Elon Musk

Because we were making very slow progress with carbon fiber, I was like, okay, we've got to try something else.

Now, for the Falcon 9, the primary airframe is made of aluminum lithium, which is a very, very good strength to weight.

And actually, it has about the same, maybe better strength to weight for its application than carbon fiber.

But aluminum and lithium is very difficult to work with.

In order to weld it, you have to do something called friction-stow welding, where you join the metal without it entering the liquid phase.

So it's kind of wild that you could do that.

But with this particular type of welding, you can do that.

But it's very difficult to like say, let's say you want to make a modification or attach something to aluminum and lithium.

You now have to use mechanical attachment with seals.

You can't weld it on.

So I want to avoid using aluminum and lithium for the primary structure for Starship.

And there was this very special grade of carbon fiber that had very good mass properties.

So with rockets, you're really trying to maximize the percentage of the rocket that is propellant, minimize the mass, obviously.

But like I said, we were making very slow progress.

And I said, at this rate, we're never going to get to Mars.

So we better think of something else.

I didn't want to use aluminum and lithium because of the difficulty of friction-steer welding, especially doing that at scale.

It was hard enough at 3.6 meters in diameter, let alone at 9 meters or above.

Then...

I said, well, what about steel?