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.

Abstract: The unknown amount or mass of liquid contained in a storage tank is measured by calibrating the tank in an empty and partially full condition. The calibration includes measuring the mass of the empty tank, and is accomplished by applying a measured amount of heater power or energy to the tank and its contents, and measuring the temperature response to determine the product of total mass multiplied by specific heat. The specific heat depends upon the mass or amount of liquid in the tank, but a linear relationship is established between total mass and the product of total mass times specific heat. The unknown mass of liquid is then determined by applying a known amount of heater power or energy to the tank and its contents, and measuring the temperature rise or rate of rise in response thereto.