Elon Musk

Now, I had a clue here because some of the early US rockets had used very thin steel.

The Atlas rockets had used a steel balloon tank.

So it's not like steel had never been used before.

It actually had been used.

And when you look at the material properties of stainless steel, especially if it's been like full-hardened stainless steel at cryogenic temperature, the strength weight is actually similar to carbon fiber.

So, if you look at material properties at room temperature, it looks like the steel is going to be twice as heavy.

But if you look at the material properties at cryogenic temperature of full hot steel stainless, of particular grades, then you actually get to a similar strength weight as carbon fiber.

And in the case of Starship, both the fuel and the oxidizer are cryogenic.

For Falcon 9, the fuel is rocket-propelled grade kerosene, basically like a very pure form of jet fuel.

But that is roughly room temperature, although we do actually chill it slightly below.

We chill it like a beer.

We do chill it, but it's not cryogenic.

In fact, if we made it cryogenic, it would just turn to wax.

But for Sasha, it's liquid methane and liquid oxygen.

They are liquid at similar temperatures.

So basically, almost the entire primary structure is at cryogenic temperature.

So then you've got a 300-series stainless that's strain-hardened.

Because it's at almost all things at cryogenic temperature, it actually has a similar strength to weight as carbon fiber, but costs 50 times less than raw material and is very easy to work with.

You can weld stainless steel outdoors.

You could smoke a cigar while welding stainless steel.