As he said in the presentation, it was because at cryogenic temperatures (which is the important part when you're building a liquid oxygen / methane tank) 301 stainless is actually _stronger_ than Al / carbon fiber. It also handles heat much better, which allows you to have a much thinner heatshield.

Both characteristics combine to give a lighter rocket than one built out of Al / carbon fiber.

The price is just the cherry on top.

the material isn't the question, it's why they had to weld it together in a patchwork quilt. Elon talked about the next version would simply have rolled coils with a single seam weld (although the cylinders would have to be welded together too), which would be better structurally and cosmetically, and likely lower cost of production. Sounds like they just didn't have the rolling machines to do the job, or perhaps there was an availability issue with the specific alloy.

It will reduce the length of the welds (for the hull) quite a bit.

The number of rings is roughly 35 for Starship, with about ten vertical seams each.

For easier thinking, line the vertical seams up and you see that this gives 50m length per vertical seam from bottom to top of Starship. So there are a total of 500m for that.

35 rings with ~28m circumference add up to about 1km of seams between them.

Now if they can use rolls twice as wide and only a single vertical weld each, then that's 500m between the rings and 50m for a single vertical seam. 550m of welds there instead of 1.5m before. This should speed things up quite a lot and reduce the cost of labour.

Raptor production is the real bottleneck, though.

The way I understood the presenation, they plan to use a single piece of steal, bent at an angle and rolled around (think one of those Pillsbury roll cans) with a single weld seem, in a giant helix.

>Raptor production is the real bottleneck, though.

They are shooting for one per day, no? Based on the differences identified in the three hanging under the Boca Chica vehicle now, it’s still in development.

They just shipped he 12th one. Definitely still in the development/ramp up process.

He said that it's about one per week. One per day will be in several months.

> although the cylinders would have to be welded together too

No, they'll do a helical weld, exactly like large pipes (like this: http://www.xysteelpipe.com/upload/201512118145340710.jpg)

You basically need to do that as the steel rolls off the press, though, which is why they didn't do that for Mk I. They'll need to cold roll the steel onsite.

They will not do a helical weld because the steel's thickness varies from the top to the bottom of the cylinder, according to Elon Musk. https://twitter.com/HarryStoltz1/status/1162595874561617920 https://twitter.com/elonmusk/status/1162601447235604480?lang...

And then you can make it a spiral of steel to eliminate the vertical welds.

He also talked about how stainless steel has a high melting point (which means their re-entry heat shield doesn't need to be as thick) and gets stronger rather than weaker at very low temperatures.

Imagine the autoclave the size of a rocket.

Composite tanks on few rockets being flown with them are made in one of a kind, purpose built autoclaves

Out-of-autoclave methods are mature and common these days.

Do they get you the same strength, fill ratio, and allow for usage with aerospace specialty epoxies?