The European space community is entering the era of composite structures with increasing ambition, and one of the most intriguing projects in this direction is Phoebus – a development initiative whose goal is to prove that the classic metal cryogenic fuel tanks on the upper stage of the Ariane 6 rocket can be replaced by lighter, more efficient, and safer tanks made of carbon fiber reinforced polymers (CFRP). At the center of the current phase of work is the liquid hydrogen (LH2) tank, which is extremely demanding as it requires an ultra-low temperature of −253 °C and absolute airtightness. For the industrial consortium led by ArianeGroup and MT Aerospace, this is a technological leap forward that could bring significant mass savings and additional flexibility in mission planning for future European launches.
Why replace metal with composite: the logic of a lighter upper stage
Classic aluminum or metal tanks have a long heritage of reliability, but they "pay" a high price in terms of mass. Every kilogram less in the upper stage structure translates into kilograms of additional payload or the ability to perform more energetic maneuvers in orbit. Composite materials – in this case, carbon fiber laminates – provide high strength with significantly lower mass, and they can be designed so that resistance and elasticity are "built-in" exactly where the load is greatest. In the context of the Ariane 6 upper stage, this means potential multiple gains: increased payload capacity, greater operational "delta-v," and reduced mission costs due to more efficient fuel utilization.
The cold that changes the rules of the game: what liquid hydrogen demands from a tank
Hydrogen is the smallest and most mobile molecule in nature. To be in a liquid state, it must be cooled to −253 °C, just twenty degrees above absolute zero. At these temperatures, many materials lose their toughness, become brittle, and susceptible to microcracks. In the world of composites, this is particularly challenging because the matrix (polymer) and the fibers (carbon) "behave" differently during contraction and expansion due to the thermal gradient. Every interface between the layers of the CT-laminate must remain stable, and the system may only leak negligible, practically immeasurable amounts of gas – otherwise, it is a leak that is unacceptable for a space system. An additional specificity of hydrogen is its tendency to diffuse through microscopic defects, making airtightness as crucial as strength.
From a 60 L "bottle" to a 2 m diameter tank: how Phoebus is growing
The engineering philosophy of the consortium is to proceed step by step. The first proof of feasibility was achieved with demonstrators of about 60 liters in volume: small "bottle" tanks showed that the composite structure could contain liquid hydrogen without measurable leakage and without degradation that would compromise safety. After these successful tests, a scale-up logically followed – towards a tank with a diameter of about 2 meters and a volume of nearly 2600 liters, which is closer in geometry and loads to the actual needs of the upper stage. This moves from the laboratory and prototype domain into an industrially relevant testing regime, where the technology must demonstrate that it is robust for multi-cycle cold-hot and pressure-decompression processes.
September–December 2025: key months in the production of the large LH2 tank
The inner liner and the main pressure vessel of the new tank are being manufactured at MT Aerospace's facilities in Augsburg, where a series of initial production steps were concluded in September 2025, including the preparation of the core and the laying of the first laminate layers. The planned completion of production is in December 2025, after which the integration of sensors and the manufacturing of accompanying test equipment will follow. In the so-called "near-net-shape" production approach, the composite is laid precisely along the designed fiber paths (fiber placement) to make maximum use of the material's anisotropic strength. This achieves thin yet extremely strong tank walls that must withstand pressure cycles and cryogenic shock.
Where it will be tested and why there specifically
ArianeGroup is taking responsibility for the tests at a specialized facility in Trauen, Lower Saxony. This is a location that already has a long tradition of working with cryogenic liquids and high-risk test procedures. Hydrogen, although supercooled, is still flammable over a wide range of concentrations in the air, which is why safety protocols – from ventilation and detection to controlled venting and fire-protection zones – are uncompromisingly strict. Work on the new test site began in February 2025, the preliminary design was confirmed in June, and the critical design review by the end of 2025 should open the way for construction work and the final integration of equipment.
Spring Campaign 2026: cold, under pressure, to the verge of fracture
The test campaign is planned for April 2026, with a clear philosophy of "increasing the load in controlled steps." The tank will be filled with liquid hydrogen, cooled to its operating temperature, and pressurized to defined intermediate points, with each loading followed by a pause for data collection and in-depth analysis. The ultimate goal is to identify behavioral thresholds – from the first micro-irregularities to phenomena that signal an approach to limit states. The test will be stopped before complete fracture, but close enough for modelers and structural analysts to validate the numerical simulations that predict where and why the first "seeds" of cracks form.
Instrumentation: how to "listen" to the tank as it breathes
A dense array of sensors is integrated into all phases of testing. A network of precise strain gauges is glued to the tank's skin to measure local deformations in the fibers and matrix, while the internal arrangement includes cryogenic thermocouples and high-sensitivity pressure sensors. Specially designed leak detection systems – adapted for temperatures close to absolute zero – measure extremely small hydrogen flows, comparable to a "molecular fog." A multi-channel data acquisition system operates at a high sampling frequency to detect transients during filling and emptying, and the data is synchronized with external conditions and valve sequences.
From measurements to models: digital twins for composite cryotanks
The digital twin of the tank – a numerical model that is continuously "fed" with data from tests – is a key tool for accelerating development. Model validation allows for more reliable estimates of service life and behavior in non-nominal situations (sudden thermal shock, partial filling, dynamic loading during flight). Since the composite is anisotropic and hydrogen is extremely diffusive, predicting the first micro-defects depends on a combination of fracture mechanics in composites and thermodynamic permeability models. With a correctly calibrated model, each subsequent design iteration can be done faster and with fewer physical prototypes.
The role of the FLPP program and the outlook towards ICARUS
Phoebus is part of ESA's Future Launchers Preparatory Programme (FLPP), whose mission is to reduce technological risk and prepare the industrial base for the next generation of European launch vehicles. In this context, a concept called ICARUS (Innovative Carbon Ariane Upper Stage) is also mentioned – a vision of a "black," composite upper stage, where the tanks, internal structures, and thermal insulation would form a coherent whole. The effect on performance would be manifold: from reducing the dry mass to more easily achieving multiple engine burns, precise injections into the target orbit, and the possibility of complex mission profiles.
Technological frontiers: linerless or with a liner, compatibility with LOX and hydrogen
While Phoebus in its current phase focuses on hydrogen, solutions for liquid oxygen (LOX) are also being developed in parallel, which is stored at about −180 °C and is chemically reactive and can be corrosive to certain polymers. The dilemma of "linerless" (without an inner lining) versus "with a liner" (e.g., a thin polymer or metal film) is not just a matter of mass, but also of long-term reliability: liners can reduce permeability and protect the matrix, but they introduce additional interfaces and potential sources of stress. A linerless approach reduces complexity and mass, but it places stricter demands on the chemical and cryogenic resistance of the matrix itself and the coatings that seal micropores.
What "no leakage" means in the world of liquid hydrogen
Unlike liquid methane or kerosene, where leakage standards are relatively more tolerant, hydrogen requires almost ideal airtightness. In practice, this means that measurement methods need to detect flows on an order of magnitude that are just above the physical background. Classic testing instruments cannot simply "go down" to −253 °C without degradation of sensitivity, which is why specially calibrated probes, cryogenic lines, and procedures have been developed to enable stable, repeatable measurements. Such a metrological step forward is not secondary, but a structural part of the project: without credible measurement, neither airtightness nor long-term functionality can be proven.
Safety above all: hydrogen handling protocols
The tanks are tested under strict procedures: multiple levels of ventilation, gas detection with redundancy, inerting the environment where necessary, and a defined "venting" schedule to prevent the accumulation of a mixture with oxygen. Grounding systems and static electricity control are standard for every connection and disconnection of cryogenic lines. Personnel undergo specialized training, and emergency scenarios (from power loss to a sudden pressure change) are worked out in detail, with clearly defined decision trees.
Industrial network and centers of excellence
Cryogenic systems technology in Europe relies on a series of specialized locations and competencies: from design and testing facilities in France, through German centers in Bremen, Ottobrunn, and Trauen, to production plants in Augsburg. This network allows for rapid development iterations: while one center is finishing fiber winding and matrix curing in an autoclave, another is preparing instrumentation and simulating the test campaign, and a third is validating models based on data from the previous test cycle.
What changes Phoebus brings to missions: from constellations to science probes
Mass savings on the upper stage open up a range of possibilities in the commercial satellite market, as well as in scientific missions. Constellation operators gain greater flexibility in distributing satellites across orbital planes, and scientific missions – especially interplanetary ones – benefit from an additional "delta-v" budget that allows for more precise gravity assists, more complex injection corrections, or mission life extension. In a world where every meter per second matters, a lighter upper stage can be the deciding factor in the competition for demanding interplanetary windows.
Challenges that remain: cyclic fatigue and long-term presence on the launch pad
An upper stage does not only exist in ideal conditions. During launch preparation, it can spend hours or longer on the pad with a full or partial cryogenic load. This means long-term exposure to thermal gradients, wind, and vibrations, as well as multiple filling and emptying cycles. The composite skin must remain stable; transitions around connections, attachments, and interfaces with insulation must not generate localized stress concentrations. That is why in Phoebus, the emphasis is on repeatable cyclic tests and on measuring noise before "acting out" the extreme – a fracture that is close, but below complete destruction.
Learning from "small" demonstrators: why the 60 L prototypes are crucial
Although miniature compared to flight tanks, the 60 L "bottles" were a laboratory for key decisions: the choice of a matrix resistant to cryogenic temperatures, the strategy for laying fibers around hoop and meridian lines, optimizing transitions around ports and valves, and formulating surface coatings that seal micropores. This work enabled faster decision-making for the larger ~2600 L tank, reducing the number of expensive iterations on a larger scale and shortening the overall time to testing in a realistic environment.
Insulation, thermal shield, and boil-off management
Cryogenic missions always lose some liquid through evaporation (boil-off). With hydrogen, which has a high latent heat, any reduction in heat flow through the wall and joints directly saves fuel. The composite tank is combined with advanced insulation systems (multi-layer MLI, aerogel substrates, selective low-emissivity coatings), and thermal bridges to the upper stage structures are minimized by the geometry and materials of the flanges. Boil-off management also includes smart venting: the hydrogen that forms from evaporation can be temporarily accumulated and disposed of with minimal loss and maximum safety.
From the test stand to the launch pad: what precedes flight
Even if the tests in Trauen show everything the models predict, a whole series of "qualification" steps follow: proof of endurance under static and dynamic loads, compatibility with liquid oxygen (for the parallel LOX tank), integration tests with piping, valves, and upper stage sensor systems, and validation of ground filling and emptying procedures. In all of this, documentation and data traceability are crucial – a robust series of measurements from all operating regimes that allow safety authorities and certification commissions to give the green light for the flight configuration.
The economics of mass: what a "few tons of savings" means in real numbers
Reducing the dry mass of the upper stage by a few tons translates into greater payload capacity or directly into an extended "operational window" for missions requiring complex orbital dynamics. In launches to GTO or MEO, this can mean tens of kilograms of additional payload; in missions to Lagrange points or deep space destinations, the value of the gained "delta-v" is often even more valuable. For operators, this is a competitive advantage that is measured in contracts.
The culture of testing: why "stopping before fracture" is a smart strategy
A complete fracture provides a dramatic video but less useful data for model calibration. That is precisely why the plan is to gradually bring the tank to the limit of brittleness and then "back off" to check for any permanent consequences and to align the stress-strain curves with predictions. This approach provides more cross-sections through the material's behavior, better illuminates transient phenomena, and helps decide where it is worthwhile to add a layer of laminate and where the material is unnecessarily massive.
What success means for European autonomy in space
Phoebus is not just "another" technological project. It is an indicator of the maturation of an industrial ecosystem that can quickly iterate, validate, and introduce advanced materials into flight systems. If composite cryotanks demonstrate full reliability, the way is opened for composite structures of a larger scope: inter-tank sections, rings, supports, and secondary walls. Synergy with other industries – aviation and hydrogen energy – further increases the return on social investment as knowledge spills over into the decarbonization of transport and energy storage.
Deadlines and the actual situation on October 17, 2025
As of today's date, the key early steps in the production of the large hydrogen tank in Augsburg have been completed, and the finalization of manufacturing is expected by the end of December 2025. In parallel, the design of the test infrastructure in Trauen is being finalized, with a critical design review planned for the end of the year – a step necessary for the construction and installation work to enter the final phase before the test campaign in April 2026. This work schedule is synchronized with the pace of sensor procurement, metrology calibration, and data acquisition software validation.
Technical details of the composite "skin": fiber orientations and transitions
Designers arrange the fiber orientations in layers so that the main sections take on the hoop and longitudinal (meridian) stress. The areas around the filling/draining ports and instrumentation are reinforced with local "patch" layers that prevent stress concentrations. The edges of the domes and the cylindrical shell are shaped so that forces are distributed smoothly, without abrupt changes in stiffness. Each transition is accompanied by a detailed fracture mechanics analysis, and NDT inspection (ultrasound, thermography) is performed to confirm homogeneity and the absence of delaminations.
Compatibility with oxygen: the other branch of Phoebus
Although this article focuses on the hydrogen tank, work on a liquid oxygen tank is also underway in the background. Oxygen, in addition to being cryogenic, is also chemically demanding: certain polymers and additives are not acceptable because they can react or degrade upon contact. Therefore, material tests are conducted under conditions that simulate closed volumes with liquid LOX and oxygen vapor, with mechanical loads corresponding to the real pressure profile. The success of the LOX tank is equally important for the complete composite architecture of the upper stage to become feasible.
How progress is measured: from "go/no-go" points to the TRL scale
The program defines clear "milestone" points: completion of the main pressure vessel production, completion of sensor integration, a cold "shakedown" without hydrogen, then full cryogenic cycles with hydrogen. Each point carries a "go/no-go" decision for the next phase. In parallel, the Technology Readiness Level (TRL) is monitored to outline how close the technology is to an operational environment. The goal is to reach a level where industrialization can begin – not just building one demonstrator, but a plan for repeatable production with quality control and repeatable performance.
The broader context: Europe, partnerships, and future spacecraft
Progress on composite tanks fits into a broader European shift towards lighter, more efficient systems. The new generation of propulsion – from reusable methane engines to electric post-injection stages – requires structures that do not "penalize" performance. Composite cryotanks for hydrogen and oxygen are also a good foundation for future configurations that consider the use of hydrogen outside of rocket systems, for example in aviation or land freight, where experience gained in the space industry helps solve issues of airtightness, insulation, and handling safety.
What follows after April 2026: the path to flight status
The results of the spring campaign will serve to finally "tighten" the design. If it is confirmed that the thresholds for the appearance of micro-cracks are predictable and under control, the focus shifts to qualification for flight use: vibration and acoustic load tests, impact resistance (e.g., ice fall), integration with valve-piping assemblies and monitoring systems. In this step, industrialization moves out of the laboratory: production tolerances are introduced, autoclaving procedures are standardized, and quality control includes statistical verification of each batch of prepregs and resins.
Why the story of Phoebus is important today
On October 17, 2025, Europe is in a phase of learning and validation that will strongly influence what launches will look like in the next decade. The answers that will come from Trauen and Augsburg – how durable the tank is, how stable, how it "breathes" in the dynamics of a launch – will draw the decision map for future generations of upper stages. This project is not just a technological curiosity, but also a strategic stake for the competitiveness and autonomy of European launch capabilities.
Related terms and contexts
- FLPP Programme – the framework from which the development of composite tanks and other upper stage technologies originated.
- Ariane 6 – the launch vehicle whose upper stage gains new capabilities thanks to lighter tanks.
- ArianeGroup – the industrial prime contractor for the testing and system integration of cryogenic tanks.
- MT Aerospace – the manufacturer of the composite pressure vessel and key tank elements in Augsburg.
What else to watch for by the end of 2025
By the end of the year, a critical design review for the test infrastructure in Trauen and the conclusion of the production of the first large LH2 tank are expected. Also in play are supplementary validations: secondary cold tests without hydrogen to verify deformation maps, calibration of strain gauges in a cryogenic regime, and checking the response of valves at low temperatures. These steps prepare the ground for the spring 2026 tests, when the tank will "meet" its real working medium – liquid hydrogen – and when the instrumentation will provide a complete picture of the structure's behavior at temperatures close to absolute zero.