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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
(Score: 1, Interesting) by Anonymous Coward on Wednesday January 23 2019, @07:53AM (4 children)
I don't claim to know much about space engineering, or engineering in general for that matter. But if you have steel and then have many many tiny little holes that you are going to "bleed" liquid out via in a temperature that is below freezing isn't that an issue? Are they going to heat the liquid before it gets pumped or what? I'm sure one of Elons geniuses already thought about that but still it would be interesting to know.
(Score: 2) by takyon on Wednesday January 23 2019, @08:28AM
https://www.reddit.com/r/SpaceXLounge/comments/aiqrk4/elon_on_twitter_yes_also_despite_high_outside/ [reddit.com]
https://www.reddit.com/r/SpaceXLounge/comments/air6oo/transpiration_cooling/ [reddit.com]
https://www.reddit.com/r/SpaceXLounge/comments/ainhml/elon_musk_why_im_building_the_starship_out_of/eep7c1d/ [reddit.com]
Can't find any more right now but I think there was a relevant plane or spaceship that used this.
[SIG] 10/28/2017: Soylent Upgrade v14 [soylentnews.org]
(Score: 4, Interesting) by choose another one on Wednesday January 23 2019, @09:05AM
> But if you have steel and then have many many tiny little holes that you are going to "bleed" liquid out via in a temperature that is below freezing isn't that an issue?
I think the whole point is to pump the liquid out when the (local) outside temp is really _hot_, in order to cool it down.
This isn't actually rocket science - jet engines have been doing this with turbine blades for ages, the blades run in engine temperatures way beyond their melting point yet they don't melt and the planes keep flying.
(Score: 2) by tangomargarine on Wednesday January 23 2019, @05:12PM (1 child)
Crazier things have happened. Apparently the SR-71's fuel tanks didn't seal fully until they warmed up enough, so on takeoff they leaked fuel everywhere, too. Granted, the SR-71 doesn't quite make it to space.
"Is that really true?" "I just spent the last hour telling you to think for yourself! Didn't you hear anything I said?"
(Score: 2) by All Your Lawn Are Belong To Us on Wednesday January 23 2019, @11:39PM
And then in flight would expand up to 6 inches in length due to the heat. The Wikipedia article also mentions how the fuel was used for both hydraulic purposes in the engine as well as a coolant of the skin.
This sig for rent.