Romanian ATD and ESA are developing a 10 kN restartable rocket engine for future European launches

Romanian ATD is developing a 10 kN restartable rocket engine: variable-thrust tests in focus of ESA’s programme for future launchers

Romanian company ATD Aerospace RS SRL is developing a 10-kilonewton (10 kN) rocket engine that can be restarted and whose thrust can be adjusted in flight. This is a technology that is increasingly important as Europe, and the global market as well, turn toward reusing rocket stages, more complex manoeuvring missions in space, and vehicles that must operate reliably after long-term propellant storage.
The development builds on ATD’s previous 1 kN engine, created with the support of the European Space Agency (ESA), and before that the company developed several smaller engines in the range from 0.5 to 1 kN. Such “small” thrust in space systems is often crucial: it is used for attitude control, precise manoeuvres, orbit corrections, or fine landing phases, where stability and repeatability matter more than raw power.
At the heart of the new project is a larger scale – an engine that in practice could fill the niche between correction thrusters and main-stage propulsion, especially in scenarios where multiple ignitions are needed, precise “metering” of force, and reliable shutdown and restart at short time intervals.

Test from 2025: water cooling and thrust cycles

According to available information about the project, in 2025 a prototype firing in a water-cooled variant was carried out in Romania. During the testing, the engine operated in a variable-thrust regime: from 100% nominal thrust it transitioned to 60% and returned to 100%, thereby verifying stability in transient regimes. Such “transitions” are among the most sensitive parts of the work because they require precise control of fuel and oxidizer flow, pressure control, and reliable maintenance of combustion without instabilities that can lead to shutdown or thermal peak loads.
In the test firings, a series of sensors was used to characterize the engine in detail: from pressure and temperature measurements, through flow and vibrations, to parameters that track flame behavior and thermal loading of critical parts. Such a dataset is usually crucial for the next step – moving from demonstration to a configuration ready for longer firings and stricter reliability criteria.
Although videos of test firings are often perceived by the public as “fireworks”, the engineering purpose of such campaigns is cold and very concrete: the collected data serve to verify calculation models, close design unknowns, and define safe operating limits before the engine enters a more demanding phase of development.

Why 10 kN is an important threshold

A thrust of 10 kN lies in a zone where the engine can already be considered a working “tool” for operations that until recently were reserved for larger systems or special propulsion concepts. If the engine is turned “upside down”, thrust of that size on Earth can support roughly a mass of about 1,000 kilograms – comparable to the weight of a typical hippopotamus. In the intended application, such an engine can serve to slow a rocket stage’s descent and achieve a controlled, soft touchdown, especially in the final phase where fine regulation of speed and altitude is important.
For reusability, variable thrust is not a “luxury” but a practical necessity. A stage returning through the atmosphere goes through phases where it needs a short, strong impulse, then a longer “hold” at lower thrust, and then an increase again. In space, on the other hand, the same principle enables extremely precise manoeuvres near the target – for example when approaching a service module, a satellite, or a platform that must be landed on without abrupt jerks.
In practice, the ability to controlled “throttle down” (throttling) and to restart after shutdown is what distinguishes a demonstrator from an engine that can be installed in a landing scenario or in operations that require multiple trajectory corrections.

Hypergolic propellants: reliable start, but demanding logistics

The engine uses hypergolic propellants – combinations of fuel and oxidizer that ignite on contact, without an external ignition system. NASA in its materials on propellant liquids describes hypergolic propellants as “self-igniting” upon component contact, which makes them particularly suitable for systems that require reliable on-demand start. At the same time, many hypergolic systems fall into so-called “storable” propulsion: the liquids can be kept in tanks at ambient conditions for a long time, which is important for vehicles that need to be ready to fire after months or years in space or in mission preparation.
But hypergolic systems also carry a cost. Traditional combinations, such as hydrazine derivatives and nitrogen tetroxide, are known for high toxicity and complex safety protocols in handling, storage, and loading. That is why European programmes increasingly investigate lower-toxicity alternatives, but hypergolic technology still remains the standard in part of space operations precisely because of reliability, restart simplicity, and proven behavior across a wide range of conditions.
For engines aiming at multiple ignitions, the hypergolic approach reduces ignition-system complexity, but increases ground-safety requirements. That is a trade-off assessed depending on the purpose: what is acceptable for propulsion operating in the late phase of a mission or on an upper stage is not necessarily the same as propulsion that is often loaded and drained or planned for use in a more intensive operational cycle.

Three designs to an autonomous cooling system

The development plan foresees three engine variants. The first is an uncooled demonstration, which serves as a basic verification of the concept and geometry, but also as a testbed for the development of valves, injectors, and thrust control. The second is a water-cooled version – as used in the test – which enables longer firings and safer testing of behavior during thrust changes, without overheating the chamber and nozzle too quickly.
The third step is a self-contained, “closed” engine that will incorporate its own cooling, i.e., transition to regenerative cooling in which heat is removed by the flow of propellant through channels in the chamber walls. Regenerative cooling in practice is one of the most demanding parts of liquid-engine design: it requires precise channel manufacturing, careful material selection, control of thermal stresses, and stable hydraulics in all operating regimes.
Why is that step decisive? Because regenerative cooling opens the door to the engine’s “longer breath”: longer firings, more cycles, a more stable thermal picture, and a lower risk of local overheating. In practical applications, especially for an engine that should operate during landing, thermal loading cannot be reduced to a short “flash” – the system must withstand continuous operation and multiple thrust changes without losing reliability.
At the same time, the transition to an autonomous cooling system means a step closer to real integration: the engine is no longer a “laboratory” assembly dependent on external infrastructure, but becomes a subsystem that can be integrated into a broader rocket-stage, module, or demonstrator system.

Broader context: ESA’s preparation for a new era of launching

The project fits into ESA’s Future Launchers Preparatory Programme (FLPP) – a programme which, according to ESA descriptions, helps European industry develop the technologies, systems, and partnerships needed for a modular, reusable, and commercially sustainable space-transport ecosystem. FLPP deals precisely with the phase of development that is riskiest for industry: technologies that are not yet mature enough to be integrated into operational rockets without great risk, but can deliver a major leap in performance, costs, or mission flexibility.
In that sense, engines with multiple-restart capability and variable thrust have a double value. First, they enable new return and landing profiles, where multiple engine phases must be performed during return – from deceleration to the final “soft” touch. Second, similar technology opens the door to in-space manoeuvres: from “tugs” that move cargo between orbits, through satellite servicing, to space-debris removal missions, where fine and reliable propulsion operation is often more important than maximum thrust.
For European industry, this is also a question of competitiveness. In the segment of reusability and rapid launch cadence, the pace of innovation is often dictated by those who turn ideas into verified demonstrators the fastest. FLPP is designed precisely as a mechanism that reduces technological risk and speeds the transition from concept to tested, documented results.

What TRL 5 means and why projects often break there

In its technical guidelines, ESA uses the Technology Readiness Levels (TRL) scale from 1 to 9 to assess technology maturity. On that scale, TRL 5 generally denotes validation of a component or demonstrator in a relevant environment – a step where laboratory proof moves to testing closer to real operating conditions. It is the level at which “hidden” weaknesses are often discovered: thermal margins, manufacturing tolerances, behavior in transients, sensitivity to vibrations, and controllability limits during rapid thrust changes.
According to available information about the project, ESA is funding development up to a successful test of the final engine with regenerative cooling, with a smaller ignition demonstration that should bring the technology to TRL 5. Such a goal means the project is expected to deliver more than a one-off firing: repeatability, control, understanding of operating limits, and documented results that enable the next phase – integration into a broader system or moving to development of a larger demonstrator.
In rocket engineering, TRL 5 is often the threshold at which it is decided whether a project will become the foundation for a new platform or will remain a “proof of concept”. Success at that level does not mean the engine is ready for flight, but it does mean risk is significantly reduced and a path can be planned toward TRL 6 and beyond, where demonstrations in even more realistic conditions follow.

Romanian niche in the European supply chain

ATD Aerospace RS SRL operates as a small and medium-sized company specialized in developing propulsion and autopilots, including hardware and software, and, according to descriptions in the Romanian space-capabilities catalogue, has participated in the development of solid rocket motors, liquid engines, and specialized guidance packages for high-altitude unmanned aircraft and related systems. In the European launch industry, dominated by large integrators, such companies are often the source of specialized solutions: from valves and injectors to control algorithms and diagnostics.
For countries like Romania, this is also a strategic opportunity. Participation in ESA programmes can mean visibility, knowledge transfer, and entry into projects that later spill over into the market – either through collaborations or through export contracts for specific subsystems. In practice, many space technologies do not grow “all at once”, but through a series of smaller proofs: from a 1 kN engine, through a jump to 10 kN, to integration into a return demonstrator or the propulsion module of some future vehicle.
For the European supply chain, this also means diversification. The more capable suppliers and development teams there are, the greater the industry’s resilience to delays, and the possibilities for parallel development – from “greener” propellants to new cooling technologies – become more realistic.

What’s next: from prototype firing to operational application

In liquid-engine development, the gap between a successful hot-fire test and real application is often the longest and most expensive part of the journey. It is necessary to repeat cycles, extend firing duration, widen the operating range, prove restart reliability, and show that the engine can withstand manufacturing variations and real integration conditions. For an engine intended for landings, this also includes control-dynamics tests: how fast and stably the system can respond to a thrust-change command, how it behaves in a vibration environment, and how it thermally “settles” between cycles.
If the planned transition to regenerative cooling is successful and if results confirm that the engine maintains stability across a wide thrust range and through multiple ignition cycles, such a system could become one of the “small but critical” pieces of the European puzzle: a technology not seen in spectacular launches, but that decides whether a stage returns safely and whether a mission has enough manoeuvring margin for precise trajectory control and a gentle end to flight.

Sources:

• European Space Agency (ESA) – official description of the FLPP programme and goals for developing technologies for future European space transportation (link)
• European Space Agency (ESA) – overview of “Future space transportation” activities and programmes for developing new propulsion and system solutions (link)
• European Space Agency (ESA) – explanation of the Technology Readiness Levels (TRL) scale and technology-maturity assessment criteria (link)
• NASA Kennedy Space Center – overview of hypergolic propellants and their basic properties (link)
• Romanian Space Catalogue (Spring 2025) – description of ATD Aerospace RS SRL and its areas of activity in propulsion and guidance (link)
• Kompass Romania – basic company information for ATD Aerospace RS SRL (headquarters and R&D activity) (link)