[mol] of xenon required for the mission. The mass of storage materials required is a function of our choice of storage pressure. Our second goal is to then find the storage pressure, for both bulkand adsorbed-xenon propellant storage systems, that minimizes the mass of storage materials. The primary performance metric of a given xenon propellant storage system is the tankage fraction: the mass of storage materials required per mass of xenon propellant we wish to store .