Cryogenic propellant recirculation for orbital propulsion systems

Abstract

In the next generation orbital propulsion systems, cryogenic propellant recirculation will be applied to reduce engine chill-down consumption by recirculation chill-down and to achieve efficient propellant utilization by active propellant cooling. To understand tank-to-tank recirculation, a recirculation test campaign was conducted using liquid nitrogen as a working fluid. An electrically-driven recirculation pump developed for this experiment has bleed holes on the impeller to bleed vapor rather than liquid so that it works under two-phase flow operation. In addition to this pump, the test facility includes a 600 mm-diameter cryogenic tank, void fraction meters, an ultrasonic flow meter, and temperature and pressure sensors. The tank has a double layered window on the top to visually observe the return flow inside. The void fraction meters and ultrasonic flow meter were used to evaluate the two-phase mass flow rate. The tests were carried out in two different return port positions in the tank, the side and bottom return port cases, and the results indicated that the pressure and temperature behaviors in the tank were affected by the return flow rate, phase of the return flow and the return port position in the tank. In the side return port case, the flow returned to tank ullage and less disturbance of liquid in the tank was observed at a low flow rate while strong mixing of the liquid was observed at a high flow rate. The tank pressure was affected by the phase of return flow rather than the ullage temperature. In the bottom return port case, less disturbance was observed in vapor return flow, while in two-phase or liquid return flow, strong disturbance was observed even at lower flow rate. The tank pressure in this case was similar to that in the side return port case: the vapor return increased the pressure while the two-phase or liquid return decreased the pressure. The bleed holes on the impeller helped the inlet flow to recover to subcooled liquid condition and to keep the impeller working under saturation condition at the inlet although the recirculation system suffered decreased pump head, loss of liquid and unsteady oscillation of pump head and flow rate.