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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: 0) by Anonymous Coward on Wednesday January 23 2019, @02:16PM (3 children)

    by Anonymous Coward on Wednesday January 23 2019, @02:16PM (#790596)

    On launch, it takes a whole booster full of fuel and chemical reactions to get enough energy to get to orbit.

    To return, this proposes to use evaporative cooling without gain from a chemical reaction to absorb that energy so as not to become a fire ball.
    How does one carry enough cooling fluid to do the job?

    It seems to me that the return problem is not one of Joules, but of Watts.
    If the rate of decent is faster than the rate the vehicle can radiate heat, it will get hot.
    So why not fly a gentle, Mach 25 decent in the extreme upper atmosphere?

  • (Score: 2) by istartedi on Wednesday January 23 2019, @05:41PM (2 children)

    by istartedi (123) on Wednesday January 23 2019, @05:41PM (#790706) Journal

    Aside from the fact that "gentle Mach 25" is an oxymoron, I suspect that there is no solution to the reentry equations that does not involve periods of intense heating. If there were, they would have used it.

    Without doing the actual math, consider that anything that slows you in the upper atmosphere will cause you to quickly descend to the lower atmosphere where even more intense heating occurs.

    Of course we could follow a low-heat trajectory by actively firing retro-rockets. I'm guessing they considered that too and found the fuel and weight penalty to be impractical.

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    • (Score: 0) by Anonymous Coward on Wednesday January 23 2019, @06:10PM (1 child)

      by Anonymous Coward on Wednesday January 23 2019, @06:10PM (#790728)

      There are downsides of course, but how about this: giant balloon

      Given a large enough balloon, the vehicle density can be taken way down. You can even float in the upper atmosphere.

      Because the cross-section is large, compressive heating will be distributed over an extremely large area. Temperatures thus do not rise very much.

      • (Score: 0) by Anonymous Coward on Thursday January 24 2019, @01:49AM

        by Anonymous Coward on Thursday January 24 2019, @01:49AM (#790978)

        Yes, SpaceX considers (or were considering, I'm not keeping that much abreast of current SpaceX "shenanigans"-) this "balloon" solution in order to make Falcon9 fully reusable i.e. bring back second stage "gently" with this method, without requiring heavy shielding in spite of its much greater velocity, after it detaches main payload into orbit.

        You can probably google previous news on this topic mentioning the idea.... hmmm... perhaps I've read it somewhere on the Wikipedia... can't remember ATM.

        Cheers.