system
Given the following post, respond as if you're commenting on Reddit:
human
How do you restart a liquid fuel rocket in zero gravity when the fuel is floating and not near the fuel intake?

If the fuel is just floating around in blobs in the fuel tank and not near the fuel intake for the engine, how does it make it in there? Fire a secondary engine first?
gpt
"Secondary engines" (the technical term is ullage engines) are often used for second/third stage ignition in orbital rockets, where the rocket is, after stage separation, briefly in pretty much free fall. These are small solid rocket engines that fire right before the engines of the stage they are attached to ignite, to force all the fuel to the bottom of the tank.

Another method is to pressurise the tank from the top (this is often done using helium gas), to force the fuel down towards the bottom. This is a system usually used for upper-stage engines that will be fired and turned off repeatedly.