A life support system (LSS) is usually defined as a system that provides elements necessary for maintaining human life and health in the state required for performing a prescribed mission. The LSS, depending upon specific design requirements, will provide pressure, temperature, and composition of local atmosphere, food, and water. It may or may not collect, dispose, or reprocess wastes such as carbon dioxide, water vapor, urine, and feces. It can be seen from the preceding definition that LSS requirements may differ widely, depending on the mission specified, such as operation in Earth orbit or lunar mission. In all cases the time of operation is an important design factor. An LSS is sometimes briefly defined as a system providing atmospheric control and water, waste, and thermal management. The major subsystems required to accomplish the general functions mentioned above are: 1 Breathing and pressurization gas storage system. 2 Temperature and humidity control system. 3 Carbon dioxide control system. 4 Trace contaminant control system. 5 Water management system. 6 Waste management system. The technical committee which originally created this document no longer exists. The documents provides good basic information on making calculations in spacecraft life support systems. This document is one of 14 documents that were part of the original SAE Applied Thermodynamics Manual, most of which have already been stabilized.