CALLISTO Reusable rocket stage demonstrator: consolidating the design

This paper describes the current status of the work in the CALLISTO (Cooperative Action Leading to Launcher Innovation in Stage Toss - back Operations) project conducted jointly by DLR of Germany, CNES of France and JAXA of Japan. The three partners are currently developing and building a reusable subscale demonstrator of a vertical take-off vertical landing rocket stage. The project is now starting with the phase D after passing the System Critical Design Review which took place in late November 2023, in Bremen, Germany. After a short introduction to the project, the design of a selection of products and subsystems under DLR responsibility and the reusability-linked challenges that led to their designs are presented. Learning about the specific impact of reusability on design and operations is determinant in order to maximize the gain brought by reusability compared to expendable launchers. A significant amount of information has already been gathered at this point in the project. Indeed, design drivers are significantly different of those of expendable launch vehicle stages. For instance, for mechanical structures and mechanisms three main new aspects have to be considered in the design: thermal loads during re-entry/powered braking, fatigue or high number of load cycles and optimization of the maintenance and repair operations. System aspects of the vehicle design are also getting more complex than for expendable vehicles. The aerodynamic and aerothermodynamic characterization domain required is extremely large with numerous vehicles configurations and variables. For what regards the Guidance Navigation and Control System: it has to cope with very different flight phases with a changing set of actuators and very high requirements in term of accuracy that need to be implemented at limited costs.