  Identifying the optimal design of a new launch vehicle is most important
since design decisions made in the early development phase limit the vehicles'
later performance and determines the associated costs. Reusing the first stage
via retro-propulsive landing increases the complexity even more. Therefore, we
develop an optimization framework for partially reusable launch vehicles, which
enables multidisciplinary design studies. The framework contains suitable mass
estimates of all essential subsystems and a routine to calculate the needed
propellant for the ascent and landing maneuvers. For design optimization, the
framework can be coupled with a genetic algorithm. The overall goal is to
reveal the implications of different propellant combinations and objective
functions on the launcher's optimal design for various mission scenarios. The
results show that the optimization objective influences the most suitable
propellant choice and the overall launcher design, concerning staging, weight,
size, and rocket engine parameters. In terms of gross lift-off weight, liquid
hydrogen seems to be favorable. When optimizing for a minimum structural mass
or an expandable structural mass, hydrocarbon-based solutions show better
results. Finally, launch vehicles using a hydrocarbon fuel in the first stage
and liquid hydrogen in the upper stage are an appealing alternative, combining
both fuels' benefits.
