On-orbit refueling of the SpaceX BFR upper stage is necessary for trips to the Moon and Mars. In the presentation at the "International Astronautical Congress (IAC) in Adelaide, Australia, SpaceX CEO and Lead Designer Elon Musk (provided) an update to his 2016 presentation regarding the long-term technical challenges that need to be solved to support the creation of a permanent, self-sustaining human presence on Mars."
In the SpaceX video Making Life Multiplanetary after
23:40 the fuel transfer process between similar-looking upper stages is illustrated, with the tanker on the left and the ship on the right. Musk explains:
"They would use the same mating interface that they use to connect to the booster on liftoff… and reuse the propellant fill lines that are used when… the ship is on the booster. […] And then to transfer propellant it becomes very simple, use control thrusters to accelerate in the direction that you want to empty… transfer propellant very easily… from the tanker to the ship."
This is analogous to "gravity feed" replacing gravitational acceleration with propulsive acceleration, but I'm guessing that the magnitude of the acceleration would be an order of magnitude or more lower.
- Have I understood this concept correctly?
- Using some estimate of a control thruster's acceleration, how long would this pump-less, pressure-driven-less milli-gravity feed take? An hour? A day? Filling rocket tanks before liftoff takes perhaps tens of minutes with all the benefits of a refueling infrastructure on the ground. With only milli-gravity and no pressure assist, wouldn't this take a lot longer?