In-space valves are required to provide precise mass flow control, wide throttling range and handle rapid on-off control. These requirements can result in significant unsteady, transient effects both on the fluid mass flow rate, as well as the torque required. However, there currently are no analytical or numerical modeling tools that can predict the unsteady/transient performance of these valves; current design tools are limited to quasi-steady models and empirical correlations. The innovation proposed here is a high-fidelity, comprehensive numerical tool that can characterize the transient performance of these flight valves and provide design support. An innovative approach to modeling valve motion in a broad range of valves designs including showerhead, ball and butterfly valves is proposed; this will permit simulations of transient valve operations and the resulting mass flow history and pressure drop. Unsteady effects at partial valve openings due to both turbulence interactions as well as multi-phase cavitation are addressed with an advanced numerical framework that incorporates both advanced LES models and real-fluid cryogenic effects. The tools and technology developed here would directly impact design support efforts for the J-2X upper-stage engine in the Ares launcher envisioned under the Constellation program for the mission to the moon.