“Today we lack a vital resource that both Wernher von Braun and Sergei Korolev took for granted: thousands of engineers with the experience of designing, building, and launching new types of rocket in a matter of years or even months. We used to have them, but some time in the past 40 years they all retired. We've got the institutions and the data and the better technology, but we don't have the experience those early pioneers had. And I'm betting that the process of rebuilding all that institutional competence is going to run over budget.”