Starship is the first stainless-steel rocket to reach space since the Atlas program in the 1950s. However, SpaceX didn’t always plan to use this material. It was a risky gamble by CEO Elon Musk that has ended up working out incredibly well.
A reusable rocket. Starship Ship 29 survived the atmospheric re-entry of Flight 4 despite friction with the air at more than 12,420 miles per hour. It lost numerous thermal tiles and endured damage to its flap when plasma began leaking from the rocket’s seals.
However, Starship’s upper stage reignited its engines, went vertical, and simulated a landing in the Indian Ocean with the help of the partly-melted flap. The re-entry failed to disintegrate the reusable rocket despite the loss of part of the ceramic heat shield. In other words, the steel held.
From BFR to Starship. At around the same time the rocket was renamed Starship, Musk announced that he had decided to abandon the carbon fiber fuselage of the Big Falcon Rocket, or BFR—speculated by some to actually stand for "Big Fucking Rocket"— the in favor of a new one made of stainless steel, specifically alloy 301.
Early spacecraft models appeared shoddy due to wrinkles in the hammer-assembled material, but Musk argued that steel was the best choice for a 100% reusable rocket.
Advantages of steel. In a 2019 interview with Popular Mechanics, Musk explained why he had decided to use steel instead of carbon fiber:
- Steel can withstand temperatures of 1,500°F, whereas carbon fiber and aluminum can only handle temperatures up to 300°F, making them brittle on re-entry.
- Steel becomes stronger at cryogenic temperatures, like those of the liquid methane used as fuel in Starship.
- Steel is more resistant to micro-perforations, making it a suitable material for a rocket designed to land and fly many times.
- Steel is cost-effective: It’s priced at $3/kg compared to carbon fiber’s $200/kg.
The heat shield. Starship's design has evolved since 2019. Initially, SpaceX planned to have it re-enter the atmosphere using a steel body actively cooled with methane instead of a heat shield. However, SpaceX engineers discarded this idea because it would’ve added too much weight to the rocket.
In the end, SpaceX opted to attach thousands of hexagonal ceramic tiles to Starship’s body to serve as a heat shield. These tiles are lightweight and resistant to cracking. Although they do release heat into the steel beneath them, the tiles can withstand atmospheric braking.
A property alloy. The material used for the rocket changed from steel type 301 to a proprietary alloy patented by SpaceX.
The change was made because commercial stainless steel had “interlaminar toughness” issues. As such, it couldn’t handle crack propagation between its layers at cryogenic temperatures, such as during liftoff when the rocket is filled with methane and liquid oxygen.
The cheapest rocket. It’s unclear how much SpaceX is investing in this material. However, there are already several dozen prototypes and Musk is very optimistic about the cost of the program.
SpaceX wants to build thousands of Starships to colonize Mars. The company is funding rocket development with launches of Starlink and other satellites, in addition to NASA contracts for lunar missions.
Because of its 100% reusable design, Musk estimates that the price per kilogram of Starship will be well under $100, which is 20 times less than it costs to launch a kilogram on SpaceX’s Falcon 9.
Image | SpaceX
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