Europe’s reusable Prometheus rocket engine restarted repeatedly in new hot-fire tests

On Oct. 29, a Prometheus engine lit its methane and liquid oxygen propellants several times in a row on a bench in Lampoldshausen, Germany, and ESA reported the results on Nov. 6. The engine reignited without a high-pressure starter system, relying on inlet pressures instead. ESA calls that capability a first for an engine of this class in Europe.

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Behind the milestone sits a simple economic bet: a booster engine that restarts reliably and costs far less to build is the foundation of any reusable European rocket.

Several hot-fire tests on the P5 bench

The campaign ran at P5, a test bench that ESA owns and the German Aerospace Center (DLR) operates at Lampoldshausen. According to ESA’s report on the improved test bench, the facility was upgraded to supply liquid methane, and Prometheus is the first engine fired there on liquid oxygen and methane. The bench can still switch back to oxygen and hydrogen, so it remains available for liquid hydrogen engines.

Prometheus is a reusable engine for future European launch systems, with power similar to the Vulcain 2.1 that drives Ariane 6. ArianeGroup is the prime contractor, ESA supported the work through its Future Launchers Preparatory Programme, and teams in Germany and France are credited with the tests. The article gives no thrust figure for this test series.

The engine’s thrust class is documented elsewhere. A 2021 ArianeGroup announcement of a 135 million euro ESA contract describes a design delivering 100 metric tons of thrust, an improved version aiming for 120 metric tons, and a digital system that regulates thrust between 30 and 100 percent of maximum.

Reignition without a starter, and why restart matters

Engines on expendable rockets only need to light once. A reusable first stage needs its engines to shut down mid-flight and then relight for the maneuvers that bring the stage back, and then do it again on the next mission, on demand and without a refit between flights. ESA’s Director for Space Transportation, Geraldine Naja, framed the point this way: “Repeated starting of engines and reduced manufacturing costs are key to expanding launcher supply.”

Dropping the high-pressure starter removes a hardware subsystem that would otherwise need to be carried, filled and recharged before each burn. In the tests, ESA says, the engine relied on its inlet pressures to restart, which means the propellant feed system itself carries the load that a starter would. The agency does not publish the number of ignitions, the burn durations or the pauses between them, so the claim of repeated restarts rests on ESA’s wording alone, and an exact count would let outside engineers judge how demanding the sequence was.

Methane is the chosen fuel because, as ESA’s page on the Themis demonstrator explains, it is denser and easier to handle than hydrogen, which lowers cost and eases in-flight reignition. The same page puts the first ignition of Prometheus at 30 September 2022.

Cold gas spray and the cost target

One new element in this campaign was the combustion chamber. ESA says it was built by cold gas spray, an additive process in which metal powder is sprayed at high speed onto a chamber body and bonds to its surface. The agency states that such processes shorten production times, simplify complex parts and cut costs, which ties directly to the second half of Naja’s sentence.

The cost aim is large. ESA’s Prometheus program page says the engine is designed to be ten times less expensive to build than the previous 1200 kN engine generation, using additive manufacturing. A Spaceflight Now report on the June 2023 burn carries the same tenfold expectation from France’s space agency CNES against the Vulcain engine, and describes a 12-second firing on the Themis stage at ArianeGroup’s Vernon site using bio-methane.

The engine’s backers form a wide European group. ESA’s program page names ArianeGroup, GKN Aerospace, Safran Aero Boosters and APP as main partners, with ESA funding and participation from France, Germany, Sweden, Belgium, the Netherlands and Switzerland. The same page notes that technologies from the effort could also be used to cut the cost of the Vulcain 2.1 on Ariane 6, so the work pays off even if no all-new rocket follows. The Spaceflight Now report places the engine within the Ariane Next program, which ESA and CNES run with ArianeGroup as prime contractor, and cites planned production of 50 engines per year.

Themis, ESA’s reusable rocket prototype, is a 28 meter-tall, 3.5 meter-wide stage designed to launch, land vertically and fly again, with first flights planned from Esrange in Kiruna, Sweden, to heights of about 100 meters. ESA’s page gives no hop date.

The next step is an improved version, called Mark 2, which ESA says will be tested first at ArianeGroup’s Vernon test center in France. Whether Mark 2 meets the 120-ton thrust goal and how many restarts it can survive are the numbers that will show how close Europe is to a reusable booster.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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