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posted by martyb on Wednesday January 23 2019, @03:39AM   Printer-friendly
from the SHINY! dept.

Popular Mechanics has interviewed SpaceX CEO Elon Musk about his decision to move to a stainless steel design for Starship Super Heavy (formerly BFR). The interview reveals new details about the design, including micro-perforations on the outside of the windward side of the rocket that can bleed water or fuel for cooling:

Ryan D'Agostino: How does stainless steel compare [to carbon fiber]?

Elon Musk: The thing that's counterintuitive about the stainless steel is, it's obviously cheap, it's obviously fast—but it's not obviously the lightest. But it is actually the lightest. If you look at the properties of a high-quality stainless steel, the thing that isn't obvious is that at cryogenic temperatures, the strength is boosted by 50 percent.

Most steels, as you get to cryogenic temperatures, they become very brittle. You've seen the trick with liquid nitrogen on typical carbon steel: You spray liquid nitrogen, you can hit it with a hammer, it shatters like glass. That's true of most steels, but not of stainless steel that has a high chrome-nickel content. That actually increases in strength, and ductility is still very high. So you have, like, 12 to 18 percent ductility at, say, minus 330 degrees Fahrenheit. Very ductile, very tough. No fracture issues.

[...] [Here's] the other benefit of steel: It has a high melting point. Much higher than aluminum, and although carbon fiber doesn't melt, the resin gets destroyed at a certain temperature. So typically aluminum or carbon fiber, for a steady-state operating temperature, you're really limited to about 300 degrees Fahrenheit. It's not that high. You can take little brief excursions above that, maybe 350. Four hundred, you're really pushing it. It weakens. And there are some carbon fibers that can take 400 degrees Fahrenheit, but then you have strength knockdowns. But steel, you can do 1500, 1600 degrees Fahrenheit.

[...] On the windward side, what I want to do is have the first-ever regenerative heat shield. A double-walled stainless shell—like a stainless-steel sandwich, essentially, with two layers. You just need, essentially, two layers that are joined with stringers. You flow either fuel or water in between the sandwich layer, and then you have micro-perforations on the outside—very tiny perforations—and you essentially bleed water, or you could bleed fuel, through the micro-perforations on the outside. You wouldn't see them unless you got up close. But you use transpiration cooling to cool the windward side of the rocket. So the whole thing will still look fully chrome, like this cocktail shaker in front of us. But one side will be double-walled and that serves a double purpose, which is to stiffen the structure of the vehicle so it does not suffer from the fate of the Atlas. You have a heat shield that serves double duty as structure.

The steel used will be about $3/kg vs. $135/kg ($200/kg assuming a 35% scrap rate) for carbon fiber.

Also at Futurism.

Previously: SpaceX's Starship Will Now be Made of Stainless Steel, With Tests Still Scheduled for Early 2019

Related: SpaceX to Purchase $2 Billion of Carbon Fiber Sheets
SpaceX Reveals Plan to Fly Yusaku Maezawa and Artists "Around the Moon" in a BFR


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  • (Score: 1, Funny) by Anonymous Coward on Wednesday January 23 2019, @05:52AM (11 children)

    by Anonymous Coward on Wednesday January 23 2019, @05:52AM (#790471)

    But steel, you can do 1500, 1600 degrees Fahrenheit.

    According to my trusted sources steel can melt at around 600 °F.

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  • (Score: 4, Funny) by takyon on Wednesday January 23 2019, @05:55AM (6 children)

    by takyon (881) <takyonNO@SPAMsoylentnews.org> on Wednesday January 23 2019, @05:55AM (#790475) Journal

    Steel memes, and only in the presence of jet fuel.

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    • (Score: -1, Troll) by Anonymous Coward on Wednesday January 23 2019, @06:00AM (4 children)

      by Anonymous Coward on Wednesday January 23 2019, @06:00AM (#790478)

      Something something the jews.

      • (Score: 0) by Anonymous Coward on Wednesday January 23 2019, @07:17AM (2 children)

        by Anonymous Coward on Wednesday January 23 2019, @07:17AM (#790495)

        Of course it was them

        • (Score: 0) by Anonymous Coward on Wednesday January 23 2019, @07:37AM (1 child)

          by Anonymous Coward on Wednesday January 23 2019, @07:37AM (#790506)

          Definitely. They used steal beams to take all of the gold.

          • (Score: 0) by Anonymous Coward on Wednesday January 23 2019, @08:09AM

            by Anonymous Coward on Wednesday January 23 2019, @08:09AM (#790516)

            Yes yes but they don't actually go to bed with it

      • (Score: 0) by Anonymous Coward on Wednesday January 23 2019, @08:08AM

        by Anonymous Coward on Wednesday January 23 2019, @08:08AM (#790515)

        What? The jet fuel was made of jews?

        The beams were made by jews?

    • (Score: 4, Funny) by DannyB on Wednesday January 23 2019, @03:22PM

      by DannyB (5839) on Wednesday January 23 2019, @03:22PM (#790622) Journal

      Steel memes, and only in the presence of jet fuel.

      Jet fuel not required to have stolen memes.

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  • (Score: 5, Informative) by bradley13 on Wednesday January 23 2019, @07:51AM (3 children)

    by bradley13 (3053) on Wednesday January 23 2019, @07:51AM (#790510) Homepage Journal

    I was surprised as well, but a little time with Google, and Musk is absolutely correct. He is refering to Austenitic stainless steel [stainlesssteelsubstrates.com], which has a melting point around 1400C. Aluminum melts at 660C. This type of steel has a crystalline structure that remains the same at all temperatures from cryogenic to the melting point.

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    • (Score: 5, Interesting) by Zinho on Wednesday January 23 2019, @11:54AM (2 children)

      by Zinho (759) on Wednesday January 23 2019, @11:54AM (#790570)

      Yep. My materials professor at University said that, if he were asked to redesign the Super-Sonic Transport plane (SST/Concorde), he'd use stainless rather than Titanium for all the reasons already mentioned. [1] He's retired now, but is probably feeling vindicated by this news item.

      [1] one more reason for stainless on the SST: fatigue cracking. Titanium, like aluminum, will eventually start cracking over time under cyclical load due to fatigue stress -- regardless of the amplitude of the stress cycles. Steel doesn't have this problem: there's a minimum threshold of stress you have to pass in steel (the fatigue limit) before cracks will start. Not as applicable in space flight, since rocket components aren't reused much; although Musk is working on changing that, too, isn't he?

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      "Space Exploration is not endless circles in low earth orbit." -Buzz Aldrin
      • (Score: 1, Informative) by Anonymous Coward on Wednesday January 23 2019, @05:55PM (1 child)

        by Anonymous Coward on Wednesday January 23 2019, @05:55PM (#790718)

        Generally no, you have that backwards, though it depends on the alloy.

        Titanium will not crack. Steel will crack.

        There are exceptions, like spring steel. While I don't know of any titanium alloy with the cracking problem, I'm quite sure they exist, simply since cracking is the norm for most metals.

        • (Score: 2) by Zinho on Thursday January 24 2019, @05:14PM

          by Zinho (759) on Thursday January 24 2019, @05:14PM (#791304)

          Great, now I'm questioning my College experience!

          Source: http://www.roymech.co.uk/Useful_Tables/Fatigue/Fatigue.html [roymech.co.uk]
          Per that, Steel and Titanium both have fatigue limits. I may have to track down my professor and figure out which Titanium alloys he was complaining about. I'll probably start with figuring out what metals were used for the Concorde frame, as the reason they were grounded was concern about fatigue cracking in the frame. No guarantee that I'll report back in via this thread, since I've got other things going on in life, and I don't expect an AC to get alerted to a reply on an old thread.

          Cheers!

          --
          "Space Exploration is not endless circles in low earth orbit." -Buzz Aldrin