If a "natural" pressure differential is the only thing that's transferring fuel, you can only ever average out the amount of fuel/pressure in both tanks. So if your craft is at 30%, and your tanker at 50%, a natural delta will result in both craft ending up with 40%.

I guess they want to settle the liquid fuel with ullage thrust, and then vent the gas in the craft, but not in the tanker, letting more flow into the craft? The article is very light on details.

[–] threelonmusketeers@sh.itjust.works 3 points 1 year ago

Oh, yeah, I'm assuming they have to vent the ullage gas in the receiving tanker. Depending on the layout of the thrusters, they might even be able to utilize this gas to produce the some of the thrust during the propellant transfer.