- Hi, it's me, Tim Dodd,
the Everyday Astronaut. SpaceX's upcoming rocket
called Starship/Super Heavy formerly known as the BFR,
will no longer be made out of lightweight carbon composites. It's gonna sweat a bunch, and it just might need a ton of WD-40. We finally actually have
a lot of good details on all this stuff, and we're
going to take take a look at all of Elon's most recent claims about stainless steel
actually being the best option and see if we can come
to the same conclusion. We're also gonna take a
look at some other rockets that are made out of
stainless steel and explain how SpaceX's use of this
material is quite a bit different as they're gonna be trying out some new manufacturing
techniques and doing some things that have never been attempted before. Let's get started. - [Ground Control]
Three, two, one, liftoff! (upbeat electronic music) - [Neil Armstrong] That's
one small step for man. I think most of us are really excited to see SpaceX's newest
rocket come to life. I mean, we're already watching them build the hopper down there
in Texas, the Starhopper, that's gonna do little propulsive flights. We already know about how big
this vehicle's going to be. We know how it's going to be powered by these Raptor engines. But there's one detail that we've had not a lot of information on, and it's left a lot of
us scratching our heads, and that's, why are they going
to be using stainless steel? Now, one little caveat
here before we get started: I'm no a physicist, I'm
definitely not a chemist, and for sure not a metallurigist? Meturlogist? However you say that word, I'm
definitely not that either. This video took me a little while longer to script and research
because I had to try and grasp a lot of concepts that I was
extremely unfamiliar with. I had to talk to a lot of experts, and I read way too many research papers. But after studying this
stuff for quite a while now, I have some really fun things to share. So let's dive right in with
why on Earth, or off Earth, is SpaceX switching to stainless steel? I mean, that's the same thing our pots and pans are made out of. It's that same heavy metal
that coats high-end appliances. This sounds much less 21st century and way more 1950s retro future. Well, there's a few really good reasons: strength at cryogenic temperatures, its characteristics at high temperatures, ease of development, and price. So let's go ahead start
off with what seems like the most controversial aspect: its weight. So, yes, of course if you
have say a cubic millimeter of carbon composites
versus stainless steel, yeah, the carbon
composite is much lighter. But weight is only half the equation in a structural material. The other big key difference is strength. And again, on paper carbon
composite seems to win out, right up until you put those materials under extreme temperature environments. And don't forget, a rocket experiences unbelievable differences in temperature. The super-chilled liquid oxygen SpaceX puts inside these
paper-thin tank walls is an unbelievably cold
-207 degrees celsius, and at the exact same time, the outside of that tank
can reach temperatures several hundred degrees celsius on ascent. And we're not even talking about the brutal reentry temperatures for now. More on that in a second. But it's the cryogenic
temperatures that's really truly the key to stainless
steel being advantageous. Most steels become brittle
at cryogenic temperatures. Perhaps you've seen videos
like this one from Cody'sLab of someone taking metal,
chilling it with liquid nitrogen, and smacking it with a hammer. Many metals become so
brittle at cryogenic temps that they're basically useless. But stainless steel with a
high chrome-nickel content like Stainless Steel 301,
at cryogenic temperatures its strength is actually increased by 50%. That's in pretty sharp
contrast to carbon composites, which become less strong
at these temperatures. And here's where the weight
difference can close. And let's throw in the
Aluminum Alloy 2219, which is probably similar to what SpaceX currently uses on their Falcon 9 rockets. While we've got this pulled up, I should probably explain it really quick. This is comparing the density
to the yield strength ratio. So, density best represents
weight since it includes volume, and the strength we're showing
here is yield strength. Since there isn't a unit of measurement that corresponds between
volume and yield strength, there isn't a nice clean number or unit to represent this ratio. The results are pretty interesting. Look how close stainless steel is to carbon composites at
cryogenic temperatures. Kinda makes you start to
wonder why stainless steel has been so underutilized this whole time. After all, stainless steel on
rockets definitely isn't new. The original Atlas rocket,
or the Convair SM-65 Atlas, started using stainless
steel way back in the 50s. On December 17th, 1957,
the first successful launch of the SM-65 Atlas missile took off. This rocket featured many high-tech firsts but probably the most interesting was its stainless steel balloon tank. Uh, why balloon tank? Well the walls of the stainless
steel were actually so thin, it required the entire vehicle to remain under pressure at all times while vertical in order to maintain its structure. As a matter of fact, when
it was under pressure, the vehicle actually grew. What's even more crazy
is if it lost pressure, it just folded in half. Yeah, we're definitely
gonna be talking about this in an upcoming episode
of Biggest Facepalms of Spaceflight History. And a fun note here. WD-40. Yeah, you know, that stuff that I'm sure literally every single one of us has sitting in their
garage that seems to be the cure-all for absolutely
everything in life. Yeah, that stuff was actually invented for Convair to protect the
outer skin of the Atlas rocket from rust and from stiffening. I wonder if SpaceX will
need to shower the Starship and Super Heavy in WD-40
before each launch. Or maybe they'll deploy a
fleet of autonomous drones with a can of WD-40 each,
and they'll just go around and clean the whole thing up. (chuckling) I have no idea. But you know what I find most interesting? Some people seem to be freaking out about using stainless steel. But don't forget, stainless
steel is still being used today by ULA on their Centaur upper stage. And it was used on the
Atlas III rocket until 2005. So it's not like stainless steel is some kind of novel concept that Elon Musk thought up over night, and it definitely has some
advantages over aluminum. So that got me thinking,
why isn't the Falcon 9 or other rockets made
out of stainless steel? And I kinda sat there and
I speculated for a bit, thinking it might have to
do with the temperatures, not being cryogenic with
RP-1 versus say hydrolocks. But I think perhaps the biggest reason is actually due to a breakthrough
in manufacturing technique known as cold forming. And cold forming is when
you chill material down to its cryogenic temperatures as you form and shape
during manufacturing. I mean, this practice has been around for a long time for
copper, brass, aluminum. But cold forming large sheets of stainless steel has been elusive. But just last year, a company
called Dawson Shanahan developed a technique
developed to cold form stainless steel, which
offers huge advantages in the strength of the material, and it's really quite easy to do. And oddly enough, Elon tweeted
about cold forming at cryo soon after the announcement
was made by Dawson Shanahan. Interesting. Ok, cool, so stainless steel
seems like a pretty good thing when the rocket's cold and fueled up, but how about when that
rocket gets really hot as it comes back in through reentry? Well, here's where things
get really interesting. When vehicles come back
in at orbital speeds, they get absolutely punished. After all, they're travelling
10 times faster than a bullet, and that's a lot of kinetic energy. In order to slow down in the atmosphere, that kinetic energy has to
actually be exchanged for heat. And that's why spacecraft
that come back in from space always have heat shields, whether it be an ablative heat shield;
that's something that flakes off material as it re-enters and takes the heat away with it; or then there's heat shields
that are able to basically soak up the heat like the
tiles on the space shuttle, which didn't let too much heat
reach the aluminum airframe by basically soaking it all
up like a really hot sponge and radiating it away very, very slowly. So what happens when you use a material that can withstand a crazy amount of heat? Well, you don't need nearly
as beefy of a heat shield, or you maybe don't need much
of a heat shield at all. Here's a few good examples of
this, take a look at this is. This is a stainless steel
tank off a Delta rocket that survived reentry basically intact. Or there's the X-15
hypersonic rocket plane which was actually made partly of Inconel, which has an extremely high melting point. This is called having a hot structure, and that's where you
basically let the structure of the vehicle get really, really hot. And there were even considerations
to build the shuttle out of titanium, which would have meant it wouldn't need nearly
as much heat shielding. Aluminum and carbon
composites can't withstand much more than about 200 degrees celsius before they start to deform,
but stainless steel can handle 800 degrees celsius and
basically keep on ticking. But reentry heating can
go well beyond that. As a matter of fact, peak heating can get up to almost
1,500 degrees celsius, and that's well beyond the point of being structurally sound. So there will still need
to be a heat shield. Starship will have a few forms
of heat shield protection. First off, since stainless steel is shiny, it'll actually reflect a good bit of the radiant heat away
instead of absorbing it. But radiating heat away
isn't enough, nope. Here's where things get even more crazy. SpaceX is looking to utilize the first regenerative heat shield for a spacecraft. Basically, on the belly of Starship will be another layer of stainless steel, but this time they'll
use 310S Stainless Steel, which can handle a
higher peak temperature. Then between those layers
of stainless steel sandwich will be some stringers
which will actually flow liquid methane when being actively cooled. Now, I know this sounds crazy. Like, what if that fails? I mean, it's your heat shield! But don't forget, the
combustion chambers of rockets have been doing this stuff
like this for decades, so it's pretty well worked out by now. Or here's another example. Take a look at the space
shuttle main engines. Here we had stainless
steel tubes brazed together and then liquid hydrogen
flowing through them to keep the nozzle cool. And don't forget the exhaust
coming out of the engine is a crazy 3,300 degrees celsius, so 1,500 degrees from reentry sounds like a walk in the park. Okay, so liquid cooling stainless steel isn't particularly new, but what is is the next step: the
sweaty, sweaty rocket. So, believe it or not, Starship
will actually bleed fuel out tiny micro pores as it reenters. These pores will be so small you probably won't even see them. The liquid-cooled methane will
take a lot of heat with it as it bleeds out, evaporates into a gas, and toots it away into
the wake of the vehicle. I know this idea sounds crazy
and like it'll never work, but this idea isn't new either. Did you know some
airplanes have tiny holes on the leading edge of the wings too? Yeah, that's right, some
planes use a system to push out an anti-freeze-type coolant
out these tiny little pores which keeps ice from
forming up on the wings. But I think my favorite example, although this one was
definitely never proven, was from the absolutely wacky Roton half rocket, half helicopter concept. That was planning to utilize a similar liquid heat shield concept as well. Okay, so sweaty metal
isn't unheard of either. But maybe the coolest
thing about the heat shield is that because it's double
layered, it's stiff enough to actually provide the
structural support of Starship so it can remain upright
even unpressurized, unlike the original Atlas rocket. (pitiful trombone music) You can almost think of it like
the backbone of the rocket, only it's on the front,
so it's really more like a chest plate of sweaty
heat shield awesomeness. Now, the last two things
we need to talk about are what probably why SpaceX
suddenly spun on their heels and totally ditched carbon composites, and that's time and money. With carbon composites,
you need to cut the fabric, impregnate it with high-strength resin, which can be pretty difficult to do, and then make 60 to 120 layers. There's also approximately a
35% scrap rate of material too, which can make carbon
composites really expensive. After all, the advanced carbon composites cost about $180 per kilogram by the time you factor
in the scrap material. So how is that compared
to stainless steel? Three. $3 per kilogram. Uh, yeah, that's 60 times
cheaper to manufacture. 60 times cheaper! I don't care what business you're in, when something is 60 times cheaper, it's readily available today, and it actually outperforms
the other material in your use case, you'd better hop on it. Which makes me wonder, how the heck does Rocket Lab get away with it? They make it look so easy. Well, these two vehicles aren't
in any way, shape or form even remotely comparable,
so let's not even go there. But best of all, since the material is so easy to work with and so well-known, they're actually getting
started on it now. Like literally right now
they're working on it. And this will certainly help achieve some of those lofty goals and timelines. Now you might be thinking, if the entire system is
reusable, the cost of the rocket doesn't really matter as much, does it? Well, of course that's
true to some degree, but forget the physical
benefits of stainless steel. If something is 60 times cheaper, it can really quickly
affect your bottom dollar. We're once again seeing
SpaceX not seem to fall into the trap of the sunk cost fallacy. I talk about this quite
a bit in a video titled "Why does SpaceX keep changing the BFR" after we saw its third
big change in design at the Dear Moon announcement in 2018. But the fact is this all checks out. It might be easy to think
this is some disappointment, a letdown or some kind of compromise, but quite frankly it is a compromise. Engineering is always a compromise, and that's not a bad thing. There's trade-offs to absolutely
every single decision, whether it be time and
money, or whether it be a flight profile where it might make sense to throttle down at a certain point, to trade-offs in strength and
weight of certain materials. There's always trade-offs. So, in summary, SpaceX
chose stainless steel over carbon composites
because it's about as light, it can handle higher temperatures, which means it needs less heat shield, which then makes it lighter, and then it reflects heat away, which means even less heat shield, which makes it even lighter, and it'll be cheaper and quicker to build, and it'll look freaking awesome! And in my opinion, when Elon gives us some updates on the vehicle,
which he promises to do soon, if the payload capacity were to be halved, which I don't think it will be, to lower from 100 tons down to 50 tons, I still think that's
a perfectly ridiculous amount of payload capability. I mean, I don't think
there's a huge market for 100 tons to low Earth orbit today. If your entire launch vehicle is reusable, that's truly the true key. Even if SpaceX could only launch the same exact payloads
as say Falcon 9 does, but it was entirely reusable, as a matter of fact as
reusable as an airliner with the same amount of
refurbishment as an airliner, that would be absolutely game-changing. But then again considering this
rocket is destined for Mars, maybe having a crazy
high payload capability isn't a bad thing for
a two-year-long trip. We'll definitely have to wait and see what the latest spec sheet looks like. So, what do you think? Are you excited about a
shiny, sweaty Starship, or do you feel like this is
just some big, bad compromise? Let me know your thoughts
in the comments below. And also be sure and let me know what other questions you have
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Earth for everyday people. (upbeat electronic music)
He missed a great example of sweating heatshields. The Atlas warhead heatshield from the 1950s: https://youtu.be/LTLA1dPby-g?t=802
Just finished the video, what I love about this is he breaks it down so you can show it to your kids and they can understand it. We need more videos like this to get young people interested.
One thing he didn't mention that I expected to see is dead weight vs useful weight on heatshield.
In a normal heat shield, that mass is dead weight that limits your mission when you land anywhere that heat tolerance is not a problem (extreme case, moonlanding).
If you are sweating fuel for heat shield, only dead weight is any tubing & 2nd layer required to allow it.
I would assume you don't sweat fuel when you don't need to, so rest of mass can actually be extra fuel for, for instance, moon landing.
For going to mars this might mean less refueling since less deadweight on launch (not sure how hot you'll get in Mar's atmosphere).
I'm guessing plumbing will mean you can only sweat methane in this system: in the scifi realm stainless starships come from, I could imagine passengers frantically dumping water/peeing into cooling tubes as the ship comes in too hot . . .
WD-40.
I don't think the payload will be less than 100 tons. Has a chance to be 150 again instead.
It was worth the long wait. Great job Tim
The Delorean was stainless steel and that thing travelled through time. Case closed.
I vaguely remember that Elon talked about possibly doing a "skip reentry" or multiple "skips" with the previous BFR. Maybe i misheard. Does anyone think that this might help the new Starship reduce the amount of methane vented while reentering the earth's atmosphere? Thanks
https://ipfs.io/ipfs/QmXoypizjW3WknFiJnKLwHCnL72vedxjQkDDP1mXWo6uco/wiki/Skip_reentry.html
Can someone explain why the payload capacity would be halved? He literally said that stainless steel would be just about as light as carbon composite.
Only part I was confused at.