In zero G, there are two ways to get the fuel to the engine:
1) They can use the maneuvering thrusters to give some forward acceleration before starting the main engine pumps to make sure the fuel is at the pickup.
or
2) they can keep the fuel in a bladder inside the tank and pressurize the tank outside the bladder to ensure that there are no gas bubbles inside the bladder to ensure the fuel is at the pickup for the pumps.
>wouldn’t the fuel float freely in the tank while sitting idle in orbit?
Yup, that’s part of why rockets always have a basic thruster on them called an Ullage Motor. The Falcon 9 uses cold gas thrusters which fire for a bit just before main engine start to force the liquid in the tank back down to the bottom where the piping is
Pressurization doesn’t help because there’s still no designated “down” for the gas to be over the liquid so every rocket expected to start up again in 0g has cold gas thrusters, monopropellant engines, or small solid rocket motors to get everything sorted out
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