←back to thread

404 points voxleone | 1 comments | | HN request time: 0s | source
Show context
cladopa ◴[] No.45660311[source]
Oh yeah. Replace the stainless steel by carbon fibre, give it to your pals of Boing and instead of being ready in 2030 for 2.3 billion it will be ready in 2050 for 50 billion.

Much better for making your friends rich.

replies(6): >>45660336 #>>45660615 #>>45661799 #>>45661926 #>>45662413 #>>45663135 #
jjk166 ◴[] No.45661926[source]
Stainless steel was a questionable choice for starship. If the pros outweigh the cons, which is yet to be seen, it will be mostly due to the peculiarities of Starship's other design choices. In general it's a terrible choice for rockets. I'm not saying Boeing would do a better job, but any actual engineer doing a ground up redesign starting today would definitely go with carbon fiber.
replies(3): >>45662185 #>>45663339 #>>45668065 #
_diyar ◴[] No.45662185[source]
> [if stainless works] it will be mostly due to the peculiarities of Starship's other design choices.

Yea but isn‘t that the point of the Starship? It has a bunch of unusual design choices regarding reusability and payload capacity, and then the rest of the owl is drawn around them.

I‘m not a rocket-scientist but I would hazard a guess they picked the best material given the options, right?

replies(1): >>45663005 #
jjk166 ◴[] No.45663005[source]
Well in the drawing analogy, they picked stainless steel while they were still trying to draw a sparrow.

Stainless steel was specifically chosen so that starship wouldn't need a heat shield and would survive re-entry with transpiration cooling. This would save substantial weight and make rapid reusability easy. The problem is that after designing starship around the stainless steel construction, they found that the transpiration cooling system wasn't workable, so now they have a stainless steel hull and a heat shield.

Further, I do not believe the drawbacks of stainless steel were fully appreciated at the time. Stainless steel on paper looks like it has better strength to weight ratio than aluminum, especially at the cryogenic temperatures of starship's fuel tanks. However a steel tank wall with the same strength as an aluminum wall is much thinner and so you wind up with different failure modes, namely buckling. In practice, a rocket made from steel is heavier than a rocket made from aluminum. This was why the Atlas rockets used stainless steel but subsequent rockets switched to aluminum in the first place.

Additionally, at the time much hooplah was made about stainless steel being cheaper and more formable which would reduce production costs. This is just nonsense. Stainless steel is expensive and tough to work with, which is why we don't use it for creating large structures despite its desirable material properties. It may be favorable compared to titanium, which was likely the only other option when transpiration cooling was the game plan, but for the current design aluminum would be far cheaper in addition to being lighter.

Now I'm sure SpaceX did some analysis after the transpiration cooling didn't work out and asked whether it made sense to start the design over and retool everything instead of continuing on with the stainless steel, and they decided at the time no. Since then they have had several further setbacks. The increased weight required them to reduce safety features, which may have contributed to some of its earlier losses. Starship has had to grow considerably and increase thrust to accommodate for these shortcomings. Would SpaceX have made the same decision to continue with the stainless with the benefit of hindsight? I can't say. But with the exception of a few chinese startups trying to carbon copy starship, other rocket manufacturers have not adopted stainless steel, likely with good reason.

replies(1): >>45664802 #
enraged_camel ◴[] No.45664802[source]
[flagged]
replies(1): >>45674564 #
1. jjk166 ◴[] No.45674564{3}[source]
> ot really, no. When SpaceX switched to stainless steel in 2019, Musk simultaneously described using ceramic hex tiles on the windward side. They showed hex-tile testing publicly in March 2019. Tiles were not an afterthought added later because transpiration "failed". Musk did initially discuss transpiration/regenerative cooling concepts for hot spots (stuff like a double wall, or fluid-cooled steel skin) but this was framed as in addition to tiles, not as a full replacement.

That's revisionist history.

https://www.teslarati.com/spacex-elon-musk-nixes-starship-sw...

Starship was switched to stainless steel in 2018. It was originally supposed to have an all-metallic heat shield. Ceramic heat shields for critical areas were added months later in march 2019, only in July of 2019 did the windward ceramic heat shield get added, which was after the starhopper prototype had flown and several more prototypes were already being built, and transpiration was still in active development at the time. Transpiration cooling was not dropped until 2020. The heat shield has been steadily growing since then, with the addition of more tiles to cover a larger area and an ablative underlayment to provide more protection to the underlying steel.

> It is not. In 2019, carbon fiber was $135/kg with 35% scrap (so effective cost was $200/kg) vs. $3/kg for stainless steel. That's a two orders of magnitude difference in raw materials.

And what did aluminum cost at the time? Yes stainless is cheap compared to the most expensive alternative, that does not make it cheap.

> 300-series stainless (301/304L) is widely used precisely because it is formable (301 work-hardens to high strength) and readily weldable (304L).

Work hardening is bad for formability.

> it's still much easier to work with than aerospace aluminum-lithium, which requires specialized friction-stir welding and tight process control.

Lithium aluminum is an exotic aluminum alloy. You would use an alloy like 7005 which is weldable.

> There's no evidence that Starship has reduced safety features to compensate for stainless steel + heat shield weight.

That is what reduced margin means. Every rocket has less safety features than it would if weight were not an issue. The more weight increases, the more everything has to give to still remain capable of completing the mission. IFT 9's failure was due to Starship relying on autogenous supercharging to save weight. No one can say how much better starship would be if it had more margin, but it undoubtedly would be better.