Typical features for open and non-open (or closed) computer systems are examined and compared, and advantages and disadvantages of each approach are discussed. Representative examples are shown for each system approach along with a trade of salient parameters. Requirements for good open system designs are then summarized as a composite interrelated set and discussed. Rationale is provided for the most significant requirements. Finally, the benefits of an open system approach to aerospace computer systems listed and discussed, also with supporting rationale. Those benefits and potentially may yield significant cost and/or schedule reduction to a project are especially highlighted. >