- 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 about space and spaceflight.
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Thanks everybody!
That's gonna do it for me.
I'm Tim Dodd, the Everyday Astronaut,
bringing space down to Earth for everyday people.
(upbeat electronic music)