Propellant Utilization System Monitors and controls propellant utilization to minimize residuals. To realize optimum performance in a liquid-fueled bipropellant space vehicle, it is necessary to control both propellants so as to exhaust them simultaneously. Such a simultaneous depletion both minimizes vehicle burnout weight (by not allowing any unusable amounts of one propellant or the other to remain in the tanks) and tends to maximize the mission total impulse (by utilizing all available propellant mass in engine reaction). Two major factors influence simultaneous propellant exhaustion. The first is accurate calibration of engine mixture ratios, flow rates, and total thrust under flight conditions. This results in an inability to predict the relative propellant masses to be loaded at liftoff. Even if such a prediction were possible, uncertainties in actually determining what has been loaded on board provide the second large error source. As an example, for the Centaur two-burn vehicle, these errors would result in a maximum mass ratio error (3-sigma) of approximately 350 pounds at burnout; resulting in a loss of 350 pounds of payload capability from a mission requiring propellant depletion. Clearly then, one way to improve total payload capability is to provide some sort of inflight system for propellant management. The first requirement of such a system (Figure 4-25) for proper propellant utilization (PU) is to accurately measure the ratio of propellants in the vehicle tanks during the entire powered flight portion of a mission. Centaur has a shaped, concentric cylinder capacitor installed in each tank. The space between the inner and outer plates is open to allow LH2 or LO2 to fill the probe. When empty, the capacitors are airdielectric types and have their minimum capacitances. As the tank is filled, the propellant (having a higher dielectric constant) displaces the gas and increases the element capacitance. Probe shaping makes the increase in capacitance directly proportional to propellant mass.