SpaceX is working on the first prototype of the Starship rocket (formerly BFR), which could eventually transport people to the Moon and Mars. CEO Elon Musk, however, recently announced a significant design revision that initially seems uninteresting. Instead of aluminum and carbon fiber, the company decided to build the Starship out of stainless steel. Now he explains why.
While the Starship prototype looks like a golden age science-fiction fantasy, this is not the ultimate goal. At the top you can see the real prototype on the left and the rendering of the final product on the right. SpaceX has a shiny steel design because it's easier to build and has the potential to work much better. The initially resin-reinforced carbon fiber frame had the potential to save a lot of mass, but was a slow process and 35 percent of the material was unusable scrap.
Stainless steel is much cheaper than carbon fiber, but it's heavier. Musk explains that the rocket can be lighter with stainless steel than with other metals. Most steel alloys become brittle at low temperatures. This is not the case with high chromium-nickel grade stainless steel. It becomes stronger in cold conditions, but also retains ductility. That is, stainless steel has a high fracture toughness, which can prevent small structural imperfections from cracking.
The use of stainless steel also allows SpaceX to fulfill Musk's dream of a regenerative heat shield. This is unlikely to happen immediately, but Musk wants to replace the tiles of the current spacecraft with a stainless steel sandwich. A stainless steel surface at 1,500 degrees Fahrenheit may be about five times higher than carbon fiber or aluminum. This allows SpaceX to inject water into the space between the two layers where it absorbs heat. The regenerative shield would bleed this water through microperforations to cool the windward side of the rocket, which is called transpiration cooling.
Musk says SpaceX will initially use 301 stainless steel – this is similar to the metal used in pots and pans. It does not sound very timely, but SpaceX seems to think it's the best option. The prototype of the rocket was recently hit by strong winds, but nothing important was damaged. In the next few weeks, the company could make its first test flight. However, this will only be a suborbital "hopping". For space missions, the Starship is paired with a booster vehicle called Super Heavy, but SpaceX has not built that yet.