On November 6, 2026, the European Space Agency (ESA) announced that Prometheus, a rocket engine under development for reuse, had completed a successful firing test on a refurbished test bench in Germany. In the test, conducted on October 29, the engine relit multiple times using engine inlet pressure rather than a high-pressure starter. Prometheus had already achieved consecutive ignitions in 2025, but this test went further: it demonstrated improved start conditions and also validated a combustion chamber made with a new manufacturing method and ground facilities adapted for liquid methane. The result confirms that the relights needed for a rocket's return and landing can be repeated under different start conditions.
Progress Since the 2025 Four-Ignition Test: Relighting Without a High-Pressure Starter
On June 20, 2025, Prometheus achieved four consecutive ignitions on the PF20 test bench in Vernon, France. That test, announced by ESA on July 2 of the same year, used the second development engine. This latest result is therefore not the first time Prometheus has relit successfully. To understand what has advanced since the earlier test, it is necessary to look not only at the number of ignitions but also at the engine's start conditions.
According to ESA's latest announcement, the biggest difference is that the engine relit without a high-pressure starter. Using a method that relies on engine inlet pressure, the team confirmed that the engine can be restarted after combustion has stopped. ESA describes this as the first such demonstration for a European engine in this class. However, what is no longer needed is the high-pressure starter; this does not mean that all auxiliary equipment and control mechanisms needed for starting have been eliminated.
The test location also changed, from PF20 in 2025 to the P5 test bench in Lampoldshausen, Germany. P5 is owned by ESA and operated by the German Aerospace Center (DLR). Prometheus development is supported by ESA's Future Launchers Preparatory Programme (FLPP), with ArianeGroup as prime contractor. The test verified not only the performance of the engine itself but also the ground facilities that support combustion and relighting.
DLR explains that engine relighting and thrust adjustment are essential if a reusable rocket stage is to return and land. However, the number of ignitions actually achieved in this test has not been disclosed. The figure of "up to five per day" cited by DLR describes the operating capacity of the test facility, not the number of ignitions in this test. It is therefore not possible to conclude that the number of relights increased compared with the four consecutive ignitions in 2025. The progress this time lies less in an updated ignition count than in improved start conditions and an expanded test environment.
First Test of a Combustion Chamber Made by Blasting Metal Powder at High Speed
The test also employed a new technology in the manufacture of the combustion chamber. According to ArianeGroup, the new chamber, used in a firing test for the first time, was made with a process called "cold spray." This technique blasts metal powder at high speed so that it strikes the surface of a base material and bonds to it. A combustion chamber built with a manufacturing process different from conventional ones was installed in an actual engine, and its operation during combustion was confirmed.
However, the fact that cold spray is a manufacturing technique using metal powder does not mean the entire combustion chamber was printed as a single piece. What the announcement describes is a process of depositing metal onto the surface of the chamber's base material; details of how the whole part is manufactured and of its structure have not been disclosed. It would therefore be inappropriate to equate it with one-piece fabrication by ordinary metal 3D printing.
ESA says that adopting cold spray can reduce manufacturing time and cost. However, no specific comparative data have been published, such as the number of days or the cost required by the conventional method, or the percentage reduction achieved by the new one. What was confirmed is that a combustion chamber made with the new method operated in an actual firing test; the cost-reduction effect has not been demonstrated quantitatively.
That a part made with the new method has progressed to a firing test is an important step toward putting the manufacturing technology into practical use. However, deciding whether to adopt it for a reusable engine will also require confirming durability after repeated firings and the stability of the manufacturing process. Once the reductions in manufacturing time and cost are available as measured values, the advantage over conventional methods can be evaluated more concretely.
P5 Test Bench Upgraded to Handle Liquid Methane Supply and Relighting
Another important result of this test is that the P5 test bench supported a firing test using liquid methane and liquid oxygen for the first time. A rocket engine test bench is not merely a facility that holds the engine in place. It supplies propellants at the appropriate temperature and pressure and measures operating data such as thrust and flow rate. In tests with repeated relights in particular, the feed system must be returned to a safe state after combustion stops and the next start conditions must be set up.
According to DLR's announcement, after each burn the engine and feed system are shut down safely and prepared for the next start. Specifically, liquid remaining in the feed system is drained, and a purge process drives out residual fluids inside. Pressure and temperature are then readjusted to conditions suited to the next test. In relight tests, multiple tasks are required of the ground facilities even while the engine is not burning.
Even if the engine itself is capable of relighting, repeated tests cannot be carried out unless propellant supply conditions can be set up properly. The P5 upgrade not only enables combustion with liquid methane and liquid oxygen but also expands the facility's ability to repeat stops and restarts. With such ground facilities in place, engine behavior can be examined continuously under different operating conditions.
During the test, DLR measured the engine's thrust and propellant flow rates and also recorded pressure ratios and temperatures. DLR says the upgraded P5 can examine long-duration burns and tests involving thrust changes, as well as the effect of those conditions on relighting. ArianeGroup also says it confirmed that P5 can support long-duration firing tests using liquid methane and liquid oxygen. However, this confirms the facility's capability to run long burns; it does not indicate how many seconds each burn in this test lasted. The duration of individual burns has not been disclosed.
The choice of liquid methane as fuel also affects the design of ground facilities. According to DLR, liquid methane is denser than liquid hydrogen and can be stored at a higher temperature, which makes tank and feed-system design and fuel handling easier. ESA, meanwhile, says the upgraded P5 can be switched back to tests using the conventional liquid hydrogen and liquid oxygen. The upgrade thus adds the functions needed to test methane engines while retaining the ability to handle the existing propellants.
Challenges Remaining on the Path to Practical Reuse
ArianeGroup describes Prometheus as a rocket engine in the 100–120 tonne thrust class. However, this figure is the engine's design thrust class and does not represent thrust measured in this firing test. In addition, this result is based on official announcements by ESA, DLR, and ArianeGroup and has not been reported as a peer-reviewed paper or preprint. Although it is a ground firing test of an actual engine, it is difficult to evaluate thrust performance, service life, or operating cost in detail from the published information alone.
To understand this result accurately, it is necessary to distinguish between relight capability, combustion chamber manufacturing technology, ground test facilities, and reuse in actual flight. Organizing the results of the 2025 test published by ESA together with the latest announcements from ESA, DLR, and others, the confirmed results and remaining items to verify are as follows.
| Item verified | Confirmed result | Undisclosed information / remaining challenges |
|---|---|---|
| 2025 PF20 test | Four consecutive ignitions on June 20 | Start conditions and test environment differ from the P5 test |
| 2026 P5 test | Multiple relights on October 29 without a high-pressure starter | Exact number of ignitions, duration of each burn, engine inlet pressure |
| Combustion chamber made with new method | First firing test of a cold-spray chamber succeeded | Extent of reductions in manufacturing time and cost; durability after repeated firings |
| Upgraded P5 | Supported firing tests using liquid methane and liquid oxygen | Detailed measurements from this test; performance data for repeated operation |
| Reuse in flight | Ground firing test conducted this time | Return and landing, post-flight maintenance, service life over repeated flights |
Note: Because the 2025 and 2026 tests differ in location and start conditions, the improvement in performance cannot be compared by ignition count alone. In addition, the announcements from ESA, DLR, and ArianeGroup concern the same test; each organization did not conduct an independent follow-up test.
Ground firing tests make it possible to measure engine operation in detail while controlling propellant supply conditions. However, evaluating the practicality of a reusable rocket requires confirming how much wear an engine suffers in flight and how much maintenance is needed before it can be used again. The announcement does not disclose the total number of engines used in testing, the cumulative number of burn cycles, or detailed measurement data. It is therefore not possible to judge service life or economics over repeated flights from this successful relight.
As the next development stage, ESA and ArianeGroup plan to test an improved engine, Mark 2, in Vernon, France. However, the announcement does not give a specific test date, first-flight date, or start of operations. Beyond further verification of relight conditions, the condition of parts after repeated burns, the maintenance work required, and measured results for manufacturing time and cost will become important. Prometheus development is moving from the stage of showing that relighting is possible to the stage of clarifying the technical and economic conditions for repeated use.