Why SpaceX ditched lightweight Carbon Composites for Stainless Steel to make a sweaty Starship

SpaceX’s upcoming rocket called Starship Super Heavy (formerly known as BFR), will no longer be made out of lightweight Carbon Composites, it’ll sweat a lot, and just MIGHT need a TON of WD-40.

SpaceX’s old 12 meter wide advanced carbon composite tanks for their previous BFR (2016)

So we’ll take a look at all of Elon’s most recent claims about stainless steel actually being the best option and see if we come to the same conclusion. We’ll also take a look at some other rockets that are made of stainless steel and explain how SpaceX’s use of this material is a little different as they’ll be using new manufacturing techniques and doing things that have never been attempted before.

I think most of us are getting really excited to see SpaceX’s newest rocket come to life. We’re already seeing their test hopper that will practice short little propulsive flights being built and then watch as the nose cone blows over in the wind…  and we’ve already got a good handle on the actual dimensions of the vehicle as well.

SpaceX’s “StarHopper”, a simple test flight vehicle being built at their Boca Chica site in Brownsville, Texas

One little caveat before we get started, I’m not a physicist, I’m not a metallurgist, I’m not a chemist. This video took me a while longer to script and research because I had to try and grasp a lot of concepts that I’m very very unfamiliar with, talk to a lot of experts, and read way too many research papers. But after studying this stuff for sometime 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… the same thing our pots and pans are made out of? The same heavy metal that coats high end appliances? That sounds much less 21st century and way more 1950’s retro future. 

Well there are four main reasons: Strength at cryogenic temperatures, its characteristics at high temperature, ease of development and price.

Let’s start off with what seems to be the most controversial aspect, the weight. Yes, if you have say a cubic millimeter of carbon composite vs stainless steel, carbon composite is much lighter. But weight is only half the equation in a structural material. The other big key is strength. And again, on paper carbon composite seems to win out. Right up until you put those materials under extreme temperature environments. Don’t forget, a rocket experiences unbelievable differences in temperature.

Condensation pouring off Falcon Heavy due to the super cold cryogenic fuels inside

The super chilled liquid oxygen SpaceX puts inside these paper thin tanks is an unbelievably cold -207 degrees celcius and at the exact same time, the outside can reach temperatures several hundred degrees celsius on ascent, and we’re not even talking about the brutal reentry temperatures… 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 of someone taking metal, chilling it with liquid nitrogen and smacking it with a hammer. Many metals become so brittle at cryo temps that it’s useless.

Cody Reeder breaking some iron that was chilled down to cryogenic temperatures using liquid nitrogen

But stainless steel with a high chrome-nickel content like stainless steel 301 actually gets stronger at cryogenic temperatures! At cryogenic temps the strength is actually increased by 50%! That’s in pretty sharp contrast to Carbon Fiber which becomes 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 quick, this is comparing the density to yield strength ratio. 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 as it’s arbitrary, but it is relevant for a comparison basis. The results are pretty interesting, look how close stainless steel is to carbon composites at cryogenic temperatures. It makes you start to wonder why Stainless Steel has been so underutilized this whole time.

I mean, after all, Stainless Steel on rockets isn’t new, the original Atlas rocket, or the Convair SM-65 Atlas, starting using stainless steel way back in the 50’s! On December 17th, 1957, the first successful launch of the SM-65 Atlas missile took off. This rocket featured many high tech firsts including an interesting stage and a half design and a stainless steel balloon tank.

Uh why balloon tank? Well, the walls of the stainless steel were so thin, it required the vehicle to remain under pressure at all times while vertical in order to maintain its structure. As a matter of fact, if it lost pressure, it folded. We’ll be talking about this in an upcoming biggest facepalms of spaceflight history.

A fun note here… WD-40, you know, that stuff almost everyone has in their garage that seems to be a cure all for just about everything. Yeah, that stuff was actually invented for Convair to protect the outer skin of the Atlas rocket from corrosion and stiffening! Crazy right? I wonder if SpaceX will need to shower the Starship and Super Heavy in WD-40 before each launch!

BUT, you know what’s interesting? Stainless Steel is still used TODAY by ULA on their centaur upper stage. So it’s not like stainless steel is some novel concept that Elon thought up over night, and it definitely has some advantages over aluminum.

A Centaur Upper Stage outside of Glenn Research Center. I knew it could fit in a trunk, but I think it was the Space Shuttle’s trunk and not a BMW 3 series…

So, that got me thinking, why isn’t the Falcon 9 or other rockets made out of stainless steel? Perhaps a reason is actually due to a breakthrough in manufacturing technique known as cold forming. Cold forming is when you chill the material down to cryogenic temperatures as you form and shape them for manufacturing. This practice has been used for a while with other metals like copper, aluminum and brass, but cold forming large chunks 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. 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 the rocket gets really hot as it comes back through reentry? Well here’s where things get really interesting. When vehicles come back in at orbital speeds, they get absolutely punished. A vehicle travelling 10 times faster than a bullet has a lot of kinetic energy. 

ESA’s ATV-1 reentry disintegrates upon reentry

In order to slow down in the atmosphere, that kinetic energy is exchanged for heat. This is why vehicles that come back from space have heat shields. Whether it be an ablative heat shield that purposefully flakes away material and which takes heat with it.

NASA’s Orion heat shield is ablative

Or there’s heat shields that are able to 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 slowly.

The silica tiles of the Space Shuttle would radiate heat 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 no heat shield at all on some parts of the ship. A good example of this, take a look at this is. This is a stainless steel tank off a Delta rocket that survived orbital reentry surprisingly intact!

This is the main propellant tank of the second stage of a Delta 2 launch vehicle which landed near Georgetown, TX, on 22 January 1997. This approximately 250 kg tank is primarily a stainless steel structure and survived reentry relatively intact. – https://www.orbitaldebris.jsc.nasa.gov/reentry/recovered.html

Or the X-15 hypersonic rocket plane which was actually made partly of inconel which has an extremely high melting point.

This is called a hot structure and there were even considerations to build the shuttle out of Titanium which would’ve meant it wouldn’t have needed nearly as much heat shielding. Aluminum and carbon composite can’t withstand much more than about 200 degrees celsius before they start to deform, but stainless steel can handle 800 degrees celsius and keep on tickin!

But, reentry heat can go beyond that. As a matter of fact, peak heating can get up to almost 1,500 degrees celsius, 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 actually will reflect a good bit of the radiant heat instead of absorbing it! Crazy. But radiating heat away isn’t enough, nope, here’s where things get even MORE interesting. SpaceX is looking to utilize the first regenerative heat shield for a spacecraft.

The starship will have a belly made of 310S stainless steel with some cooling channels sandwiched between the 301 stainless steel tanks

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 will be stringers which will house some methane when being actively cooled. Now I know this sounds crazy and scary, but don’t forget, the combustion chamber of rocket engines have been doing stuff like this for decades!

A combustion chamber is basically just hundreds of cooling channels squeezed together!

Or 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.  Don’t forget, the exhaust coming out of the engines is a crazy 3,300 degrees celsius! So 1,500 degrees from reentry sounds like a walk in the park!

Ok, liquid cooling stainless steel isn’t particularly new, but what is is the next step. The sweaty sweaty rocket. 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 cool liquid 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.

And again, I know this idea sounds crazy and like it’ll never work… but this idea also isn’t new. Did you know some airplanes have tiny holes on the leading edge of the wings too? Some planes use a system to push out an anti-freeze type coolant out the tiny pores which keeps ice from building up on the wings.

TKS anti-icing system on the leading edge of a plane

And another fun example, although not ever proven, was the absolutely wacky Roton half helicopter half rocket concept. It was planning to utilize a similar liquid heat shield concept.

The wacky Roton Rotary Rocket on display at the Mojave Air and Space Port in California

Ok 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 provide the structural support of Starship so it can remain upright even unpressurized, unlike the original Atlas. So you can almost think of it like the backbone, only, it’s on the front… so more like a chest plate of sweaty heat shield awesomeness.

Now the last two things we need to talk about are what probably really made SpaceX spin on their heels and totally ditch carbon composites… and that’s time and money.

With carbon composite, you need to cut the fabric, impregnate it with high-strength resin, which can be difficult and then make 60 to 120 layers! There’s also approximately a 35% scrap rate of material too, which makes it so carbon composites are terribly expensive. As a matter of fact, the advanced carbon composites cost about $180 per KG by the time you factor in the scrap material. So how’s that compare to stainless steel? $3. $3 per KG….

The tail section of the Boeing 787 Dreamliner manufactured out of Carbon Composites

Uhhh yeah… 60 times to cheaper to manufacture. SIXTY TIMES CHEAPER. I don’t care what business you’re in, when something is 60 times cheaper, readily available today, and outperforms the other material, 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 comparable… so let’s not even do it haha

The Electron rocket by Rocket Lab manages to be lightweight and inexpensive

But best of all, since the material is so easy to work with and well known, they’re actually getting started on it NOW. Like right now! 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. And SpaceX is, after all, a private company looking to make a profit at the end of the day.  Sure, they could spend 10 years developing the most advanced carbon composite fuselage ever that costs insane amounts to produce, but let’s look at where that’d get them in 20 years. Now they lost a lot of time of potentially profiting off the development of a fully reusable vehicle, and THEN they’ll have a rocket that’s even more expensive to build!

We’re once again seeing SpaceX not fall into the trap of the sunk cost fallacy. I talked 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 a disappointment, a letdown or a 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 are 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 less heat shield is need, which then makes it lighter, it reflects heat which means even less heat shield which again makes it even lighter, it’ll be cheaper and quicker to build AND it’ll look FREAKING AWESOME.

Gorgeous render by Reese Carges – https://twitter.com/AstroReeseW

And in my opinion when Elon gives us the new updated specs on the vehicle, if the payload capacity were to be halved from 100 tonnes to Low Earth Orbit down to 50 tonnes, I still think that’s a perfectly ridiculous amount of payload capability. I don’t think there’s a huge market for 100 tonnes to LEO, and if your entire launch vehicle is reusable, that’s really the true key. Even if Starship could only launch the same payloads as Falcon 9 but was entirely reusable with just air liner like maintenance, that would be absolute game changing.

Although… considering this rocket is destined for Mars… maybe having a crazy payload capability isn’t a bad thing for a 2 year trip… We’ll just have to wait and see what the latest spec sheets look like.

So what do you think? Are you excited about a shiny Starship or do you feel like this is just a big bad old compromise? Let me know your thoughts in the comments below.

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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.

Tim Dodd – The Everyday Astronaut