system
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human
How to rocket engines use liquid fuel while in zero gravity?

With the liquid fuel just floating around, how can a rocket light up its engines again?  Is there some sort of RCS thruster activation that needs to occur before the fuel is in its proper place?
gpt
Yes, this is indeed a problem. Aside from the gas pressure methods already given there are procedures to settle the fuel at the bottom of the tank before firing the engine. Because the rocket needs to be accelerating up small thrusters can be used after a stage has flamed out. Another possibility is lighting the next stage before the previous stage flames out which is called hot staging. In that case the exhaust gasses from the next stage need a way to escape. This is the reason why some (mostly soviet) rockets have struts between two stages and exposed engines rather than a closed fairing.