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 German Aerospace Center (DLR), the French Space Agency (CNES), and the Japanese Aerospace Exploration Agency (JAXA).
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 was since postponed to 2024 at the earliest. 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.
During the 2010s, various aerospace entities, both in the public and private sector, became increasingly interested in the aspect of reusability, in particular Vertical takeoff, vertical landing (VTVL) rocket elements, as a means of reducing the cost of space-based activities. Companies such as SpaceX had demonstrated their own progress in the field, leading to agencies in Russia, China, and Europe announcing their own reusable rocket projects. By the mid-2010s, development of the pan-European Ariane 6 launcher was well-underway, and incorporated a reusable engine. However, some officials in both France and Germany felt that greater emphasis on reusability was necessary, and that future rockets needed to go beyond what was then underway. According, these nations begun to collaborate on a tentative research effort that would crystalise as CALLISTO; a test rocket aimed at investigating, demonstrating, and maturing relevant technologies in the field of reusability.
A major focus area for CALLISTO is to evaluate the maintenance, repair, and overhaul of the vehicle between flights; it is hoped that it could perform at least eight launches within six months. It is also sought to perform at least one landing with a minimum non-gravitational acceleration of 1.3g, as well as to demonstrate a large and rapid manoeuvre at low dynamic pressure and a vertical landing which includes both a boostback manoeuver and an unpowered aerodynamic manoeuver. The economics involved in all such operations are also to be stringently evaluated.
Upon the project's launch in 2015, CALLISTO was not driven by the European Space Agency (ESA); instead, it was originally a joint project between the German Aerospace Center (DLR) and the French Space Agency (CNES) alone. During June 2017, the Japanese Aerospace Exploration Agency (JAXA) opted to join the CALLISTO partnership; around this time, it was agreed that Japan will provide the engine for the rocket, that being the 40 kN re-ignitable LOx/LH2 engine developed by JAXA for its Reusable Sounding Rocket programme. It will be modified to generate 15 percent greater thrust and to provide a throttling range from 16 to 46 kN, permitting adequate control of the vehicle during the landing phase.
Various aspects of the project have been divided between the partner agencies; DLR is responsible for the fairing, navigation, fins, equipment bay structure, hydrogen tank, and the landing system. CNES is responsible for the hydrogen peroxide thrusters, telemetry, neutralisation system, ground segment, and will also perform final assembly of the equipment bay. JAXA is responsible for the oxygen tank, aft bay structure, power supply, and propulsion. All three organisations will work on the onboard computer and flight software. Experience and technologies are being drawn from multiple existing programmes, including the Vega expendable launch system and the Space Rider lifting body spaceplane.
Following the project's preliminary design review,[when?] the vehicle's configuration was frozen. Possessing a height of 13.5 m and a diameter of 1.1 m, CALLISTO is a relatively compact rocket, a feat aided by its role as a demonstrator rather that an end product in and of itself. It will have a dry mass of 1,520 kg and a take-off mass of 3,600 kg. The flight control system incorporates a total of four deployable fins, along with two electromechanical actuators to gimble the engine, and eight hydrogen peroxide thrusters amongst other elements. It is furnished with four deployable landing legs for use during the landing phase.
In comparison to the established Ariane programme, CALLISTO has been described as being a relatively austere effort, operating on a budget between 1 and 2 percent of that allocated to Ariane. During 2018, it was stated that the project's timetable had scheduled the first flight of the rocket to take place sometime in late 2020, as well as for flight testing to be completed by the end of 2021. The flight test programme was set to be entirely performed at the newly-built commercial launch facility at the Guiana Space Centre, although considerations were made towards the use of alternative landing sites on both land and sea (the latter using barges).
By mid 2019, flight tests were expected to start in 2022.
In 2021, CALLISTO reportedly passed its System Design Key Point, confirming the definition of the preliminary system design review. Numerous flight-worthiness reviews are anticipated to take place in 2024 ahead of a decision to proceed with flight testing.