Sure, SpaceX promised to reuse a rocket sometime in September or October... but which rocket? At last, we know. As part of a NASA presentation, SpaceX has revealed that it will be using the second Falcon 9 to make a successful landing (in this case, on a drone ship). Why not the first, you ask? If you'll recall, Elon Musk sees it as a museum piece. He doesn't want to lose the most historically important machine.