Selling the dream of SpaceX was the easy part. Now Elon Musk has to hang on.

SpaceX may be a rocket company, but nowadays it feels more like a roller coaster. This past week alone, Chief Executive Elon Musk watched as its shares plummeted almost 14% in one day—which wasn’t even the worst performance in SpaceX’s short tenure as a publicly traded company. By Friday, the shares had soared more than 20% from where they began the week. Put another way, that was a more than $300 billion market-value swing. For most CEOs, that would be a vomit-inducing ride. For Musk, it’s a nagging reminder that he’s back in grind mode, having to deliver on all…