Carbon fiber is lighter and stronger than stainless steel, making it a popular material, especially in the last two decades. We now have extensive experience in building and using it for various purposes. For example, it is commonly used in the bodywork of Formula 1 cars, including the airbox, wings, engine cover, steering wheel, and suspension. Additionally, space shuttles utilize carbon fiber composite materials for their excellent heat resistance, essential for withstanding the extremely high temperatures encountered during atmospheric re-entry.
Given its exceptional strength-to-weight ratio, carbon fiber is an extremely strong material. So, why did SpaceX engineers decide to use stainless steel instead of carbon fiber when building the Starship?
As a cyclist, when I first learned that SpaceX was planning to build their interplanetary spacecraft, the Starship, out of carbon fiber, my initial reaction was, “No way.” Most serious cyclists are familiar with the saying “steel is real” – a common phrase in the cycling world.
Steel is, of course, a real material, but what do cyclists truly mean when they say that, and what makes steel so “real”?
SpaceX’s Starship
SpaceX’s Starship spacecraft and Super Heavy rocket (collectively called Starship) form a fully reusable transportation system designed to carry crew and cargo to Earth orbit, the Moon, Mars, and beyond. Starship will be the world’s most powerful launch vehicle ever developed, capable of carrying over 100 metric tonnes to Earth orbit.
The Super Heavy rocket was previously known as the BFR (Big Falcon Rocket).
The initial design of Starship planned to use carbon fiber for everything, from the spacecraft’s body to the pressurized liquid oxygen tanks. SpaceX even built prototypes of these parts, and founder Elon Musk shared their photos on social media.
So, why did SpaceX build Starship from stainless steel instead of carbon fiber?
In fact, we largely received the answer to this question during the OceanGate implosion. On June 18, 2023, Titan, a submersible operated by the American tourism and expeditions company OceanGate, imploded during an expedition to view the wreck of the Titanic in the North Atlantic Ocean off the coast of Newfoundland, Canada.
The 6.7-meter-long (22 feet), 10,432 kg (23,000 lb) vessel was constructed from carbon fiber and titanium. The entire pressure vessel consisted of two titanium hemispheres with matching titanium interface rings bonded to the 142 cm (56 inches) internal diameter, 2.4-meter-long (7.9 feet) carbon fiber-wound cylinder. On June 30, Insider published an analysis of the recovery photos by Plymouth University professor Jasper Graham-Jones. He concluded that a failure of the carbon-fiber hull was the most likely cause of the loss.
In summary, in environments where the reliability of the material is crucial (such as the depths of the oceans or deep space), you should not build your vehicle’s body with an unreliable material like carbon fiber (at least compared to steel and aluminum). Unfortunately, the passengers of the Titan submersible proved this once again at the cost of their lives.
![I race with a carbon fiber bike [Ozgur Nevres during a Time Trial startup]](https://i0.wp.com/ozgurnevres.com/wp-content/uploads/2024/08/Ozgur-Nevres-carbon-bike.webp?resize=640%2C870&ssl=1)
Carbon fiber is brittle and fails catastrophically
Despite being lighter and stronger, carbon fiber is very brittle under certain conditions, meaning it shows very little yielding and almost always fails catastrophically. Metals, especially steel, are more ductile and therefore yield significantly before fracturing (think of yielding as irreversible deformation, like when a spring is stretched past its elastic limit).
Steel is a more “forgiving” material than carbon fiber. A damaged steel part of a spacecraft can still function or protect the people inside, whereas a carbon-fiber part most likely cannot. It tends to fail catastrophically.
Carbon fiber is brittle at low temperatures, especially at the cryogenic temperatures needed for LOX and liquid methane. The last thing you want in space is your tank becoming brittle when it’s cold enough to hold these substances.
Additionally, carbon fiber does not perform well at high temperatures. Above about 400°F (200°C), the resins that bind the carbon fibers together begin to break down and melt, causing the carbon fiber to lose all strength.
Therefore, if you want a carbon fiber rocket that can re-enter the atmosphere, where it needs to withstand temperatures as high as 1750°F (954°C), you need to apply a significant amount of heat shielding on the outside. This shielding must be a solid covering, as even a small gap can result in a burn-through of the carbon fiber, necessitating the replacement of the entire rocket.
Carbon fiber is weak against impact
Carbon fiber displays the greatest strength lengthwise through the fibers. This means it is very strong when stretched or bent but weak when compressed or exposed to high shock (for example, a carbon fiber bar is extremely difficult to bend but will crack easily if hit with a hammer).
Certain types of forces, like sharp impacts, can damage the fibers and epoxy, weakening the material -something that is less likely with metal. Additionally, a small clamp can crush a carbon fiber tube if enough force is applied. While you can also crush thin-walled aluminum or steel tubing, it requires significantly more effort.
But, carbon fiber is light… or is it really?
Carbon fiber transfers heat very well, necessitating a double-walled container with tanks inside a barrel. This effectively doubles the amount of carbon fiber needed for your rocket. The exterior must be strong enough to withstand the pressure of reentry and landing, while the interior must be robust enough to handle the loads of pressurized tanks, and they must be insulated from one another.
Doubling the amount of carbon fiber increases the spacecraft’s weight, negating the material’s theoretical weight advantage.
Additionally, you will still need structural elements to tie everything together, which means incorporating some kind of aluminum or steel frame inside the carbon fiber to distribute the loads.
Compared to steel, carbon fiber is slow to manufacture
Compared to steel, carbon fiber is very expensive.
Carbon fiber is costly to manufacture. Elon Musk estimated that a carbon fiber BFS (Big Falcon Ship, the former name of Starship) would cost $150 per kilogram to build. With all the additional supports and heat shielding needed, it would still have a mass of between 80 and 100 tons.
So, the best you can achieve with carbon fiber is a ship weighing between 80 and 100 tons, which is extremely expensive to manufacture—up to 100,000 kg at $150 per kilogram amounts to at least $15 million just for the carbon fiber, not including other crucial parts like engines, flight hardware, computers, software, etc.
By September 2018, SpaceX was already assessing the costs, speed, and maintenance issues and concluded, “This is not going to work to build a rocket when we want to send 100 of them to Mars every two years.”
And that’s where stainless steel comes in.
Stainless steel is cheap and plentiful
Unlike carbon fiber, where the talent pool is limited to automotive and aerospace specialists, there are large industries that use stainless steel and thousands of workers skilled in working with it.
With stainless steel, you could hire, for example, a team of workers who normally weld together water towers, place them in a field in Texas, and see how long it would take them to weld together a prototype for just a few tens of thousands of dollars.
Elon Musk famously stated that the cost of Starship construction using carbon fiber was $150 per kilogram, while the cost of using stainless steel was $3 per kilogram. This makes a 100-ton Starship cost just $300,000 in stainless steel. Instead of hiring specialists at $250 an hour to build it, you can employ a team of tank welders at $50 an hour to assemble it.
With steel, maintenance is possible
If you make a mistake, you can simply pound it out with a hammer or cut it out and weld over it.
This flexibility is crucial when building a reusable spaceship. If you poke a hole in it, you can patch it. If a piece breaks off, you can weld on a new one.
Additionally, you can change the design on the fly because of this. There’s no need to alter the original mold. When you learn something new, you can make adjustments to an existing object.
Steel is still strong at low and high temperatures
Stainless steel, particularly the 301, 304, and experimental 30x variants that SpaceX is using, becomes stronger at cryogenic temperatures. When you load the fuel and LOX, the tanks are actually stronger than at room temperature. It took time to perfect the welding techniques, but now the tanks can hold 8.5 atmospheres, providing a 40% safety margin above the planned 6 atmospheres of pressure expected during flight, meeting the safety level required for human flight.
Additionally, stainless steel has no problem withstanding environmental exposure, so you don’t need to paint it.
While it conducts heat, stainless steel can still serve as both the wall of the ship and the wall of the tank, reducing the need for separate tanks and barrels and eliminating extra weight for insulation.
Finally, 301 stainless steel doesn’t begin to lose strength until 1500°F (815°C). This means you can nearly reenter from orbital velocity without any heat shielding at all. You only need light heat shielding on the side exposed to re-entry. Even if some heat leaks through due to stagnation, there’s no real danger of melting through the ship.
And if any damage occurs, you can patch the ship on the ground-something impossible with carbon fiber.
In practice, a stainless steel spacecraft is as light as a carbon fiber one – without all the headaches the latter brings in
In practice, a stainless steel spacecraft is as light as a carbon fiber one—without all the associated headaches.
Thanks to the fact that stainless steel doesn’t require insulation, double tanks, or heavy heat shielding, and because they have determined that much thinner stainless steel is needed compared to what was used on Starhopper, the actual mass of Starship SN8 (made of 304 stainless) was only about 100 tonnes. Elon Musk has already mentioned that they expect the 30X follow-up serial numbers to potentially have a launch mass of only 85 tonnes.
By ditching the “ultra-light” carbon fiber for the far more utilitarian stainless steel, they’ve managed to match the mass of a carbon fiber Starship, without all the complications.
Moreover, they can already produce a Starship in Boca Chica in about two weeks, whereas it might take that long just to build a single barrel section of a carbon fiber spaceship.
Why not use aluminum or titanium?
Anything lighter than steel, like aluminum or titanium, won’t stand up to high temperatures or becomes brittle at cryogenic temperatures.
Composite or ceramic materials are nearly impossible to work with on this scale or would require decades of research to be viable for this application.
Overall, stainless steel hits the perfect “sweet spot.” It has numerous favorable properties—ease of use, affordability, temperature resistance, strength, corrosion resistance, etc. While it may not excel in any single category, it is the only material that meets all these criteria effectively.
This post is the extended and edited version of Jeffrey Naujok‘s perfect answer to “What comparable material to stainless steel could be more effective and lower cost for the SpaceX Starship rocket? What gives stainless steel the advantage? Is it just cost? What grade of stainless steel is used presently for Starship?” on Quora.
Related post: The Evolution of SpaceX’s Raptor Engine
Sources
- What comparable material to stainless steel could be more effective and lower cost for the SpaceX Starship rocket? What gives stainless steel the advantage? Is it just cost? What grade of stainless steel is used presently for Starship?” on Quora.com
- Carbon Fiber on the F1 Technical website
- “We can’t talk about space development without mentioning carbon fiber composite materials” on Torayca.com. Torayca is one of the most important producers of carbon fiber. Their products are widely being used from the aerospace industry to high-end racing bicycles.
- Starship on SpaceX’s official website
- Titan submersible implosion on Wikipedia
- SpaceX Starship on Wikipedia
- Carbon Fibers on Wikipedia
- “Why is carbon fiber inherently weak? Or is it?” on bicycles.stackexchange.com
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