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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, Funny) by Anonymous Coward on Wednesday January 23 2019, @05:52AM (11 children)
According to my trusted sources steel can melt at around 600 °F.
(Score: 4, Funny) by takyon on Wednesday January 23 2019, @05:55AM (6 children)
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)
Something something the jews.
(Score: 0) by Anonymous Coward on Wednesday January 23 2019, @07:17AM (2 children)
Of course it was them
(Score: 0) by Anonymous Coward on Wednesday January 23 2019, @07:37AM (1 child)
Definitely. They used steal beams to take all of the gold.
(Score: 0) by Anonymous Coward on Wednesday January 23 2019, @08:09AM
Yes yes but they don't actually go to bed with it
(Score: 0) by Anonymous Coward on Wednesday January 23 2019, @08:08AM
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
Jet fuel not required to have stolen memes.
Universal health care is so complex that only 32 of 33 developed nations have found a way to make it work.
(Score: 5, Informative) by bradley13 on Wednesday January 23 2019, @07:51AM (3 children)
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.
Everyone is somebody else's weirdo.
(Score: 5, Interesting) by Zinho on Wednesday January 23 2019, @11:54AM (2 children)
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?
"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)
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
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
(Score: 0) by Anonymous Coward on Wednesday January 23 2019, @06:56AM (4 children)
Sell it to Space Force. They won't ask a lot of questions if it sounds cool.
(Score: 2) by takyon on Wednesday January 23 2019, @07:36AM
Giving money to SpaceX doesn't enrich the usual suspects.
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(Score: 2) by c0lo on Wednesday January 23 2019, @07:37AM (2 children)
Steel - so past millenium, how can it be cool? I mean, look, no nano- meta- or -phobic material, no blockchain, no CRISPR-x, no nothing, not even jobs.
https://www.youtube.com/watch?v=aoFiw2jMy-0 https://soylentnews.org/~MichaelDavidCrawford
(Score: 0) by Anonymous Coward on Thursday January 24 2019, @12:16AM
Orangium.
(Score: 2) by realDonaldTrump on Thursday January 24 2019, @05:06AM
I hear that Steel is the future. We love Steel -- so long as it's American Steel. Our Steel Industry is the talk of the World. It has been given new life, and is thriving. Because of me. I hired Wilbur Ross, one of our biggest Steel Men. He said, do the Tarriffs, we need Tarriffs. And I moved very strongly on those. Tarriffs have had a tremendous positive impact on our Steel Industry. Plants are opening all over the U.S., Steelworkers are working again, and big dollars are flowing into our Treasury. Into the coffers of our Country. Other countries use Tariffs against, but when we use them, foolish people scream. America First!!
(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.
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(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.
(Score: 0) by Anonymous Coward on Wednesday January 23 2019, @08:35AM (2 children)
Great Scott!
(Score: 0) by Anonymous Coward on Wednesday January 23 2019, @08:41AM
What the hell is going on here?
(Score: 2) by DannyB on Wednesday January 23 2019, @03:24PM
Heavy!
The spaceship, that is.
Universal health care is so complex that only 32 of 33 developed nations have found a way to make it work.
(Score: 0) by Anonymous Coward on Wednesday January 23 2019, @09:49AM (2 children)
Why does this Elon Musk talk just like the president trump of SN? is it Musk posing as trump here? is Trump posing as Musk in the interview?
(Score: 0) by Anonymous Coward on Wednesday January 23 2019, @10:30AM (1 child)
That is a huge compliment to president Trump.
(Score: 2) by realDonaldTrump on Thursday January 24 2019, @09:50AM
Yes it is, and I'm very honored! Elon is one of our wonderful African Americans. One of our Legal Aliens. He came to our Country, he wanted to become American. So he learned our beautiful language -- from the best people, the ones that talk like me. His tremendous achievements continue to show American ingenuity at its very best. And hopefully we can have many more incredible Africans like him coming in. Through that big beautiful Door!
(Score: -1, Troll) by Anonymous Coward on Wednesday January 23 2019, @12:27PM (1 child)
Bruce Perens is mad about this and mocks Elon
(See Bruce Perens' twitter).
What a fucking faggot.
Elon should sue him for libel.
(Score: 0) by Anonymous Coward on Wednesday January 23 2019, @03:09PM
Not seeing enough clicks Bruce?
(Score: 0) by Anonymous Coward on Wednesday January 23 2019, @02:16PM (3 children)
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)
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.
Appended to the end of comments you post. Max: 120 chars.
(Score: 0) by Anonymous Coward on Wednesday January 23 2019, @06:10PM (1 child)
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
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.
(Score: 3, Interesting) by aclarke on Wednesday January 23 2019, @03:21PM (1 child)
What surprised me most about this article is that apparently SpaceX measures temperature in Fahrenheit. Or does Musk only do this when talking to the American press?
(Score: 4, Insightful) by Immerman on Wednesday January 23 2019, @04:26PM
Probably for the press - He's an immigrant, and his staff are mostly engineers, so it seems very unlikely that Fahrenheit is used internally at SpaceX.
(Score: 0) by Anonymous Coward on Wednesday January 23 2019, @08:45PM (1 child)
Reading the blurb. It looks like we've found TheRealDonaldTrump.
(Score: 0) by Anonymous Coward on Thursday January 24 2019, @01:04AM
The common denominator is having done shitloads of cocaine.