Source: Elon Musk Interview: Why the Starship Is Built of Stainless Steel . . . On the windward side, what I want to do is have the first-ever regenerative heat shield. A double-walled stainless shell—like a stainless-steel sandwich, essentially, with two layers. You just need, essentially, two layers that are joined with stringers. You flow either fuel or water in between the sandwich layer, and then you have micro-perforations on the outside—very tiny perforations—and you essentially bleed water, or you could bleed fuel, through the micro-perforations on the outside. You wouldn’t see them unless you got up close. But you use transpiration cooling to cool the windward side of the rocket. So the whole thing will still look fully chrome, like this cocktail shaker in front of us. But one side will be double-walled and that serves a double purpose, which is to stiffen the structure of the vehicle so it does not suffer from the fate of the Atlas. You have a heat shield that serves double duty as structure. . . . This technique was used in some rockets to keep the nozzle throat from being eaten away by the welding flame of the engine. In effect he is turning the rocket engine inside-out and keeping the interior cool. Bob Wilson
The shell of the Apollo Command Module was composed of two layers of stainless steel with a separate steel "honeycombed" layer brazed in between. The heat shield or "ablation layer" was a phenolic based material harking back to the original synthetic phenol formaldehyde polymer invented by Leo Hendrik Baekeland. The original Mercury and Gemini capsules were cladded by "shingle" like overlapping plates made from a nickel alloy designated Rene 41. Stainless steel is used because it is less likely to deform in temperatures of 1200 degrees F plus, while aluminum alloys start to deform at around 500.
Examination of man made objects like satellites that degrade in orbit reveals that the majority of debris that survives is made of steel, stainless steel and titanium. Aluminum vaporizes on reentry with a few large dense lumps surviving. Stainless steel is less expensive than titanium and easier to fabricate. Titanium alloys like 6Al4V or Type 5 weigh about .6 as much as stainless steel. The metal in the structure is virtually "paper thin." Stainless steel is chosen on the basis that it can be brazed which results in less distortion than welding. Titanium basically needs to be welded.
The last syllable -ium is used to name elements. If these are undiscovered, they would be exceptionally heavy and unstable. There are no magical elements in the periodic table. Most all high performance metals would be alloys of existing elements. Most all carbon fiber is basically carbon fiber reinforced plastic. They begin to soften at around 200 degrees F. The most temperature stable plastic is PEEK or polyetheretherketone which can operate at around 500 degrees. PEEK is very very expensive. Stainless steel is basically cheap and can operate at 1200+ degrees all day long.
Actually the Wiki article on turbines gives a lot of clues about what can work: Turbine blade - Wikipedia One open question is dealing with radiant heat. Unless the film coolant is significantly opaque to infrared or the surface reflective, the heat transfer to the surface will be a challenge. Bob Wilson
Turbine blades are made of "super alloys." Super alloys have special qualities due to controlled cooling and special grain structure that conforms to the physical structure of the turbine blade. The rate of cooling determines the crystalline structure of the alloy. Faster cooling creates finer grained crystals, while slower cooling forms larger crystals. Physically working alloys like steel increases hardness and strength and toughness, but also increases "frangibility" or likeliness to fracture and reduces ductility.
Good reason to keep the ISS going.....just in case. Hey....waitaminute!!! Whataya mean FICTIONAL?? un·ob·tain·i·um /ˌənəbˈtānēəm/ noun informal noun: unobtainium; plural noun: unobtainiums; noun: unobtanium; plural noun: unobtaniums a highly desirable material that is hypothetical, scientifically impossible, extremely rare, costly, or fictional, or has some of these properties in combination.
Maybe Elon Musk is a Neil Young fan. "...flying Mother Nature's silver seed to a new home in the sun..." (from After the Goldrush)
