Modern engineering systems are increasingly software intensive. There is pressure from customers to reduce costs and development times whilst increasing quality. Finding appropriate techniques for achieving this are a major concern for systems engineers. Increased reuse addresses this concern. Domain Modelling is seen as a way of extracting the common features in a domain. The problems are: how to build a domain model; populate it and reuse from it.. In this paper we show how to build an Object Oriented Domain Model using discriminants. Discriminants are modelled using patterns. The resulting architectural model can then be used to build new systems in the domain by selecting features.This paper presents the results of a case study where the method has been applied to the generation of a Domain Model for Spacecraft Mission Planning Systems. The method has been successful where classical attempts have failed.