Carbon fiber is lighter and stronger than stainless steel. It is widely used especially in the last two decades, and now we have a lot of experience of building and using it for different purposes. For example, it is most commonly used in the bodywork of a Formula 1 car and also the air box, the wings, the engine cover, the steering wheel, and suspension. Space shuttles also use carbon fiber composite materials offering excellent heat resistance in order to withstand the extremely high temperatures they receive while atmospheric re-entry.
Carbon fiber is an extremely strong material compared to its weight. So, why SpaceX engineers changed their minds and decided to use stainless steel instead of carbon fiber while building the Starship?
I am a cyclist and when I first learned that SpaceX was planning to build their interplanetary spacecraft, the “Starship”, out of carbon fiber, my first thought was “no way”. Because most serious cyclists know that (at least think that) “steel is real” – it is a common saying in the cycling world.
Steel is of course real, it exists, but what cyclists really mean when they say that, and what makes steel so “real”?
SpaceX’s Starship spacecraft and Super Heavy rocket (collectively referred to as Starship) represent a fully reusable transportation system designed to carry both crew and cargo to Earth orbit, the Moon, Mars, and beyond. Starship will be the world’s most powerful launch vehicle ever developed, with the ability to carry in excess of 100 metric tonnes to Earth orbit.
The Super Heavy rocket was previously known as the BFR (stands for “Big Falcon Rocket”).
The initial design of Starship was to use carbon fiber for everything – from the body of the spacecraft to the pressurized liquid oxygen tanks. They even built prototypes of the parts and SpaceX founder Elon Musk shared their photos on social media.
Why SpaceX built Starship from stainless steel instead of carbon fiber?
So, why did SpaceX engineers change their minds and decide to build Starship from stainless steel instead of carbon fiber?
Carbon fiber is lighter and stronger than stainless steel, but it has certain issues.
Carbon fiber is brittle
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 quite a bit before they fracture (think of yielding as irrecoverable deformation, like when a spring is stretched past its elastic limit).
Steel is more “forgiving” material than carbon fiber. A damaged steel part of a spacecraft can still work, or still can protect people inside. A carbon-fiber part, on the other hand, most probably cannot do that. It tends to fail catastrophically.
Carbon fiber is brittle at low temperatures, especially at the cryogenic temperatures you need for LOX and Liquid Methane. The last thing you want is your tank getting brittle when it’s cold enough to hold the stuff that you’re putting in it.
Carbon fiber is also no good when it gets too hot. Above about 400 degrees F (200°C), the resins that bind the carbon fibers together begin to break down and melt, and your carbon fiber loses all strength.
So, if you want to have a carbon fiber rocket that can re-enter the atmosphere, where it needs to stand up to temperatures as high as 1750°F (954°C), you need to put a serious amount of heat shielding on the outside of the carbon fiber, and it has to be a solid covering – even a small gap will result in burn-through of the carbon fiber, and then, as mentioned above, the whole rocket has to be replaced.
Carbon fiber is weak against impact
Carbon fiber displays the greatest strength lengthwise through the fibers. That means it is very strong when stretched or bent, but weak when compressed or exposed to high shock (e.g. 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 weaken the material, something that is less likely with a metal. And a small clamp can crush a carbon-fiber tube, given enough force (you can do this with thin-walled aluminum or steel tubing too but it takes more effort).
Carbon fiber is light… or is it really?
Carbon fiber transfers heat very well, which means you need a double-walled container, with tanks inside a barrel. This effectively doubles the amount of carbon fiber you need for your rocket. The outside has to be strong enough to withstand the pressure of reentry and landing, the inside has to be strong enough to withstand loads of pressurized tanks, and they have to be insulated from one another.
Doubling the amount of carbon fiber means your spacecraft is getting heavier, and that erases the material’s theoretical weight advantage.
And, you’ll still need structural elements to tie this all together, which means some kind of aluminum or steel frame inside the carbon fiber distributing the loads.
Compared to steel, carbon fiber is slow to manufacture
Carbon fiber is also slow to manufacture. It requires massive mandrels to wind the pre-cursor sheets around, and then, you have to bake the whole thing in a controlled-atmosphere oven for hours or days to develop the carbon-fiber lattice.
These mandrels are also very expensive. The one shown above was apparently close to $20M on its own, so buying more mandrels isn’t really an option. This brings us to the next point:
Compared to steel, carbon fiber is expensive… a lot
Carbon fiber is expensive to manufacture. Elon Musk gave an estimate that a carbon fiber BFS (Big Falcon Ship or Big Fraggin’ Ship – the former name of the Starship) would cost $150 per kilogram to build, and with all the extra supports and heat shielding needed, would still end up with a mass of between 80 and 100 tons.
So, the best you can do with carbon fiber is a ship that weighs between 80 and 100 tons, that’s extremely expensive to manufacture – at up to 100,000 kg x $150 per kilogram, and you’re at least $15M just in carbon fiber cost, without other crucial parts like engines, or flight hardware, computers, software, etc.
So, in September of 2018, SpaceX was already looking at the costs, and the speed, and the issues with maintenance, and they said, “This is not going to work to build a rocket where 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 to draw from is still automotive and aerospace specialists, there are huge industries that use stainless steel, there are also thousands of workers who know how to work with it,.
With stainless steel, you could literally hire, for example, a bunch of guys who normally weld together water towers, stick them in a field in Texas, and see how long it would take them to weld together a prototype for a few tens of thousands of dollars.
Elon Musk’s big quote was that the price of Starship construction using Carbon Fiber was $150 a kilogram, but the cost of using stainless steel was $3 per kilogram. That makes that 100-tonne Starship cost just $300,000 in stainless steel. And rather than hiring specialists at $250 an hour to build it, you hire a bunch of tank welders for $50 an hour to put it together for you.
With steel, maintenance is possible
And if you make a mistake… you can pound it out with a hammer, or cut it out and weld over the top of it.
That’s the real kicker when you want to build a reusable spaceship. Poke a hole in it and you can patch it. Break a piece off and you can weld a new piece on.
And you can change the design on the fly because of this. No more having to change the original mold. When you learn something, you can make an adjustment to an existing object.
Steel is still strong at low and high temperatures
Stainless steel (at least the 301, 304, and 30x experimental variants that SpaceX is using) gets stronger at cryogenic temperatures. So, now when you load the fuel and Lox, the tanks are actually stronger than when they are at room temperature. It took a while to get the welding techniques up to snuff, but now they have no issues with the tanks holding 8.5 atmospheres, enough for the 40% safety margin above the planned 6 atmospheres of pressure expected during flight, and the safety level required for human flight.
Plus, stainless steel has no problem sitting out in the environment, which means you don’t need to paint it.
And, while it conducts heat, it’s still fine to use as both the wall of the ship and the wall of the tank, meaning you’re back down to needing one layer of material. No separate tanks and barrels, and no extra weight for insulation.
Finally, 301 Stainless doesn’t even begin to lose strength until 1500°F (815°C). That means you can very nearly reenter from orbital velocity without any heat shielding at all. It means you only have to put light heat shielding onto the side exposed to reentry, and that, even if some heat leaks through, thanks to an effect known as stagnation, there’s no real danger of melting through the ship.
Even if it did, you could still patch the ship on the ground, something that’s 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
So, thanks to the fact Stainless steel doesn’t need the insulation, or the double-tanks, or the heavy heat shielding, and, as they’ve worked out that they actually need much thinner stainless than they used on Starhopper, the actual mass of Starship SN8 (made of 304 stainless) was only about 100 tonnes, and Elon has already said they expect the 30X follow-up serial numbers to potentially get down to a launch mass of only 85 tonnes.
Effectively, by ditching the “ultra-light” carbon fiber for the far more utilitarian stainless steel, they’ve actually managed to match the mass of a carbon fiber Starship, without all the headaches.
And they can already crank a Starship out in Boca Chica in about 2 weeks – while it might take that long to build a single barrel section of a carbon fiber ship in that time.
Why not using aluminum or titanium?
Anything lighter than steel, like aluminum or titanium, won’t stand up to the high temperatures or embrittles at cryogenic temperatures.
Anything in the wild composite or ceramic materials is nearly impossible to work with on this scale or would require decades of research to make work in this application.
Overall, stainless steel is just a total “sweet spot” in the curve. It has so many good properties – ease of use, price, temperature resistance, strength, corrosion resistance, etc – that even though it doesn’t win any particular category, it’s the only one that checks them all off.
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.
- “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 the SpaceX website
- Carbon Fibers on Wikipedia
- “Why is carbon fiber inherently weak? Or is it?” on bicycles.stackexchange.com