The European Space Agency (ESA) and its partners have made a crucial breakthrough in the development of next-generation space technologies. At the end of June 2025, the second series of tests for the Prometheus rocket engine was successfully completed, demonstrating capabilities that put Europe on the map for the development of reusable propulsion systems. The tests, which included multiple ignitions, confirmed the optimization of the engine and its subsystems, paving the way for a cheaper and more sustainable access to space.

This significant development phase took place at the PF20 test stand at the ArianeGroup facility in Vernon, France. Engineering teams conducted a series of tests with the second manufactured model of the Prometheus engine, systematically increasing the total operating time and analyzing performance under various thrust profiles. The campaign's highlight occurred on June 20, 2025, when the engine successfully performed four consecutive ignitions in a single day. Such a feat is a first of its kind in Europe for an engine of this class and is a clear indicator of the technology's maturity and the teams' operational readiness.

Prometheus: The heart of Europe's future rockets

Prometheus is not just another rocket engine; it is the cornerstone of Europe's strategy for the future of space transportation. Named after the Titan from Greek mythology who brought fire to humanity, this engine symbolizes a new era of innovation. It is being developed as part of ESA's Future Launcher Preparatory Programme (FLPP), with the goal of drastically reducing production and operational costs. The estimated production cost of a single Prometheus engine is about one million euros, which is a tenfold reduction compared to existing engines of similar power, such as the Vulcain 2 engine that powers the Ariane 5 rocket.

The key to achieving such a low price lies in its innovative design and the application of advanced manufacturing technologies, primarily metal 3D printing. A large portion of the engine's complex components are made using additive manufacturing, which allows for faster production, a reduced number of parts, and simplified supply chains. Furthermore, Prometheus was designed from the outset for reusability, which is a key factor in reducing launch costs in the future.

One of the engine's most important technical characteristics is the use of liquid oxygen (LOX) and liquid methane (LCH4) as propellant. This choice offers numerous advantages over traditional fuels like kerosene (RP-1) or the combination of liquid hydrogen and oxygen. Methane burns cleaner, leaving significantly less soot and residue in the engine, which greatly simplifies and speeds up the inspection and refurbishment process between flights. Additionally, methane is cheaper, has a higher density than liquid hydrogen which allows for more compact fuel tanks, and its boiling point is closer to that of liquid oxygen, simplifying system design and insulation. In the long term, the possibility of producing methane off-Earth, for example on Mars, makes it the fuel of choice for future interplanetary missions.

Themis: The demonstrator paving the way

Prometheus is currently demonstrating its full potential as the propulsion unit for Themis, a prototype of a reusable first stage of a rocket. Themis, named after the Greek goddess, mother of Prometheus, represents a key European demonstrator for technologies of rocket stage recovery and reuse. Developed under the leadership of ArianeGroup, this program aims to test all aspects of vertical takeoff and vertical landing (VTVL).

Through a series of tests, Themis will allow engineers to gather invaluable data on aerodynamics, guidance and control systems, as well as the performance of landing legs and other key systems. The first model of the Themis demonstrator, equipped with a single Prometheus engine, has already been transported to the Esrange space center in Kiruna, Sweden, where so-called "hop" tests will be conducted – short, low-altitude flights that simulate the final phase of landing. The success of these tests will be crucial for the further development of more complex models, including a variant with three Prometheus engines, which would form the basis for future operational launch vehicles.

The Themis program, along with Prometheus, is part of a broader European effort to ensure autonomous, competitive, and sustainable access to space. In a world dominated by new private actors with innovative solutions, Europe is developing its own capabilities through these programs to remain a relevant and reliable partner in the global space arena.

A look into the future: New tests and further development

Although the second test campaign in Vernon has concluded with outstanding success, the work on perfecting Prometheus is far from over. The next step in the rigorous validation process involves the third engine model, which will soon be subjected to a new series of tests. This time, the tests will take place in Germany, at the German Aerospace Center (DLR) facility in Lampoldshausen.

The P5 test stand in Lampoldshausen, historically used for testing the Vulcain engine for Ariane rockets, has recently been modernized and adapted specifically for testing methane-powered engines. At this advanced facility, Prometheus will be subjected to tests with significantly longer firing durations. These long-duration tests are crucial for verifying the engine's endurance and reliability under conditions that faithfully simulate its actual operation during a rocket launch into orbit. The goal is to confirm the engine's performance over longer operational cycles and to collect data on the thermal and mechanical stress on key components.

The flexibility of the Prometheus engine's design allows it to be adapted for different types of future European launch vehicles. Although Themis is its primary testbed, Prometheus is being considered as the propulsion basis for future rockets, including potential successors to the Ariane 6 rocket, as well as for smaller, commercially oriented launchers. The success of this program will directly shape Europe's capabilities for launching satellites, sending probes into deep space, and supporting future human missions, ensuring strategic independence for decades to come.