Expulsion Dynamics of Saturated Cold-Gas Propellant in Microgravity

CubeSat cold-gas propulsion systems increasingly rely on two-phase, self-pressurizing propellants in conformal tanks. However, state-of-the-art propellant management strategies suffer from volumetric and power inefficiency, limiting the ΔV budget and mission lifetime. These limitations motivate the development of a more compact propellant management approach. The VAPOR (VAporization for PrOpellant Repositioning) experiment demonstrated the feasibility of using thermally induced phase change to generate and reposition an ullage within various forms of propellant management devices (PMDs). However, the dynamics and thermodynamic evolution of the ullage gas volume during propellant expulsion in microgravity remain largely unexplored, posing a risk to orbital operations. Building on VAPOR, the VAPOR-VENT (Vapor Expulsion in Near-weightlessness Testing) experiment investigates the venting process of a self-pressurizing R-236fa conformal tank during parabolic flight tests. Venting ullage from a 95% mass-filled propellant tank results in an averaged mass flow rate of ~0.8 g s-1 at a mean tank pressure of 35 psia, generating an estimated peak thrust of 0.34 N under vacuum conditions. The system delivers a total impulse of ~0.16 N s over 0.5 s of gaseous propellant venting, corresponding to a specific impulse of 41 s. Video analysis reveals that PMD cavity refill within 3 s, which supports ~7 s of continuous gas venting under orbital operational conditions at a mass flow rate of 0.057 g s-1. Localized vaporization is also observed at hotspots outside the PMD volume. These findings provide new insights into the design considerations of thermal-induced phase-change propellant management strategies, supporting the development of a compact and precise impulse-delivery system for future CubeSat missions.