How does a rocket keep fuel flowing in low gravity?


Rockets have big tanks for liquid fuel, but wouldn’t the fuel float freely in the tank while sitting idle in orbit? Like when one is aligned and waiting to burn, what is making sure that the fuel is being piped into the engine?

In: Engineering


You can either use pumps to force the fuel/oxidizer out of the tanks and into the engine or you can have an extra tank of some high pressure, inert gas that you can bleed into the fuel/oxidizer tanks to maintain the flow.

This is why rockets often have what are called ‘ullage motors’, which are small pressurised-gas thrusters that give the ship a bit of acceleration so that the fuel in the main tank will all get caught at the ‘bottom’ as the ship moves.

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.


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