Starship Was Engineered Backward From One Objective: Moving millions of tons to Mars at the lowest possible cost per ton to build a self-sustaining civilization. Five first-principles decisions make it possible: - Produce propellant on Mars. Methane and oxygen made from local CO2 and water ice via Sabatier. No Earth return fuel required. - Use 301 stainless steel. 67× cheaper than carbon fiber and survives both cryogenic propellants and 1,700°C reentry without heavy shielding. - Refuel in orbit. Tankers reset the rocket equation, enabling 100–150 ton payloads to Mars. - Belly-flop reentry. Maximize drag area so the atmosphere does most of the braking. - Full reusability. Both stages fly again rapidly. Marginal cost collapses to propellant only. This is the minimum architecture that makes multi-planetary life feasible.