Like complex projects in other sectors, space projects frequently exceed cost and schedule performance targets. Reasons frequently cited for this include excessive optimism at the start of projects, political interference, technology development challenges such as design flaws and rework, changes to the work content during the project, and integration issues. Problems with progress can rarely be isolated to just one aspect of a project or system, however, and decisions taken in one part of a project to remedy a perceived problem may have unanticipated consequences later, elsewhere in the project. Based on data from a space science institute, this research presents a model of project progress to understand the effectiveness of the strategies available to managers of complex instrumentation projects. The paper focuses on the decision making around staffing when progress falls behind schedule and finds that practical challenges in expanding capacity in a team may mean that schedule slippages experienced early in the project lifecycle are unlikely ever to be reversed, even if additional resources are made available. This reinforces the importance of comprehensive risk analysis, thorough cost and schedule estimating at the start of the project, and the availability of realistic funding from the outset.