Elon Musk shares SpaceX will use a new stainless-steel alloy for Starship

Featured Image Source:  SpaceX

SpaceX aims to develop a Starship capable of carrying one-hundred passengers plus tons of cargo, during long duration voyages to the moon and Mars. Earlier this year, the founder and Chief Engineer of SpaceX Elon Musk, announced his company will run 24/7 operations to build a Starship factory at South Texas facilities located in Boca Chica Beach. Their goal is to manufacture a Starship flight vehicle every week -eventually one every 72 hours. Engineers are currently building Starship prototypes to conduct cryogenic pressurization tests, that are performed to check weld and structure strength. They will also conduct a 20 kilometer test flight with a fully assembled Starship in a couple of months. SpaceX expects to built at least 20 Starship prototypes this year and aim to conduct Starship's first orbital-flight test before this year ends. In February, Musk called an all-hands meeting at his SpaceX facility in Boca Chica at 1:00 a.m. on a Sunday to ask his teams why they were not running 24/7 operations to finish a Starship vehicle. His engineering teams explained they needed more people to fill out shifts, 48 hours later - SpaceX hired 252 workers to double the work force. Musk is determined to colonize the Red Planet by the year 2050, a high-production rate is required to develop a space-ready craft. He envisions a fleet of 1,000 Starships departing from Earth, embarking on a journey to Mars every 26 months. Mass departures with hundreds of people and tons of cargo is needed to establish a permanent, sustainable settlement on the Red Planet. Engineers are designing a stainless-steel Starship that will be capable of being reused, Musk says it needs "to be relaunched an hour after landing, with zero nominal work. The only thing you expect to change on a regular basis is propellant."

On Monday (March 9) during a keynote conversation at the Satellite 2020 conference in Washington, D.C. Musk announced that SpaceX will change the stainless-steel alloy used in the construction of Starship. Currently, Starship prototypes are built from the 301 stainless-steel alloy. He said on March 9:

"We should be able to do better in the 2020s than they did in, like, the 50s, you know? So, I think we'll start switching away from 301 maybe in the next month or two."

The 301 stainless-steel alloy uses a mix of nickel, chromium and iron, this kind of material is very resistant to corrosion and cheaper than carbon fiber -that other rockets are built from. At the conference, Musk did not give any specifics as to what type of stainless-steel alloy mix the company will be using in future.

Today (March 14), I asked him via Twitter, "...What is the new stainless-steel alloy mix that will be used?" To which Musk responded: 

"Some parts will use 304L, as it has higher toughness at cryo temps. Will move to internally developed alloys probably end of year."

Some parts will use 304L, as it has higher toughness at cryo temps. Will move to internally developed alloys probably end of year.

— Elon Musk (@elonmusk) March 14, 2020

When Musk unveiled a Starship prototype in 2019, he explained that the reason they chose to design a stainless-steel flight vehicle is because "it’s obviously cheap, it’s obviously fast -but it’s not obviously the lightest. But it is actually the lightest. If you look at the properties of a high-quality stainless steel, the thing that isn’t obvious is that at cryogenic (cryo) temperatures the strength is boosted by 50 percent." He explained to Popular Mechanics reporters "Most steels, as you get to cryogenic temperatures, they become very brittle. You’ve seen the trick with liquid nitrogen on typical carbon steel: You spray liquid nitrogen, you can hit it with a hammer, it shatters like glass. That’s true of most steels, but not of stainless-steel that has a high chrome-nickel content. That actually increases in strength, and ductility is still very high. So you have, like, 12 to 18 percent ductility at, say, minus 330 degrees Fahrenheit. Very ductile, very tough. No fracture issues."

"I think is quite important, when you consider this as a reentry vehicle. See, here’s the other benefit of steel: It has a high melting point."

"Much higher than aluminum, and although carbon fiber doesn’t melt, the resin gets destroyed at a certain temperature." He added, "So typically aluminum or carbon fiber, for a steady-state operating temperature, you’re really limited to about 300 degrees Fahrenheit. It’s not that high. You can take little brief excursions above that, maybe 350."

Starship is designed to become a fully reusable launch vehicle that must be capable of withstand the highest temperatures upon the fiery atmosphere reentry as it returns from space, in order to be reused.

In 2019 a reporter asked him where the stainless-steel comes from, Musk responded:

"It’s just 301 stainless. Let me put it this way: 304 stainless is what they make pots out of. There’s plenty of it."

This makes Starship production a bit more cost effective compared to the production of other rockets that are made of carbon fiber. "The carbon fiber is $135 a kilogram, 35 percent scrap, so you’re starting to approach almost $200 a kilogram. The steel is $3 a kilogram," he said. So, stainless-steel is the most affordable option that offers high  strength and heat-shield properties. Musk expects to reduce manufacturing costs to $5 million per stainless-steel Starship vehicle as they scale-up production.