Moses says. In addition, the analysis of this valve brought to light results of past tests in which cruddy valves worked through at least 200 open-shut cycles, although they made some unsettling noises as they opened and closed. Taking all these tests into account, "we've come to understand the problem, and we can now relax that rule," Moses says, and revert to an earlier version that allowed controllers to directly test the valve, even though the fuel tank is full. To some, the move triggers memories of a criticism investigators into the 1986 Challenger disaster leveled at NASA ? its increasing willingness to grant waivers to flight-readiness requirements in order to keep launches on schedule. But Moses notes that while mission managers are likely to revert to the old rule, "we're probably going to make it a little stricter." He explains that the valve would have to pass the secondary tests and the preflight screening in order to give controllers confidence that, in the absence of a direct indication of the valve's status, they can verify it by opening and shutting the valve with a full load of fuel in the external tank.
原文来自：http://www.csmonitor.com/Science/2009/0828/p02s14-usgn.html

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译文：阀门控制燃料流量
经过两个为期4天的延误，发现号航天飞机以及7名船员被设置为推出13天的太空任务周五晚上到国际空间站。 船员们将能够为喜剧演员命名的跑步机，讽刺作家斯蒂芬科尔伯特到车站，以及实验室的硬件，一个新的船员睡车站，和其他用品。宇航员预计为维护任务进行三次太空行走。和核查团将返回空间站成员蒂莫Kopra，使空间站上的宇航员尼科勒斯托特，作为一个普通船员轮换的一部分。 这次发射，为东部夏令时间11:59举行，原本有发生，26日凌晨，但在{zh1}一分钟恶劣天气被迫发射导演彼得尼克伦科25，xx尝试。 一个星期三上午发射目的是团队。但是，企图而被取消控制器无法判断一个关键的阀门关闭了，因为这是应该做的，在加油过程中，结束了。 阀门控制燃料流量时，航天飞机的主油箱正在加载或清空。飞行规则支配，当传感器没有信号阀的状态，控制器不能直接测试阀门通过打开和关闭的超低温燃料箱中的满负荷了。如果不愿支付阀是因为它gunked与碎片磨损，它可以得到在错误的位置停留。 相反，航天飞机的外部燃料箱，必须清空。与温度和阀门的管道周围的压力，必须在环境前的水平控制器可以尝试打开和关闭阀门几次，以确保它工作。 因此，技术人员空箱和控制器的运行测试。阀门通过。因此，他的指标，没有信号阀的状态时，油箱满。该车队在第二次的，间接的测试，以确保阀门已经关闭。该阀通过，一个也。 但这也给了一些工程师暂停，迈克承认摩西，谁负责特派团的管理团队。 有些人认为：“我们应该不走”，在一个发射前的简报，他说，星期四下午。原因：如果是气的燃料箱和传感器再次失败，没有人知道是否已关闭的阀门。 小组成员已经提出了几个间接的方式来验证阀门的地位 - 他们可以比较测量与阀门以前的飞行性能。如果这些当时的，现在的测量匹配，这将给发射团队的信任高水平的阀门行为不当，智先生说。 此外，提请当中眉头，阀门，至少有200启闭周期根据以往的测试工作，结果这种阀的分析，虽然取得了一些，因为它们打开和关闭不安的噪音。 考虑到所有这些测试中，“我们已经认识到这个问题，我们现在可以放松这一规则，”摩西说，并恢复到较早的版本，允许控制器直接测试阀，即使油箱满的。 对一些人来说，此举引发了1986年进入美国航天局挑战者夷为平地的批评灾害调查的回忆 - 它越来越愿意给予豁免，以飞行准备的需要，以保证工程如期发射。 但摩西指出，虽然任务管理人员有可能恢复到旧的规则，“我们可能会使其更严格一点。”他解释说，阀门必须通过二级测试和飞行前检查，以使控制器的信心，在一个阀门的地位直接显示的情况下，他们可以通过打开和关闭的阀门满载它燃料在外部燃料箱。

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