Prometheus engine for Themis flight model during assembly

Prometheus engine being installed on the reuseable rocket stage demonstrator Themis at ArianeGroup site in Les Mureaux, France in October 2024.

Themis comes in at 28 m tall, and its Prometheus engine is almost as powerful as the Ariane 6 Vulcain 2.1 engine. Unlike all European cryogenic rocket engines to date, Prometheus will use liquid methane at –162 °C as a fuel instead of hydrogen. This has advantages as methane is denser and easier to handle than hydrogen, so it will reduce costs in engine and launcher design and will ease in-flight vehicle reignition. This ability to reignite mid-flight is key as Themis will need to restart its Prometheus engine several times as it descends back to Earth to land.

Themis encompasses all the elements for a reusable rocket stage. The Themis programme includes developing the flight test model shown here that requires new technologies from European countries such as the vehicle landing legs, grid-fin aerodynamic stabilisers, light-weight fuel tanks, distributed power systems, avionics and reduced-diameter multi-engine bay.  New flight algorithms, derived from previous European projects, will be key to make Themis land safely after flight.

Having finished its testing programme on the main systems at ArianeGroup’s nearby Vernon test facility in France, the reuseable rocket stage is now being built up with all the components necessary to launch, land vertically, and be reused for another mission.