CALLISTO

thumb|Overview of the architecture of CALLISTO CALLISTO (Cooperative Action Leading to Launcher Innovation in Stage Toss-back Operations) is a reusable VTVL demonstrator propelled by a small 40 kN Japanese LOX-LH2 rocket engine. It is being developed jointly by the French (CNES), German (DLR), and Japanese (JAXA) national space agencies. thumb|Simulation of mass flow around CALLISTO thumb|Early illustration of CALLISTO thumb|Model of CALLISTO at Paris Air Show 2025 The goals for CALLISTO are to mature and demonstrate the technologies which are necessary to build and operate a reusable launch vehicle, but also to better assess the operational cost of such a vehicle. The first flight was originally planned to occur in late 2020; it has since been postponed multiple times to 2027. Following the programme's conclusion, the experiences and technologies gained through CALLISTO will be harnessed to benefit other ongoing and future space programmes, such as the development of the European reusable launcher Ariane Next.

CALLISTO is distinct from a similar project Themis under development by ESA and from the DLR-developed Reusable Flight Experiment (ReFEx), which is a demonstrator of a winged reusable rocket first stage.