Elon Musk was right in all his predictions when he abandoned the carbon fiber Starship to remake it in steel

Starship is the first stainless steel rocket to reach space since the Atlas program of the 1950s. SpaceX has not always had plans to use this material: it was a risky bet by Elon Musk that ended up working out incredibly well.

A reusable rocket. Starship 29 survived atmospheric reentry on Flight 4 despite friction with the air at 20,000 kilometers per hour. She lost numerous thermal tiles and much of a spoiler when plasma began leaking through the rocket’s joints.

However, it restarted its engines, went vertical and simulated a landing in the Indian Ocean, with the help of the half-melted aileron. Reentry failed to disintegrate the reusable rocket despite the loss of part of the ceramic heat shield. The steel resisted.

From BFR to Starship. Around the same time that the rocket changed its name to Starship, Elon Musk announced that he had decided to abandon the carbon fiber fuselage of the Big Fucking Rocket for a new one made of stainless steel, specifically alloy 301.

Early models of the ship looked shoddy due to wrinkles in the hammered material, but Musk argued that steel was the best choice for a 100% reusable rocket.

The advantages of steel. In a 2019 interview with Popular MechanicsMusk listed the reasons for the change:

• Steel withstands temperatures of 810ºC, compared to 149ºC for carbon fiber and aluminum, which would have been brittle upon re-entry
• Steel becomes stronger at cryogenic temperatures, such as those of the liquid methane that Starship uses as fuel
• Steel better resists microfractures, such as those that can occur in a rocket designed to land and fly many times
• And above all, steel costs $3/kg compared to $200/kg for carbon fiber

The heat shield. Starship’s design has continued to evolve since 2019. Instead of a heat shield, it was planned to re-enter its own steel body actively cooled with methane, but that idea was scrapped because it made the rocket heavier.

In the end, SpaceX added thousands of hexagonal ceramic tiles over Starship’s body as a heat shield. They are very light and resistant to cracks, but capable of withstanding atmospheric braking, although they transpire heat to the steel beneath them.

An alloy of its own. Another important change is that they stopped using type 301 steel in favor of their own alloy patented by SpaceX.

Commercial stainless steel gave problems of “interlaminar toughness”: it did not tolerate the propagation of cracks between its layers well at cryogenic temperatures; like those at takeoff, when the rocket is full of methane and liquid oxygen.

The cheapest rocket. It is not clear how much SpaceX is investing in this material, but there are already several dozen prototypes, and Elon Musk is very optimistic in his projections about the cost of the program.

SpaceX aims to manufacture thousands of Starships to colonize Mars, financing the development of the rocket with launches of Starlink and other satellites, in addition to NASA contracts for lunar missions.

Thanks to its 100% reusable design, Musk estimates that the price per kilogram of Starship will be well under $10020 times less than what it costs to launch a kilogram on a Falcon 9.

Image | SpaceX

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