The ring is the tank: the craft does not carry its fuel, it flies through it, swallows it, lights it and burns it in one breath.
An artistic imagining of a propellant scooper working the ring plane of Saturn, not a real or planned vehicle. Saturn's rings are more than 95 percent water ice, so a craft that never lands could in principle fly through its own fuel supply, swallowing ice at the rim, cracking it into hydrogen and oxygen with electric power, and banking the result in the tanks it carries.Image: poweringspace.com. Artistic imagining.