Our primary focus in this paper is on open engineering systems which are readily adaptable to changing design requirements. Designing an open engineering system allows a family of products to be developed around a common baseline model. This entails increasing design freedom and design knowledge during the early stages of design. Toward this end, developing ranged sets (as opposed to points sets) of top-level design specifications provides a means to improve system flexibility by increasing design knowledge while maintaining design freedom. Consequently, our secondary focus in this paper is on metrics for assessing the design freedom and information certainty associated with a ranged set of top-level design specifications. As a demonstration, these metrics are applied to an example problem, namely, the conceptual design of a family of aircraft. Our emphasis in this paper is on introducing open engineering systems and metrics for design freedom and information certainty, not on our example, per se.