These improvements have involved several areas, such as increased specific impulse (I sp ), reduced stage "dry" mass (e.g., lighter engines, tanks, valves, etc., or higher propellant density), engine throttle control, increased storage lifetime, or improved safety. In each case, the improvements have been sought to address a particular mission performance need. The accompanying picture illustrates two examples of multi-stage, earth-to-orbit shuttle vehicles employing reusable,...