A strong need has emerged in the aerospace industry for a well-founded and effective failure prevention technique or process that can be employed during the conceptual design of high-risk aerospace systems. In this paper, we describe a methodology to support risk assessment and failure prevention during the earliest stages of spacecraft design and mission planning. The three-step procedure, an extension of the Function-Failure Design method, utilizes detailed information from existing subsystems and systematically abstracts this information to a level that can be used during conceptual design. During this process, we address the issue concerning the level at which systems containing a combination of electrical and mechanical components should be functionally modeled. The complete method is described in general before being applied to two major unmanned spacecraft subsystems.