Free liquid in orbit is weightless, so it doesn't drain to a tank outlet on its own — it floats as a wobbling blob held together only by surface tension. Real depots fire small RCS thrusters ("ullage burns") to push the whole vehicle, which settles the liquid against the outlet by inertia, exactly like tipping a bottle.
Whether it actually settles is set by the Bond number:
Bo = ρ · a · L² / σ
- ρ — propellant density, a — ullage acceleration, L — tank radius, σ — surface tension.
- Bo ≫ 1: thrust dominates surface tension — liquid settles flat against the outlet. Safe to pump.
- Bo ≈ 1: the two compete — liquid clings to the wall in a slow-draining film. Marginal.
- Bo ≪ 1: surface tension wins — liquid stays a floating blob that drifts off the outlet. Pumping ingests vapor, which can cavitate the pump or stall the transfer.
Try zero thrust with high surface tension: the blob never commits to the outlet. Then push thrust up and watch the same propellant pin itself flat against the bottom within a few seconds.