Multistage Rocket Optimization, Geophysics, and the Spacefaring Envelope of Habitable Super-Earths

Habitability is typically defined by whether a planet can support life, rather than whether it can support a technological civilization capable of escaping its gravity well. We introduce spacefaring capability as a technological axis of habitability, defined by the ability to place a 1000 kg payload on an escape trajectory using chemical propulsion. We develop a coupled geophysical--atmospheric--astronautical model that maps this ``spacefaring envelope'' as a function of planetary mass and surface pressure. Building on Hippke (2018) and Gonzalez (2020), we optimize multistage chemical rockets by minimizing the reliability-weighted expected launch mass while determining the optimal stage count, first-stage engine number, and mission reliability. Assuming F-1-class first-stage engines, the model reproduces the Saturn V gross lift-off mass to within about!30% and the F-1 turbopump power to within about!18%. Over 0.1 -- 10 bar, atmospheric pressure changes the required launch mass by up to about!35% on 0.5,M planets, where drag contributes substantially to the ascent v, but by only a few percent for M 4. Gravity, rather than atmospheric drag, therefore sets the primary limit on chemical escape from super-Earths. Imposing a post-optimization limit of about!100 F-1-class first-stage engines renders escape of the benchmark payload impractical above about!11.5,M. This engine-counting constraint independently corroborates the about!10,M limit derived by Hippke (2018) from an engine-independent fuel-ratio argument. These results provide a physically motivated framework for assessing whether rocky exoplanets are capable of supporting technological civilizations that can escape their planetary gravity wells.