> Starship is incredibly ambitious but if it doesn't reach orbit before 2025 I won't be surprised. Can someone elucidate what are the major technical challenge

> Starship is incredibly ambitious but if it doesn't reach orbit before 2025 I won't be surprised. Can someone elucidate what are the major technical challenges related to Starship that can cause such a delay? Powered descent and landing I assume is a largely solved problem from Falcon rockets. Re-entry would require heat shielding tech but isn't that a fairly mature technology? Also the fact that SpaceX isn't trying to maximize payload per launch and compensate with launch frequency argues they can use more robust tech than NASA used. The engines are completely new and the only one of it's kind that's ever flown but I've not heard that they are a problem now. Re-launch so quickly may not happen initially but that's ok and SpaceX does have experience with the other rocket. So I'm wondering what's the big unknown left?