tokens: The propulsion system shall be designed to avoid the effects of internal contaminants , including propellant vapours , by : 1 . Preventing intrusion , internal generation and circulation of contaminants . 2 . Preventing or controlling accumulation of contaminants throughout the various parts of the system . 3 . Preventing accumulation of contaminants during the various steps of production , verification and operation of the system . NOTE 1 The presence of contaminants inside the propulsion system can lead to the loss of performance of some components or even to catastrophic failures . NOTE 2 For example , propellant vapours can be considered as contaminants in a pressurisation system .