Stainless steel made Starship easier to build and lighter

Transcript

Transcribed by machine (Whisper) from the episode’s audio, so a word may be off here and there.

1. 1:55:45

   carbon fiber is pretty expensive. You can generally, when you do volume production, you can get any given thing to start to approach its material cost. The problem with carbon is that Material cost is still very high. So it's about 50 times, particularly if you go for a high-strength specialized carbon fiber that can handle cryogenic oxygen, it's like roughly 50 times the cost of steel. And at least in theory it would be lighter.

2. 1:56:23

   People generally think of steel as being heavy and carbon fiber as being light. And for room temperature applications, like say, more or less room temperature applications like a Formula 1 car, static aerostructure, or any kind of aerostructure really, you're going to probably be better off with carbon fiber. Now, the problem is that we were trying to make this enormous rocket out of carbon fiber, and our progress was extremely slow.

3. 1:56:53

   And it had been picked in the first place just because it's light? Yes. Like at first glance, like most people would think that the choice for making something light would be carbon fiber. Now, the thing is that when you make something very enormous out of carbon fiber, and then you try to have the carbon fiber be efficiently cured, meaning not room temperature cured, because sometimes you've got like 50 plies of carbon fiber.

4. 1:57:32

   And a carbon fiber is really carbon, string, and glue. And in order to have high strength, you need an autoclave, so something that's essentially a high-pressure oven. And if you have something that's a gigantic, the oven's got to be bigger than the rock one. So we're trying to make the autoclave that's bigger than any autoclave that's ever existed. Or do room temperature cure, which takes a long time and has issues.

5. 1:58:02

   But the fundamental issue is that we're just making very slow progress with carbon fiber. I think the meta question is why it had to be you who made that decision. There's many engineers on your team. Yeah, how did the team not arrive at this deal? Yeah, exactly. This is part of a broader question of understanding your comparative advantage at your companies. So because we were making very slow progress with carbon fiber, I was like, OK, 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 lithium is very difficult to work with in order to weld it you have to do something called friction

6. 1:58:55

   steel welding where you join the you join the metal without it entering the liquid base so it's kind of wild that you could do that but with this particular type of welding you can do that um but uh it's very difficult to like say let's say you want to make a modification or attach something to um aluminum lithium you now have to use a mechanical attachment with seals You can't weld it on. So I wanted to avoid using aluminum lithium for the primary structure for Starship.

7. 1:59:30

   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. And like I said, we were making very slow progress. And I said, at this rate, we're never going to get it to Mars. So we're going to have to think of something else. I didn't want to use...

8. 2:00:00

   aluminum and lithium because of the difficulty of friction still welding, especially doing that at scale was hard enough at 3.6 meters in diameter, let alone at nine meters or above. Then I said, well, what about steel? And so 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.

9. 2:00:32

   It actually had been used. And when you look at the material properties of stainless steel, especially if it's been like full-hard, strain-hardened stainless steel at cryogenic temperature, the strength weight is actually similar to carbon fiber. So if you look at the material properties at room temperature, it looks like the steel is uh it's 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 uh then the you actually get to a similar strength weight as carbon fiber and and in the case of starship both the fuel and the oxidizer are cryogenic so for for uh falcon 9 the fuel is rocket propellant grade kerosene basically pure like a a very pure form of jet fuel um which is but but that is that is roughly room temperature um although we do actually we do actually chill it slightly below

10. 2:01:37

    we chill it like a beer um we do chill it but um but it's not cryogenic in fact if we made it cryogenic it would just turn to wax so um but for starship the 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, actually has a similar strength the weight as carbon fiber. It costs 50 times less than normal material and is very easy to work with. You can weld stainless steel outdoors. You could smoke a cigar while welding stainless steel. It's very resilient. You can modify it easily. If you want to attach something, you just weld it right on. So very easy to work with, very low cost. And like I said,

11. 2:02:51

    at cryogenic temperature, similar strength to weight to carbon fiber. Then when you factor in that, we have a much reduced heat shield mass because the melting point of steel is much greater than the melting point of aluminum. It's about twice the melting point of aluminum. So you can just run the rocket much hotter? Yes. So especially for the ship, which is coming in like a blazing meteor, you can greatly reduce the mass of the heat shield.

12. 2:03:27

    So you can cut the mass of the windward part of the heat shield maybe in half, and you don't need any heat shielding on the leeward side. So the net result is actually the steel rocket weighs less than the carbon fiber rocket