How is propellant managed in microgravity?

Without gravity, propellant does not settle to tank bottom, requiring special management for consistent thruster feed. Techniques: Bladders/diaphragms - Elastomeric barrier separates propellant from pressurant, positive expulsion; Surface tension devices - Vanes, sponges, or channels use capillary forces to retain propellant at outlet; Propellant Management Devices (PMD) - Combination of screens, traps, and channels; and Settling burns - Use thrusters to accelerate spacecraft, settling propellant before main burn. Challenges: Gas ingestion (causes combustion instability), Slosh (affects attitude control), and Propellant gauging (difficult without settled liquid level). Pressurant systems: Regulated (constant pressure) or blow-down (decreasing pressure), and Pressurant solubility in propellant affects gauging. Design validation: Ground testing (drop towers, parabolic flight), Fluid dynamics modeling, and Flight heritage. Propellant mass tracking uses book-keeping, PVT (pressure-volume-temperature), or thermal methods.