This webinar demonstrates how Flownex can be used to model and analyze pressure fluctuations that occur during in-space propellant transfer operations. A detailed simulation was developed for transferring liquid oxygen between two docked spacecraft in low Earth orbit, incorporating tanks, pipes, valves, pumps, and artificial gravity created through spacecraft rotation. The results showed that opening a transfer valve into an evacuated line generated a large pressure spike and flow surge, similar to a water hammer event, which could place significant stress on system components. A second scenario introduced controlled valve opening, successfully reducing the peak pressure from 7.6 MPa to 1.77 MPa while only slightly increasing the overall transfer time. The webinar concludes that transient simulations are valuable tools for optimizing rocket fuel transfer systems and can also be applied to a wide range of spacecraft and rocket propulsion applications.