在太空中重新启动引擎有什么困难之处?Why is it challenging to restart rocket engines in space?

【翻译自quora, George Ushakov 的回答】

Mainly because in a Zero-G environment (like space), propellant starts ‘sloshing’ – floating around inside the propellant tank.

主要是因为在零重力环境(如太空)中,推进剂会开始“晃动”-也就是漂浮在推进剂罐内。

You know how astronauts float around inside the space station? Well, propellant does the same.

您知道宇航员如何在空间站内漂浮吗?好吧,推进剂也一样。

This is demonstrated in the following video, which shows the fuel inside the Falcon 9 second stage floating around inside its tank after the engine cuts off and the rocket enters a Zero-G environment (hint: it happens at about 24 seconds in the video):

在下面的视频中对此进行了演示,该视频显示了猎鹰9第二级内部的燃料在发动机熄火并且火箭进入零重力环境后在其油箱内漂浮(提示:它在视频中大约在24秒时发生) :【youtube的视频需要梯子才能看】

If the propellant floats away from the suction point of the tank, no propellant will be able enter the engine. Obviously, you can’t restart an engine without propellant.

推进剂本来应该进入引擎,但它现在却可能漂浮走。显然,如果没有推进剂的话,你启动不了火箭。

Luckily, there’s a solution to this problem – Ullage motors.

幸运的是,我们有解决办法—— Ullage 发动机。

Ullage motors are small, independently-fuelled rocket motors that are used to provide a small amount of acceleration to get the rocket out of a Zero-G environment.

Ullage 电动机是小型的,使用独立燃料的火箭发动机,用于提供少量加速,使火箭脱离零重力环境。

This acceleration causes the propellant inside the rocket to move to the bottom of their tanks, wherefrom they can be sucked and pumped into the rocket engine.

启动了Ullage motors后,火箭的推进剂将移动到燃料箱底部,然后它们就能进入火箭引擎了。

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