Thermal fuel transfer and tank isolation to reduce unusable fuel

Abstract

A spacecraft includes a pair of pressurized propellant tanks. The pressure may be provided from a common pressurant tank. Each propellant tank has a propellant output port. A manifold includes a first branch coupled between a common juncture and the output port of the first tank, and a second branch between the juncture and the output port of the second tank. At least the second tank is associated with a heater. A controllable valve, such as a normally open pyrovalve, is coupled in the second branch. In order to fully utilize the propellant in the tanks, the second tank is differentially heated relative to the first when the tanks are nearly empty, to create a tendency toward a pressure difference, which drives propellant from the second tank to the first. When all or most of the propellant is transferred, the valve is operated to close off the second branch of the manifold, to isolate the now-empty tank. The thruster continues to be operated from time to time, drawing the remaining propellant from the first tank. The heating may be performed at times of day and/or seasons of the year when the appropriate tank receives solar radiation. The fuel remaining can be directly measured, or the spacecraft is launched with a known amount of propellant in the tanks. The thrusters are operated from time to time for attitude control and/or stationkeeping, and records are kept of the cumulative fuel use.