Earlier this fall I had an opportunity to head down to a windy piece of land near the Mexican border to tour the Starship Mk1 This is the first of many prototypes that will one day become the ship that carries us to the moon and Mars and back. Seeing the gleaming 165-foot high rocket at night, standing at the end of the lonely nondescript Texas State Highway 4, seemed at once like a Hollywood movie set and the world's greatest manifestation of optimism. It was also just beautiful.
Starship is 30 feet in diameter and will extend its height to a remarkable 387 feet when supported by the Falcon Heavy booster that will hold up to 37 Raptor engines, with a force necessary to push Starship out of Earth's gravitational pull, and into far orbit. Starship will be refilled with methane and liquid oxygen in orbit and so will be able to carry 100 passengers and 150 tons of cargo to the moon and also, as part of SpaceX's plan to develop a self-sustaining colony, onward to Mars.
I used to write a regular materials science blog for this magazine years back, and so the thing about Starship that lingered with me is this: The decision to build Starship using steel. Musk and his SpaceX engineers intended to build the rocket with carbon fiber but then last year the team switched to steel, a decision that Elon Musk described as maybe his "…best idea ever." And during his public presentation of Starship on September 28, he proclaimed, "Honestly, I'm in love with steel."
The first reason for this is its thermal properties. Steel isn't brittle at extremely cold temperatures and it won't melt at extremely high temperatures. But recently I mentioned this quality to a few industrial engineers and they responded skeptically insisting, "steel cannot maintain stability at extremes." These structural engineers were steel experts, so I needed to figure out what I was missing.
The first thing we need to be clear on is that SpaceX is using stainless steel. And this makes a huge difference. As most of us know, steel is used everywhere and there are many kinds. Steel isn't just steel. Steel is an alloy, a combination of iron and lots of other elements like manganese, sulphur and carbon. And if you add elements like nickel, titanium and specifically chromium you can really alter steel's properties.
Stainless steel has more chromium than other steels (requiring a minimum of 10% chromium), and Starship is using stainless steel 301. This has 17 percent chromium and 7 percent nickel. It remains solid until 1,500 degrees Centigrade which allows it to handle the insane heat of re-entry, without a heat shield. And this makes Starship much lighter.
On the other end of the temperature spectrum stainless steel has higher cryogenic toughness due to the nickel content—meaning that at crazy low temperatures (i.e., -150 to -273 degrees Celsius) it has high ductility and high tensile strength, so it can be stretched thin without snapping. In fact, at cryogenic temperatures the tensile strength of stainless steel is higher than at ambient temperatures. This is the underlying secret. If you rely on basic steel manuals that assume relatively normal ranges in temperature, you might miss the magic of stainless steel. At cryogenic temperatures 301 stainless steel is as strong as any other advanced composite or aluminum-lithium.
Additionally it is less corrosive, easier to maintain, with a zero need for paint and with its brilliant mirror-like sheen steel is very attractive.
Most stainless steel alloys have excellent resistance to corrosion in normal conditions. Stainless steel alloys tend to possess a strong and thin layer of oxide that prevents rusting, hence the name "stainless" steel. Starship is evidence of this corrosive resistance, as it was welded with no factory protecting it. The crew of engineers build it outdoors by the highway, with a strong salt wind constantly whipping.
The chromium in the alloy forms a self-healing protective clear oxide layer. Even if the material surface is cut or damaged, it will self heal and corrosion resistance is maintained. This is why I love materials science…it's pure magic!
Carbon fiber is considered to be ideal for industrial applications because of its strength-to-weight ratio. But it is expensive. Turns out this stuff is a major pain in the ass to make. Before carbon fiber becomes carbon fiber, it starts as a base material—usually an organic polymer with carbon atoms binding together long strings of molecules called a polyacrylonitrile. Which brings us to the coup de grace: carbon fiber is $130K per ton while stainless steel is $2.5K per ton. For a company that is paying its own way to Mars this detail is a game changer.
Like so many breakthrough discoveries stainless steel was an accident. In the early 1900s Harry Brearley explored different alloys for gun barrels and he noticed that some never rusted. These were the combinations with higher amounts of chromium. Brearley went on to use these in cutlery and in the 1920s Sheffield become world famous as the birth place of mass-produced cutlery. And now rockets!
The speed at which SpaceX plans to iterate on Starship Mk1 seems impossible, with a plan to have Starships Mk4 and Mk5 built in about six months. And the aspirational goal is to head to Mars (without humans) by 2022. Crews will follow in 2024.
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