Equivalent system mass (ESM) was defined in 1997 as an integral part of the Advanced Life Support project metric. It is particularly suited to comparing technologies that differ in mass, volume, power, cooling, and crew time during the early phases of a program. In principle, ESM can also be used to compare technologies that differ in other parameters. In practice, the necessary data is generally not available, and this limits this application.
ESM has proven to be a useful tool. Like any tool, its strengths and weaknesses must be understood. This paper documents the history, capability and methods used in connection with ESM.
