Why Starship Loves Stainless Steel...

I've had many people ask about using other materials than stainless steel on Starship the three options that are most commonly mentioned are titanium aluminum and carbon fiber note that for titanium and aluminum I'll be talking about Alloys and different Alloys behave very differently I've tried to pick the most common ones used for Aerospace the analysis I'm going to do will make a little more sense if you watched my engineering trade-off analysis video if you want to go watch it now we'll wait for you here we'll start by talking about Starship titanium is our first candidate titanium is widely reputed to be as strong as steel and 45% lighter but it loses that Advantage at higher temperatures 304 stainless is heat resistant up to about 870 de centigrade and the best titanium Alloys only go up 600° that is a significant disadvantage for Starship because of re-entry heating you may be able to keep the titanium from getting too hot but that will likely involve a heavier thermal protection system you may be able to keep the titanium from getting too hot but that will involve a heavier thermal protection system which will likely negate any weight savings there are also issues with contamination in high heat environments under high heat titanium wants to grab onto other elements and that can degrade its strength you don't want a material that sees high heat and appears to be fine but actually has significantly reduced strength one of the sources I came across described titanium as notoriously difficult to machine welding is possible and NASA has a 236 page document that tells you how to do it properly it says the following about the process contamination that arises either from the open atmosphere or from dirt on the filler metal or surfaces to be joined must be strictly avoided for the successful joining of titanium if you don't weld it exactly right the impurities you get lead to embrittlement and the weld is not as strong as it should be cost is is also an issue if we assume that Starship has 60 tons of steel in it we can make a guess at material costs I found suppliers of 304 stainless en rolls for about $1,500 per ton I'm sure SpaceX pays less with the quantities they use and that means that Starship has about $90,000 of stainless steel in it if we assume titanium is 45% lighter for a given strength which may not be true at high temps that would mean we would need 33 tons of titanium it's hard to find prices for coils of titanium but