Twitter/SpaceX

So SpaceX is making a huge rocket out of stainless steel. As far as we know, this marks the first time the material has been used in spacecraft construction since some early, ill-fated attempts during the Atlas program in the late 1950s.

We know he is doing this this because, after weeks of rumors about a tweak to the design, a few days before Christmas Musk revealed that there would be much more than a tweak. The state-of-the-art carbon fiber forming the body of the Starship rocket (formerly known as the BFR, or Big Falcon Rocket, or Big F-other-word Rocket) and its Super Heavy booster would be replaced by 300-series stainless.

On January 10, Musk tweeted a photograph of a test version of Starship—essentially a prototype that can be used for suborbital VTOL (vertical takeoff and landing) test flights, reaching around 16,400 feet. He is calling these “hops.”

Since the quasi-unveiling, Musk has briefly answered some direct questions from the curious space-watchers of cyberspace via Twitter. But two weeks before the announcement he sat down with Popular Mechanics editor in chief Ryan D’Agostino at SpaceX headquarters in Hawthorne, California, for an exclusive interview in which he discussed, in great detail, the thinking behind the change. He talked about a lot more than that—we’ll be bringing you more soon. For now, here’s what he said about the big change.