By applying a statistical environment definition at the initiation of multifuel capsule radioisotope thermoelectric-generator development, the continual high cost expenditure attributed to redesign common to programs advancing the state-of-theart can be minimized. The design synthesis is accomplished by first establishing environmental conditions (e.g., peak reentry temperature) based upon statistically combining the characteristic variables required to define the environment. Selected values from this range of probable environmental conditions along with the structural characteristics are then applied to establish the design requirements of the system. The technique to acquire this result is illustrated by an example system design. A description of the many environmental factors and design requirements imposed on these systems are also discussed in this paper.