Trajectory Optimization of a Partially Reusable Rocket–Scramjet–Rocket Launch System Including Fly-Back

The integration of high-speed airbreathing engines into dedicated small satellite launchers may allow for some of the stages of the launch system to be reused, resulting in improved cost efficiency and turnaround times. Designing a trajectory of this type is challenging, due to the high number of competing performance drivers and design constraints, which are often in opposition with each other between the different modes of flight (i.e. airbreathing, rocket, and glide). In this work the design of the trajectory for a partially airbreathing rocket–scramjet–rocket multistage launch system is studied by generating optimal trajectories using the pseudospectral method of optimal control for maximum payload-to-orbit. This optimization simultaneously determines the ascent of all three stages, as well as the return trajectory of the scramjet stage within the trajectory optimization. It is found that the scramjet accelerator is able to achieve fly-back to its initial launch site, but that it must ignite its scramjet engines during its return flight, causing this to become an important consideration in the optimal trajectory design. This work shows that the return of the scramjet stage of a rocket–scramjet–rocket launch system may be feasible, and indicates the trajectory features that may heavily influence future partially airbreathing launch system designs.