Re:SpaceX test flight of Starship SN-5 [video]
by Robotbeat on Thursday August 06, 2020 @10:06AM
(#24070980)
They would be using a pressure differential to transfer the fuel, not relying on just ullage thrust.
You'd be surprised at how low of an acceleration is required for propellant settling and transfer. On the order of 0.0001-0.01m/s^2. At a 310s Isp for the ullage thruster, that's between 0.01kg and 1kg of propellant usage per second. Even if it takes an hour to transfer fuel (on the ground, fuel is transfered in about 20 minutes and in flight, all the propellant is transfered from the tanks to the engines in ~8 minutes), we're only talking about maybe 1% of the propellant needing to be used for ullage thrust and perhaps as low as 0.01%.
You'd be surprised at how low of an acceleration is required for propellant settling and transfer. On the order of 0.0001-0.01m/s^2. At a 310s Isp for the ullage thruster, that's between 0.01kg and 1kg of propellant usage per second. Even if it takes an hour to transfer fuel (on the ground, fuel is transfered in about 20 minutes and in flight, all the propellant is transfered from the tanks to the engines in ~8 minutes), we're only talking about maybe 1% of the propellant needing to be used for ullage thrust and perhaps as low as 0.01%.
ULA has published a paper on the topic: https://www.ulalaunch.com/docs/default-source/extended-durat...